DE102018105791A1 - Electrical control of an optical transition in a coherent quantum system - Google Patents

Abstract

A method for the electrical control of an optical transition in a coherent quantum system (1), wherein a quantum state is excited by means of laser pulses (2), is to be further developed so that the chirp necessary for a robust state preparation does not belong to the laser pulse (2) but to the quantum system (1) is imprinted. This is achieved in that the laser pulses (2) have a constant energy and the quantum system (1) has a chirp, wherein the chirp of the quantum system (1) is generated by transient energy detuning by means of Stark effect in time synchronism with the laser pulses (2) ,

Description

The invention relates to a method and a device for the electrical control of an optical transition in a coherent quantum system by means of laser pulses.

Also, the present invention relates to the use of the method for a semiconductor single photon emitter, as well as the use of the device in a semiconductor single photon emitter.

The coherent state preparation and state control of individual quantum systems is an important prerequisite for the implementation of functional quantum devices. Prominent examples of such systems are semiconductor single-photon emitters, which are constructed, for example, from quantum dot photodiodes with semiconductor quantum dots and are used in particular in quantum information processing and quantum cryptography. A fault-tolerant optical excitation of these single-photon emitters is of great importance here.

Methods are known from the prior art in which a robust state preparation in atoms or quantum dots is carried out by means of chirped laser pulses. These methods known from the prior art have the disadvantage that the instrumental outlay for providing chirped laser pulses is very high. The spectral properties of such laser pulses are determined by optical components such as gratings. The laser pulses are therefore not controllable or changeable on short time scales. Another disadvantage of the prior art methods is that when the optically active quantum systems are in photonic structures such as microresonators, the spectral properties of the laser pulses are modified and filtered before interacting with the quantum system.

Based on the above-mentioned prior art, the object of the invention is thus to provide a method for the electrical control of an optical transition in a coherent quantum system, in which the chirp necessary for a robust state preparation is impressed not on the laser pulse but on the quantum system becomes.

The object is achieved by the features of the independent claims. Advantageous embodiments of the invention are specified in the subclaims.

According to the invention, therefore, a method for the electrical control of an optical transition in a coherent quantum system is indicated, wherein a quantum state is excited by laser pulses, characterized in that the laser pulses have a constant energy and the quantum system has a chirp, wherein the chirp of the quantum system by transient energy detuning is generated synchronously in time to the laser pulses by means of Stark effect.

The basic idea of the present invention is therefore to impose the necessary chirp on the quantum system by transient energy detuning by means of Stark effect.

In an advantageous embodiment of the invention, the energy detuning is generated by an externally controllable, transient electric field. The Stark effect can be generated by applying an external electric field, it is advantageous that the Stark effect can be controlled exactly by a controllable electric field.

In a further advantageous embodiment of the invention, the quantum system is at least a single exciton in a quantum dot and for excitation, an electrically induced rapid adiabatic passage (RAP) is used. The Rapid Adiabatic Passage is a complete occupancy transfer process in a two-level system. The advantage here is that the rapid adiabatic passage can be used in a two-level system to generate a complete population inversion that is robust to the excitation amplitude.

In an advantageous embodiment of the invention, the quantum system is at least one bending exciton in a quantum dot and for excitation a sequential rapid adiabatic passage (RAP) is used, the excitation via the ground state | 0> to the one exciton state | X> to the bending exciton State | XX> occurs, whereby the excitation occurs within the duration of the laser pulse. The sequential rapid adiabatic passage is the implementation of two consecutive RAP processes by means of an externally controllable transient electric field, whereby the excitation of a bending citon by excitation of the ground state | 0> via a first RAP process to the one-exciton state | X> and the Excitation to the Biexziton state | XX> via a second RAP process in a sequential manner.

In a further advantageous embodiment of the invention, a cw laser is used in combination with the externally controllable transient electric field to excite the quantum state. The design can be applied particularly advantageously in quantum systems with a long service life and coherence time.

In an advantageous embodiment of the invention, the quantum system is embedded in a | μ-resonator. In this case, it is advantageous that a robust excitation of a quantum system embedded in a photonic structure, such as in a microresonator by rapid adiabatic passage, is only possible by means of the electrical chirp according to the invention, since with high resonator grades (typically over 10000) an optically chirped laser pulse the resonator mode would be heavily filtered and therefore lose its advantageous chirp properties.

In a further advantageous embodiment of the invention, a resonator is used for state control and / or for subsequent tuning of the energy of the quantum system. In a particularly advantageous embodiment of the invention, a | μ-resonator is used, in which an electrically tunable quantum system in the form of a p-i-n structure with a quantum dot is embedded within the i-layer.

