GB2466299A - Single photon detection using variable delay component to cancel periodic signal variations - Google Patents

Abstract

A photon detection system, method and conditioning circuit for cancelling periodic signal variations to detect single photons (e.g. using an avalanche photodiode (APD) 51) comprises: dividing a signal into first and second parts, wherein the second part is delayed with respect to the first part; combining the first and delayed second parts of the signal such that delayed second part cancels periodic variations in the first part. A variable path length component 58 (e.g. co-axial line stretcher) provided in the path of the first and/or second signal parts enables in-situ variation of the delay. A further stage (150) for dividing the combined signal into first and delayed second parts and recombining the parts to cancel periodic variations may be incorporated. A stub (175) (e.g. transmission line, strip line or waveguide) to reflect part of a signal directed along it may be used as a delay line for the second part of the signal. The detection system may be used as part of a receiver of a quantum communication system which also includes a sender comprising a pulsed radiation source (207) and an encoder.

Description

A Photon Detector, a Method of Photon Detection and a Conditioning Circuit The present invention relates to the field of photon detectors and methods for detecting photons which are configured to detect the presence of a single photon. The present invention also relates to the field of conditioning circuits for extracting fluctuations from a periodic signal.

Detectors which are capable of detecting a single photon so-called single photon detectors are an important component of any system which operates using the principles of quantum cryptography. Such systems rely upon the transmission of data bits as single particles, in this case, photons, which are indivisible.

Single photon detection is also useful as a low level light detection means for time of flight ranging experiments, spectroscopy, medical imaging or astronomy. Both in medical arid astronomical applications the high energy photons (X-ray etc) or high energy particles are converted in scintillators into many (10-100) low energy photons.

These low energy photons are then detected by avalanche photodiodes or photomultiplier tubes. As the low energy photons that are produced are scattered in space there is a need for large area detectors which are very sensitive. Also arrays of such detectors allow the spatial distribution of low energy photons to be obtained in order to gain information about the original photon.

One problem which many single photon detectors suffer from is that the signal which is outputted due the detection of a single photon is often weak and sometimes difficult to distinguish from other artefacts of the detector output.

For example, a particularly popular type of single photon detector is the avalanche photo-diode (APD) operating in gated mode. In gated mode, to detect a photon, a short duration of high reverse bias is applied across the APD which is above the breakdown voltage of the diode. An absorbed photon generates an electron-hole pair in the APD, which upon separation can trigger an avalanche of excess carriers. This avalanche of excess carriers causes a macroscopic and detectable current flow thorough the APD.

Although this macroscopic current is detectable, it is usually buried within artefacts of the output signal of the APD caused by the capacitance of the APD in reaction to the biasing gates. One solution to this problem is to bias the APD to an extent where the avalanche current dominates the output of the detector. However, this has the disadvantage that the APD needs to be operated at relatively low frequencies.

US 6 218 657 describes a system for distinguishing a photon induced avalanche from the large electrical transient caused by the bias voltage using a transmission line system wherein an input pulse is divided into first and second input pulses separated in time and the output from the APD is divided in two output signals separated in time using transmission lines, wherein the second output from the first input pulse is inverted and combined with the first output from the second input pulse which has not been inverted to partially cancel the large electrical transient.

GB publication number GB 2447254 describes a system which addresses some of the above problems where a periodic input signal is used and the output of the APD is divided into two equal parts and the second part of the output is inverted and shifted in time by one period and combined with the first part to cancel the large electrical transient.

The present invention is an improvement of the device of GB 2447254 and in a first aspect provides a photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal and wherein the system further comprises a variable path length component provided in the path of the first and/or second parts of the signal such that the delay in the second part of the signal is variable in-situ.

Thus the delay between the first and second parts of the signal may be varied after the system has been constructed such that the system may be used with input signals of different frequencies and the system can also cope with variations in the input signal or system itself over time, for example due to temperature fluctuations etc. The variable delay line may be provided in the path of the first part of the signal, the second part of the signal or even both parts.

The variable path length component may comprise a variable electrical path length element such as a line stretcher, for example, a co-axial line stretcher, PCB strip line and adjustable wiper contact, telescopic wave guide or any other method of extending the electrical path length.

The system preferably further comprises means to apply a periodic gating signal to said detector and wherein said variable path length component is configured to tune the delay of the second part of the signal by an integer multiple of said period.

By dividing the output of the detector into two parts and combining the signal from a period with the signal from later periods, periodic variations in the output of the detector are removed.

Better cancellation may be achieved if there are multiple stages, thus the system may further comprise at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.

A plurality of such further stages may be provided arranged such that the combined signal outputted from one stage is provided as the input for the next further stage.

The above system of multiple stages does not require the presence of a variable delay line, thus, in a second aspect, the present invention further provides a photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal, the system further comprising at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.

The delay line may be a stub configured to cause reflection of a part of the signal directed along said stub.

Jn a third aspect, the present invention provides a photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal and wherein the delay line is a stub configured to cause reflection of a part of the signal directed along said stub.

The stub may be grounded at its end to cause the reflected signal part to be inverted.

The stub is provided is a transmission line, strip line, micro strip line or waveguide.

The stub may be adjustable in situ to allow tuning of the delay.

The signal divider and combiner may be the same component, wherein the stub allows the second part of the signal to be reflected back to the divider for recombination. A "T" piece may function as both the divider and combiner. For example, the first and second parts of the signal may be input into the central arm" of the "T" such that the second part exits the T through one arm to the stub and the first part exits though the other arm, the second pat being reflected back to the "T" such that it can be combined with the first part of the signal from the next period.

The photon detector in any of the above systems may be an avalanche photodiode, but may also be other photon detectors.

Preferably, the detector receives a periodic signal and said delay means are configured to delay the second part of the signal by an integer multiple of said period. However, it is possible for the system to isolate a single period or a multiple of periods from the second part of the signal and repeatedly use this isolated signal to cancel periodic variations in the first part of the signal. For example, the period of the waveform may be stored digitally; the stored waveform may then be used to perform cancelling using digital processing.

