GB2378575A - A photon source with higher order excitons - Google Patents

Abstract

A photon source comprising a quantum dot has a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier, and a supply means for intermittently supplying at least one carrier to the energy levels to create an exciton in the quantum dot such that as the exciton decays either a single photon or a plurality of single photons each having different distinct energy levels (fig 5) are emitted during a predetermined time interval. The source is able to produce a desired type of exciton to exploit higher order excitons such as biexcitons and triexcitons which have a shorter radiative decay time (fig 4) thus reducing jitter and allowing a higher clock rate. This is achieved by fixing the position of energy levels relative to the Fermi level (E<SB>f</SB>, fig 8), blocking or filtering photons arising from single exciton emission, and/or by separating photons having different energy levels (eg. fig 11). The source may also be used as an entangled photon source.

Description

A Photon Source and a Method of Operating a Photon Source The present

invention relates to a photon source which is capable of emitting single photons which are spaced apart by a predetermined time interval or pulses of n photons, where n is an integer which can be controlled to the accuracy of a single photon. More specifically, the present invention relates to such a source with enhanced control over the jitter time of the emitted photons. The photon source can also be configured as an entangled photon source.

There is a need for so-called 'isingle photon sources" for use in optical quantum cryptography where, for example, the security key for an encryption algorithm is formed by sending a stream of single photons which are regularly spaced in time. It is essential for the security of this technique that each bit is encoded on just a single photon. This is because an eavesdropper trying to intercept the communication will be forced to measure and thereby alter the state of some photons. Therefore, the sender and the intended recipient can determine if their communication has been intercepted.

Such a source is also useful as a low-noise source for optical imaging, spectroscopy, laser ranging and metrology. Normal light sources suffer from random fluctuations in the photon emission rate at low intensities due to shot noise. This noise limits the sensitivity of many optical techniques where single photons are detected. A single photon source which produces photons at regular time intervals has a reduced shot noise. Recently, advances have been made in making such single photon sources from semiconductor quantum dot structures. Michler et al in "A Quantum Dot Single-

Photon Turnstile Device" Science 290 p 2282 to 2284 (2000) and Santori et al "Triggered Single Photons from a Quantum Dot" Physics Review Letters 86 p 1502 to 1505 (2001) also describe single photon sources which operate by optically pumping a single quantum dot.

The above devices all concentrate on the production of a photon from the decay of a single neutral excitor (simple excitor) where prior to emission of the single photon there is a single electron in the ground state of the conduction band of the quantum dot and a single hole in the valence band of the quantum dot.

However, photons can also arise from the decay of more exotic excitors such as charged excitors or higher order excitors such as bi-excitons, triexcitons, quad excitors etc. Warburton et al, Nature 405 p926 to 929 describes photons arising from neutral and single, double, triple, quadruple and quintuple negatively charged single excitors in a quantum ring.

Findeis et al, Phys Rev B 63 p 121309=1 to 121 09-4 (2001) mid Finley et al, Pnys Rev B 63 073307-1 to 073307-4 (2001) report observations of charged excitors from quantum dots.

Finally, Hartmann et al, Phys Rev Lett 84 p 5648 to 5651 describes experiments where they observe the presence of bi-excitons and higher order excitors in quantum dots.

In every single photon source where a photon is emitted due to recombination of an electron and a hole, there is always some uncertainty in the actual time when the photon will be emitted. This is usually referred to as the "jitter". The applicants have surprisingly found that by using photons resulting from the decay of higher order excitors or charged excitors, the jitter of the photon source can be reduced.

Thus, in a first aspect, the present invention provides a photon source comprising a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; fixing means configured to fix the number of excess carriers in the energy levels by fixing the relative position of the energy levels with respect to the Fermi level of the source; and supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that

as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval.

For the avoidance of doubt and as used hereinafter, the term 'excitor' is used to describe any combination of electrons and holes in the dot, and the term 'simple excitor' is used to describe the state of just one electron and just one hole. Further, the term 'excess carriers' is used to refer to the number of the more populous carrier (which may be either electrons or holes) minus the number of the other type of carrier.

The present invention is capable of emitting a single or n photons at a predetermined time (where n is an integer). The present invention may be used to produce a stream of single or n photons at predetermined time intervals. For instance, it would produce a cyclic generation of single or e-photons spaced by constant time intervals.

The present invention is capable of being configured to produce a single photon or a group of n photons at a predetermined time. It may also be configured to repetitively produce single photons or groups of n photons at a predetermined time or times. The time between successive photon emissions can be controlled, successive photon emissions (be it single photons or n photons) will preferably be regularly spaced in time.

Although, the source may be configured to irregularly space successive photon . emissions. The supply means may comprise a trigger means in order to trigger the supply of carriers at the required time or for the required time interval. The supply means will generally be a pulsed supply means delivering a supply of carriers per pulse such that an excitor is created in the quantum dot per pulse.

Charged excitors are produced when there is an excess of electrons or holes in the quantum dot prior to recombination. The ability to vary the conduction and valence band edges with respect to the Fermi level allows the actual number of excess electrons or holes in the quantum dot to be fixed. Thus, the source can be configured to produce photons from neutral, singly charged, doubly charged etc excitors as required. Without

a means to fix the excess carrier density, it is likely that a photon source may produce a random mixture of charged and neutral excitors.

Also, an advantage in fixing the number of excess carriers in the dot is that it increases the emission efficiency of the source by concentrating more of the emission into a particular excitor line. As each of the different types of charged excitors has a different emission energy, by choosing a different excitor emission, the emission wavelength of the source is altered.

The fixing means may be configured to fix a finite number of excess carriers in the quantum dot or even to ensure that there are usually no excess carriers in the dot. When there are no excess carriers in the quantum dot, the probability of producing neutral excitors substantially increases. In this cases the Fermi Energy lies fin the energy gap between the valence and conduction bands of the dot. Excess charges may be removed using a electric field provided across the quantum dot.

If the fixing means is configured to ensure that there is a single electron in the conduction band of the quantum dot, then the probability of forming excitors having a single negative charge is greatly increased. Such excitors may be single excitors, bi-

excitons, tri-excitons etc. Similarly, the fixing means may be configured to fix the number of excess carriers in the valence band of the quantum dot so that the valence band of the quantum dot comprises one hole prior to introduction of carriers by the

supply means. This will enhance the probability of creating excitors having a single positive excess charge. By increasing the number of excess carriers further than one, then the probability of creating excitors having a double, triple charges etc is enhanced.

The fixing means preferably comprises at least one electrode configured to apply an electric field across the quantum dot. This electrode may be provided in the form of a

Schottky gate. At least one layer of the photon source may comprise a doped layer so that the potential applied to the Schottky gate can be applied with respect to the dot layer of the photon source. Alternatively, a potential can be applied between highly doped layers on either side of the dot. Also, it is possible to provide ohmic contacts to the device to provide a DC bias across the source in order to vary the energies of the

conduction and valence bands with respect to the Fermi level. The dot may even be placed between two electrodes of opposing polarities i.e. a ptype layer and an e-type layer. The supply means may preferably be configured to enhance the creation of bi-excitons and higher order excitors. If the supply means uses irradiation in order to excite electron hole pairs to create excitors in the quantum dot, then the power of the laser may be increased to intermittently excite more than one electron hole pair. Typically this can be achieved if the laser intermittently supplies at least two photons to the dot during any predetermined time interval.

When higher order excitors are excited, at least two photons will be emitted for each pulse supplied from the supply means. These photons will have different energies.

Therefore, preferably, the photon source comprises a blocking means to block photons arising from emissions which are undesirable or not of interest. The blocking means allows the transmission of the desired photons to a receiver or to intermediate processing equipment which is placed between the photon source and a receiver. For example, the intermediate equipment may introduce changes in the polarisation or phase of the photons. The photons which are blocked may be annihilated or may be directed away from the desirable photons.

The blocking means may be provided at or adjacent to the quantum dot. The blocking means may also be provided remote from the quantum dot. For example, the blocking means may be provided at a detector or receiver for the photons leaving the dot.

For example, the photon source may form part of a photon transmission apparatus which also comprises a receiver, where the blocking means is located at the receiver.

The blocking means may be a filter configured to block photons of a certain undesirable energy or energies. The filter may be tuneable in wavelength.

The filter may be an interference filter comprising alternating dielectric layers, or a prism or a grating spectrometer, or a fibre optic filter. The filter may be integrated

within the photon source itself or it may be provided on the photon source. For example, the filter may be provided by a resonant cavity in which the quantum dot is disposed. The dimensions of the cavity may be chosen such that emission at the desired photon emission energy is enhanced.

As an alternative to wavelength filtering, it is also possible to distinguish between the emission from some excitors using time. A single excitor will be emitted after a bi-

exciton or higher order excitor, thus either the single excitor or higher order excitor emission can be blocked by blocking the output of the source at certain times. For example, if bi-exciton emissions are of interest, then a blocking means may be provided which is configured to block emission from the source except at the times when there is a high probability of a bi-exciton emission. The blocking means may comprise means to time-gate a receiver.

The emission time can also be used to distinguish between charged and neutral excitors as they will have different emission times.

When a bi-exciton is formed, a first photon is emitted due to the decay of the bi-exciton.

A second photon is then emitted when the single excitor decays. The blocking means may be configured to block such lower order emissions such as the single excitor decay. The blocking means may also be configured to block the single neutral excitor . emission. Also, as charged excitors may be advantageous due to their quicker decay time than that of the neutral excitors, the blocking means may be configured to block out photons at any neutral excitor frequency.