In an advantageous embodiment of the invention, the quantum system is dynamically switched between the regions of the radiative recombination and of the photocurrent generation. By measuring the deterministic photocurrent, the completeness of the occupation transfer can be quantitatively verified.

In a further advantageous embodiment of the invention, a BiCMOS chip is used to generate the externally controllable, transient electric field. On the SiGe: C and CMOS technology-based chips are characterized by a particularly high operating frequency, very good driver characteristics and advantageous operating parameters for controlling quantum systems in terms of current, voltage and temperature.

In an advantageous embodiment of the invention, a quantum dot photodiode is used as the quantum system. In a particularly advantageous embodiment of the invention, the quantum dot photodiode can consist of an n-i Schottky diode based on GaAs with InGaAs quantum dots, the InGaAs quantum dots being embedded in the i-layer. In a further particularly advantageous embodiment of the invention, a pin diode is used.

Furthermore, an apparatus for carrying out the method is specified according to the invention.

Also according to the invention, the use of the device is given in a semiconductor single photon emitter.

Furthermore, according to the invention, the use of the method for a semiconductor single photon emitter is specified.

As essential building blocks of quantum information technology, efficient emitters of individual photons are of fundamental interest. Therefore, the use of the device according to the invention in a semiconductor single photon emitter, as well as the use of the inventive method for a semiconductor single photon emitter is particularly advantageous.

The invention will be explained in more detail with reference to the accompanying drawings with reference to preferred embodiments. The illustrated features may represent an aspect of the invention both individually and in combination. Features of various embodiments are transmittable from one embodiment to another.

• 1a ein Energie/Zeit Diagramm eines gechirpten Laserpulses und eines nichtgechirptes Quantensystems aus dem Stand der Technik,
• 1b ein Amplituden/Zeit Diagramm eines gechirpten Laserpulses aus dem Stand der Technik,
• 2a ein Energie/Zeit Diagramm eines nicht-gechirpten Laserpulses und eines gechirpten Quantensystems gemäß einem Ausführungsbeispiel der Erfindung,
• 2b ein Amplituden/Zeit Diagramm eines nicht-gechirpten Laserpulses gemäß einem Ausführungsbeispiel der Erfindung,
• 3 den prinzipiellen Aufbau einer Vorrichtung zur Durchführung des Verfahrens gemäß einem Ausführungsbeispiel der Erfindung,
• 4a ein Energie/Zeit Diagramm eines nicht-gechirpten Laserpulses und die Erzeugung eines Biexzitons durch sequenzielle RAP gemäß einem weiteren Ausführungsbeispiel der Erfindung,
• 4b ein Amplituden/Zeit Diagramm eines nicht-gechirpten Laserpulses gemäß einem weiteren Ausführungsbeispiel der Erfindung,
• 5 den prinzipiellen Aufbau einer Vorrichtung zur Durchführung des Verfahrens gemäß einem weiteren Ausführungsbeispiel der Erfindung, wobei das Quantensystem in einer photonischen Struktur eingebettet ist,
• 6a ein Energie/Zeit Diagramm eines nicht-gechirpten Laserpulses und eines gechirpten Quantensystems gemäß einem Ausführungsbeispiel der Erfindung, dessen Energie nach erfolgter Anregung eingestellt wird, und
• 6b ein Amplituden/Zeit Diagramm eines nicht-gechirpten Laserpulses gemäß einem Ausführungsbeispiel der Erfindung.

Show it:

• 1a an energy / time diagram of a chirped laser pulse and a non-chirped quantum system of the prior art,
• 1b an amplitude / time diagram of a chirped laser pulse of the prior art,
• 2a an energy / time diagram of a non-chirped laser pulse and a chirped quantum system according to an embodiment of the invention,
• 2 B an amplitude / time diagram of a non-chirped laser pulse according to an embodiment of the invention,
• 3 the basic structure of an apparatus for carrying out the method according to an embodiment of the invention,
• 4a an energy / time diagram of a non-chirped laser pulse and the generation of a bending cite by sequential RAP according to another embodiment of the invention,
• 4b an amplitude / time diagram of a non-chirped laser pulse according to a further embodiment of the invention,
• 5 the basic structure of an apparatus for performing the method according to another embodiment of the invention, wherein the quantum system is embedded in a photonic structure,
• 6a an energy / time diagram of a non-chirped laser pulse and a chirped quantum system according to a Embodiment of the invention, whose energy is adjusted after excitation, and
• 6b an amplitude / time diagram of a non-chirped laser pulse according to an embodiment of the invention.