Preferably, the system comprises means to apply a periodic gating signal to said detector. The gating signal may be a square wave signal or a sinusoidal signal etc. As the present invention enhances the presence of the signal due to a single photon without increasing the bias required across the detector, the detector of the present invention may operate at higher frequencies than those of the prior art. The present invention may therefore have a gating signal with a frequency of 50MHz or more, preferably 70MHz or more, even more preferably 100MHz or more.

As the present invention can be used at higher gatirig frequencies, it can achieve quasi-continuous operation. In quasi-continuous operation, there is no synchronization required between the source of photons and the detector. Quasi continuous operation is possible because for very high gating frequencies, the period at which the detector is incapable of detecting photons is not prohibitive to the overall detection efficiency.

To enhance quasi continuous operation, it is possible to vary the period of the gating signal to broaden the detection window. The period may be varied randomly or as noise.

Generally, the two signals which arrive at the combiner will be balanced. However, the system may further comprise means to balance the amplitudes of the two signals arriving at the combiner. For example, the system may further comprise a tunable attenuator. The divider may comprise a tunable RF splitter which is used to balance the two voltage amplitude in each part.

The system may also comprise means to invert one part of the signal with respect to the other part. Inversion may be achieved at either the divider, combiner or during transfer between the divider and combiner. Inversion may be achieved by many methods, for example, using a hybrid junction which performs divisionlcombination and inversion.

It is also possible to use a differential component, for example, a differential amplifier, to combine the signals.

The above system may be used in a quantum communication system. Thus, in a fourth aspect the present invention provides a quantum communication system comprising a sender and a receiver, said sender comprising a source of pulsed radiation and an encoder for encoding information on said radiation pulses, said receiver comprising a detection system according to any of the above first to third aspects.

The above system may also be supplied without a photon detector or used for a variety of applications other than photon detection. The fifth, sixth and seventh aspects may be used in any situation where it is necessary to extract a small signal from a larger signal, for example, radio where a small signal needs to be extracted from a carrier frequency, or a small signal on a mains voltage. Other examples, include small sounds on a large carrier for example sonar. In other words, the present invention can be used anywhere where a large carrier wave is used in any medium, for example, electrical, water, sound, light, and there is a small signal imposed on the carrier wave which needs to be extracted.

Thus, in a fifth aspect, the present invention provides a conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal and wherein the system further comprises a variable path length component provided in the path of the first andlor second parts of the signal such that the delay in the second part of the signal may be varied in-situ.

In a sixth aspect, the present invention a conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, , where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal, the system further comprising at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.

In a seventh aspect, the present invention provides a conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal and wherein the delay line is a stub configured to cause reflection of a part of the signal directed along said stub.

In an eighth aspect, the present invention provides a photon detection method comprising: providing a photon detector configured to detect single photons; providing a divider for dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; tuning the delay between the first and second paths in situ; dividing the output signal using said divider into a first part and a second part; delaying the second part with respect to the first part; and combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal.

In a ninth aspect, the present invention provides a photon detection method comprising: providing a photon detector configured to detect single photons; dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; delaying the second part with respect to the first part; combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal; dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; delaying the second further part with respect to the first further part; and combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.

In a tenth aspect, the present invention provides a photon detection method comprising: providing a photon detector configured to detect single photons; dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; delaying the second part with respect to the first part by directing the second part of the signal into a stub; and combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal.

The present invention will now be described with reference to the following non-limiting embodiments in which: Figure Ia is a schematic of a prior art detection system using an avalanche photodiode (APD), figure lb is a schematic plot of voltage against time for the input signal of the APD of figure la and figure ic is a plot of the output signal as voltage against time for the detection system of figure la; Figure 2a is a schematic of a known detection system comprising an avalanche photodiode, figure 2b is a plot of the input signal to the device of figure 2a, figure 2c is a plot of a first part derived from the output signal of the APD of figure 2a, figure 2d is a plot of the second part derived from the output signal of the APD of figure 2a which has been delayed and figure 2e is a plot of the self-differenced output signal produced by the device of figure 2a; Figure 3a is a schematic of a detection system in accordance with an embodiment of the present invention and figure 3b is a plot of the input signal to the system of figure 3a; Figure 4a is a variation on the device of figure 3a, figure 4b is a first part of the output signal of the device of figure 4a, figure 4c is a delayed second part of the output signal of the APD in Figure 4a and figure 4d is a plot of the output signal of the detection system of figure 4a; Figure 5a is a schematic of a further variation on the detection system of figure 3a and figure 5b is a plot of the input signal to the detection system of figure 5a; Figure 6 is a schematic of a detection system which is a variation of the detection system of figure 3a; Figure 7 is a plot of the dark count probability and afterpulse probability against photon detection efficiency for the detection system of figure 3a; Figure 8 is a plot of the count rate against photon flux; Figure 9a is a schematic of a photon detection system in accordance with an embodiment of the present invention having multiple self differencing stages; and figure 9b is a plot of an input signal for the system of figure 9a; Figure 1 Oa is a plot of the output waveform of the system of figure 9a from the first self differencing stage and figure lOb is a plot of the output from the second self differencing stage of the system of figure 9a; Figure 11 a is a schematic of a photon detection system in accordance with an embodiment of the present invention having two stub self differencing stages; and figure lib is a plot of an input signal for the system of figure ha; Figure 12a is a quantum communication system having a detection system as previously described with reference to figure 3a, figure 12b is the clocking signal for the quantum communication system of figure 12a, figure 12c shows the outputted laser pulse for the transmitter system of figure l2a, figure 12d is a plot of the signal arriving at the detector for the detection system of figure 12a and figure 12e shows a potential gating system for the detector used in figure 1 2a; Figure 13 a is a schematic of a quantum communication system which uses a protocol based on phase coherence between adjacent pulses, figure 13b is a plot of the pulse sequence which maybe sent using the system of figure 13a and figure 13c is a schematic of the gating requirements of the detectors of figure 13a; and Figure 14a is a schematic of a quantum communication system which uses a detector in accordance with the present invention and which is based on a differential phase shift protocol, figure 14b is a schematic of the pulse train sent from the transmitter to the detector; figure 14c is a schematic of the pulse train which passes through the short arm of the receiving interferometer, figure 14d is a schematic of the pulse train which passes through the long arm of the receiving interferometer, figure 14e is a schematic of the possible photon detection times by detector Dl of figure 14a and figure 14f is a schematic of the possible photon detection time s by the detector D2 of figure 14a.