Although, the present invention can be used to essentially choose between the formation of charged excitors or neutral excitors, it should be noted that even when charged

excitors are chosen, there is a chance that some neutral excitors (or be it a small number) may be produced. For a single photon source, all excitors of the same order may be collected together e.g. single excitors, i.e. charged and neutral may be collected together or biexcitons both charged and neutral may be collected together as the dot cannot produce a neutral excitor and a charged excitor of the same order. The blocking means may be provided to transmit excitors of a predetermined order regardless of whether they are charged or not.

Although it is highly desirable to have a device where you can choose between what order of excitor e.g. single excitor, bi-exciton, triexciton etc and the charge state of that excitor, a photon source which allows the higher excitor states to be utilised, still provides considerable advantages over the previous devices where only the single neutral excitor decay has been utilised.

Therefore, in a second aspect, the present invention provides a photon source comprising a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; and blocking means configured to block photons at the neutral single excitor frequency.

The blocking means may also be located in a receiver, thus, in a third aspect, the present invention provides a photon transmission apparatus comprising: a photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier, and supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; a receiver

for receiving photons emitted by the source; and blocking means configured to block photons arising from the neutral single excitor emission.

The blocking means can be of the type previously described. For example, the supply means may preferably excite a bi-exciton.

As previously mentioned, when higher order excitors are generated, there will be at least two photons emitted. This provides a basis for providing an entangled photon source. In the situation where a bi-exciton is excited, a first photon is emitted due to the decay of the bi-exciton to a single excitor and then a second photon is emitted due to the decay of the single excitor itself. The first and second photons will have either polarisation or phase entangled states. In other words, a measurement of a quantum parameter of one of the photons wit! directly affect a rneas', er.ent Blade on the other photon of the entangled photon pair.

These entangled photon pairs have been found to be particularly useful in the field of

quantum cryptography. Here, a single source is used to send one photon of the entangled photon pair to a receiver "Alice" and the other state to receiver "Bob". If a key is transmitted to both Alice and Bob, Alice knows what Bob should have measured since Alice's measurements actually fix the measurements which Bob should make.

However, if an eavesdropper "Eve" intercepts the photon on the way to Alice, then Alice's measurements are suddenly completely independent of Bob's. By comparing a small part of the key, it will be clear to either Alice or Bob that an eavesdropper has intercepted at least part of the message.

Therefore, in a fourth aspect, the present invention provides a photon source comprising a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; supply means for intermittently supplying carriers to the energy levels to create a bi-exciton or higher order excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays a plurality of single photons each having different distinct

energies are emitted during a predetermined time interval; and means to separate the photons having different distinct energies.

The means to separate the photons having different distinct energies allows one photon to be sent to Alice and one photon to be sent to Bob etc. This is not only limited to bi-

excitons, the principle can be extrapolated to photons arising from the decay of triexcitons or higher order excitors where again, means can be provided to separate at least two of the photons having different distinct energies and use the correlation of their states in order to allow transmission of a quantum key.

The means to separate the photons may comprise means to separate the photons by using time or energy.

The supply means may be radiation which is configured to excite one or a predetermined number of electrons and holes within the first and second energy levels respectively. The fixing means of the type described in relation to the first aspect of the invention.

As previously mentioned, there may be many conduction band levels into which electrons can be excited, similarly, there may be many valence band levels. If an electron is supplied to an excited energy level of the conduction band then the electron will almost certainly relax into a lower energy state or the ground state of the conduction band prior to recombining with a hole. The time which a carrier spends in a energy state before relaxing to a lower energy state is known as the 'relaxation time'.

As used hereinafter, the relaxation time will be taken to mean the average time which a carrier remains in an energy state before releasing to another energy level. As used hereinafter, the recombination time will be taken to mean the average time it takes for an electron and hole to recombine. Usually carriers will undergo relaxation more quickly than recombination. This is especially true when the carriers are created in the excited levels of the quantum dot, as in this case, the carriers usually relax to the ground states of the quantum dot before recombination. However, in some situations, relaxation may occur by recombination.

An electron from the ground state may recombine with a hole to emit a photon.

Alternatively, an electron from an excited state may recombine with a hole to emit a photon. It should be noted that although it is possible that an electron from a higher energy state to combine with a hole, the probability of this transition occurring is very small, as it is far more likely that the electron will first relax into the ground state prior to recombination.

When forming charged excitors, the conduction or valence bands may already be populated with electrons or holes respectively. In this situation, it is necessary to excite the photo-excited carriers with enough energy to populate the excited energy levels as the lower energy levels may be occupied by excess carriers. It should be noted that the photo-excited Carriers chin only relay if there is a free!o..,er energy state available in the same band.

To excite the carriers into the conduction and valence bands for recombination, the source is illuminated with radiation which is configured to excite one or more electron-

hole pairs in the ground or excited states of the quantum dot, or in the surrounding barrier material.

This is achieved by irradiating the quantum dot with radiation having an energy equal to or higher than that of the appropriate optical transition energy of the quantum dot or its surrounding material. The required transition energy may be capable of exciting an electron into the ground state of the conduction band and a hole into the ground state of the valence band of the quantum dot. Alternatively, it may be required to supply the electron and/or hole into one of the excited energy levels of the quantum dot. In the quantum dot, the confined conduction band (valence band) levels can only accommodate a maximum of two carriers. One of these carriers is of a spin up and the other electron is of a spin down configuration.

The radiation may also excite one or more electron-hole pairs in the surrounding material, after which they will quickly relax into the quantum dot before recombination.

It is possible to configure the structure so that only one electron is excited into a level of the quantum dot by each pulse of the radiation. This can be done by either polarising the incident light so that it circularly polarised with a single orientation direction (e.g. either left or right circular polarization). Or, a magnetic field can be applied to the device to

lift the spin degeneracy of the conduction band level. Only one electron can then be accommodated in a single conduction band level.

Also, the absorption of the first electron hole pair by the dot results in a shift in the transition energy of the dot. The spectral line width of the laser can be tuned to be smaller than this shift in energy. Thus, it is possible to excite one single electron hole pair at a time.

For the purpose of explaining the operation of the device, it will be assumed that there is only one electron in the conduction band of the quantum dot. A hole will also be excited into the valence band layer when the electron is supplied to the conduction band layer. The electron and the hole will recombine to emit a photon during a characteristic time called the recombination time. If the incident radiation has an energy equal to that of the lowest energy transition energy of the quantum dot and the radiation is pulsed with a duration much shorter than the recombination time, then by the time the electron and hole have recombined, no radiation is present to excite a second electron and hole.

Therefore, only a single photon is emitted.

Also, the time between the leading edges of the incident pulses should be longer than the recombination time. This is to prevent a second pulse arriving at the quantum dot before a photon from the desired excitonic emission has been emitted.

If the radiation photon energy is tuned to the energy of an excited (i.e. not lowest energy) transition of the quantum dot, the photoexcited electron and hole will relax to their ground energy levels in the dot before recombination. If the duration of the pulse is much shorter than the relaxation time, then only a single election and hole can be excited per pulse of the exciting radiation. Therefore, only a single photon is emitted.

However, if the pulse duration is longer than the relaxation time, a second electron hole pair might be excited after relaxation of the first electron hole pair forming a bi-exciton.

However, the bi-exciton decay will emit a photon having a different energy to that of the single excitor decay. Thus, the two photons may be distinguished as they have different emission energies and times.

If the conduction band contains two electrons which recombine with two holes, the two photons emitted will have different polarizations, different photon energies and be emitted sequentially. Therefore, a filter selecting a particular photon energy, polarisation, or emission time will allow a regular stream of single photons to be produced. The detection of the single photons may also be gated in time so as to detect just one of the emitted photons in average.

Also, if the degeneracy of the levels is lifted by a magnetic field, the emitted photons

will have different energies depending on the specific transition. Hence, it is possible to filter out photons emitted at the other energies to obtain a stream of single photons.

In order to excite a particular transition in the dot, i.e. in order to populate a certain conduction band level (i.e. the ground state or an excited level), the dot is irradiated with radiation having an energy substantially equal to that of the required energy level.

Often it is convenient to prepare a device with more than one quantum dot. Often these quantum dots possess different transition energies due to fluctuations in their size or composition. In this case, the emission from a single quantum dot may be isolated by filtering the wavelengths of the emitted light. By allowing only the light in a narrow bandpass to pass it is possible to collect the emission of a single dot and exclude that of the others.

Preferably, the area of the source from which light is collected contains at most one thousand optically active quantum dots.

Alternatively, it is possible to selectively excite a single quantum dot by using a laser with a narrow wavelength spectrum. The laser will excite only the quantum dot with the appropriate transition energy.

It is also possible to produce an e-photon source if a dot is produced which has transition energies which lie close enough to one another such that it is possible to excite more than one transition at a time. Another possible way to produce an e-photon source is to use several excitation wavelengths in the incident pulse.

In these devices with more than one quantum dot, it is possible to change the quantum dot from which light is collected by changing the wavelength bandpass energy or the laser energy.

The selected population of an electron or hole level can also be achieved using continuous radiation (i.e. non-pulsed). If the photon source is irradiated with radiation corresponding to a particular transition energy within the quantum dot, the population of the quantum dot levels can be controlled by periodically varying the transition energy of the quantum dot. This can be done in many ways, for example, the electric field across

the dot may be varied by an applied AC voltage. Also, the carrier density of the dot or surrounding layers, the magnetic field applied to the quantum dot and even the

temperature of the quantum dot can all be modulated to vary the transition energy of the quantum dot.

Where the photon source is modulated in order to inject carriers or vary the transition energy of the dot, the fixing means can be realised by modulating the potential so that excess carriers or no excess carriers are provided in the dot when the transition energy of the dot is equal to or less than the laser energy.

