Device for generating single photons and entangled photon pairs

Abstract

An embodiment of the present invention provides a device for generating photons (300) comprising a quantum box (301) inserted into a micropillar-type optical cavity (302) having at least one optical mode, the quantum box (301) having at least one ground state and two states with one elementary excitation, the optical cavity (302) having a bottom surface and a top surface, the bottom surface having an electrical contact (305), and the photon generating device (300) advantageously comprising at least three electrical bonding pads (304a, 304b, 304c) electrically isolated from one another and arranged around the periphery of the top surface of the cavity (302).

Description

Technical Field

[0001] The present invention generally relates to sources of photons, and more particularly to devices for generating single photons and entangled photon pairs, and methods of implementing such devices.

Background Art

[0002] The deployment of quantum communication networks requires the provision of single photon sources capable of generating a series of optical pulses each containing one (just one) photon. Such sources can provide secret keys that enable subsequent communications to be encrypted to two remote nodes. Imperfections in the propagation channel, whether optical fiber or free space, limit the range of such applications to a few hundred kilometers. The range of quantum communication networks can be extended by quantum repeaters that rely on entanglement of pairs of photons. The generation of pulses with single photons or entangled photon pairs can be performed by nanometer-sized emitters such that the transitions between their electronic states correspond to the emission of a single quantum dipole.

[0003] Furthermore, the generation of quantum computers depends on systems that can be operated as qubits. Qubits are coherent superpositions of two basic states, usually represented as |0> and |1>, such that they can adopt an infinite number of possible values, contrary to binary bits that are operated by conventional computers with values of 0 or 1. Qubits can be of solid state or photonic nature. The former type offers flexibility in terms of storage and processing, while the latter photonic qubits enable quantum information to be carried over long distances. To enable the processing and transport of quantum information, it is common practice to use both qubits in solid state and photonic qubits associated with the interface between two elements.

[0004] The use of quantum boxes in applications including the deployment of quantum communication networks and the creation of quantum computers has seen significant growth in recent years. The interest in such nanometer structures is explained by their similarities to atoms in terms of charge carrier confinement and the quantization of their energy levels. These properties allow quantum boxes to generate single photons via a process of spontaneous radiation in response to favorable light adaptation, which enables the movement of electrons from the valence band to the conduction band. This mechanism can be adapted to place two electrons at the same energy level in the conduction band and generate entangled photon pairs. Quantum boxes also enable reliable interactions of light with the properties of confined charge carriers.

[0005] Isolated quantum boxes used as sources of single photons or entangled photon pairs are generally limited in terms of luminance and bitrate. Luminance quantifies the probability of having a single photon or entangled photon pair per optical pulse, while bitrate measures the number of photons emitted per second and is equal to the product of the above-defined luminance and the rate of the emission clock that governs the optical pulses exciting the quantum box. To improve the performance level of quantum boxes in terms of luminance and bitrate, it is a known method to insert the quantum box into a resonant optical cavity. Such a configuration utilizes the Purcell effect (also known as the "low light-material coupling system") to increase the initial spontaneous generation ratio of quantum boxes in cavity mode and to allow for more effective collection of photons emitted by the quantum box. Encapsulating quantum boxes within optical cavities also allows for reduced sensitivity of the quantum boxes to environmental disturbances (mechanical, electrical disturbances, etc.) and enables the generation of indistinguishable (i.e., identical in terms of quantum state) single photons (same frequency, same polarization state, same spatial and temporal distribution).

[0006] Quantum boxes are typically semiconductor-like structures obtained by molecular jet epitaxy, which essentially involves growing layers of semiconductors that differ in terms of gap energy and mesh parameters. Such differences create nanometer-sized quantum boxes that can confine charge carriers in three-dimensional space.

[0007] Electrons in the valence band of a quantum box can cross the conduction band to form electron-hole pairs (also called excitons) in relation to holes remaining in the valence band. Exciton formation can occur by absorbing photons with energies above the stopband of the material forming the quantum box. The exciton state corresponding to the formation of an exciton is an unstable state, and the reconstruction of the neutral state, also called the ground state, occurs through electron-hole recombination accompanied by the emission of photons via a spontaneous radiation process characterized by a radiative lifetime on the order of nanoseconds and radiation wavelength (or frequency) corresponding to the stopband of the material forming the quantum box.

[0008] A quantum box can confine a twin exciton, formed by two excitons coupled by Coulomb interaction while sharing the same ground state. The energy level of the twin exciton state is higher than that of a single exciton. The formation of a twin exciton can occur by the sequential absorption of two photons. The relaxation of a twin exciton also occurs according to a spontaneous emission process that produces two photons emitted sequentially. The two photons are emitted at different wavelengths because the Coulomb interaction differs depending on whether one electron-hole pair or two electron-hole pairs are excited in the quantum box. Therefore, the photons emitted in response to the transition of the system from a twin exciton state to an exciton state have different wavelengths than those emitted in response to the transition of the system from an exciton state to a neutral state.

[0009] The polarization states of photons absorbed to form excitons and the polarization states of photons emitted in response to electron-hole recombination follow optical selection rules governed by the Pauli exclusion principle and the conservation of angular moments. Such selection rules depend on the spin states of electron-hole pairs, and explicitly state that the “bright” exciton states that emit photons in response to their recombination correspond to spin states with opposite signs. If the quantum box has cylindrical symmetry, the system presents two energy-degenerate “bright” exciton states. Figure 1 illustrates the relaxation of a twin exciton 103 formed in a semiconductor quantum box 100 having cylindrical symmetry (or “isotropy”), where only “bright” exciton states are realized. The relaxation of a twin exciton 103 confined within the isotropic quantum box 100 occurs within a system with three energy levels by sequentially emitting two photons. Therefore, two paths are possible: -Relaxation of the twin excitons 103 to exciton 1020 by emission of right-rotating polarized photons, then relaxation of exciton 1020 to ground state 101 by emission of left-rotating polarized photons; or, -The twin excitons 103 relax to exciton 1021 due to the emission of left-rotating polarized photons, and then the excitons 1021 relax to ground state 101 due to the emission of right-rotating polarized photons.

[0010] Given that the two pathways are indistinguishable, excitons 1020 and 1021 are degenerate, which leads to the emission of a pair of entangled polarization photons.