Figure 1 a is a schematic of a known detection system which may be used to detect single photons. It comprises an avalanche photodiode 1 and a resistor 3. The avalanche photodiode (APD) 1 is configured in reverse bias. An input signal which acts as a gating signal is shown in figure lb is applied between input 5 and ground 7.

The input voltage shown in figure lb is a periodic series of rectangular voltage pulses, which varies between a first value V1 and a second value V0. V1 is selected to be above the breakdown voltage VBR of the avalanche photodiode 1. When such a voltage is applied to the avalanche photodiode, the detector becomes sensitive to incoming photons. An absorbed photon generates an electron-hole pair in the APD, which are separated by the electric field inside the APD. Due to the high electric field within the avalanche region of the APD, the electron or hole may trigger an avalanche of excess carriers causing a macroscopic and detectable current flow thorough the APD.

The macroscopic current is usually detected by monitoring the voltage drop across a resistor 3 as shown in figure lc. A voltage spike 13 indicates that a photon has been detected. However, as the APD has a finite capacitance, typically one pico-Farad, the output also contains a charging pulse 9 due to the charging of the APD capacitance when reacting to the rise edge of the gating pulse, followed by a discharging dip 11 due to the discharging of the APD capacitance when reacting to the falling bias at the falling edge of a gate pulse. The charging pulse is positive, and often obscures a photon induced avalanche.

It is clear from the results shown in figure 1 c that it is difficult to isolate peak 13 which is due to absorption of a single photon. For an avalanche due to photon absorption to be detectable, one method is to increase the APD bias voltage so that amplitude of an avalanche spike exceeds that of the charging pulse. An avalanche can then be detected by setting discrimination level above that of all charging pulses. However, such a method has serious shortcoming. APDs usually contain crystallographic defects, and those defects act as traps to confine electrons from the macroscopic avalanche current flow during a detection event. Trapped electrons will be released spontaneously after some delay causing a second, spurious avalanche when the following gate is applied.

Such a spurious pulse is called "afterpulse", and its probability depends on the size of the avalanche current. To limit the afterpulse probability, APDs are typically operated at low gating frequencies (up to a few MHz) if biasing is used to achieve avalanches larger than charging pulses.

To increase the APD operation frequency, it is necessary to remove or limit the amplitude of the charging pulse 9.

Figure 2a schematically illustrates a system which has been developed to address some of the above problems and is described in GB publication number 2447254). As before, the device comprises an avalanche photodiode 51 provided in series with a resistor 53.

An input signal as shown in figure 2b may be applied. This is identical to the input signal described with reference to figure lb. The voltage dropped across the resistor 53 is first input to power splitter 55. Power splitter 55 divides the output signal into a first part as shown in figure 2c and a second part which is identical to the first part shown in figure 2c. These two signals are then output via ports 57 and 59 of power splitter 55, The signal which is output via port 59 enters a delay line 56 which serves to delay the signal by a duration equal to the gating period. The delayed signal is shown in figure 2d. The first part of the signal and the delayed second part are then fed into hybrid junction 61. Hybrid junction 61 combines the first and the delayed second parts of the signals with 1800 phase shift to give the output shown in figure 2e.

As can be seen in figure 2c, the detection of a photon by APD 51 produces an avalanche signal 73. This avalanche signal is then repeated one period later in the trace of figure 4d. By combining 4c and 4d, a peak 77 and a dip 75 are seen in the trace of figure 2e which indicates the presence of a photon. The provision of a positive peak followed by a negative dip (or a negative dip followed by a positive peak dependent on the configuration of the equipment) allows a clear signature indicating the detection of a photon.

A preferred mode of detection is to test for both the presence of positive peak 77 and negative dip 75 in the output signal. Alternatively, however it may be more convenient to test for just the positive peak alone or just the negative dip alone. The peaks or dips may be detected using discrimination techniques. Discrimination techniques use a discriminator level. Voltage signals which are larger than said discriminator level are assumed to be due to detection of a photon.

The power splitter 55 may be of the type which is sold under part number ZFRSC-42+ from Mini-circuits and the hybrid junction is also available under part number ZFSCJ- 2-4 which is also available from Mini-circuits. The exact delay can be realised by using two co-axial cables with two different lengths that link the power splitter and the hybrid junction. It should be noted that the combination of the power splitter 55, the delay line 56 and the hybrid junction 61 may be integrated onto a single printed circuit board.

Figure 3 a is a schematic of a photon detection system in accordance with an embodiment of the present invention. The system is based on that described with reference to figure 2a. Therefore to avoid any unnecessary repetition, like reference numerals will be used to denote like features.

As described with reference to figure 2a, power splitter 55 divides the signal form the APD 51 into two equal parts. These two signals are then output via ports 57 arid 59 of power splitter 55. The signal which is output via port 59 enters a delay line 56 which serves to delay the signal. However, the signal which is output via port 57 flows through a path and encounters variable delay line 58. Variable delay line 58 allows the system to be tuned to allow the delay between the signal passing from output 59 to be delayed by exactly one period (of the input signal as shown in figure 3b, or an integer multiple thereof) with respect to the signal which passes from output 57.