The transition energy of the quantum dot can be modulated so that the confined energy level is only capable of being populated by carriers for a certain time. This should be less than the recombination time of the photoexcited electron-hole pair. Therefore, although the radiation intensity is constant, light can only be absorbed by the quantum

dot for the short time that the transition energy is equal to or less than that of the irradiating radiation. The electron and hole can then recombine to emit a photon in the same way as described with reference to excitation by the pulsed laser. Sometime later, the transition energy of the quantum dot will be swept through the laser energy again and the dotis able to absorb an electron and hole again. Again, the degeneracy of the level may be lifted by application of a magnetic field or, a single electron may be

introduced into the level by polarization of the incident radiation. As before, the emitted radiation can be filtered to remove emitted light from the specific polarization or to remove photons which arise due to recombination within other quantum dots.

Previously, the discussion has concentrated on illuminating the device in order to supply carriers for the conduction valence band. However, the present invention may also operate by populating either of the conduction or Thence blinds by injection of c,;ers into the conduction or valence band level. In such a structure, in order to obtain fine control, it is preferable if either the conduction band levels or the valence levels are continually populated with excess carriers. The remainder of the discussion will concern a device where the valence band levels are populated with excess holes and electrons are injected into the conduction band. However, it will be appreciated that the inverse device can be fabricated where the conduction band levels are populated with excess electrons, and holes are injected into the valence band. The fixing means may be provided to fix the excess carrier concentration.

1 he holes are preferably provided to the valence band of the quantum dot via a doped barrier. Such a doped barrier will preferably be a remotely doped or modulation doped barrier which is separated from the quantum dot by a spacer layer.

Preferably the quantum dots are placed within a two dimensional excess carrier gas so as to provide the excess carriers.

Preferably at least one ohmic contact is made to the two dimensional carrier gas so as to repopulate the carrier gas after recombination of an excess carrier with an injected carrier.

Electrons are injected into a conduction band level of the dot. As with the previously described optically excited sources, the injected electron and a hole recombine and emit a photon. To avoid more than one photon emission, the electrons are preferably injected one at a time. A particularly preferable way of injecting the electrons is to use resonant tunnelling through a barrier layer. Here, the energy of the injected electrons is matched to the energy of a conduction band level in the quantum dot. To achieve selected injection of electrons, the energy of the electrons to be injected is periodically varied so that the device is switched between an ON state (where the energy of the injected electrons aligns with that of the conduction band level within the quantum dot) and an OFF state (where the injected electrons do not align with the conduction bands within the quantum dot). In the OFF state, electrons cannot tunnel into the quantum dot.

The present invention may comprise a plurality of quantum dots. In such a device, it is virtually impossible to make a plurality of quantum dots which will have identical transition energies. Therefore, it is possible to select emission from a single quantum dot by filtering the wavelengths of the collected light. There is also a variation in the transition energy from dot to dot which may be due to fluctuations in the size or composition of the dots for instance. Thus, it is possible to selectively inject carriers into just one of the quantum dots.

In the above described device, this can be achieved by precise control of the voltage in the "ON" state. Alternatively, it may be possible to control the energy of the illuminating radiation to excite a transition in a single dot.

Preferably, the area of the source from which light is collected should contain, at most 1000 optically active quantum dots.

Once the photon is emitted from the quantum dot, it can be collected by an optical fibre.

Preferably, the device is provided with a coupling means to allow the photons to be efficiently collected by a fibre optic cable. Such coupling means may comprise antireflection coating located on the surface of the device through which the emitted photons are collected. Also, the antireflection coating could be located on the optical fibre itself.

The coupling means may also comprise a lens to collect emitted photons.

A particularly preferable arrangement of the device is achieved if the source has a mirror cavity which has two mirrors located on opposing sides of the quantum dot.

Preferably, one of the mirrors (ideally the mirror closest the output surface) is partially reflective such that it can transmit the emitted photons. More preferably, the energy of the cavity mode for said mirror cavity is preferably substantially equal to that of the emitted photons. Further, it is preferable if the distance between the two mirrors Lcav Of the cavity is defined by m] Lcav = 2nc where m is an integer, ncav is the average refractive index of the cavity and the emission wavelength (in vacuum).

The advantage of using a cavity is that it allows more of the emitted light to be coupled into the numerical aperture of the collecting fibre or optic. The cavity mode of the resonant cavity is emitted into a narrow range of angles around the normal to the mirrors. The fibre/collection optic is arranged to collect the cavity mode.

The resonant wavelength of the cavity can be chosen to be that of the wavelength of the desired transition. Thus, the cavity may also act as a filter means.

At least one of the mirrors may be Bragg mirror comprising a plurality of alternating layers where each layer satisfies the relation: na ta = rib tb \/4 Where one dielectric layer (A) has a refractive index of na and a thickness of ta and second dielectric layer (B) has a refractive index of rib and a thickness of tb.

At least one of the mirrors may also comprise a metal layer. A phase matching layer should also be located between the cavity and the metal layer, so that an antipode is

produced in the cavity mode at the interface between the cavity and phase matching layer. At least one of the mirrors may even be a semiconductor/air or semiconductor/dielectric interface.

A three dimensional cavity may also be provided by forming a photonic band gap structure within the plane of the layers of the two dimensional cavity.

In the device with multiple quantum dots, the cavity is preferably designed so that only one of the quantum dots has an emission energy which couples to the cavity mode. This can be used to ensure that emission from only one dot is collected producing just single photons. The energy of the cavity mode is controlled largely by the optical thickness of the cavity layer.

In this case, the energy width or band-pass of the cavity mode should be approximately equal to the line width of the emitting quantum dot. This can be achieved by configuring the design of the cavity as required - in particular the reflectivities of the cavity mirrors.

The width of the cavity mode decreases with increasing mirror reflectivity. The mirror reflectivities can be increased by increasing the number of periods in a Bragg mirror.

The device has been described with one optical fibre. However, it will be appreciated that the device can be fabricated with more than one quantum dot emitting into more than one fibre.

A particularly preferable method for fabricating the quantum dot(s) of the present invention is by use of a self-assembling growth technique (such as the Stranskii Krastinow growth mode).

Typically, the first material will be InAs, InGaAs or InAlAs and can be grown to a thickness of preferably less than 50 non. The second material will preferably be GaAs or AlGaAs.

Preferably, a layer of a third material overlying the first material. The third material may be the same as the second material.

The areal density of the quantum dots is preferably less than 3x1 07cm 2.

Methods of operating the photon sources according to the first, second and fourth aspects of the invention are provided as fifth, sixth and seventh aspects of the invention.

The present invention will now be described with reference to the following preferred, non-limiting embodiments in which Figure 1 shows a schematic band structure of a single quantum dot; Figures 2a to 2m schematically illustrate some of the types of excitors which may be formed; Figure 3 is a photo luminescence spectrum where luminescence intensity is plotted against photon energy illustrating the spectra from a single neutral excitor (X), a neutral bi-exciton (X2), a charged biexciton (X2) and a charged single excitor (X); Figure 4 illustrates time resolved photo luminescence traces for the excitor transitions of Figure 3; Figures 5a to 5d illustrates a plot of photo luminescence intensity against photon energy for a laser power of 3.2 nW (Figure Sa), 0.1 IlW (Figure 5b), 0.25 LOW (Figure 5c) and 1 uW (Figure Sd); Figure 6 is a plot of logarithmic photo luminescence intensity against logarithmic laser power for the single neutral excitor emission, the neutral bi- exciton emission, the charged single excitor emission and the charged bi- exciton emission;

Figures 7 a to f show results from correlation measurements illustrating suppression of two photon emission from a device in accordance with a preferred embodiment of the present invention; Figures 8a to c illustrate a schematic band structure of a photon source where the excess carrier concentration in the quantum dot can be controlled using a gate bias of + 0.3 volts (Figure 8a), 0 volts (Figure 8b) and -0.3 volts (Figure Sc); Figure 9 shows an embodiment of a device in accordance with the present invention where the supply means comprises a pulsed laser diode and where a filter is provided; Figure 10 shows a source in accordance with a further embodiment of the present invention where the supply means comprises a pulsed laser diode and where means are provided for fixing the excess carrier concentration in the dot; Figure 11 shows a source in accordance with an embodiment of the present invention configured as an entangled photon source; Figure 12 illustrates a further embodiment of the present invention where the supply means comprises means to vary the transition energy of the quantum dot and a continuous wave laser; Figures 1 3a and 1 3b are schematic plots illustrating the variation in the quantum dot transition wavelength with applied AC bias in the embodiment of Figure 12; Figure 14 shows a band structure of an electrically injected quantum dot single photon emitter in accordance with an embodiment of the present invention in an off condition; Figure 15 shows a band structure of the device of Figure 1 4 when in an on condition; Figure 16 shows a schematic layer structure of a device in accordance with an embodiment of the present invention;

Figure 17 illustrates a single photon emitter in accordance with a preferred embodiment of the present invention; Figure 18 illustrates a single photon emitter in accordance with another preferred embodiment of the present invention; Figure 19 illustrates a photon source in accordance with a further embodiment of the present invention where light emitted from the photon source is confined using a photonic band gap structure; Figure 20 illustrates a single photon emitter according to an embodiment of the present invention coupled to an optical fibre; Figure 21 illustrates a single photon emitter having a plurality of quantum dots in accordance with an embodiment of the present invention; Figure 22 illustrates a schematic absorption spectra of the embodiment of Figures 21, Figure 23 illustrates a wavelength filter for filtering the wavelength of the embodiment of Figure 21; and Figure 24 illustrates an absorption spectra for a single photon emitter having a plurality of quantum dots with wavelength filtration.