[0011] It should be noted that single-photon generation can be achieved by utilizing the recombination of exciton 1020 or 1021 to the ground state 101, and that in this case, passing through the double exciton state 103 is irrelevant. The generation of double exciton 103 is necessary for the generation of entangled photon pairs.

[0012] However, in reality, semiconductor quantum boxes involved in the lifting of exciton-level degeneracy (and thus the loss of entanglement of emitted photons) are anisotropic. Such lifting of degeneracy is primarily induced by anisotropic effects of the quantum box form, piezoelectric effects, mechanical constraints, etc. In anisotropic quantum boxes, the "bright" exciton states are separated by energy (called "fine structure splitting") and are linear combinations of fundamental states as defined in symmetric quantum boxes. Exciton states in anisotropic quantum boxes present light emission that is linearly polarized along two specific axes "x" and "y" corresponding to crystallographic directions (axis "z" is the growth axis). Figure 2 shows the recombination of twin excitons 203 in an anisotropic semiconductor quantum box 200. The relaxation of the twin exciton 203 to the ground state 201 can occur according to a linear polarization state on one of the specific axes "x" or "y" of the quantum box, leaving excitons 2020 or 2021 that relax according to the same polarization state as twin exciton 203. The release of exciton-level degeneracy leads to the total or partial loss of entanglement between the two generated photons.

[0013] Using an anisotropic quantum box to generate single photons with improved performance levels by resonantly exciting a superposition of two "bright" exciton states and by utilizing the time-dependent phase change ΔΦ between the two exciton states polarized according to "x" and "y", is a known method, and its speed (called the "transition rate" as it governs the transition from the linearly polarized state to the orthogonal state of the emitted photon, as will be explained in more detail with reference to Figure 10) is proportional to the value of the fine structure resolution by the following relation:

number

[0014] The use of the fabricated anisotropic quantum box remains unsuitable for generating entangled photon pairs due to the degeneracy of the exciton states being lifted. Therefore, in order to obtain entangled photon pairs, it is necessary to statically reduce the fine structure resolution values ​​(in particular, to cancel out the fine structure resolution) in order to reconstruct the indistinguishability of the emission pathway from the twin excitons.

[0015] Reducing and / or controlling the microstructure partitioning values ​​within a quantum box by applying a vertical electric field along with one or more strain fields in two or three directions of the environment in which the quantum box is inserted is a known method (in this case, a mechanical method). Such a solution was demonstrated in reference [1]. The application of such strain fields leads to thinning of the substrate in which the quantum box is created in order to place the piezoelectric material as close as possible to the quantum box and to ensure that the applied mechanical tension is successfully transferred to the realized quantum box. The thinning required to implement such a method presents a technical challenge for fabricating a three-dimensional (3D) optical cavity to enclose the quantum box. Furthermore, the proximity of the quantum box to the surface makes the quantum box sensitive to environmental disturbances that induce a loss of indistinguishability of the generated single photons.

[0016] Another known approach involves applying an electric field within a two-dimensional space into which the quantum box is inserted to control the fine-structure partitioning value. Such solutions are implemented by inserting several (usually four) bonding pads (on surfaces very close to the faces of the quantum box) (reference [2]). The proximity of the quantum box to the surface in which such bonding pads are defined is known to increase charge noise and reduce the performance level of emitted photons in terms of indistinguishability. [Prior art documents] [Non-patent literature]

[0017] [Non-Patent Document 1] Trotta, Rinaldo et al., “Highly Entangled Photons from Hybrid Piezoelectric Semiconductor Quantum Dot Devices.” Nano Letters, 14.6(2014):3439-3444. [Non-Patent Document 2] K Kowalik, O Krebs, A Lemaitre, S Laurent, P Senellart, P Voisin, JA Gaj, “Influence of an in-plane electric field on exciton fine structure in InAs-GaAs self-assembled quantum dots.”, Applied Physics Letters, 86(4), 041907

Summary of the Invention

Problems to be Solved by the Invention

[0018] Therefore, there is a need for a source of single photons or entangled photon pairs based on semiconductor quantum boxes in which fine structure splitting can be controlled statically or dynamically without the drawbacks of the prior art.

Means for Solving the Problems

[0019] General Definition of the Invention To achieve this object, the present invention provides a device for generating photons, the device including a quantum box inserted into a micropillar optical cavity having at least one optical mode, the quantum box having two states with at least one ground state and one elementary excitation, the cavity having a bottom surface and a top surface, the bottom surface having an electrical contact. The device is characterized in that it includes at least three electrically insulated electrical bonding pads arranged around the top surface of the cavity.

[0020] In one embodiment, the electrical bonding pads can be linked to the top surface of the cavity by semiconductor arms oriented radially with respect to the cavity and having a width smaller than the width of the cavity in the tangential direction and at their ends closest to the cavity.

[0021] In another embodiment, the electrical bonding pads may be linked to semiconductor arms radially oriented in the direction of the cavity, and both ends of the semiconductor arms may be separated from the top surface of the cavity by a gap with a sub-micrometer width or by a gap filled with a dielectric.

[0022] Advantageously, the micropillar optical cavity may form a PIN-type diode, and the quantum well is within the intrinsic region of the PIN-type diode.

[0023] As a variant, the electrical bonding pads may be borne by respective pillars optically and electrically decoupled from the cavity.

[0024] In one embodiment, the device may further include at least three adjustable voltage sources for applying respective variable potential differences between each of the electrical bonding pads and an electrical contact borne by the bottom surface of the optical cavity.

[0025] In another embodiment, the micropillar optical cavity may have at least one first pair and at least a second pair of modes in which each pair is polarization degenerate, and the quantum well may also have one state with two elementary excitations.

[0026] Advantageously, the device may further include a second optical cavity coupled to the optical cavity in which the quantum well is inserted, and the geometries of the first cavity and the second cavity and the strength of their coupling are such that the assembly composed of the two coupled cavities presents a first pair of modes that are polarization degenerate and resonate with the transition between the state with two elementary excitations of the quantum well and the two states with one elementary excitation; and a second pair of modes that are polarization degenerate and resonate with the transition between the two states with one elementary excitation and the ground state of the quantum well, and the radiation patterns of each pair of modes of these pairs overlap by 70% or more.