The system of figure 3a allows the delay between the two paths to be tuned in situ.

Thus, the system does not need to be matched to a specific input signal frequency when it is assembled. The system, through the use of the tuneable or variable delay line can be used with a range of input signal frequencies. Also, if during use, the input signal frequency drifts or the delay in the system drifts due to temperature or other effect, the delay of the system can be corrected to match that of the period of the input signal or an integer multiple thereof The variable delay line may be implemented by a number of techniques, for example with a co-axial line stretcher, PCB strip line and adjustable wiper contact, telescopic wave guide or any other method of extending the electrical path length.

The variable delay may be positioned in either or both of the paths of the first and second parts of the signal.

Figure 4 shows a variation on the device described with reference to figure 3. The device of figure 4a takes the output from an APD and resistor (not shown) and provides it to hybrid junction 81. Hybrid junction 81 splits the output into a first part and a second part is described with reference to the power splitter 55 of figure 4a. However, hybrid junction 81 also introduces a 1800 phase shift between the first part and the second part of the signal. The first part of the signal is output via output 83 and the second part is sent via output 85 into delay line 87. The delay between the first part of the signal and the second part of the signal is tuned using variable delay line 84.

The first part of the signal is shown in figure 4b and the second part of the signal which has passed through delay line 87 and is identical to figure 4b except being inverted and delayed by a clock period is shown as figure 4c.

The two signals shown in figures 4b and 4c are then combined to produce the output signal of figure 4d. The signals are combined during power combiner 89 which does not need to allow a phase shift to one of the signals since this has been already performed by hybrid junction 81.

The systems of figure 3a and figure 4a have both used combinations of power splitters/combiners and hybrid junctions. However, the hybrid junction may be replaced by a combination of phase shifters and power combiners. For example, a power combiner and a 180° phase shifter, or two power combiner/splitters with a 900 phase shift etc. In a further variation on the systems of figure 3a and figure 4a, a tuneable RF attenuator is provided which may be used in either of the inputs to hybrid junction 61 (figure 3a) or power combiner 89 (figure 4a) to ensure that the two signals reach the hybrid junction or power combiner with equal amplitudes.

In a yet further variation on the systems of figure 3a and 4a, a tuneable RF slitter is provided which can be used to adjust the voltage amplitude in each arm thus improving the cancellation of the periodic waveform.

Typically, all the hybrid junctions and power splitter/combiners have finite response frequency range. For example, hybrid junction, Mini-circuits ZFSCJ-2-4 has a frequency range of 50MHz to 1 GHz. It may not work well when signal contains frequency components outside of the range, and the cancellation may not be perfect. To improve signal to noise ratio, further bandpass filters may be used to filter out those frequency components. In Figure 3a, for example, a low bandpass filter may be placed after the hybrid junction output.

Figure 5 shows a further variation on the system described with reference to figures 3 and 4.

The system of figure 5a has an avalanche photodiode 51 and a resistor 53 as described with reference to figure 3a. Further, the voltage dropped across the resistor 53 is taken to power splitter 55 which splits the signal into a first part and a second part. The first part being outputted via output 57 and the second part via output 59 into delay line 56.

The second part of the signal also passes through variable delay line 58 which allows the delay between the first and second parts of the signal to be tuned as described above.

The first part of the signal and the delayed second part are then fed into hybrid junction 61 which combines the two parts of the signal with 1800 phase difference.

However, in the apparatus of figure 5a, the input voltage signal is a sinusoidal voltage signal as shown in figure Sb and not the periodic train of rectangular pulses as shown in figure 3b. It is possible to bias the detection system of figure 5a with a sinusoidal signal as long as the signal has sufficient voltage swing to bias the APD above and below the threshold for avalanche breakdown. In fact, the detector may be biased with any periodic voltage signal.

In response to the sinusoidal gating voltages, the APD output is also sinusoidal.

Superimposed on the sinusoidal output are occasional avalanche spikes due to photon detection. The amplitude of avalanche spikes is typically much smaller than that of the sinusoidal output. However, as described previously with reference to Figure 3a, by using a power splitter, delay line, and hybrid junction, the sinusoidal components can be largely cancelled and the avalanche spikes become clearly visible.

Furthermore, any small remaining components of the sinusoidal signal may be removed from the output of the hybrid junction 61 by a band rejection filter 63 which is tuned to the frequency of the sinusoidal signal. The signal is passed to amplifier 65 and then discriminator 67 in order to determine the presence or absence of photon induced spikes in the outputted signal.

Figure 6 shows a further variation in the system of figure 3, the configuration is the same as that described with reference to figures 3a, 4a and 5a. The output signal is then fed into power splitter 101. Power splitter 101 divides the signal into a first part and a second part. The first part is output via output 103 and the second part via output 105 which is further fed into delay line 107. The second part of the signal is also passed through variable delay line 108 which is used to tune to delay between the two parts of the signal as described above. The two parts of the signal are then fed into differential amplifier 109. Due to the configuration of amplifier 109, only the difference of the two inputs is amplified. The output signal is not shown but will be similar to that shown in figure 3e.

Figure 7 is a plot of the output of the detection system of figure 3a using an APD driven with a square wave operating at -1 GHz. The dark count probability per gate is plotted against photon detection efficiency. Also shown is the efficiency versus afterpulse probability. The APD is cooled at -30°C. To measure the detection efficiency, the APD was illuminated with laser pulses with a wavelength of 1550nm, a duration of 100 ps, a repetition rate of-16MIHz, and an intensity of 0.1 photons per pulse on average.

In the measurement, the output of hybrid junction 61 (figure 3a) was fed into first a broadband amplifier, and then a discriminator which identifies each individual avalanche spike and converts the spike into a TTL pulse output. The TTL pulses are then counted by a pulse counter. The APD gating signal is square wave with 6-7V amplitude, superimposed on a DC bias voltage typically 2V below the avalanche breakdown voltage of the APD (which is 52.6 V). The points in Figure 7 were recorded by varying the DC bias level. The higher the DC bias, the higher the detection efficiency. However, the detector dark count probability of the detector, defined as the output probability in the absence of any input light, also increases with the DC bias.