1a shows an energy / time diagram of a chirped laser pulse 2 and a non-chirped quantum system 1 from the prior art. In the current state of the art, the known concept of robust state preparation in atoms or quantum systems 1 by means of chirped laser pulses 2 carried out. 1a shows the energy evolution of a quantum system 1 over time, being the quantum system 1 is non-chirped and has a constant energy. The energy of the laser pulse 2 is relative to the quantum system 1 initially lower in energy over time and thus red-shifted 3. Within the interaction time between the laser pulse 2 and the quantum system 1 caused by the duration of the laser pulse 2 is given, makes the laser pulse 2 an energetic transition from a red-detuning of energy 3 to a blue-mood of energy 4 ,

In 1b is the chirped laser pulse 5 shown with its oscillatory temporal field profile from the prior art, wherein the temporal change of the frequency can be seen.

In 2a is an energy / time diagram of a non-chirped laser pulse 2 and a chirped quantum system 1 shown according to an embodiment of the invention. The energy of the laser pulse 2 is constant. When the laser pulse arrives 2 the quantum system makes an energetic transition from a blue detuning of the energy 3 to a red-upset 4 relative to the energy of the laser 2 , This case can be considered analogous to the optically chirped excitation, in which the laser pulse 2 has a chirp, the quantum system 1 however, it has a constant energy. In 2 B is the non-chirped laser pulse 6 represented with its oscillatory temporal field profile, wherein the frequency does not change with time.

In 3 the basic structure of an apparatus for carrying out the method for the electrical control of an optical transition in a coherent quantum system by means of laser pulses 6 shown according to an embodiment of the invention. The laser pulses are in a laser source 16 For example, in a titanium: sapphire laser produced. For every laser pulse 6 gets from the laser 16 a trigger signal 15 output. The optical laser pulse 6 is via a beam splitter 17 to a QD photodiode 7 guided. The central wavelength of the laser pulse 6 is set so that the quantum dot 8th resonantly excited.

As mentioned above, each laser pulse 6 from the laser source 16 an electrical trigger signal 15 output and to a controllable electrical delay unit 14 forwarded. The delay unit 14 generates a relative to the optical pulse 6 adjustable delay of the electrical signal. The output is fed to a BiCMOS chip 13. This chip generates to every electrical trigger signal 15 the laser 16 an electrical pulse 12 which is connected to the photodiode 7 is created. The rise and fall times of the electrical pulse 12 are less than 100 ps and significantly shorter than the coherence time of a quantum dot exciton.

The delay unit 14 is set so that the rising or falling edge of the electrical signal 12 in time with the optical pulse 6 is synchronized. The resulting electrically induced chirp of the energy of the quantum system 8th thus results in excitation with a non-chirped optical pulse 6 to a complete population inversion via Rapid Adiabatic Passage. This arises in the quantum dot 8th exactly one exciton whose radiative recombination is the emission of exactly one photon 18 entails. The resulting photon is at the beam splitter 17 decoupled.

In 4a is an energy / time diagram of a non-chirped laser pulse 2 and the creation of a bending citation 21 by sequential rapid adiabatic passage according to another embodiment of the invention. By means of the sequential Rapid Adiabatic Passage, two excitatory RAP processes, the excitation of an exciton first 20 by excitation of the ground state | 0> via a first RAP process to the one exciton state | X> 20 and then the excitation from the one exciton state | X> 20 to the exciton state | XX> 21 via a second RAP process , In this sequential process, they initially become laser energy 2 Red-shifted 4 exciton 20 and Biexziton 21 Transition energies electrically in the region of the blue shift 3 chirped. As for the excitation from the ground state to the exciton 20 usually higher energy is required than excitatory excitement 20 to the Biexziton 21 (positive Biexziton binding energy in the quantum dot), in a chirp from the red to the blue shift, the excitation of the exciton 20 through the non-chirped laser pulse 2 shortly before the excitement from exciton to biexciton 21 , The sequential excitation for bending excitement can be carried out so advantageously in the correct order. For quantum dots with negative Biexziton binding energy, the chirp can be carried out analogous to the blue to red shift.

In 4b is the non-chirped laser pulse 6 shown with its oscillatory temporal field profile according to another embodiment of the invention, wherein the frequency does not change with time.