Figure 1 shows a schematic band structure of a single quantum dot 1. The quantum dot forms a minimum in the conduction band 3 and a maximum in the valence band 5. A plurality of quantised conduction band levels 7 are formed in the minimum and a plurality of valence band levels l l are formed in valence band maximum. Due to the Pauli exclusion principle each of these levels 7, 11 can accommodate two carriers corresponding to the two possible spin states.

Recombination of a single electron 15 and a single hole 17 within the dot results in the emission of a single photon 13. This recombination occurs over a characteristic time called the recombination time.

It is impossible using attenuation of a laser beam alone to obtain a stream of single photons regularly spaced in time or at predetermined times. However, in the quantum dot shown in Figure 1, a photon is emitted only when an electron and hole recombine.

Therefore, providing that a predetermined number of electrons and holes can be supplied to the dot 1 at regular time intervals and that the recombination time is shorter than the time between successive supply events, a stream of single photons can be produced. Alternatively (or additionally), a single photon can be collected after each supply event by filtering the emission in photon energy or time to collect the emission from only one excitor in the dot.

If the electron and hole in the quantum dot are excited optically, then the level occupied by the electron and hole initially, is dependent on the wavelength (or energy) of the incident light. The electron and/or hole may be excited into the ground state levels from which they will recombine or they may be excited into excited levels. From such excited levels, the electron and hole will most probably relax into a lower energy level before recombination. The time which a carrier takes to transfer from an excited level to a lower level is known as the "relaxation time". Generally, it is more statistically favourable if the carrier relaxes from an excited state to the ground state conduction band before recombination.

The electrons and holes may also be excited in the surrounding material and then relax into the quantum dot. This relaxation process typically occurs much more rapidly than recombination of the electrons and holes.

Figures 2a to m illustrate some possible excitors which are formed when electrons and holes are excited into the conduction band 3 and valence band 5 of the dot. The relative separations between the energy levels are not to scale.

Figure 2a shows the simple single excitor. Here, a single electron 1 S is located in the lowest conduction band level l 9 and a single hole 17 is excited into the lowest valence band level 21. This represents the simplest case. If the dot is weakly illuminated with radiation which has an energy larger than that of the bandgap, then a single excitor is formed. It may be possible in some devices to add an excess electron 23 to the conduction band level 19 of the quantum dot 1 prior to illuminating the device. In this situation, when a photon is absorbed, an electron 15 is excited in conduction band level 19 and a hole 17 is excited in valence band level 21, and a negatively charged single excitor is formed in the dot. In the negatively charged single excitor, there are two electrons in the conduction baud alla one hole in the valence band. The To electrons are identical and cannot be distinguished, but we give them different labels for convenience.

In Figure 2b, a single electron 23 is added to the conduction bands prior to illumination to excite an electron hole pair. In Figure 2c, two electrons 23 and 25 are added to the conduction band 19 prior to illumination of the device to excite an electron 15 hole 17 pair. The maximum number of electrons which can be accommodated in any confined energy level of the quantum dot is 2. Thus, the photo-excited electron 15 occupies the second conduction band energy level in the quantum dot 25. This arrangement where there are three electrons in the conduction band and one hole in the valence band fortes what is known as a double negatively charged single excitor (X2-). Figure 2d illustrates a further extension of this principle where the conduction band contains three electrons 23, 25 and 27 where electrons 23 and 25 are contained in the first conduction band level 19 and one in the second conduction band level 26. Upon illumination, an electron hole pair is excited so that the hole 17 populates valence band level 21 and the electron 15 populates second confined energy level 26. This forms the so-called triple negatively charged single excitor (X3-).

Figure 2e shows a further variation. Here, neither the conduction band 3 nor the valence band 5 are populated prior to illumination. Here, upon illumination, two electrons 31 and 33 are excited into lowest conduction band level 19 and two holes 35

and 37 are excited into lowest valence band level 21. This arrangement forms the neutral biexciton (X2).

As explained with reference to the single excitor of 2b, the conduction band may be populated with electron 23 prior to irradiation. When two electrons 33 and 31 and two holes 35 and 37 are excited by the incident illumination. In Figure 2f, the first excited electron occupies first confined energy level 19. As this energy level is now full due to the presence of excess carrier 23, the second excited electron occupies second confined conduction band level 26. As there are no excess holes, photo-excited holes 35 and 37 occupy the ground valence band energy level 21. This arrangement fonns the negatively charged biexciton (X-2).

Figure 2g shows schematically the doubly charged biased excitor (X2 2) where there are four electrons, the two photo-excited electrons 31 and 33 located in upper conduction band level 26 and the two excess electrons 23 and 25 located in the ground conduction band level 19. Two holes 35 and 37 are photo-excited into the valence band level 21.

Similarly, the triply charged biexciton may be formed. In this case, there will be five electrons and two holes. As each conduction band level can only accommodate two electrons, then the triply charged biexciton would need to have three confined levels formed within the quantum dot conduction band.

In addition to the biexciton illustrated schematically in Figure 2e, it is also possible to excite three electrons and three holes as shown in Figure 2h to form the neutral triexciton (X3). Here, two of the three photo-excited electrons 39 and 41 are located in the lowest conduction band level 19 whereas the third photo-excited electron is located in the upper conduction band level 26. Similarly, because the confined energy levels of the valence band can only accommodate two holes, two of the photo-excited holes 35 and 37 are located in the first confined valence band energy level 21 whereas the third photo-excited hole is located in the second confined valence band energy level 47. It should be noted that electrons and holes are referred to as first excited, second excited, this does not mean to suggest the order in which they are excited, it is just used as a nomenclature to distinguish between the electrons and holes populating the different levels within the dots.

Similarly, as explained in relation to the single and bioxcitons, the conduction band can be doped with one, twos three etc more electrons in order to form singularly, doubly, triply etc charged triexcitons.

Previously, the explanation has concentrated on the idea of adding excess electrons to the quantum dot in order to form charged excitors. However, it is also possible to positively charge excitors by adding excess holes to the valence band. In Figure 2i, an excess hole 51 is added to valence band level 21. The photo-excited electron hole pair 15 and 17 are excited so that the electron 15 populates the lowest conduction band level l 9 and the hole 17 populates the valence band level 21. Due to the excess hole 51 in the valence band level, the dot contains a positively charged single excitor (aft). Figure 2j shows a qll nhlm dot cony a double positively charged single excitor t^2+). Here, the quantum dot 1 is doped with two excess holes 51 and 53 in the first confined energy level of the valence band 21. An electron hole pair is then excited so that an electron 15 occupies the first energy level 19 of the conduction band and a hole 17 occupies the second energy level 55 of the valence band. Thus, the dot contains three positively charged holes and one negatively charged electron.

Figure 2k shows a triple positively charged single excitor (X3+). A single electron 15 is located in the first energy level 19 of the conduction band and four holes 17, 51, 53 and 57 located with two holes 51 and 53 in first valence band level 21 and hvo holes 17 and 57 in second valence band level 55.

In addition to the singularly charged excitors, it is also possible to produce positively charged bi-excitons as shown in Figures 21 and 2m. In Figure 21, there are two electrons 31 and 33 located in the first conduction band level 19. There are also three holes 61, 63 and 65 located in the first 21 and second 55 levels of the valence band.

In order to achieve this structure, the valence band is populated with one excess hole prior to excitor of two electron hole pairs to forth a positively charged bi-exciton (x2+).

Figure 2m shows the doubly positively charged bi-exciton (X2 2+) where there are two electrons 31 and 33 in the conduction band 19 and there are four electrons in the valence bands 21 and 55. The charged bi-exciton is formed by introducing two excess holes into conduction band level 21 prior to illumination of two electron hole pairs. The multiple electron and hole excitors shown in Figures 2b to 2m decay in a similar manner to that of a simple excitor shown in Figure 2a. In Figure 2a, the electron 15 and the hole 17 eventually recombine to emit a photon having the single excitor wavelength. Figure 3 shows the emission spectrum of a quantum dot. Peak 101 arises from decay of the single excitor shown in Figure 2a. Figure 2b previously described shows the negatively charged single excitor and Figure 2I shows the positively charged single excitor. Both of the singularly charge excitors decay by a single electron 15 combining with a single hole 17 to emit a photon. Due to the presence of the excess electron 23 (as shown in Figure 2b) or the excess hole (51) as shown in Figure 2i, the transition energy of this dot is modified and the energy of a photon derived from a charged single excitor is higher than that from the single neutral excitor. In Figure 3, the charged excitor peak is shown as 103.

Figure 2e illustrates a dot containing a bi-exciton. Peak 105 arises from the recombination of a single electron with a single hole in the biexciton state (figure 2e) to leave a single electron in the conduction band and a single hole in the valence band level 21. It is seen at a slightly higher energy than that of the simple single excitor decay 101. Thus, the bi-exciton radiatively decays to leave behind a single excitor.

When the remaining electron and hole combine a further photon is emitted at the energy of peak 101.

Peak 107 is due to the decay of a singularly charged bi-exciton, for example, the negatively charged bi-exciton of Figure 2f or the positively charged bi-exciton of Figure 21. When a charged bi-exciton decays, a single electron from the conduction band level

19 combines with a single hole from the valence band level 21. The decay leaves behind a charged single excitor which will decay at the energy of peak 103.