For an efficiency of 10%, a dark count probability of 3x 1 06 is measured which is very similar to a "gold standard" detector. The afterpulse probability is better than 2% for 10% detector efficiency.

The results from figure 7 are surprisingly good since the APD is being driven with a frequency 1GHz. This is considerably higher than the usual gating frequencies for APDs which are typically a few MBz. At the same time, little deteriotation has been found in the efficiency of the same device and dark count probability as compared to low frequency operations. The ability to drive an APD at this frequency is possible due to seif-differencing technique described in the present invention.

The high gating frequency allowed by the present invention means that the detection system can operate quasi-continuously. In quasi-continuous operation, there is no synchronization required between the source of photons and the detector. Quasi continuous operation is possible because for very high gating frequencies, the period at which the detector is incapable of detecting photons because its bias is below the breakdown voltage of the APD, is not prohibitive to the overall detection efficiency.

In order to avoid any unintentional synchronisation of the source and the detector it is desirable to vary the frequency of the signal used to gate the APD, for example the drive frequency may be varied randomly by applying some noise to the frequency.

In such a scheme, the delay line which introduces a time delay between the first part and the second part of the signal remains constant. However, the gating frequency may vary by a small amount, for example, 50 kHz, which essentially broadens the time window over which the detector is capable of detecting single photons.

Figure 8 shows the experimental raw count rate (squares) of the detector versus j.iv (photon flux x detector efficiency product). This shows the detector response is linear over a wide range before it saturates and then starts to drop. The solid curve is a simulation showing the expected count rate. As expected, the count rate drops for a certain photon flux as the avalanches are starting to be cancelled by subsequent avalanches.

Figure 9a shows a system in accordance with a further embodiment of the present invention with a cascade arrangement. The input signal is shown in figure 9b.

In figure 9a, there are two self differencing stages. The use of cascading self differencing stages allows for removal of residual signal variations which were not removed by the first or preceding stages. This eases the stringent requirements for perfect matching of the delay to the period of the input signal in a single stage.

In figure 9a, two cancellation stages 50 and 150 are seen, but three or more stages may be provided.

The APD 51 and the first stage 50 are arranged in the same manner as described with reference to figure 3a. Therefore., to avoid unnecessary repetition, like reference numerals will be used to denote like features.

The combined signal output from first cancellation stage 50 is then directed into second stage 150. Power splitter 155 divides the signal from the first stage 50 into two equal further parts. These two signals are then output via ports 157 and 159 of power splitter 155. The signal which is output via port 159 enters a delay line 156 which serves to delay the signal. However, the signal which is output via port 157 flows through a path and encounters variable delay line 158. Variable delay line 158 allows the system to be tuned to allow the delay between the signal passing from output 159 to be delayed by exactly one period (of the input signal as shown in figure 9b, or an integer multiple thereof) with respect to the signal which passes from output 157. The first further part of the signal and the delayed second further part are then fed into hybrid junction 161.

Hybrid junction 161 combines the first and the delayed second parts of the signals with 180° phase shift.

Figure lOa is a schematic of the output of the combined signal from a first stage where the cancellation between the two parts of the signal is not perfect. Figure lOb is the output from two cancellation stages in the system of the type described with reference to figure 9a.

The cascading system has been explained with reference to the self differencing stage of figure 3a. However, it could also be used with the stages described with reference to figures 4 to 6.

Also, in figure 9a, a variable delay line is seen in both cancellation stages. However, the system may also function without this component. Of course, in the absence of a variable delay line, a stage cannot be tuned to the frequency of the input signal after the system has been assembled.

Figure 11 shows a further example of a cancellation stage which is based on stubs.

As before, in Figure 11 a the system comprises an avalanche photodiode 51 provided in series with a resistor 53. An input signal as shown in figure 1 lb may be applied. This is identical to the input signal described with reference to figure lb. The output from the junction between the APD 51 and the first resistor 53 is fed into second resistor 171 and split using a simple "T" piece 173 into a first part which follows path 1 and a second part which follows path 2 and is sent down first stub 175. The stub 175 terminates in a short circuit to ground 177 which causes the signal to be inverted and reflected. The reflected signal follows path 2 on the figure and combines with the first part of the signal at "T" piece 173.

The length of the stub 175 is chosen such that the second part of the signal is delayed by exactly one period with respect to the first part. The first stub 175 is variable such that it can be tuned to introduce a delay of a period after the system has been assembled.

The combined signal is then directed into second stage 179. The signal is then fed into third resistor 181 and split using second "T" piece 183 into a first part which follows path 1' and a second part which follows path 2' and is sent down second stub 185. The stub 185 terminates in a short circuit to ground 187 which causes the signal to be inverted and reflected. The reflected signal follows path 2 on the figure and combines with the first part of the signal at "T" piece 183.

The length of the stub 185 is chosen such that the second part of the signal is delayed by exactly one period with respect to the first part. The second stub 185 is variable such that it can be tuned to introduce a delay of a period after the system has been assembled.

The combined signal 189 is then output.

Third or fourth stages may be added. Alternatively, there may just be a single stage.

Although the system has been described with T-pieces, it is possible to use other components which can divide an electrical signal.

The first and or second stubs do not have to be variable in situ. They may both be fixed.

However such a system would have the disadvantage that the delay time could not be tuned once the system had been assembled.

Also, in figure ha, the first 175 and second 185 stubs tenninate in short circuits.

However, the stubs could also terminate in open circuits. If the stubs terminate in open circuits the signal will be reflected, but not inverted. Therefore an inverting component will need to be added in order to achieve cancellation.