In 5 the basic structure of an apparatus for performing the method according to another embodiment of the invention is shown, wherein the quantum system 1 in a photonic structure 22 is embedded. In the device shown, the electrically tunable quantum system is a pin diode with a laterally contacted p-layer 23 and n-layer 9 executed. The quantum dot 8th is in the center of the i-layer. In 5 is shown that the quantum system in a μ-resonator 22 is embedded. Due to the low longitudinal fill factor resonator mirrors 24 . 25 necessary with high reflectivity. Therefore, and because of the vertical structure offer here especially Bragg mirrors 24 . 25 which consist of several layers with alternately high and low refractive index. The in 5 shown μ-resonator 22 consists of a semitransparent Bragg mirror 24 , which allows the coupling of light, and a fully reflecting Bragg mirror 25 , However, a high quality of the resonators above about 10,000 leads to spectrally narrow resonator modes and to a strong, unfavorable spectral filtering of optically chirped laser pulses. Therefore, an excitation of a quantum system embedded in a photonic structure is 8th by rapid adiabatic passage only possible by means of the electrical chirp according to the invention.

In 6a is an energy / time diagram of a non-chirped laser pulse 2 and a chirped quantum system 1 according to an embodiment of the invention in which a complete population inversion is generated by means of rapid adiabatic passage. In the time domain after the excitation, the energy of the quantum system becomes by electrical tuning 26 by means of strong effect spectrally AE tune. This method makes it possible to tune the comparatively slow emission compared to the excitation in energy, for example to achieve a predetermined target energy or the energy of a resonator mode.

In 6b is the non-chirped laser pulse 6 shown with its oscillatory temporal field course according to an embodiment of the invention, wherein the frequency does not change with time.

The invention underlying this patent application was developed in a project which was funded under the grant mark SFB TRR 142, TP C04 by the Deutsche Forschungsgemeinschaft.

LIST OF REFERENCE NUMBERS

1

Energy quantum system

2

Energy laser pulse

3

Quantum system blue-shifted to the energy of the laser pulse

4

Quantum system red-shifted to the energy of the laser pulse

5

Chirped laser pulse

6

Non-chirped laser pulse

7

Quantum dot photodiode

8th

quantum dot

9

n ⁺ contact of the quantum dot photodiode

10

Schottky front contact of the quantum dot photodiode

11

Semitransparent Schottky contact

12

Electric signal

13

BiCMOS chip

14

Controllable delay unit

15

trigger signal

16

Laser Source

17

beamsplitter

18

Emission of single photons

19

Photocurrent measurement

20

| X> exciton condition

21

| XX> Biexciton Condition

22

μ resonator with quantum dot photodiode

23

p ⁺ contact of the quantum dot photodiode

24

Partly transparent Bragg mirror

25

Fully reflective Bragg mirror

26

Electrical adjustment of the emission energy of the quantum system

Claims (13)

Method for electrically controlling an optical transition in a coherent quantum system (1), wherein a quantum state is excited by means of laser pulses (2), characterized in that the laser pulses (2) have a constant energy and the quantum system (1) has a chirp, the chirp of the quantum system (1) is generated by transient energy detuning by means of Stark effect in time synchronism with the laser pulses (2). Method according to Claim 1 characterized in that the energy detuning by a externally controllable transient electric field is generated. Method according to Claim 1 or 2 characterized in that the quantum state is at least a single exciton and for the excitation of an electrically induced rapid adiabatic passage (RAP) is used. Method according to Claim 1 or 2 characterized in that the quantum state is at least a Biexziton and for excitation a sequential Rapid Adiabatic Passage (RAP) is used, wherein the excitation of the ground state | 0> to the one-exciton state | X> (20) to Biexziton state | XX> (21) takes place, wherein the excitation within the duration of the laser pulse (2) takes place. Method according to one of the preceding claims, characterized in that for excitation of the quantum state, a cw laser is used in combination with the externally controllable transient electric field. Method according to one of the preceding claims, characterized in that the quantum system (1) is embedded in a μ-resonator (22). Method according to one of the preceding claims, characterized in that a resonator (22) is used for state control and / or for subsequent tuning of the energy of the quantum system (1). Method according to one of the preceding claims, characterized in that the quantum system (1) is dynamically switched between the regions of the radiative recombination and of the photocurrent generation. Method according to one of the preceding claims, characterized in that a BiCMOS chip (13) is used to generate the externally controllable transient electric field. Method according to one of the preceding claims, characterized in that a quantum dot photodiode (7) is used as the quantum system. Apparatus for carrying out the method according to one of Claims 1 to 10 , Use of the device according to Claim 11 in a semiconductor single photon emitter. Use of the method according to one of Claims 1 to 10 for a semiconductor single photon emitter.

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