[0027] In particular, the second optical cavity may also be of the micropillar type, and the two cavities are arranged side by side.

[0028] A method has been proposed for generating entangled photon pairs using a photon generation device, and this method may include the following steps: - A process of applying a potential difference between each of the electrical bonding pads and the electrical contacts supported by the bottom surface of the optical cavity, wherein the potential difference is: ○ Minimize the energy difference between two states having one excitation in the quantum box, and A process in which the transition between a state with two elementary excitations and a state with one elementary excitation is brought into resonance with a first pair of modes of the cavity, and the transition between a state with one elementary excitation and the ground state is brought into resonance with a second pair of modes of the cavity; and - A process of filling a quantum box with a state having two elementary excitations by inputting energy.

[0029] A method for generating single photons using a photon generation device has also been proposed, and this method may include the following steps: - A step of applying a potential difference between each of the electrical bonding pads and the electrical contacts supported by the bottom surface of the optical cavity, and - A step of illuminating the quantum box of the device with a light pulse that presents a polarization spectrum and direction corresponding to the mode of the optical cavity, wherein the potential difference is selected to modify the energies of two eigenstates having one elementary excitation in the following way: ○ The state resonates with the wavelength of the light pulse spectrum; and ○The probability of emission by the quantum box of a single photon with linear polarization perpendicular to the polarization of the illumination is maximized.

[0030] In one embodiment, the potential difference may also be selected to orient a specific axis of the quantum box according to an intermediate direction between the directions of two specific axes of polarization of the optical cavity.

[0031] In another embodiment, the potential difference may be variable over time, and: -During the first period, for at least a portion thereof, the quantum box is illuminated and takes a first value selected with respect to the energies of two eigenstates having one elementary excitation in order to have a non-minimum deviation, the first period having a period selected to induce a phase change of 45° to 135° (preferably 80° to 100°) between the two states; and, -During the second period immediately following the first period, a value is chosen to minimize the difference between the energies of two eigenstates having one elementary excitation.

[0032] Other features and advantages of the present invention will become apparent from the following detailed description and accompanying drawings. [Brief explanation of the drawing]

[0033] [Figure 1] This demonstrates the relaxation of twin excitons within the isotropic quantum box described above. [Figure 2] This demonstrates the relaxation of twin excitons within the anisotropic quantum box described above. [Figure 3A] This represents a device for generating single photons and entangled photon pairs according to one embodiment of the present invention. [Figure 3B] This represents a device for generating single photons and entangled photon pairs according to one embodiment of the present invention. [Figure 4] This example demonstrates the generation of an electric field by three components in space. [Figure 5] This shows a portion of the top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 6] This shows a portion of the top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 7] This shows a portion of the top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 8] This represents a micropillar-type optical cavity that incorporates a quantum box according to one embodiment of the present invention. [Figure 9]This shows a portion of a top view of a device for generating single photons and entangled photon pairs to realize two optically coupled optical cavities according to another embodiment of the present invention. [Figure 10] This represents the time-dependent change in the excited state of a quantum box enclosed within a micropillar-type optical cavity. The quantum box exhibits non-zero exchange anisotropy, while the optical cavity exhibits birefringence. [Figure 11] This illustrates the use of a device for generating single photons according to one embodiment of the present invention. [Figure 12] This describes a method for calibrating an anisotropic quantum box. [Figure 13A] This illustrates a method for generating a single photon according to one embodiment of the present invention. [Figure 13B] This illustrates a method for generating a single photon according to one embodiment of the present invention. [Figure 14] This illustrates a method for generating entangled photon pairs according to one embodiment of the present invention. [Figure 15] This shows the measured results of the fine structure division of an anisotropic quantum box exposed to an electric field generated according to an embodiment of the present invention. [Modes for carrying out the invention]

[0034] Figures 3A and 3B show a top view and a cross-sectional view, respectively, of a device 300 for generating single photons and entangled photon pairs according to one embodiment of the present invention. The device includes a quantum box 301 inserted into a micro-pillar-type optical cavity 302. The optical cavity 302 has a cylindrical shape and is connected to three pillars 303 by semiconductor arms 308a, 308b, and 308c.

[0035] The quantum box 301 may be a semiconducting material obtained by molecular jet growth, which is essentially a method for growing semiconductor layers that differ in terms of gap energy and mesh parameters. The layer forming the quantum box 301 may be a layer of indium arsenide (InAs). The layer forming the substrate and capsule may be a layer of gallium arsenide (GaAs). The quantum box 301 may have a frustoconical shape with a height of several nanometers in the growth axis direction and a base diameter of approximately 20 nanometers. Such features give the quantum box 301 properties that are close to those of an atom in terms of charge carrier confinement and energy level quantization.

[0036] The quantum box 301 can be in a neutral state (also called the ground state), an excited state corresponding to the formation of an electron-hole pair (also called an exciton), or a state with one elementary excitation. The formation of an exciton in the presence of charge carriers already confined within the quantum box 301 corresponds to a trion, which can be quantified as positive or negative depending on the sign of the already confined charge carriers. A double exciton (also called a state with two elementary excitations) is formed within the quantum box 301 when two excitons are confined simultaneously.

[0037] Charge carriers, electrons, and holes are fermions characterized by half-integer spins (±1 / 2 for electrons and ±3 / 2 for heavy holes). The exciton states of the symmetric quantum box 301 are bright if electron-hole recombination occurs according to a spontaneous radiation process whose nature is to emit photons. Bright exciton states (also called fundamental exciton states) correspond to total angular moments equal to ±1 and are relaxed by emitting left or right circularly polarized photons depending on the sign of the sum of spins.

[0038] In fact, quantum box 301 is anisotropic (meaning the exciton-level degeneracy of quantum box 301 is lifted). The "bright" exciton states within an anisotropic quantum box 301 (also called eigenstates with one fundamental exciton) are linear combinations of fundamental exciton states defined in a symmetric quantum box 301, and are separated by non-zero energy quantities called fine structure partitioning.

[0039] The micropillars forming the optical cavity 302 are cylindrical in shape and consist of two stacks of layers 3020 and 3022 surrounding a central region 3021. Such layers are typically semiconducting and can be P- or N-doped according to a given configuration. For example, the top of cavity 3022 may have P-type (or N-type) doping, while the bottom 3020 may have the opposite type of doping, so that cavity 3020 forms a PN diode from an electrical standpoint. Each of the two stacks of layers 3020 and 3022 forms a Bragg mirror and is obtained by alternating two layers of two different materials in terms of refractive index. The thickness and refractive index of each of the two layers forming each of the two stacks 3020 and 3022 may be selected such that their product is equal to λ / 4, where λ is the operating frequency. The number of pairs of layers forming the bottom stack 3020 in contact with the substrate 305 may differ from the number of pairs of layers forming the top stack 3022. Such differences allow for the preference of upward emission of photons generated by, for example, the quantum box 301 to increase the brightness of the photon source 300. Each of the two Bragg mirrors 3020, 3022 allows incident light to be refracted along the stack axis over a wide range of wavelengths centered on the operating frequency λ. A central region 3021 placed between the two stacks 3020, 3022 may have a thickness equal to λ and is configured to contain the quantum box 301. The presence of the central region 3021 allows for the modification of the refractive index of the micropillar so that the two Bragg mirrors 3020, 3022 are transparent at the operating frequency. The radius of the micropillar is on the order of several micrometers and is chosen to allow at least one optical mode, called the fundamental mode, to propagate through the micropillar with minimum energy. Advantageously, the radius of the micropillar may be chosen to allow the propagation of several optical modes. The micropillar may have rotational symmetry, meaning two degenerate polarization states for each propagating mode. Alternatively, micropillars are birefringent media through which light propagates anisotropically. This birefringence property eliminates the polarization degeneracy of the light propagation modes defined by the micropillar's small and large axes (denoted by H and V, respectively).

[0040] Advantageously, the optical cavity 302 has at least one first pair and at least one second pair of optical modes, each of which is polarization-degenerate. The optical cavity 302 having two pairs of polarization-degenerate modes is particularly used in the generation of entangled photon pairs, for example, by matching the wavelength of relaxation from a biexciton (a state with two elementary excitations) to an exciton (a state with one elementary excitation) with the energy of the first pair of modes, and by matching the wavelength of relaxation from an exciton to a neutral state with the energy of the second pair of modes.

[0041] Pillar 303 may have a height equal to, higher than, or lower than the pillar of the optical cavity 302. The optical cavity 302 may be placed at the center of an assembly formed by three pillars 303, with a radial spacing sufficient to optically decouple the optical cavity 302 from the pillars 303. Furthermore, the pillars 303, which correlate with the electrical contact 305 to be discussed later, must be electrically insulated from each other except at their bottom surfaces. The pillars 303 may be formed by the same lamination as the micropillars forming the optical cavity 302. Such a structure offers the advantage of being compatible with countless cleanroom manufacturing methods known to those skilled in the art. The process for fabricating such a structure may include a thin-layer deposition method, a lithography method, and a subsequent dry etching method that allows the optical cavity 302 and the three pillars 303 to be defined.

[0042] The top surface of each pillar 303 (defined as the surface opposite to the surface in contact with the substrate 305) includes electrical bonding pads 304a, 304b, and 304c, the dimensions of which are selected such that the electrical bonding pads 304a, 304b, and 304c are fully supported by the top surface of the corresponding pillar 303. The three electrical bonding pads 304a, 304b, and 304c may have the same electrical properties and the same geometric shape, which may be triangular, rectangular, or other shapes. The lateral dimension of such electrical bonding pads 304a, 304b, and 304c, defined in a plane perpendicular to the lamination axis (growth axis), is preferably at least 10 greater than the thickness of such pads 304a, 304b, and 304c, defined according to the lamination axis. As an example, the thickness of the electrical bonding pads 304a, 304b, and 304c is selected to be less than 50 nanometers, and their lateral dimensions are selected to be greater than 50 micrometers, allowing for their connection to different voltage sources 306a, 306b, and 306c by a wiring method also known as "wire bonding". The electrical bonding pads 304a, 304b, and 304c are arranged symmetrically around the optical cavity 302 (corresponding to the same radial spacing between each of the electrical bonding pads 304a, 304b, and 304c and the optical cavity 302, and the same angular spacing between two adjacent electrical bonding pads).

[0043] The bottom surface of pillar 303 is connected to another electrical contact 305, and thus the electrical contact 305 is supported by the bottom surface of the optical cavity 302. The electrical contact 305 is fabricated on a semiconducting surface, and its conductivity is increased via P or N doping. The electrical contact 305 may have a disc shape, a rectangular shape, or another 2D geometric shape that allows it to completely cover the bottom surface of the assembly formed by the optical cavity 302 and pillar 303. Such a configuration allows a potential difference to be applied between each of the electrical bonding pads 304a, 304b, 304c positioned around the top surface of the optical cavity 302 and the electrical contact 305 supported by the bottom surface of the optical cavity 302 by using three independent voltage sources 306a, 306b, 306c that deliver the respective voltages Ta, Tb, and Tc. The voltages delivered by each of the voltage sources 306a, 306b, 306c may be constant or variable over time.

[0044] Semiconductor arms 308a, 308b, and 308c are used to connect the electrical bonding pads 304a, 304b, and 304c, respectively, to the top surface of the optical cavity 302. The material forming the semiconductor arms is doped to increase their conductivity, but this conductivity should not be that of a metallic type (degenerate doping). The semiconductor arms 308a, 308b, and 308c may have the same height as the optical cavity 302, their length is long enough to connect the outer surface of the optical cavity 302 to the inner surface of the pillar 303 (on which the electrical bonding pads 304a, 304b, and 304c are placed), and their width is small enough that two adjacent semiconductor arms 308a, 308b, and 308c are electrically insulated from their contact with the outer surface of the optical cavity 302. Each of the semiconductor arms 308a, 308b, and 308c extends radially toward the corresponding pillar 303 such that a 120-degree angle is formed between two adjacent semiconductor arms 308a, 308b, and 308c. The use of dielectric or semiconductor arms allows for amplification of the horizontal component of the applied electric field by concentrating the lines of force.

[0045] Photons generated by the quantum box 301 according to one of the optical modes, and photons that excite the quantum box 301, each exit from or enter into the optical cavity 302 according to the same or two different emission patterns 307 characterized by a given aperture angle.

[0046] The potential difference between each of the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 supported by the bottom surface of the optical cavity 302 generates a fundamental electric field, whose characteristics are controlled in terms of intensity and orientation. The resulting electric field, which is dominant in the central region 3021 and to which the quantum box 301 is exposed, is the vector sum of three fundamental electric fields derived from the potential differences between each of the three electrical bonding pads 304a, 304b, and 304c, arranged around the top surface of the optical cavity 302, and the electrical contact 305 supported by the bottom surface of the optical cavity 302. Such a resulting electric field has three components: a vertical component defined in the same direction as the growth axis (stack axis) of the optical cavity 302 and two horizontal components defined within the stacking planes forming the two Bragg mirrors 3020, 3022. The characteristics of such a resulting electric field in terms of orientation and intensity can be regulated in a deterministic manner by acting on one or more fundamental electric fields via the associated potential difference.

[0047] Figure 4 shows the generation of an electric field with three components defined in an orthonormal trihedron by introducing four electrical contacts according to the configuration described above, with two voltage sources 306a and 306b shown, and the fundamental and resulting electric fields represented by dotted and continuous lines, respectively. The various components of the resulting electric field are: - Adjusting the fine structure partitioning value of quantum box 301; - Adjusting the emission wavelength associated with the relaxation of that "bright" exciton state; and, - This makes it possible to define the orientation of a specific axis of the quantum box 301 (in which case it can emit photons or be excited by photons).

[0048] To adjust these three parameters, it is necessary to have at least three degrees of freedom, and therefore, at least three independent voltage sources are linked to three non-collinear electrical bonding pads.

[0049] From an experimental standpoint, the intrinsic parameters of quantum box 301 (such as fine structure resolution) are not precisely known in advance due to imperfections in the fabrication method. The parameters of interest of quantum box 301 (such as fine structure resolution, emission wavelength, and orientation of specific axes) are typically measured after the fabrication of the device for generating single photons and entangled photon pairs 300.

[0050] Figure 5 shows a top view of a portion of a device for generating single photons and entangled photon pairs 300 according to another embodiment of the present invention. In such an embodiment, the semiconductor arms 308a, 308b, and 308c described above are not in direct contact with the optical cavity 302. The isolation of each of the semiconductor arms 308a, 308b, and 308c from the optical cavity 302 is ensured by a dielectric layer 309. The dielectric layer 309 is configured to ensure electrical isolation between the optical cavity 302 and the semiconductor arms 308a, 308b, and 308c, and its thickness is on the order of tens of nanometers to several micrometers. The dielectric material forming the dielectric layer 309 can be a cavity. The electrical isolation of the optical cavity 302 from its adjacent environment makes it possible to avoid any strong currents passing through the optical cavity 302 and disrupting the operation of the quantum box 301.

[0051] Figure 6 shows another embodiment of the present invention in which no semiconductor arm is used to connect the optical cavity 302 to the realized pillar 303. In such an embodiment, the pillar 303 is arranged symmetrically around the optical cavity 302, and the radial spacing between the optical cavity 302 and each of the pillar 303 is selected to, for example, less than 10 micrometers. Such a radial spacing allows the quantum box 301 to be exposed to an electric field strong enough to act on the characteristic parameters of the quantum box 301, including the microstructure partitioning value. Isolation between the optical cavity 302 and the pillar 303 can be ensured by a dielectric material or cavity.

[0052] Figure 7 shows another embodiment of the present invention in which the electrical bonding pads 304a, 304b, and 304c are supported by a cylindrical hollow structure 3030 surrounding the optical cavity 302. The hollow structure 3030 may be formed of a dielectric material that enables electrical insulation between the electrical bonding pads 304a, 304b, and 304c. The hollow structure 3030 may have the same height as the optical cavity 302, and its top surface is selected to be broad enough to include the electrical bonding pads 304a, 304b, and 304c. Furthermore, semiconductor arms 308a, 308b, and 308c related to the dielectric layer 309 described in the embodiment of Figure 5 are used to connect the electrical bonding pads 304a, 304b, and 304c to the optical cavity 302. Alternatively, the connection between the electrical bonding pads 304a, 304b and the optical cavity 302 can be ensured by bringing the electrical bonding pads 304a, 304, 304c close enough to the optical cavity 302 as described in Figure 6, or by using the semiconductor arms 308a, 308b, 308c as described in Figures 3A, 3B.

[0053] Figure 8 shows the structure of a micropillar-type optical cavity 302 according to one embodiment of the present invention. In this embodiment, the two apical Bragg mirrors 3022 and basal Bragg mirrors 3020 forming the micropillar are doped with P and N, respectively. The central region 3021, including the quantum box 301, remains intrinsic so that the micropillar forms a PIN-type diode from an electrical standpoint. The concentration of the impurity used to generate the doping (P or N) may be uniform over the entire length of each of the two Bragg mirrors 3020, 3022. Alternatively, the concentration of the impurity may vary in a way that decreases to a minimum in the region in contact with the central region 3021. The change in the impurity concentration may be linear, logarithmic, or otherwise. Advantageously, the various voltages applied to the optical cavity 302 doped according to one of the configurations described above correspond to the reverse polarity of the PIN junction formed by the optical cavity 302. Such reverse polarity ensures that no large current passes through the quantum box 301 and does not disturb its emission of photons. Alternatively, the optical cavity 302 is forward-biased (which is likely to generate a large current through the quantum box 301 and disrupt its operation).

[0054] According to some embodiments of the present invention, each of the electrical bonding pads 304a, 304b, and 304c surrounding the top surface of the optical cavity 302 is connected to one of the terminals of an adjustable voltage source 306a, 306b, and 306c. Each of the adjustable voltage sources 306a, 306b, and 306c is configured to apply a potential difference between the electrical bonding pads 304a, 304b, and 304c to which it is connected and the electrical contact 305, which is provided by the bottom surface of the optical cavity 302 and is common to all the adjustable voltage sources 306a, 306b, and 306c realized. The voltages generated by each of the adjustable voltage sources 306a, 306b, and 306c measured between the corresponding electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 may be positive, zero, or negative.

[0055] Figure 9 shows a top view of a device for generating single photons and entangled photon pairs 300, comprising two optical cavities 302, 310, according to another embodiment of the present invention. One of the two optical cavities 302 includes a quantum box 301 and may be configured according to one of the embodiments described above. The other optical cavity 310 does not include a quantum box and may have different optical geometric parameters (size, shape) than the first optical cavity 302. The two realized optical cavities 302, 310 are configured to be optically coupled according to a given coupling force. The assembly formed by the two optical cavities 302, 310 has two pairs of polarization-degenerate optical modes. The first pair of polarization-degenerate optical modes is configured to resonate with the transition between a state having two elementary excitations (dexcitons) and two states having one elementary excitation (exciton). The second pair of polarization-degenerate optical modes are configured to resonate with the transition between the two states of the quantum box 301 having one elementary excitation (exciton) and the ground state, also called the neutral state. Each pair of optical modes has a radiation pattern 307 that superimposes more than 70% of the modes. The coupling between the two optical cavities 302, 310 can also be configured according to the embodiments described in patent application document [3].

[0056] According to some embodiments of the present invention, the number of electrical bonding pads arranged around the top surface of the optical cavity 302 is greater than 3. Such a number of electrical bonding pads may be even or odd and may be less than 20. The electrical bonding pads may be arranged asymmetrically around the optical cavity 302 such that at least three electrical bonding pads (and possibly their associated semiconductor arms) are not oriented in a pairwise parallel direction. This means that "the angular and radial spacing between adjacent electrical bonding pads measured from the optical cavity 302 may not be constant with respect to all the electrical bonding pads realized." Furthermore, the electrical bonding pads may be placed at various distances from the electrical contacts 305, in other words, the electrical bonding pads may be placed in various planes perpendicular to the stacking axis.

[0057] According to some embodiments of the present invention, a device for generating single photons and entangled photon pairs 300 may include a cooling unit (e.g., by the Peltier effect or by nitrogen or helium cryogenic physics) configured to control and maintain a constant operating temperature of the quantum box 301. Such an operating temperature can be between 4 Kelvin and 100 Kelvin. Such a low operating temperature limits the interaction between the quantum box 301 and its neighboring environment and allows the consistency between the excited and ground states to be maintained over a sufficiently long time interval to manipulate the state of the quantum box 301.

[0058] Figure 10 shows the time-dependent change 400 of the “bright” exciton state within a quantum box 301 inserted into a micropillar-type optical cavity 302, where the quantum box 301 and optical cavity 302 are characterized by non-zero anisotropy. The anisotropy of the optical cavity 302 signifies the release of degeneracy between the modes of the optical cavity 302, which have low-energy modes, and the high-energy modes, which are aligned with a specific axis (H, V) of polarization of the micropillar. The anisotropy of the quantum box 301 signifies the release of degeneracy between two excited states separated by a non-zero energy quantity called fine-structure decomposition. Each of the two excited states (also called eigenstates) of the quantum box 301 is associated with a given linearly polarized state X or Y, which can thereby emit or be excited by photons. Generally, the modes (H, V) of the optical cavity 302 do not coincide with the polarization direction (x, y) of the quantum box 301, and an angle represented by θ can be defined to quantify such mismatch. Resonant exciton-level excitation of the quantum box 301 by linearly polarized photons according to one of the modes (H or V) of the optical cavity 302 leads to the excitation of two eigenstates according to a weighting dependent on the value of the angle θ. Such results of excited states 4021 or 4020 are not stable, and their phase changes over time with a transition rate proportional to the fine structure resolution value. Such behavior can be exploited to collect only single photons generated by the quantum box 301, for example, by excitation with linearly polarized photons according to the axis H of the optical cavity 302, and by collecting only photons generated according to the linearly polarized state perpendicular to the angle used during excitation (in this case, the axis V of the optical cavity 302). In such a configuration, increasing the fine structure resolution value allows the photon to be emitted in the direction of detection polarization before it is naturally emitted in the direction of excitation polarization. In the second stage, the fine structure resolution value should be reduced to limit the probability that two photons, rather than one, are emitted within the same excitation region.

[0059] Figure 11 represents a device for generating a single photon 300 according to one embodiment of the present invention. Such an embodiment realizes an anisotropic quantum box 301 enclosed within a micropillar-type birefringent optical cavity 302. The quantum box 301 is exposed to an electric field whose intensity and orientation are adjustable via electrical bonding pads 304a, 304b, 304c, and 305 arranged according to one of the embodiments described above. Pad 304c and associated voltage source 306c are hidden in Figure 11. The device for generating a single photon 300 includes an excitation laser source 501 configured to generate an optical pulse intended to optically excite the quantum box 301 to convert its state from a neutral state to an excited state. The polarization state of the optical pulse is linear according to one of the particular axes of the optical cavity 302, and the associated wavelength is selected such that the energy carried by the photon is large enough to excite the quantum box 301. The duration and repetition frequency of the optical pulse determine the ratio of photons per second generated by the device for generating single photons 300. The device for generating single photons 300 further includes a polarization splitter cube 502 and a focusing lens 503, configured to separate photons generated in the optical cavity 302 according to a linear polarization state in a direction perpendicular to the direction of the excited photon and to improve the optical coupling between the polarization splitter cube 502 and the optical cavity 302, respectively. The specific axis (H, V) of the optical cavity 302 is configured not to be collinear with the specific axis (x, y) of the quantum box 301 according to an angle θ (ideally equal to 45 degrees).

[0060] Figure 12 illustrates a method 600 for calibrating an anisotropic quantum box 301 exposed to an electric field as shown in Figure 4. The three spatial components of the resulting electric field are independently adjustable by at least three adjustable voltage sources. The method is repeatable. The method consists of two steps: in each iteration, modifying the intensity of at least one of the three components of the resulting electric field (step 601) and measuring a characteristic parameter or set of parameters of interest of the quantum box 301 associated with the applied resulting electric field in a second step (step 602). The method can be stopped once a desired value of the characteristic parameter of interest is obtained, and the values ​​of the voltages applied to various electrical bonding pads can be restored. The first iteration of the method may correspond to a zero-intensity resulting electric field.

[0061] A calibration method may be used to identify the operating point of a device for generating entangled photon pairs 300. The characteristic parameters of interest for such a device include the fine structure resolution, which must be significantly reduced (ideally set to zero), and the emission wavelengths corresponding to the twin exciton relaxation and exciton relaxation.

[0062] This calibration method can further be used to find the optimal operating point of the device for generating a single photon 300 operating in static mode, thereby statically adjusting the fine structure resolution characterizing the realized anisotropic quantum box 301 before the generation of any exciton. The fine structure resolution must allow for a transition rate fast enough so that the excited state can relax according to a linearly polarized state perpendicular to the angle of the excited photon at the end of the transition time, which is less than the duration of the excitation light pulse. The fine structure resolution is further optimized as follows: - The transition time is less than a factor of 3 (preferably at least a factor of 5) shorter than the lifetime of the exciton state due to its spontaneous emission, and the limit is greater than the lower limit to which the excited state can relax according to a linearly polarized state perpendicular to the angle of the excited photon; and - The quantum box 301 is kept below the upper limit of two or more photons that can be emitted during the duration of the excitation light pulse.

[0063] Figure 13A is a flowchart of a method 700a for generating a single photon using a device for generating a photon 300 according to one embodiment of the present invention. In such an embodiment, the fine structure division characterizing the realized anisotropic quantum box 301 is statically tuned. The first step 701a of the method is to apply a potential difference between the electrical bonding pads 304a, 304b, and 304c, respectively, and the electrical contacts 305, which are carried by the bottom surface of the optical cavity 302, to generate a resulting electric field to which the quantum box 301 is exposed. The components of such a resulting electric field are tuned to optimize the fine structure division value as described above. In addition to matching the wavelength of the single photon generated by the quantum box 301, the application of the resulting electric field in three directions of space makes it possible to define the orientation of a specific axis of the quantum box 301 so as not to be collinear with a specific axis of the optical cavity 302 into which the quantum box 301 is inserted. Advantageously, the specific axis of the quantum box 301 is oriented at 45 degrees with respect to the specific axis of the optical cavity 302.

[0064] The second step 702a of this method is to illuminate the quantum box 301 with a series of light pulses whose wavelength spectrum and polarization state correspond to one of the modes of the optical cavity 302 into which the quantum box 301 is inserted. The duration of the light pulses is selected to be long enough to excite the quantum box 301 and to emit at least one photon according to the spontaneous radiation process.

[0065] Figure 13B is a flowchart of a method for generating a single photon 700b by a device for generating photon 300 according to another embodiment of the present invention. In such an embodiment, the fine structure resolution characterizing the anisotropic quantum box 301 is dynamically controlled. Such dynamic control is achieved through modulation of the electric field as a result of the exposure of the quantum box 301, so that the fine structure resolution value can be modified within a time interval shorter than the spontaneous emission time characterizing the quantum box 301.

[0066] The first step 701b of this method essentially involves illuminating the quantum box with a series of light pulses whose wavelength spectrum and polarization state correspond to the modes of the optical cavity 302 that encloses the quantum box 301. The duration of the light pulses is selected to be long enough to excite the quantum box and to emit at least one photon according to the spontaneous radiation process.

[0067] The second step 702b of this method is essentially to apply a potential difference between the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305, which is supported by the bottom surface of the optical cavity 302, to generate an electric field to which the quantum box 301 is exposed. The fine structure resolution value is increased in proportion to the strength of the resulting electric field, which must remain below a threshold corresponding to damage to the quantum box 301 (i.e., a threshold to which the electrical and optical characteristics determined during the calibration of the quantum box 301 can no longer be reconstructed). The strength of the resulting electric field is selected to be sufficiently high so that the time-dependent change in the phase of the generated excited state is between 45 and 135 degrees (preferably between 80 and 100 degrees). Such a time-dependent change in the phase must also occur within a time shorter than the lifetime due to the spontaneous emission of the exciton state (by at least a coefficient of 3, preferably at least a coefficient of 5).

[0068] A third step 703b of this method can be triggered immediately after the time-dependent phase change of the initial exciton state, as requested, is completed. Such a step of this method is essentially to modify the strength of the resulting electric field to minimize the energy difference between two eigenstates having one elementary excitation.

[0069] Figure 14 illustrates a method for generating entangled photon pairs 800 using a device for generating photons 300 according to one embodiment of the present invention. Such a device for generating photons 300 implements an anisotropic quantum box 301 enclosed within an optical cavity 302 having at least one first pair of optical modes and at least one second pair of optical modes, each of which is polarization degenerate. The entanglement of the generated photons is obtained thanks to static control of the fine structure partitioning that characterizes the implemented anisotropic quantum box 301. The first step 801 of the method is essentially to apply a potential difference between each of the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 carried by the bottom surface of the optical cavity 302. Such a potential difference is optimized to reduce the energy deviation between two states having one elementary excitation, as defined by the fine structure partitioning of the quantum box 301. In particular, step 801 is essentially about canceling out the fine structure decomposition that makes quantum box 301 a system with three energy levels related to the relaxation of twin excitons. The potential difference is also set as follows: - The transition between a state with two elementary excitations and a state with one elementary excitation becomes a resonant state with the first pair of modes of the optical cavity 302, - The transition between the state with one elementary excitation and the ground state is made such that it resonates with the second pair of modes of the optical cavity 302.

[0070] The second step 802 of this method is essentially to fill the quantum box 301 with a state having two elementary excitations by inputting energy. Such a dual-excitation state XX can be generated by exciting the quantum box 301 with two consecutive photons of the following preferred energies: energy ω sufficient to excite the transition of the first exciton. X ω to generate the first photon and the second exciton, and thus obtain a bi-excitation state XX X A different energy ω XX The second photon, or alternatively, energy (ω XX +ω X Two photons of ) / 2.

[0071] Figure 15 shows experimental measurement results of the fine structure resolution characterizing an anisotropic quantum box 301 enclosed within a micropillar-type optical cavity 302, where the top surface of the optical cavity 302 is surrounded by two electrical bonding pads 304a and 304b according to an embodiment of the present invention. The experimental measurements were performed for two voltage ranges applied to the electrical bonding pads 304a and 304b, demonstrating the technical advantages of the embodiment of the present invention in terms of deterministic control of the fine structure resolution value. Such measurement results are obtained with respect to the following configuration: the quantum box 301 is excited by a laser with energy greater than the optical transition corresponding to the exciton. This non-resonant excitation allows exciton states 2020 and 2021 to be filled by the energy input, and then exciton states 2020 and 2021 emit photons at two different wavelengths to be collected. The difference in wavelength between the two photons allows for the extraction of the fine structure resolution value. Figure 15 shows the results of such measurements, where the color scale reflects the absolute values ​​of the microstructure division corresponding to the two voltages applied to contacts 304a and 304b. The figure shows that the microstructure division changes in absolute value depending on the applied voltage, and that the microstructure division changes sign by canceling each other out over the entire range of voltage 1 and voltage 2 combinations.

[0072] According to one embodiment of the present invention, the formation of excitons within the quantum box 301, which is neutral in the initial stage, is carried out by non-resonant optical pumping, which is essentially the excitation of the quantum box 301 by photons with energies greater than the stopband of the material forming the quantum box 301.

[0073] According to another embodiment of the present invention, the geometric shape of the micropillar forming the optical cavity 302 defined in a plane perpendicular to the growth axis may be elliptical, polygonal, or otherwise.

[0074] The present invention is not limited to the embodiments described above as non-limiting examples. The present invention encompasses all modified embodiments that may be conceived by those skilled in the art.

[0075] References [1]Trotta,Rinaldo et al.”Highly Entangled Photons from Hybrid Piezoelectric Semiconductor Quantum Dot Devices.”Nano Letters 14.6(2014):3439-3444. [2]K Kowalik,O Krebs,A Lemaitre,S Laurent,P Senellart,P Voisin,JA Gaj”Influence of an in-plane electric field on exciton fine structure inInAs-GaAs self-assembled quantum dots.”Applied Physics Letters 86(4),041907 [3]WO / 2011 / 089336”SOURCE OF POLARIZATION-ENTANGLED PHOTON PAIRS AND METHOD FOR MANUFACTURING SAME”.

Claims

1. A device for generating photons (300) comprising a quantum box (301) inserted into a micropillar optical cavity (302) having at least one optical mode, The quantum box (301) has two states, each having at least one ground state and one elementary excitation, the optical cavity (302) has a bottom surface and a top surface, the bottom surface having an electrical contact (305); The device includes at least three electrical bonding pads (304a, 304b, 304c) electrically isolated from one another and arranged around a periphery of a top surface of the cavity (302), The device also includes at least three adjustable voltage sources (306a, 306b, 306c) for applying a variable potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the optical cavity (302).

2. 2. The device (300) of claim 1, wherein the electrical bonding pads (304a, 304b, 304c) are linked to the top surface of the cavity (302) by semiconductor arms (308a, 308b, 308c) oriented radially relative to the cavity (302) and having a width smaller than the cavity (302) in the tangential direction and at their ends closest to the cavity.

3. 2. The device (300) of claim 1, wherein the electrical bonding pads (304a, 304b, 304c) are linked to semiconductor arms oriented radially toward the cavity (302), both ends of which are separated from the top surface of the cavity (302) by air gaps (309) or by sub-micrometer-wide dielectric-filled air gaps (309).

4. The device (300) of any one of claims 1 to 3, wherein the micropillar optical cavity (302) forms a PIN-type diode, and the quantum box (301) is in the intrinsic region (3021) of the diode.

5. The device (300) according to any one of claims 1 to 4, wherein the electrical bonding pads (304a, 304b, 304c) are carried by respective pillars (303) that are optically and electrically decoupled from the cavity (302).

6. The device (300) of any one of claims 1 to 5, wherein the micropillar optical cavity (302) has at least one first pair and at least one second pair of modes, each of the pair of modes being polarization degenerate, and the quantum box (301) also has one state with two elementary excitations.

7. 7. The device (300) of claim 6, also comprising a second optical cavity (310) coupled to the optical cavity (302) into which the quantum box (301) is inserted, the geometry of the first and second cavities (302, 310) and the strength of their coupling are selected so that the assembly consisting of the two coupled cavities has a first pair of polarization-degenerate modes that resonate with the transitions between the state with two elementary excitations of the quantum box (301) and the two states with one elementary excitation; and a second pair of polarization-degenerate modes that resonate with the transitions between the two states with one elementary excitation of the quantum box (301) and the ground state, The device (300), wherein the modes of each pair have radiation patterns that overlap by 70% or more.

8. 8. The device (300) of claim 7, wherein the second optical cavity (310) is also a micropillar type, and the two cavities (302, 310) are arranged side by side.

9. A method (800) for generating entangled photon pairs by a device (300) according to any one of claims 6 to 8, said method comprising: applying a potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the optical cavity (302), said potential difference being: so as to minimize the energy difference between the two states with one excitation (301) of the quantum box; and selected to cause the transition between the state with two elementary excitations and the state with one elementary excitation to be in resonance with the first pair of modes of the cavity (302), and the transition between the state with one elementary excitation and the ground state to be in resonance with the second pair of modes of the cavity (302); and - filling said state with two elementary excitations of said quantum box (301) by inputting energy.

10. A method (700a) for generating single photons by a device (300) according to any one of claims 1 to 6, said method comprising: applying a potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the optical cavity (302); and - illuminating the quantum boxes (301) of the device with light pulses exhibiting a spectrum and direction of polarization corresponding to the modes of the optical cavity (302), The potential difference is chosen to modify the energies of the two eigenstates with one elementary excitation in the following manner: the state is resonant with a wavelength in the spectrum of the light pulse; and A method in which the probability of emission by the quantum box (301) of a single photon exhibiting a linear polarization orthogonal to the polarization of the illumination is maximized.

11. 11. The method of claim 10, wherein the potential difference is also selected to orient a particular axis of the quantum box (301) according to an intermediate direction between directions of two particular axes of polarization of the optical cavity (302).

12. The potential difference is variable with time: - during a first period, during at least a portion of which the quantum box (301) is illuminated, it takes on first values ​​selected for the energies of the two eigenstates having one elementary excitation to exhibit a non-minimal deviation, the first period having a duration selected to induce a phase change between 45° and 135° (preferably between 80° and 100°) between the two states; A method (700b) according to any one of claims 10 and 11, wherein during a second period immediately following the first period, the energy of two eigenstates having one elementary excitation takes a value selected to minimize the deviation between the energies of two eigenstates.

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