In figure 11 a, the signal is transmitted along coax cables and variable delay lines are provided by coax line stretchers. However, other systems may be used such as transmission lines, strip lines, micro strip lines or waveguides.

The values of the second and third resistors are selected to impedance match the circuit and to minimise reflections back to the input. In one example of the above system, resistors of 50 Ohms were used.

The detection systems described in figures 3 to 11 can be used in quantum cryptography systems, for example, the system of figure 12.

In figure 12a, a sender (Alice) 201 sends photons to a receiver (Bob) 203. The sender and receiver are linked by an optical fibre 205.

Alice generates single photons, which she encodes and sends to Bob, along with a bright laser pulse to act as a clock signal.

Alice's equipment comprises a single photon source 207. The single photon source is made from a pulsed laser diode 209 and an attenuator 211. The laser produces a single optical pulse for each clock signal with a repetition period of TCOCk. Typically each laser pulse has a duration of diaser = 5Ops. The level of attenuation is set so that the average number of photons per pulse which are sent by Alice are much less than 1 (t <<1), for example t = 0.1 is typical. Alternatively, the level of attenuation may be varied from pulse to pulse as described in GB2404 103.

A clock signal is provided to the laser 209 by bias electronics 210. The bias electronics may comprise a timing unit, a driver for the signal laser 209, a driver for the clock laser 227 which will be described later and a driver for the phase modulator 223 which will be later described.

The photon pulses from the photon source 207 are then fed into an imbalanced Mach-Zender interferometer 213. The interferometer 213 consists of an entrance fibre coupler 215, a long Arm 217 with a delay ioop of fibre 219 designed to cause an optical delay, a short arm 221 with a phase modulator 223, and an exit fibre coupler 225 which combines the fibres 217 and 221 from the long and short arms respectively. The length difference of long and short arms corresponds to an optical propagation delay Of tdeiay.

Typically the length of the delay ioop 219 is chosen to produce a delay tjelay 0.5ns. A photon travelling through the long arm will lag that travelling through the short arm 221 by a time of tdelay at the exit of the interferometer 213.

The output of Alice's interferometer 213 is multiplexed with the output from a bright clock laser 227 at a WDM coupler 229. The clock laser 227 operates imder the control of the biasing circuit 210. The clock laser 227 may emit at a different wavelength from that of the signal laser 209, so as to facilitate their easy separation at Bob's 203 end.

For example the signal laser 209 may operate at 1.3 m and the clock laser 227 at 1.55 im or vice versa.

The multiplexed signal and clock pulses are transmitted to the recipient Bob 203 along optical fibre link 205.

Bob's equipment 203 is similar to Alice's equipment 201. Bob's equipment 203 comprises a WDM coupler 231 which is used to de-multiplex the signal received from Alice 201 into a signal from Alice's bright clock laser 227 and the pulses from Alice's signal laser 209.

The bright clock laser 227 signal is routed to an optical receiver 233 to recover the clock signal for Bob to synchronise with Alice. The optical receiver 233 transfers this signal to biasing circuit 255. Biasing circuit 255 synchronises various parts of Bob's equipment 203.

The signal pulses are fed into a polarisation controller 235 to restore their original polarisation.

The signal pulses then pass Bob's interferometer 237. Bob's interferometer 237 is similar to Alice's interferometer and has a long Arm 239 which comprises an optical fibre delay loop 241 and a variable fibre delay line 243. The short arm 45 of the interferometer 39 comprises a phase modulator 247. Phase modulator 247 is controlled by biasing circuit 255 in accordance with the signal received from clock laser 227.

The long arm 239 and the short arm 245 of the interferometer are connected to a 50/50 fibre coupler 249 with a single photon detector 251 and 253 attached to each output arm of the fibre coupler 249. The single photon detector 251 attached to one arm of the coupler 249 will be referred to as detector A and the single photon detector 253 attached to the other arm of the output coupler 249 will be referred to as detector B. Photon detectors 251 and 253 are controlled by biasing circuit 255 in accordance with the signal received from clock laser 227.

The variable delay line 243 at Bob's interferometer is adjusted to make the optical delay between its two arms 239 and2 45 identical as that between the arms of Alice's interferometer 213, tdelay.

There are four possible paths for a signal pulse travelling from Alice's signal laser 209 to Bob's single photon detectors 251 and 253: i) Alice's Long Arm 217 -Bob's Long Ann 239 (Long-Long); ii) Alice's Short Arm 221-Bob's Long Arm 239 (Short-Long); iii) Alice's Long Arm 219--Bob's Short Arm 245 (Long-Short); and iv) Alice's Short Ann 221-Bob's short arm 245 (Short-Short).

Bob's interferometer 237 is balanced by adjusting the variable delay 243 so that photons taking paths (ii) and (iii) arrive at nearly the same time, at the exit coupler 249 of Bob's interferometer. Nearly the same time means within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode.

Figure 1 2b is a trace of the clock which is output from laser 209 to the receiver 203.

Typically the clock signal has a repetition of 1 GHz. Figure 12c is a trace of the laser pulse which is used to generate the signal pulses.

Figure 12d is a plot of the optical signal seen by the detectors 251 and 253 of receiver 203. Photons taking paths (ii) and (iii) corresponds to the large central peak in Figure 1 Od. Photons taking path (i) have a positive delay tdelay (later arrival time), and those taking path (iv) have a negative delay tdclay (early arrival time) compared to paths (ii) and (iii). These form the smaller satellite peaks of figure lOd. Only photons arriving in the central peak shown in figure 1 Od undergo interference and are encoded by both Alice and Bob. Thus only these photons are of interest.

Figure 1 2e is a plot of the gating bias which is in synchronization with the clock bias shown in figure 12b. Bob gates his detectors 251, 253 to record only photons in the central peak and not those in the earlier or later satellite peak.

By controlling the voltages applied to their phase modulators 223, 247, Alice and Bob determine in tandem whether paths (ii) and (iii) undergo constructive or destructive interference at detectors A and B, 251, 253.