The photoluminescence was excited by a laser energy of 1.55 eV and power of 0.4 LOW focussed to a spot of 1 Am in diameter. The sample was maintained at a temperature of 5K. Figure 4 shows time resolved luminescence spectra recorded for the various excitonic transitions after excitation by a lps laser pulse and at an energy of 1.55eV and a laser power of 3.2 FEW. Trace 111 corresponds to the single simple excitor decay. It displays an exponential decay with a time constant of 1.36 ns. The time resolved photo luminescence due to the singularly charged single excitor is shown as line 1 13, for which the decay irne is 1.07 us. Trace i iS corresponds to the time resolved photo luminescence of the neural bi-exciton. The decay time here is 0.59 ns. The photo luminescence spectra for the singularly charged bi-exciton is shown as trace 1 17, with a decay time of 0.52 ns.

The decay time for both of the bi-excitons is seen to be much shorter than that of the single excitors 1 1 1 and 113. This has serious physical ramifications for a single photon emitter operating at the biexciton photon energy. In such single photon devices, there is always a jitter which occurs due to the uncertainty in the time after excitation that the photon is emitted. However, a shorter radiative decay time, as observed from the biexciton, reduces this jitter. This is advantageous because it means that the single photon detector can be gated on for a shorter time and will hence be less susceptible to noise. Another advantage of the faster recombination time is that the single photon emitter can be triggered at a higher clock rate.

Figures Sa to Sd show four photo luminescence spectra recorded at different laser powers. To avoid unnecessary repetition, the same reference numerals are used to identify the peaks as those in Figure 3. The single simple excitor peak 111 can be seen clearly in each of the diagrams. The neutral biexciton peak is only really identified in Figure 5b where the energy is raised to 0.1 qW. In Figure Sd, (where the laser power is

1 1W) the bi-exciton peak is seen to be nearly the same strength as the single neutral excitor peak. Furthermore, the charged bi-exciton peak 117 is seen to strengthen also.

Figure 6 shows a plot of photo luminescence intensity versus incident power. The intensity of the simple excitor is shown as solid squares, the neutral biexciton as hollow squares, the singularly charged single excitor as solid circles, and the singularly charged biexcitons are shown as open or solid triangles.

As the laser power is increased, the intensity of the single neutral excitors (full squares) increases linearly with the incident power, the dotted line indicates a linear dependence of the intensity on the power. On the other hand, the biexciton peak shows a quadratic (dashed line) dependence on laser power.

It can be seen that these two lines converge, indicating that at higher laser pulse powers, the intensity of the single excitor peak is substantially equal to that of the bi-exciton peak. Eventually the signal from both the single excitors and the bi-excitons saturate.

In this particular example, this is seen to occur around 0.4 LOW. In this regime, it is believed that essentially the laser is operating at a power where there are at approximately 2 photons per laser pulse, absorbed in the region close to the quantum dot, so that the photo excited carriers are captured by the dot.

Therefore, from Figures 5 and 6, it can be seen that using higher incident laser powers causes excitation of the higher order excitonic states such as the biexciton. From Figure 4, it can be seen that the higher excitonic states have more potential for producing commercially useffil quantum devices than the single neutral excitor which has been previously disclosed.

The results shown in Figures 5 and 6 illustrate how higher laser power can be used in order to favour biexcitons and higher order excitors. Also, singularly and doubly charged single excitors can also be observed using a higher power laser.

Figure 7 is a plot of the second order correlation function of photons emitted from a quantum dot.

The experimental system that was used was as follows. The emission from a quantum dot was spectrally filtered using a grating spectrometer to pass only the emission line of interest, for example the neutral bi-exciton, and block all other lines, for example the charged excitors, and the neutral simple excitor.

The filtered emission was then used in a Hanbury-Brown and Twiss experiment, whereby the stream of photons is split into two separate paths by a 50/50 beam splitter, and detected by two single photon detectors. Onedetector is used to start a timer, and the second detector is used to stop a timer. This experiment allows the time between the detection of two photons, T. to be measured. The second order correlation function is determined from the frequency of start-stop events as a function of T. Figure 7 shows the second order correlation function of photons emitted by (a) the neutral excitor, (b) the neutral bi-exciton and (c) the charged excitor, under CW laser excitation, (lines showing dip) and pulsed laser excitation (bars showing peaks). The laser energy was set to 1.55eV, and the powers used for pulsed excitation were 0.2pW, 0.8pW, and 0.21lW for (a), (b) and (c). The powers used for CW excitation were 1.5 LW, 15,uW and 1.5,uW for (a), (b) and (c).

At zero delay (T=O) the dip in the CW correlation demonstrates that it is relatively unlikely to detect two photons at the same time. In the pulsed correlation, the fact that the zero delay peak is much weaker than the others demonstrates that the emission of two photon pulses is strongly suppressed for emission from the excitor, bi-exciton and charged excitor.

Figures 7(d) to 7(e) shows the time distribution, or jitter, of emission from the excitor (d), biexciton (e) and charged excitor (f), relative to the centre of the peaks. The narrow peak for the bi-exciton emission relative to the excitor peak shows a reduction in timing jitter for the bi-exciton by a factor of 1.9. In addition, the charged excitor shows a smaller reduction in peak width compared to the simple excitor, by a factor of 1.3.

The sharp peaks associated with the bi-exciton also make it particularly desirable for use with a time-gated detector. As there is much noise between the peaks, generally detectors are switched off when no photon is expected. As the peaks are sharp for the bi-exciton, the detector can be switched off for longer between the peaks. Also, as the peaks are sharper, it is possible to modulate the source at a higher frequency without the peaks overlapping. Thus, a faster transmission rate can be achieved.

These measurements demonstrate the improved performance of a single photon emitter using photon emission from the bi-exciton, rather than the excitor, in a quantum dot.

The measurements were taken using a simplified device where about 2 monolayers of InAs were grown on a GaAs buffer and substrate. The InAs layer was capped with 300 nm of GaAs. The source was then etched into 0.8 Em mesas to try and produce one dot per device. The laser was focussed onto the dot using a microscope lens and collected using a microscope lens.

In addition to modulating the laser power in order to enhance certain transitions, it is also possible to modulate the photon source using a schottky gate or an electric field in

order to fix the excess charge density of the conduction band or valence band of the quantum dot. This is shown schematically in Figures 8a to 8c.

Figure 8a shows a schematic band structure illustrating the conduction band 201 and the valence band 203 of a photon source comprising quantum dot 205. The device has a gate 207 which can bias the device with respect to device base 209. Device base 209 is a n+ contact. This n+ contact also sets the Fermi layer Ef of the device. In Figure 8a a negative bias of -0. 3V is applied to the gate 207 relative to the base 209 so that the conduction band edge of the quantum dot 205, which is illustrated by reference numeral 211, is located above the Fermi layer Ef. Similarly, the valence band edge 213 of quantum dot 205 is located well below the Fermi layer Ef. In this situation, as the Fermi layer which defines the energy of the carriers in the device is below the conduction band edge 211 and above the valence band 213, then the conduction band of dot 205 usually contains no excess electrons and the valence band of dot 205 contains no excess holes.

The situation is changed in Figure 8b. Here, the voltage applied between metal contact 207 and n+ contact 209 has been changed to OV in order to move the conduction band edge 211 below the Fermi layer Ef. The conduction band edge 211 lies only just below the Fermi level Ef SO that only one electron is contained within quantum dot 205. This allows the formation of the singularly charged negative excitor of Figure 2b or the singularly charged biexciton of Figure 2f.

Figure 8c shows the device when the bias has been further changed to +0. 3V and the conduction band edge 21 1 lies even further below the Fermi level Ef. Thus, the quantum dot 205 is now populated with two or even more electrons such that it can form the double and triple charged negative excitors shown in Figure 2c and 2d or the doubly charged biexcitons.

It is also possible to bias or configure the device so that the valence band edge 213 is raised above the Fermi level Ef. This allows the number of excess carriers within the valence band to be fixed in order to produce the positively charged excitors or bi-

excitons illustrated schematically in Figures 2i to 2m.

Previously, we have discussed the device with reference to exciting electrons and holes using a pulsed laser. A typical arrangement for this apparatus is shown in Figure 9.

Figure 9 shows an embodiment of the present invention. In this simplified example, single photon source 401 comprises a single quantum dot 411. The single photon source is driven by pulsed laser 403 which produces pulsed beam of radiation 405 having an energy sufficient to excite a desired transition within the quantum dot 41 1.

Lens 409 focuses the pulsed radiation 405 onto quantum dot 41 1 and at least one electron hole pair are excited in the conduction and valence bands per laser pulse. A first pulse of radiation arrives at the quantum dot 411 and excites one, two or more electron hole pairs. As previously explained, the number of electron hole pairs excited is dependent to a large extent on the chosen power of the laser. In this example, it is presumed that a biexciton has been excited since the biexciton emission has less jitter

than the single excitor emission and thus may be more desirable for a commercial photon source.

The length of the laser pulse is desirable shorter than the recombination time of the emission process of interest. In this example, the biexciton emission is the desirable emission, hence, the pulse length of the laser is less than the biexciton emission time. If the biexciton is excited, then photon emission from the bi-exciton will occur first, followed by a second photon emitted by the single excitor. In this example, we are not interested in the single excitor emission and since single photon emission is required, the emission from the excitor is blocked using filter 413 to avoid. Filter 413 can be an energy filter (configured to allow the transmission of photons at the biexciton emission energy or to allow transmission of charged and neutral bi-exciton, but block charged and neutral excitor), a polarization filter (since the two photons emitted due to the biexciton emission and the single excitor emission will have opposing polarizations) or a time filter (since the biexciton emission will occur before the single excitor emission).

The photons due to the biexciton emission are then directed into fibre optic cable 415.

If the single excitor emission is blocked, then theoretically a single photon 417 with the biexciton frequency will be directed into fibre optic 415 for each pulse from laser 403.

Although the filter 413is shown as an addition component the filter may be integrated into or onto photon source 401 as will be described with reference to figures 16 to 19.

Figure l O shows a further embodiment of the present invention which represents a variation on the system of figure 9. To avoid unnecessary repetition, like reference numerals will be used to denote like features.

In the system of figure lO, the photon source is provided with upper 416 and lower 418 electrodes which are configured to apply an electric field across quantum dot 411. The

electrodes 416 and 418 are connected to voltage source 419. The voltage applied by source 419 is selected in order to enhance the probability of n excess carriers within the quantum dot. For example, if the source 419 applies a voltage such that there should be a single excess electron within the quantum dot 411, then a negatively charged single

excitor should be produced for each laser pulse. The emission from these excitors is then directed into fibre optic cable 415.

The system of figure 10 is illustrated without a filter 413. However, the output from such a photon source can be filtered using energy, time or polarisation as described with reference to figure 9. Further, the apparatus of figure 10 can also be used to fix the probable number of excess carriers in the quantum dot 41 1 at zero to enhance the production of neutral excitors. The laser power can be chosen depending on the order of the excitors required.

Figure 11 illustrates a further variation on the system of figure 9. To avoid unnecessary repetition, like reference numerals will be used to denote like features.

Filter 413 of figure 9 was used to block certain emissions. The apparatus of figure 11 comprises a separator 414 which directs the photons emitted from photon source 401 either down first fire optic cable 415 or down second fibre optic cable 421.

Taking the example described in relation to figure 9 where a biexciton emission occurs, the separator may be a wavelength separator such as a dichoric mirror which is configured to direct biexciton emissions into first cable 415 and single excitor emissions 423 into second fibre 421.

This type of source is an entangled photon source, where the polarization and/or phase state of a biexciton is entangled with the polarization and or phase state of the single excitor emission which follows from the biexciton emission. Measuring the phase and or polarisation of one of the states of the entangled photon pair (i.e. biexciton emission and subsequent single excitor emission) will set the respective state of the other photon of the pair.

The separator may alternatively use the difference in time between the biexciton and single excitor emissions in order to separate the entangled pair or a polarization measurement may be used.

The apparatus of figure lO may be used with the voltage source 419 and contacts 416 and 418 of figure l O in order to enhance the probability of emission from a particular charged state of an excitor. Also, the filter 413 of figure 9 may also be used in order to block unwanted emissions.

Figures 9 to 11 related to supplying the carriers to the dot using a pulsed laser. Figure 15 shows a variation on this arrangement where a continuous wave (COO) laser is used in order to irradiate the dot.

Figure 12 shows an electrically triggered quantum dot filter. The single photon source 401 is illuminated by a CW (continuous wave) laser 439 with a narrow spectral line width. The CW laser 439 provides, as the name suggests, a continuous intensity and does not emit a pulsed signal. The CW laser excites an optical transition of the quantum dot411. The output from the CW laser 439 is focused by lens 409 onto the quantum dot 411 of photon source 401.

The single photon source 401 has a quantum dot 411 which is interposed between a top contact 431 and a bottom contact 433. An AC voltage source 435 is connected across the top contact 431 and bottom contact 433 such that on application of an AC voltage, the field across the quantum dot 411 is periodically varied. This periodic modulation

varies the transition energies of the quantum dot.

The CW laser 439 is capable of exciting an optical transition of the quantum dot. The applied periodic modulation varies the transition energy of the dots. Therefore, the input radiation is only capable of exciting an electron and hole into the relevant levels at certain voltage levels applied to electrodes 431, 433. Hence, the period modulation of the voltage applied to electrodes 431 and 433 has a similar effect to pulsing the laser radiation. The emission may then be filtered using filter 413 and directed into optical fibre 415.

As explained with reference to figure 9, the laser may be used to excite higher order

excitors. Also, the apparatus of figure 10 may be used to fix the excess carrier density.

Although the photon source of figure 10 has contacts which are used to apply an electric field, it should be noted that it is still possible to modulate the transition energy of the

dot and fix the excess carrier concentration at the dot transition energy.

An entangled photon source of the type described with reference to figure 11 can also be provided using the electrically pulsed source of figure 12.

Figure 13 illustrates the relationship between single photon emission and applied AC perturbation in the device of Figure 1 1. In Figure l 3a, applied AC perturbation on the y-axis (arbitrary units) is plotted against time on the x-axis (arbitrary units). In Figure 1 3b, the quantum dot transition wavelength (related to the quantum dot transition energy by E=hc!) is plotted on Me y-zxis alla the elapsed time is plotted agamst We x-

axis (arbitrary units).

The time axis in both Figures 13a and 13b are identical. It can be seen, that a quantum dot transition wavelength varies periodically with that of the applied AC perturbation.

The laser wavelength in this case (that is the applied radiation) is tuned to the transition energy of the quantum dot transition in the absence of any modulation. Photon absorption occurs at the times that the quantum dot transition energy equals that of the laser which occurs twice per period of the modulation. Subsequent recombination results in the emission of a photon. Photons are thereby emitted at time intervals detennined by the period of the applied modulation.

The single photon emitter does not always require excitation by illumination. It is possible to introduce the electrons and the holes into the energy levels for recombination by an applied voltage.

Figure 14 illustrates the band structure for such an electrically operable device. The device comprises a buffer layer 503 formed overlying an upper surface of an n+ injection gate 501. The buffer layer separates the injection gate 501 from an injection layer 505. Electrons can be induced in to injection layer 505. A tunnel barrier 507 is formed overlying an upper surface of injection layer 505. A quantum dot layer 509 is

then formed overlying an upper surface of tunnel barrier 507. A p-type doped barrier layer 513 is formed overlying an upper surface of an edoped spacer layer 511 such that barrier layer 513 separated from the quantum dot layer SO9 via spacer layer 511. The structure is finished with a cap layer 515 which overlies an upper surface of the doped barrier layer 513.

Contacts are made to the dot layer 509 and the injection gate 501 such that the injection gate 501 can be biased with respect to dot layer 509. In operation, the device is configured so that doped barrier layer 513 supplies holes to dot layer 509 so that the quantum dots are always populated by holes. Injection gate 501 can be biased with respect to dot layer 509 such that electrons are induced in injection layer 505. Electrons can be injected into the quantum dot layer 509 due to resonant tunnelling through tunnel barrier 507 by varying the bias between the injection gate and the hole gas 525.

The injection of electrons into the dot layer 509 is regulated by applying a periodic voltage between the dot layer 509 and the injection gate 501.

The bias consists of a periodic stream of pulses between two levels Von and Voff. Figure 14 illustrates the band structure for the situation of Voff i.e. no carries are injected into the quantum dot 509 and Figure 15 illustrates the situation for Von where carriers are injected from the injection layer 505 to the quantum dot 509.

The voltage level Voff is chosen so that the electron energy level 521 in the injection layer 505 is lower than the level 523 in the quantum dot 509.

The electrons in the electron injection layer 505 have an energy 521. In order to resonantly tunnel through barrier 507 into quantum dot layer SO9, the electrons must have an energy equal to that of level 523 shown in the quantum dot. In the Voa state, the electrons do not have this energy. Therefore, no tunnelling can take place and hence, no recombination of electrons with holes in the dot can occur.

Figure 15, shows the state where the potential of the injection gate 501 is raised to Von.

Under these conditions, the band structure of the device changes so that energy level

521 in the electron injection layer 505 aligns with energy level 523 of the quantum dot and resonant tunnelling of a single electron can occur from injection layer 505 through tunnel layer 507 into quantum dot layer 509. Thus, recombination can occur and a photon can be emitted. It is clear, that as the tunnelling is controlled by switching the potential between Von and Voff, the control of single photons can be achieved.

The device of figures 14 and 15 can be used to produce higher order excitors and/or can be used to fix the number of excess carriers as described in relation to figures 9 and 10.

Also, the entangled photon source of figure 11 can also be achieved using the photon source of figures 14 and 15.

Figure 16 illustrates a schematic layer structure for a single photon source which can be used to enhance the desired excitonic transition The device Comprises In n doped GaAs substrate 301. A distributed Bragg reflector 303 is then formed overlying and in contact with the upper surface of said substrate 301. Alternatively, the distributed Bragg reflector 303 may be deposited on a buffer layer which is formed overlying and in contact with said upper surface of the GaAs substrate 301.

The distributed Bragg reflector 303 comprises an alternating sequence of n doped GaAs layers having a thickness of 95.46 nm 305 and n doped AlAs having a thickness of 111.04 nm 307.

The thickness of the layers of the distributed Bragg reflector 303 are chosen in order to make the distributed Bragg reflector 303 highly reflected at the intended device operation wavelength it. The composition of each adjacent layer is chosen so that there is a high refractive index contrast. This can be achieved by alternating AlAs and GaAs layers. The layers are doped n type by silicon doping during the epitaxial growth process. The optical thickness of each pair of layers is chosen to be as close to \/2 as possible and the optical length of each layer should be \/4 for maximum reflectivity.

The number of repeats increases the reflectivity and 10 to 20 pairs of layers should be sufficient for the device operation. In the specific device of Figure 16, the bi-exciton

emission occurs near 1.3 lam and the thickness of the above layers is chosen to enhance emission at this wavelength.

An optical cavity 309 is then formed overlying and in contact with the upper surface of the distributed Bragg reflector 303. The bottom of the cavity comprises an n doped GaAs cavity layer having a thickness of 34.4 nm 311. This layer is then followed by 156.5 nm layer of undoped GaAs at 313 that extends the total GaAs region i.e. n doped GaAs layer 31 1 and undoped GaAs layer to an optical thickness of \/2. A self assembled quantum dot layer is grown by depositing a thin layer (approximately 1.5 to 4 monolayers) of InAs. There is a large difference in the lattice parameters between InAs and GaAs. This results in the InAs forming quantum dots as it grows. Preferably the layer is grown to the equivalent of about 2 monolayers. A 190.9 nm thick undoped layer of GaAs is then formed overlying and in contact with the quantum dot layer. This layer is again chosen to be of a thickness of \/2. This completes cavity structure 309.

A final undoped 70 rim GaAs layer 319 is then formed overlying and in contact with the top layer 317 of the cavity region 319. A metal top layer comprising 50 nm of aluminium 321 is then formed overlying and in contact with GaAs layer 319. GaAs layer 319 is provided in order to phase match with the metal top layer. The metal top layer may be deposited on NiCr in order to add adhesion to the top of the structure.

Hole 323 is then etched in the top of the metal layer and the hole extends through the whole of metal layer 321. The hole will be defined by a technique such as photo or electron beam lithography and may be wet edged or RIE etched.

In this structure, it is possible to bias metal contact layer 321 with respect to n doped substrate 301 via contacts (not shown). This means that it is possible to electrically fix the potential of the quantum dot conduction and valence bands with respect to the Fermi level of the device which will be to a large extent determined by the carrier concentration of the n doped layers underlying the cavity region 309.

The quantum dot is located within resonant cavity 309. The resonant cavity also acts as a wavelength filter. This is because the resonance condition ( Lean - cap) is only 2ncav satisfied for a narrow range of emission wavelengths. Thus, the wavelength of the cavity mode can be matched to the wavelength of the desired excitonic transition such as the biexciton or charged excitor transition. This will greatly suppress the collection of other excitor lines such as the simple neutral excitor line.

Thus, only these wavelengths are emitted into a narrow cone normal to the lower DBR 303 and upper mirror 321. The bandpass which can be thought of arising from the lifetime of the photon in the cavity, is largely determined by the reflectivity of the lower DBR 303 and upper mirror 321. Thus, increasing the reflectivities of these layers leads to a sharper cavity mode. The spectral bamdpass of the cavity (or fin other words, the width of the cavity mode) should ideally be designed to be roughly equal to the spectral width of the relevant excitor line of the emitting quantum dot.

The above example is formed in a GaAs substrate. It is also possible to form the device on a InP substrate. Taking Figure 16, substrate 301 has a InP substrate. The lower distributed Bragg reflector 303 comprises alternate layers of 1 17.4 nm AlAs0 5$bo 5 and 98.6 nm Ale Gas sAso 5Sbo 5. Again, this structure is e-doped The cavity region 309 is constructed from an e-type In0.s2Al0.4sAs layer which is 0.18 x thick, a 0.82 x thick undoped In0 s2Alo 4sAs layer. A layer of InAs quantum dots 315 is then formed overlying and in contact with undoped layer 313. The upper part of the cavity comprises a 1.0 x thick undoped Ins s2Al0 4sAs layer 317. A 0. 73 x thick In0.s2Alo.4 As phase matching layer 319 is then formed overlying and in contact with upper cavity layer 317 where x is calculated from x =-for] _ l.SS,wn.

2ncav Figure 17 illustrates a variation on the structure of Figure 16. To avoid unnecessary repetition, like reference numerals are used to denote like features.

The processing of the device of Figure 17 is identical to that of Figure 16 up to the formation of phase matched GaAs layer 319. Once this layer has been formed, mesas

are etched into the structure by photo lithography followed by wet etching. The mesas have a diameter in the region of 0.2 to 10,um. The etch is allowed to proceed into GaAs layers 3 11 and 313. The etch does not proceed into the distributed Bragg reflector 303.

After etching, 50 nm of alu ninium is evaporated over the pillar so that it is completely covered. This allows a top electrical contact to be made.

* The structure of Figure 18 is a pillar p-i-n structure. Essentially the cavity is the insulating region and the upper and lower distributed Bragg reflectors are p and n doped respectively. The lower distributed Bragg reflector 303 is identical to that described with reference to Figure 16. To avoid unnecessary repetition, like reference numerals will be used to denote like features.

A first layer of 190.9 nm of undoped GaAs 312 is then formed overlying and in contact with the upper surface of lower reflector 303. InAs quantum dot 315 is then formed by depositing a few monolayers of InAs. Upper cavity layer 316 comprising 190.9 nm of undoped GaAs is then formed overlying and in contact with lower cavity layer 312 and dot 315.

A second distributed Bragg reflector 318 is then formed overlying and in contact with the upper surface of the cavity 309. The upper distributed Bragg reflector 318 is formed in an identical manner to that of the lower distributed Bragg reflector 303. However, the upper Bragg reflector is p type doped whereas the lower Bragg reflector is n type doped, and should have fewer repeats to match the reflectivity of the bottom distributed Bragg reflector.

Pillar 320 is then defined by etching through the upper distributed Bragg reflector 318 and through most of the cavity 309 and into layer 312. This is achieved by defining the pillar using photo lithography and etching using reactive ion etching. The etch should be taken as close as possible to the bottom of the distributed Bragg reflector 303. The mesa has a diameter in the range 0.2 to 10 m.

Metal is then evaporated onto the top surface so that an ohmic electrical contact can be made with some part of the p type top distributed Bragg reflector 318 and an ohm electrical contact (not shown) is made to e-type layer 301.

The dimensions of the cavity 309 are important as they define the energy of the cavity mode for the device. The cavity mode energy allows photons with the same energy to escape from the device. For example, if the neutral bi-exciton is the preferred emission then it is necessary for the cavity mode to correspond to the bi-exciton emission.

Figure 19 shows a photonic bandgap structure. Here, the device of Figure 16 is taken and the device is fabricated up to the top contact layer 321.

A series of holes is then etched doe. thorough the structure into He distributed Bragg reflector 303. A plan view of the whole arrangement is shown in Figure 1 9b.

The holes are etched using reactive ion beam etching which could define deep holes which can extend up to a micron, or preferably move into the semiconductor. Missing holes in the centre of the pattern define the cavity in the third dimension. About 1-7 holes are suggested for this example. This array of holes acts as a two dimensional distributed Bragg reflector with air acting as the material with the low refractive index.

This creates an optical cavity surrounding the quantum dot confined by a metal top mirror, a DBR and an in-plane photonic crystal lattice.

Again, it is important to define the energy of the cavity mode and to equate that to the energy of the desired excitor emission. For example, the bi-exciton emission.

Typically, the whole spacing will be from 300 to 700 rim with a hole diameter from 100 to 500 urn.

Figure 20 illustrates schematically a photon source of the type described with reference to figure 16 connected to a fibre optic cable.

The fibre optic 322 is positioned so that its core 324 is located so that it overlays one quantum dot. Hence, the emission from one quantum dot is collected by the fibre. The

optical fibre also has a relatively high numerical aperture to collect as much of the emitted light as possible.

Other optical components such as a microscope objective lens may also be used. The elements of the objective may be coated for maximum transmission of the desired emission wavelength of the quantum dot.

The above examples have mainly concentrated on photon sources comprising a single quantum dot. However, it is often desirable to form a multi-dot device of the type illustrated in figure 21.

Figure 21 shows a photon source where photons emitted from a plurality of quantum dots. Except for the density of the quantum dot layer the structure is similar to that of figure 16. Therefore, to avoid unnecessary repetition like features are denoted by the same reference numerals. In this type of structure, the emission from a large number of quantum

dots (1 to 1000) can be collected. However, in this case, it is possible to extract the emission from a single quantum dot by spectrally filtering the emitted light. Spectral filtering also selects a particular transition within the quantum dot as previously described.

As described with reference to figure 16, the quantum dots are located within a resonant cavity 309. The resonant cavity will also act as a wavelength filter. This is because the resonance condition ( LCY71 = 2) is only satisf ed for a narrow range of emission wavelengths. Thus, the wavelength of the cavity mode can be matched to the wavelength of the desired excitonic transition such as the biexciton or charged excitor transition. This will greatly suppress the collection of other excitor lines such as the simple neutral excitor line.

Thus, only these wavelengths are emitted into a narrow cone coronal to the lower DBR 303 and upper mirror 321. The bandeaux which can be thought of arising from the lifetime of the photon in the cavity, is largely determined by the reflectivity of the lower

DBR 303 and upper mirror 321. Thus, increasing the reflectivities of these layers leads to a sharper cavity mode. The spectral bandpass of the cavity (or in other words, the width of the cavity mode) should ideally be designed to be roughly equal to the spectral width of the relevant excitor line of the emitting quantum dot.

Figure 22 shows a schematic optical absorption spectrum of the plurality of quantum dots shown in Figure 21. Absorption of the quantum dot is plotted along the y-axis (arbitrary units) photon wavelength of the emitted photons are plotted along the x-axis (arbitrary units). The optical spectrum of each quantum dot consists of a series of sharp lines whose width are determined by the homogenous broadening due to the finite lifetime. However, because of unavoidable fluctuations in the size and composition of the dots in a plurality of dots, the transition energies vary from dot to dot. Thus, the three ahso lotion peaks 371, 373, and 375 are ir or 'ogenously broadened.

If the quantum dots are excited by a laser with a broad wavelength spectrum, a large number of quantum dots will be expected in the active region. However, because each of these emits a different wavelength, it is possible to filter the collected light in order to see the emission from a single dot. This is shown schematically in Figures 23a and 23b.

Figure 23b shows a plot of emission intensity on the y-axis (arbitrary units) and photon wavelength on the x-axis (arbitrary units). Each of the plurality of spikes is due to emission from a particular transition within a single quantum dot. Figure 23b shows the results where filter 377 has filtered all but one of the photon wavelengths 379.

Therefore, using pulsed excitation, it is possible to generate period emission of a single or multiple photons from a dot in a similar way to that described above. Preferably, the emitting area of the sample from which light is collected should contain a limited (<1000) number of optically active quantum dots. In this case, the spectral filter selects the quantum dot as well as a particular transition within the dot.

Alternatively, a spectrally narrow laser may be used to excite a transition in just or a few quantum dots. Such a configuration is illustrated with reference to Figure 24.

Figure 24 shows an optical absorption spectrum similar to that of Figure 22. Peaks 371, 373 and 375 are due to excitation of different quantum dot transitions. The incident laser wavelength 370 is much narrower here than the laser wavelength described with

reference to Figure 22. Hence, the laser excites only a fraction of the quantum dot plurality: those with a transition energy equal to the laser energy. Hence, it can be seen that the emitted wavelength 372 is also much narrower than the emitted wavelength (before filtering) of the spectrum shown in Figure 22. A laser of sufficiently narrow wavelength spectrum will excite just one of the quantum dots. Preferably, the emitting area of the sample from which light is collected should contain a limited (<1000) number of optically active quantum dots.

Claims (58)

CLAIMS:

1. A photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; fixing means configured to fix the number of excess carriers in the energy levels by fixing the relative position of the energy levels with respect to the Fermi level of the source; and supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval.

2. A photon source according to claim 1, wherein the fixing means is configured to cause at least one excess carrier to populate the said energy levels.

3. A photon source according to either of claims 1 or 2, wherein the fixing means comprise at least one electrode configured to apply an electric field across the quantum

dot.

4. A photon source according to any of claims 1 to 3, wherein the supply means are configured to enhance creation of bi-excitons and/or higher order excitors.

5. A photon source according to any preceding claim, further comprising a blocking means to block undesirable photons from certain predetermined transitions.

6. A photon source according to claim 5, wherein the blocking means comprises filter means to block the said undesirable photons by blocking photons having a particular energy.

7. A photon source according to claim 5, wherein the blocking means is configured to block undesirable photons by blocking photons emitted at a predetermined time.

8. A photon source according to any of claims 5 to 7, wherein the blocking means is configured to block photons at the neutral single excitor frequency.

9. A photon source according to any of claims 5 to 8, wherein the blocking means is configured to block photons at a neutral excitor frequency.

10. A photon source according to any of claims S to 7, wherein the blocking means is configured to transmit photons arising from an emission from a predetermined excitonic order.

11. A photon source according to any preceding claim, further comprising separating means for separating photons having different distinct energies.

12. A photon source according to claim 11, wherein the separating means is configured to use the difference in energies between the photon emissions to separate the photons.

13. A photon source according to claim 12, wherein the separating means is a dichoric mirror.

14. A photon source according to claim 11, wherein the separating means comprises means to separate the photons according to their emission time.

15. A photon source according to any of claims 11 to 14, wherein the separating means is configured to separate photons arising from excitor transitions of different orders.

16. A photon source comprising:

a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; and blocking means configured to block photons arising from the neutral single excitor emission.

17. A photon source according to claim 16, wherein the blocking means comprises filter means to block photons at the neutral single excitor energy.

18. A photon source according to claim 16, wherein the blocking means comprises means to block the emission of photons at the time when the emission due to a single neutral excitor is expected.

19. A photon source according to any of claims 16 to 18, wherein the blocking means are configured to block photons due to any neutral excitor emission.

20. A photon source according to any of claims 16 to 19, wherein the blocking means are configured to allow the transmission of photons arising from an emission from a predetermined excitonic order.

21. A photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier; supply means for intermittently supplying carriers to the energy levels to create a bi-

exciton or higher order excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays a plurality

of single photons each having different distinct energies are emitted during a predetermined time interval, and separating means for separating the photons having different distinct energies.

22. A photon source according to claim 21, wherein the separating means is configured to use the difference in energies between the photon emissions to separate the photons.

23. A photon source according to claim 22, wherein the separating means is a dichoric mirror.

24. A photon source according to claim 21, wherein the separating means comprises means to separate the photons according to their emission time.

25. A photon source according to any preceding claim, wherein the supply means comprises incident radiation configured to excite a predetermined number of carriers into the energy levels.

26. A photon source according to claim 25, wherein the supply means comprises pulsed radiation.

27. A photon source according to claim 26, wherein the pulse has a duration which is less than the recombination time of an electron and hole in the quantum dot which combine to emit a photon of a desired predetermined energy.

28. A photon source according to either of claims 26 or 27, wherein the time between leading edges of successive pulses is greater than the recombination time of the electron and hole in the quantum dot which combine to emit a photon of a desired predetermined energy.

29. A photon source according to any of claims 25 to 28, wherein the incident radiation has an energy which is substantially equal to that of the quantum dot transition energy.

30. A photon source according to any preceding claim, wherein the supply means comprises modulation means configured to repetitively modulate the transition energy of the quantum dot between an energy which allows carriers to be excited by incident radiation and an energy which blocks the absorption of the incident radiation.

31. A photon source according to claim 39, wherein the modulation means comprises an AC voltage applied to vary the electric field across said dot.

32. A photon source according to any of claims 30 or 31, wherein the modulation means comprises means to vary a magnetic field applied to the said quantum dot.

33. A photon source according to any of clairlls 1 to 24, wherein the supply means is configured to electrically inject carriers into the quantum dot.

34. A photon source according to any preceding claim, wherein the source has an output surface through which the emitted photons are collected, the source further comprising coupling means for coupling the emitted photons to a fibre optic cable.

35. A photon source according to any preceding claim, wherein the source has an output surface through which the emitted photons are collected and comprises an anti-

reflection coating located on said output surface.

36. A photon source according to any preceding claim, wherein the source further comprises a microscope optic for collecting emitted photons.

37. A photon source according to any preceding claim, wherein the source comprises a mirror cavity having a mirror located on opposing sides of said quantum dot.

38. A photon source according to claim 37, wherein the source has an output surface through which the emitted photons are collected and said mirror closest to said output surface is partially reflective such that it can transmit the emitted photons.

39. A photon source according to either of claims 37 or 38, wherein the energy of the cavity mode for said mirror cavity is substantially equal to that of the desired emitted photons.

40. A photon source according to any of claims 37 to 39, wherein the distance between the two mirrors Lcav bounding the cavity is defined by the equation: m] 2ncav where is the wavelength of the emitted photons, m is an integer and ncav is the refractive index of the cavity.

41. A photon source according to any of claims 37 to 40, wherein the spectral band-

pass of the cavity is substantially equal to the spectral width of the radiation emitted from the dot in the absence of a cavity.

42. A photon source according to any claims 37 to 41, wherein the quantum dot is positioned at an anti-node of the standing wave pattern caused by said mirrors.

43. A photon source according to any of claims 37 to 42, wherein at least one of the mirrors is a Bragg mirror comprising a plurality of alternating layers wherein each layer satisfies the relation nt =- wherein is the wavelength of the emitted photons, n and t are the refractive index and thickness respectively of a layer within the mirror.

44. A photon source according to any of claims 37 to 38, wherein the mirror comprises a metal layer and a phase matching layer.

45. photon source according to any of claims 37 to 44 when dependent on any of claims 5 to 10 or 16 to 20, wherein the blocking means comprises the said cavity.

46. A photon source according to any preceding claim, wherein the source further comprises an optic fibre cable for collecting the emitted light.

47. A photon source according to claim 46 when dependent on claim 37, wherein the cut off wavelength of the fibre optic cable is greater than the wavelength of the cavity mode.

48. A photon source according to either of claims 46 or 47, wherein the optical fibre has a non-reflective coating.

49. A photon source according to any preceding claim, comprising a plurality of quantum dots.

50. A photon transmission apparatus comprising: a photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one cattier and a confined valence band energy level capable of being populated by at least one carrier; and supply means for intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot, wherein the supply means are configured to regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; a receiver for receiving photons emitted by the source; and blocking means configured to block photons arising from the neutral single .. excitor emission.

51. An apparatus according to claim 50, wherein the blocking means is provided at the receiver.

52. An apparatus according to claim 51, wherein the blocking means is integral with the receiver.

53. A method of operating a photon source, the photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier, the method comprising: fixing the number of excess carriers in the energy levels by fixing the relative position of the energy levels with respect to the Fermi level of the source; and intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot and regulating the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval.

54. A method of operating a photon source, the comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier, the method comprising: intermittently supplying at least one carrier to the energy levels to create an excitor in the quantum dot; regulate the supply of carriers such that as the excitor decays either a single photon or a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; and blocking photons arising from the neutral single excitor emission.

55. A method of operating a photon source, the photon source comprising: a quantum dot comprising a confined conduction band energy level capable of being populated with at least one carrier and a confined valence band energy level capable of being populated by at least one carrier, the method comprising: intermittently supplying carriers to the energy levels to create a bi-exciton or higher order excitor in the quantum dot;

regulating the supply of carriers such that as the excitor decays a plurality of single photons each having different distinct energies are emitted during a predetermined time interval; and separating the photons having different distinct energies.

56. A photon source as substantially hereinbefore described with reference to any of figures 2 to 24.

57. A photon transmission apparatus as substantially hereinbefore described with reference to any of figures 2 to 24.

58. method of operating a photon source as substantially hereinbefore described X. th reference to any of f Ares 2 to 24.