The variable delay 243 can be set such that there is constructive interference at detector A 251 (and thus destructive interference at detector B 253) for zero phase difference between Alice and Bob's phase modulators. Thus for zero phase difference between Alice's and Bob's modulators and for a perfect interferometer with 100% visibility, there will be a negligible count rate at detector B 253 and a finite count rate at A 251.

If, on the other hand, the phase difference between Alice and Bob's modulators is 1800, there should be destructive interference at detector A 251 (arid thus negligible count rate) and constructive at detector B 253. For any other phase difference between their two modulators, there will be a finite probability that a photon may output at detector A 251 or detector B. In the four-state protocol, which is sometimes referred to as B384, Alice sets the voltage on her phase modulator to one of four different values, corresponding to phase shifts of 0°, 90°, 180°, and 270°. Phase 0° and 180° are associated with bits 0 and 1 in a first encoding basis, while 90° and 270° are associated with 0 and 1 in a second encoding basis. The second encoding basis is chosen to be non-orthogonal to the first.

The phase shift is chosen at random for each signal pulse and Alice records the phase shift applied for each clock cycle.

Meanwhile Bob randomly varies the voltage applied to his phase modulator between two values corresponding to 0° and 90°. This amounts to selecting between the first and second measurement bases, respectively. Bob records the phase shift applied and the measurement result (i.e photon at detector A 251, photon at detector B 253, photon at detector A 251 and detector B 253, or no photon detected) for each clock cycle.

In the BB84 protocol, Alice and Bob can form a shared key by communicating on a classical channel after Bob's measurements have taken place. Bob tells Alice in which clock cycles he measured a photon and which measurement basis he used, but not the result of the measurement. Alice then tells Bob the clock cycles in which she used the same encoding basis and they agree to keep only those results, as in this case Bob will have made deterministic measurements upon the encoded photons. This is followed by error correction, to remove any errors in their shared key, and privacy amplification to exclude any information known to an eavesdropper.

Detectors 251 and 253 of receiver 203 in figure 12a may conveniently be provided by the detection systems discussed with reference to figures 3 to 11.

Figure 12a is a quantum communication system which may be used for the well-known BB84 protocol or B92 protocol. Recently, other quantum communication systems have been developed which use different protocols. The detector of the present invention with its higher frequency gated operation or quasi-continuous operation is particularly advantageous for these new types of quantum communication systems.

Figure 13 shows quantum communication apparatus for another type of one-way quantum key distribution scheme. A sender's apparatus 301 comprises a coherent laser 303 which outputs a continuous pulsed beam into intensity modulator 305. Intensity modulator 305 either transmits the pulse or blocks it almost completely. The output from the intensity modulator 305 is then passed into attenuator 307 which attenuates the beam so that each pulse contains less than 1 photon on average. The output is then passed along fibre 309 to receiver 311.

At receiver 311, the beam first encounters beam splitter 315. Beam splitter 315 is configured to pass most of photons along path 317 and the remainder along path 319 into interferometer 320.

Path 317 extends to qubit detector 320 which is a detection system as described with reference to any of figures 4 to 9. Path 319 directs photons to 50150 beam splitter 321 which sends photons either along long arm 323 or short arm 325. Photons which pass along long arm 323 encounter phase shifter 327. The photons from path 323 and 325 are then combined by a beam splitter 329 which outputs photons either into detector 331 or into detector 333 dependent on the phase correlation between the photons.

Figure 13b shows how bits may be encoded by the sender 301. Each bit consists of two "pulses". Bit 0 is encoded by a pulse sequence which comprises a first pulse which has intensity and a second pulse with intensity 0 and bit 1 is encoded with a first pulse which has intensity 0 and a second pulse which has intensity p.. In addition to sending bit 0 or bit 1, decoy states are sent which comprise two pulses with intensity p..

Due to the coherence of the laser 303, there is a well defined phase relationship between any two neighbouring non-empty pulses. Therefore, within each decoy sequence, there is coherence. There is also coherence between some of the sequences, for example in the case where bit 0 is followed by bit 1 If an eavesdropper intercepts the pulses, the coherence of adjacent non-empty pulses will be affected. This loss of coherence can be determined by interferometer 320.

The interferometer is configured (by applying an appropriate phase shift and an appropriate difference between the length of the long 323 and short 325 arms) to ensure that photons exit into just one of the two detectors 331 and 333 when interference takes place. Thus by monitoring the count rate in the other detector it is possible to detect eavesdropping. Detectors 331 and 333 are described with reference to Figures 4 to 9.

Alice sends a stream of pulses as shown in figure 13b. The receiver 302 will then inform Alice 301 for which pulses the qubit detector 320 is fired. Alice will then advise Bob which bits should be thrown away as they are due to decoy states. It should be noted that Bob only informs Alice in which sequence a count was received and does not inform Alice whether these bits were measured as bit 0 or bit 1.

The detectors 331 and 333 are used to monitor coherence. When two adjacent non-empty pulses pass through the interferometer 322, interference will take place between the early pulse passing thorough the long arm 323 and the late pulse passing through the short arm 325. Interference determines that there is a finite probability for detector 331 to fire, but negligible probability for detector 333 to fire, at this particular detection time bin. Violation of this means the loss of coherence. In the QKD, the receiver tells the transmitter when and which of the detectors 331 and 333 fires, and this allows the sender to establish whether or not coherence was broken.

The sender and receiver will then run error correction and privacy amplification on the results dependent on the loss of coherence which is well-known in the art to determine the secret key.

The detection systems described in figures 3 to 11 can conveniently be used with the communication system of figure 13 since they provide higher speed operation and can also work in CW mode. Figure 1 3c shows the gating voltage which is supplied to the APD's of the detection system. The gating pulse is synchronised with the expected arrival time of each incoming pulse. This gating system would be applied to the three detectors of the receiver 302 in figure 13a.

Figure 14 shows a further quantum communication system. The communication system of figure 14a also uses coherence between adjacent pulses in order to communicate a key. The sender 401 comprises a pulsed coherent laser 403 which outputs to a phase modulator 405. The phase modulator is randomly varied to apply a phase modulation of either 0 or 180 degrees. It is possible to apply some other phase shifts providing that the difference between the two phase shifts is 180. The signal is then attenuated by attenuator 407 which ensures that there is less than one photon per pulse. This is then transmitted down fibre 409 to receiver 411. Receiver 411 comprises an interferometer 413. Interferometer 413 comprises a first beam splitter 415 which directs photons either down short arm 417 or long arm 419. Long arm 419 comprises a phase shifter 421.

The long arm 419 and the short arm 417 are recombined at second beam splitter 423 which then outputs to a first detector 425 and a second detector 427.

The sender 401 sends a pulse train where each pulse is modulated by either a 00 phase shift or a phase shift of 180 degrees. This is shown in figure 14b. At the receiver's side, the received pulse train is split into two paths: the short arm 417 and the long arm 419. Figure 14c shows the pulse train at the short arm 417, and figure 14d shows the pulse train at the long arm 419. Notice that the pulse train in long arm is delayed by exactly a clock period. The interferometer 413 is configured by varying the phase shift or the relative length or arms 417 and 419 to essentially introduce a 1 bit delay.

Therefore, the interferometer 413 can be thought of as interfering the pulse train shown in figure 14c with the pulse train shown in figure 14d. The pulse train in figure 12d is the same as that of figure 1 4c but has a 1 bit delay. This results in the detection of pulses shown in figure 14e at detector 425 and the detection of pulses shown in figure 14f at detector 427.

The security relies upon the uncertainty in the detection time of a photon. Due to coherence, the photon's wavefunction is spread over a number of adjacent time bins. An eavesdropper eve's detection of a photon causes its wavefunction to collapse into a single time bin, and she is only able to know the phase difference between two particular time bins. Therefore she is not able to re-produce the original state which is coherent over a number of time bins. This prevents intercept-and-resend attack. It is also inherently secure against photon number splitting attack. Eve may split a photon from the pulse train, however, it will not necessarily collapse into the same time bin as that detected by Bob.

Previously, the differential phase shift QKD is implemented using a CW detector, and discrimination between neighbouring time bins is done in the post processing with a time-resolution of 10-ps. Such a post-processing requires either sophisticated hardware or software data processing. With detectors rulming in gated mode, time discrimination is automatic requiring no post processing.

Claims (20)

1. CLAIMS1. A photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal and wherein the system further comprises a variable path length component provided in the path of the first ancL/or second parts of the signal such that the delay in the second part of the signal is variable in-situ.
2. 2. A photon detection system according to claim 1, wherein the variable path length component comprises a variable electric path length element.
3. 3. A photon detection system according to claim 2, wherein the variable electric path length element is a co-axial line stretcher.
4. 4. A photon detection system according to any preceding claim, comprising means to apply a periodic gating signal to said detector and wherein said variable path length component is configured to tune the delay of the second part of the signal by an integer multiple of said period.
5. 5. A photon detection system according to any preceding claim, further comprising at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.
6. 6. A photon detection system according to claim 5, comprising a plurality of further stages arranged such that the combined signal outputted from one stage is provided as the input for the next further stage.
7. 7. A photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal, the system further comprising at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.
8. 8. A photon detection system according to any preceding claim, wherein the delay line is a stub configured to cause reflection of a part of the signal directed along said stub.
9. 9. A photon detection system comprising a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal and wherein the delay line is a stub configured to cause reflection of a part of the signal directed along said stub.
10. 10. A photon detection system according to either of claims 8 or 9, wherein the stub is grounded at its end to cause the reflected signal part to be inverted.
11. 11. A photon detection system according to any of claims 8 to 10, wherein the stub is provided is a transmission line, strip line, micro strip line or waveguide.
12. 12. A photon detection system according to any of claims 8 to 11, wherein the stub is adjustable in situ to allow tuning of the delay.
13. 13. A photon detection system according to any preceding claim, wherein the photon detector is an avalanche photodiode.
14. 14. A quantum communication system comprising a sender and a receiver, said sender comprising a source of pulsed radiation and an encoder for encoding information on said radiation pulses, said receiver comprising a detection system according to any preceding claim.
15. 15. A conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal and wherein the system further comprises a variable path length component provided in the path of the first and/or second parts of the signal such that the delay in the second part of the signal may be varied in-situ.
16. 16. A conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal, the system further comprising at least one further stage, wherein said further stage comprises: a divider for dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; a further delay line for delaying the second further part with respect to the first further part; and a combiner for combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.
17. 17. A conditioning circuit for extracting fluctuations from a periodic signal, the circuit comprising a signal divider to divide the periodic signal into a first part and a second part, where the first part is substantially identical to the second part, a delay line for delaying the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the signal and wherein the delay line is a stub configured to cause reflection of a part of the signal directed along said stub.
18. 18. A photon detection method comprising: providing a photon detector configured to detect single photons; providing a divider for dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; tuning the delay between the first and second paths in situ; dividing the output signal using said divider into a first part and a second part; delaying the second part with respect to the first part; and combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal.
19. 19. A photon detection method comprising: providing a photon detector configured to detect single photons; dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; delaying the second part with respect to the first part; combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal; dividing said combined signal into a first further part and a second further part where the first further part is substantially identical to the second further part; delaying the second further part with respect to the first further part; and combining the first and delayed second further parts of the signal such that the delayed second further part is used to cancel periodic variations in the first further part of the output signal.
20. 20. A photon detection method comprising: providing a photon detector configured to detect single photons; dividing the output signal of said photon detector into a first part and a second part, where the first part is substantially identical to the second part; delaying the second part with respect to the first part by directing the second part of the signal into a stub; and combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal.