Apparatus, system and method for providing a light-matter interface for enhancing transitions that generate single photons - Patents.com

Abstract

According to an aspect of the disclosure, an apparatus, system, and method are provided for providing a light-matter interface for enhancing a transition that generates a single photon. The apparatus includes a first reflecting surface, a locking recess formed in the locking reflecting surface, the first reflecting surface and the locking recess forming a locking cavity for receiving light from a locking laser, and a portion of the light output from the locking cavity indicating whether the locking cavity is resonant with the locking laser, an emission recess formed in the emission reflecting surface, the first reflecting surface and the emission recess forming an emission cavity for receiving photons from a single-photon emitter material, the single-photon emitter material being located in the emission cavity for emitting single photons in response to optical or electrical excitation of the material, and an actuator for tuning a length of the locking cavity based on the portion of the light output from the locking cavity to resonate with the locking laser to lock the locking cavity to the locking laser. The locking reflecting surface and the emission reflecting surface are mechanically coupled to tune and stabilize the emission cavity at a single-photon emission wavelength by locking the locking cavity. The system includes an apparatus for generating single photons and an optical system for transmitting light into a locking cavity of the apparatus. In a method of operation, light is received from a locking laser in the locking cavity, and the length of the locking cavity is tuned using an actuator based on a portion of the light output from the locking cavity to resonate with the locking laser to lock the locking cavity to the locking laser. Single photons are then emitted by a single-photon emitter material located in the emission cavity in response to optical or electrical excitation of the material.

Description

[Technical field]

[0001] The present disclosure relates to devices, systems, and methods for providing a light-matter interface for enhancing transitions that generate single photons, as well as methods for fabricating such devices. In particular, the present disclosure relates to devices that include a single-photon emitter material, such as a solid material, trapped ions, or neutral atoms, disposed within an optical cavity. The present disclosure finds utility, for example, in providing a source of single photons, or in interfacing and manipulating quantum objects, such as qubits in a quantum computer.

[0002] background Sources of single photons are predicted to be essential for a variety of near-future applications, such as quantum computing and information processing, cryptography and communications.

[0003] Several possible sources that can be used to generate single photons have been investigated, including single emitters such as trapped ions and single atoms, solid-state emitters including quantum dots, color centers in solids, or preceding single-photon sources such as spontaneous parametric down-conversion. Fluorescent quantum defects, such as optically active quantum dots of tightly confined electrons or holes in semiconductor materials that can be excited and decay to emit single photons, can be formed in solid three-dimensional materials, such as defects in diamond, or by epitaxial growth of layers with lattice constant mismatch at their interfaces, such as InGaAs quantum dots in GaAs. Such quantum defects have also been shown to be created in two-dimensional materials, such as graphene and tungsten diselenide, where atoms form crystalline structures in a single layer or a few multilayers, and quantum dots occur naturally, especially at the edges of the material, or quantum dots are artificially formed using various engineering techniques. Such quantum dots in two-dimensional materials naturally have out-of-plane emission, which facilitates the collection and use of single photons, and such materials are typically robust and can be easily integrated into photonic devices.

[0004] The effectiveness of such materials as sources of single photons for these applications depends on their ability to be harnessed to stably, reliably, and easily generate pure, indistinguishable single photons in large numbers and at suitably high emission rates. Furthermore, the ability to operate these single photon sources without the need for cryogenic cooling, as well as the ability to manufacture such sources at scale to consistent quality and performance, will facilitate their adoption and enable the development of quantum computing and information processing technologies.

[0005] Single photons can also be emitted by quantum objects such as trapped ions and neutral atoms, which can be used as physical qubits in quantum computers. In this case, the physical qubits act as single-photon generating materials. Transitions that emit single photons from these quantum objects can provide a mechanism for interfacing with physical qubits in order to manipulate or interrogate them. However, these transitions to generate single photons do not occur reliably at a high enough rate.

[0006] The present disclosure is conceived in the foregoing context. overview Viewed from one aspect, the present disclosure provides an apparatus for generating single photons. In particular, the apparatus can be considered to provide a light-matter interface for enhancing a transition that generates a single photon. The apparatus comprises a first reflective surface; a locking recess formed in the locking reflective surface, where the first reflective surface and the locking recess form a locking cavity for receiving light from a locking laser, and a portion of the light output from the locking cavity indicates whether the locking cavity is resonant with the locking laser; an emission recess formed in the emission reflective surface, where the first reflective surface and the emission recess form an emission cavity for receiving photons from a single-photon emitter material, the single-photon emitter material being located in the emission cavity for emitting single photons in response to optical or electrical excitation of the material; and an actuator for tuning a length of the locking cavity based on the portion of the light output from the locking cavity to resonate with the locking laser to lock the locking cavity to the locking laser, and the locking reflective surface and the emission reflective surface are mechanically coupled to tune and stabilize the emission cavity at a single-photon emission wavelength by locking the locking cavity to the locking laser.

[0007] Viewed from another aspect, the present disclosure provides a method for generating single photons. In particular, the method can be considered to provide a light-matter interface for enhancing a transition that generates a single photon. The method includes receiving light from a locking laser in a locking cavity, the locking cavity being formed from a first reflecting surface and a locking recess formed in the locking reflecting surface, a portion of the light output from the locking cavity indicating whether the locking cavity is resonant with the locking laser, tuning a length of the locking cavity based on the portion of the light output from the locking cavity to resonate with the locking laser to lock the locking cavity to the locking laser, and emitting a single photon in response to optical or electrical excitation of the material by a single-photon emitter material located in an emission cavity, the emission cavity being formed from a first reflecting surface and an emission recess formed in the emission reflecting surface, the locking reflecting surface and the emission reflecting surface being mechanically coupled to tune and stabilize the emission cavity at a single-photon emission wavelength by locking the locking cavity.

[0008] According to the above aspect of the disclosure, a single-photon emitting material is provided that is optically coupled into a resonant cavity to enhance emission and increase its photon generation efficiency, and a separate mechanically coupled locking cavity is provided that, together with a suitable stable laser, can be used to control the actuator to maintain the locking cavity in resonance with the stable laser using a suitable locking technique. By maintaining the locking cavity in an optically tuned state to the stable laser, the mechanically coupled emission cavity containing the single-photon emitting material can also be maintained in a stable tuning state, which can be configured to resonate with the emission wavelength of the single-photon emitting material, its zero phonon line, etc., allowing suppression of its phonon sidebands. In this way, the device according to the above aspect of the disclosure can be controlled to maintain long-term emission cavity mode stability at the single-photon emission wavelength. Furthermore, since the single-photon emitting material can be maintained in a stable tuning using a locking cavity that is mechanically coupled to the emission cavity, the device does not require a specific single-photon emitting material because the locking cavity, and therefore the tuning, can be adjusted based on the material. Furthermore, because tuning of the locking cavity tunes the emission cavity via a mechanical coupling within the device, the locking cavity can be tuned using light separate from the emission cavity, meaning that the light from the locking laser is decoupled from the photons from the single-photon emitter. That is, the light from the locking laser can be used to tune the emission cavity used to enhance the emission of single photons from a material without the light entering the emission cavity or needing to be separated from the emitted single photons. Thus, the device can be used to reliably generate a stable source of indistinguishable photons from any suitable single-photon emitting material.

[0009] In an embodiment, the portion of light output from the locked cavity may be the portion of light reflected and / or transmitted from the locked cavity. Mode-locking of the cavity may be performed using the portion of light reflected from the cavity or the portion of light transmitted through the cavity, although using reflected light may provide improved performance.

[0010] In an embodiment, the locking and emitting reflective surfaces may be mechanically supported by the same substrate. For example, the locking and emitting reflective surfaces may be provided in a surface of the same substrate, such as formed in the same substrate layer using a suitable technique, such as focused ion beam (FIB) milling or CO2 laser machining. Alternatively, the locking and emitting reflective surfaces may be provided in different layers of the substrate, or in layers of material deposited on or secured to the substrate, or in recesses formed in the substrate, for example by etching. Nevertheless, the locking and emitting reflective surfaces may be supported by the same substrate. In an embodiment, the locking and emitting reflective surfaces may be on different substrates. In an embodiment, the substrate comprising the emitting reflective surface may be mechanically coupled to the locking reflective surface of the substrate comprising the locking reflective surface. In an embodiment, the emitting recess and the locking recess may have different depths. In an embodiment, the locking and emitting reflective surfaces may be at different distances from the first reflective surface, such that the locking cavity has a different length relative to the emitting cavity. For example, the locking cavity may have a longer length than the emitting cavity. The locking and emitting reflecting surfaces may be at different depths or heights within the substrate, which allows the locked laser to be decoupled from the single photon emission. By allowing the locking cavity to be made to have different coupling characteristics to the emission cavity, this allows the locking cavity to be optimized for a given mode-locking technique, and the emission cavity to be optimized to match the resonant wavelength of the single photon emission, while tuning the locking cavity also allows tuning of the emission cavity.

[0011] In an embodiment, the emission cavity may receive light from an excitation laser, and the single-photon emitter material may emit a single photon in response to the light received from the excitation laser being incident on the material. In this manner, the single-photon emitting material in the cavity may be stimulated to emit a single photon. The single-photon emitting material may be any material capable of emitting a single photon in response to a stimulus.

[0012] In an embodiment, the emitter cavity and the lock cavity may be laterally spaced apart and substantially axially aligned. In this way, tuning of the lock cavity by the actuator can also tune the emission cavity, while the lock cavity is separated from the emission cavity, so that the locking laser light enters the lock cavity instead of the emission cavity. In an embodiment, the lock cavity and the emission cavity may operate in different modes and have different resonant wavelengths. In this way, the locking laser light is not resonant with the emission cavity and is transmitted through it.

[0013] In an embodiment, the emission cavity may have a length in the range of 0.3-10 μm and the lock cavity may have a length in the range of 10-1000 μm. In an embodiment, the emission cavity may have a linewidth in the range of 10-1000 GHz and the lock cavity may have a linewidth in the range of 10-1000 MHz. In this way, the lock cavity may be optimized for use as a lock cavity using a Pound Drever Hall (PDH) technique, where the lock cavity linewidth is smaller than the emission cavity linewidth and the lock laser light is phase modulated using an electro-optic modulator at a frequency significantly larger than the lock cavity linewidth, for example in the GHz range. Alternatively, the lock cavity may be optimized for use as a lock cavity using a side locking technique, where the locking criterion is set at the steepest part of the flank of the cavity mode and the intensity change is monitored as the cavity mode deviates from the set point. This intensity change is then used to correct for frequency drift.

[0014] In an embodiment, the device may operate within a temperature range of 4 K to room temperature. The cavity locking mechanism provided by the locking and emission cavities is insensitive to temperature and therefore may operate at any temperature. The temperature range of the device may depend on the single photon emitter material used. Additionally, while the device may operate within a particular temperature range, the selected temperature at which the device operates may be the temperature at which the device is most effective, i.e., the temperature at which the device most reliably produces a steady source of indistinguishable photons. For example, the device may operate at low temperatures, e.g., below 0° C., to achieve a particular performance metric.

[0015] In an embodiment, the single photon emitter material may be a two-dimensional material having fluorescent defects or quantum dots. In an embodiment, the single photon material may emit a single photon at a temperature in the range of 0-20°C. In an embodiment, the single photon emitter material may include hexagonal boron nitride, hBN. When the single photon emitter material is hBN, this allows the device to operate in a temperature range of 0-20°C. hBN also emits single photons in a much wider temperature range than the 0-20°C range, so the device can operate in a wide temperature range, including temperatures significantly lower than 0-20°C, but being able to emit photons at room temperature enhances the usefulness of the material. In other embodiments, the single photon emitter material may include nanodiamonds or layered transition metal dichalcogenides. In other embodiments, the single photon emitter material may include trapped ions or neutral atoms, or quantum objects. In an embodiment, the single photon emitter material may be a quantum object, such as a qubit in a quantum computer.

[0016] In embodiments, the single photons emitted from the single-photon emitter material may have a wavelength of about 400 nm or greater. For example, the single photons emitted from the single-photon emitter material may have a wavelength of 550 nm to 800 nm.

[0017] In an embodiment, tuning the length of the locking cavity may include adjusting the first reflective surface and / or the locking reflective surface. In an embodiment, the actuator may be a piezo actuator. In an embodiment, the actuator may tune the length of the locking cavity based on the phase of the portion of the light output from the locking cavity. In an embodiment, the locking cavity and the emission cavity may be Fabry-Perot cavities. In an embodiment, the locking cavity may receive light from a phase modulated locking laser. In this manner, the actuator may finely tune the length of the locking cavity to lock it to a stable locking laser. In an embodiment, the actuator may actuate a portion of the apparatus providing the first reflective surface, a portion of the apparatus providing the locking reflective surface, and / or a portion of the apparatus providing the emission reflective surface.

[0018] Viewed from another aspect, the present disclosure provides a system for generating single photons. In particular, the system can be considered to provide a light-matter interface for enhancing a transition that generates a single photon. The system includes an apparatus for generating single photons according to an aspect of the present disclosure and an optical system for transmitting light into a lock cavity of the apparatus.

[0019] In an embodiment, the optical system may comprise a locking laser for generating light for transmission into a locking cavity of the device. In an embodiment, the locking laser may generate light having a wavelength different from that of the single photon emission wavelength. In an embodiment, the optical system may further comprise a modulator for modulating the phase of the light generated by the locking laser and transmitting the modulated light into the locking cavity of the device. In an embodiment, the modulator may be an electro-optic modulator. In an embodiment, the system may further comprise an RF driver coupled to the electro-optic modulator and configured to output a modulation signal for driving the modulator to modulate the phase of the locking laser at a phase modulation frequency substantially greater than the linewidth of the locking cavity. In an embodiment, the phase modulation frequency may be at least 1 GHz. In an embodiment, the optical system may further comprise a coherent detector for detecting a portion of the light output from the locking cavity of the device and mixing the detected light signal with the modulation signal from the RF driver to output a feedback signal based on the detected portion of the light from the locking laser indicative of drift of the locking cavity. In an embodiment, the system may further comprise a controller configured to receive the feedback signal and output an error signal to an actuator of the device based on the feedback signal, the actuator tuning a length of the lock cavity based on the error signal to maintain the lock cavity in resonance with the lock laser. In an embodiment, the controller may output an error signal to an actuator of the device based on a phase of a detected portion of the light output from the lock cavity. In an embodiment, the controller may be a proportional integral derivative (PID) controller. In an embodiment, the system may be configured to control the actuator to lock the lock cavity in resonance with the lock laser using a Pound Drever Hall technique. In an embodiment, the system may be configured to maintain a fundamental mode of the emitter cavity in resonance with a single photon emission frequency of the single photon emitter material by locking the lock cavity in resonance with the lock laser.In an embodiment, the system may further comprise a pulsed excitation laser and optics for directing light from the pulsed excitation laser to illuminate a single-photon emitter material in the emitter cavity to trigger emission of single photons at a fixed single-photon emission frequency. In this manner, the device may be operated to maintain the single-photon emitting material in the enhancement cavity at a stable long-term frequency that is held in resonance with the single-photon emission frequency, and the locked laser light is not coupled or mixed with the single photons, facilitating their detection.

[0020] In an embodiment, the system may further comprise at least one additional device for generating single photons according to aspects of the present disclosure. In an embodiment, the optical system may be configured to transmit light from the same locking laser into the locking cavity of each device. In an embodiment, the optical system may be further configured to transmit light from the same pulsed excitation laser into the emission cavity of each device, the photon emitter material of each device emitting a single photon in response to light received from the excitation laser being incident on the material. In an embodiment, multiple emitter cavities in the multiple devices may be for emitting single photons, each emitter cavity emitting a single photon at substantially the same fixed single photon frequency in response to illumination by the excitation laser. In this manner, multiple single photon emitters provided in separate tunable devices can all be tuned together to remain in resonance over time with their single photon emission wavelength based on the same common stable locking laser.

[0021] Viewed from another aspect, the present disclosure provides a method of manufacturing an apparatus for generating single photons, the method including: providing a lock substrate with a lock reflective surface; forming a recess in the lock reflective surface to form a lock cavity with a first reflective surface of the first substrate; providing an emission substrate with an emission reflective surface, the emission substrate and the lock substrate being mechanically coupled; forming a recess in the emission reflective surface to form an emission cavity with the first reflective surface of the first substrate; providing a single photon emitter material for emitting single photons located in the emission cavity; providing a first substrate with a first reflective surface; and tuning a length of the lock cavity based on a cavity length of the lock cavity to tune the length of the emission cavity to a fixed configuration to be finely tuned by an actuator during use. In an embodiment, the recess may be formed using a focused ion beam.

[0022] In light of the teachings presented herein, many modifications and other embodiments of the inventions described herein will occur to those skilled in the art to which these inventions pertain. It will therefore be understood that the disclosure herein is not limited to the specific embodiments disclosed herein. Moreover, while the descriptions provided herein provide example embodiments in the context of specific combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the scope of the invention.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like reference symbols are used to refer to like parts, and in which: [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram of an apparatus for generating single photons according to an aspect of the present disclosure. [Figure 2A]FIG. 1 illustrates a first embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. [Figure 2B] FIG. 2 illustrates a second embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. [Diagram 3] FIG. 1 is a schematic diagram of one embodiment of a system for use in generating single photons in accordance with aspects of the present disclosure. [Figure 4A] FIG. 13 is a further schematic diagram of another embodiment of a system for generating single photons according to an aspect of the present disclosure. [Figure 4B] FIG. 13 is a further schematic diagram of another embodiment of a system for generating single photons according to an aspect of the present disclosure. [Diagram 5] FIG. 13 is a further schematic diagram of another embodiment of a system for generating single photons according to aspects of the present disclosure. [Figure 6] FIG. 1 illustrates a method for generating a single photon according to an aspect of the present disclosure. [Figure 7] 1 illustrates a method of manufacturing an apparatus for generating single photons according to an aspect of the present disclosure. [Figure 8A] FIG. 1 illustrates a third embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. [Figure 8B] FIG. 13 illustrates a fourth embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Detailed Description Hereinafter, the embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that the present disclosure is not limited to the embodiments, and all modifications and / or equivalents or replacements thereto also belong to the scope of the present disclosure. The same or similar reference symbols may be used throughout this specification and drawings to refer to the same or similar elements.

[0026] As used in this specification, the terms "have," "may have," "include," or "may include" of a feature (e.g., a number, function, operation, or component such as a part) indicate the presence of that feature and do not exclude the presence of other features.

[0027] As used herein, the terms "A or B," "at least one of A and / or B," or "one or more of A and / or B" can include all possible combinations of A and B. For example, "A or B," "at least one of A and B," or "at least one of A or B" can refer to all of: (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0028] As used herein, the terms "first" and "second" can modify various components, regardless of importance, and do not limit the components. These terms are used only to distinguish one component from another. For example, a first user device and a second user device can refer to different user devices, regardless of the order or importance of the devices. For example, a first component may be referred to as a second component, and vice versa, without departing from the scope of the present disclosure.

[0029] When an element (e.g., a first element) is referred to as being "coupled to" (mechanically, operatively, or communicatively) or "connected to" another element (e.g., a second element), it will be understood that it may be coupled or connected to the other element directly or through a third element. In contrast, when an element (e.g., a first element) is referred to as being "directly coupled to" or "directly connected to" another element (e.g., a second element), it will be understood that the other element (e.g., a third element) is not intervening between the element and the other element.

[0030] As used herein, the term "configured (or configured) to" may be used interchangeably with the terms "suitable for," "capable of," "designed to," "adapted to," "made to," or "capable of," depending on the context.

[0031] It is understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0032] The terms used in this specification are provided only to describe some embodiments thereof, and do not limit the scope of other embodiments of the present disclosure.All terms used in this specification (including technical and scientific terms) have the same meaning as those commonly understood by those skilled in the art to which the embodiments of the present disclosure belong.Terms such as those defined in commonly used dictionaries should be interpreted to have a meaning consistent with their meaning in the context of the relevant art, and it will be further understood that they are not interpreted in an idealized or overly formal sense unless expressly so defined in this specification.

[0033] As used throughout the figures, features or method steps are outlined with dashed lines to indicate that such features or method steps are optional features for providing in some embodiments, but may not be provided in all embodiments for practicing aspects of the present disclosure, that is, aspects of the present disclosure do not require that these optional features be included or steps be performed, but rather that they are merely included in example embodiments to provide further optional implementation details.

[0034] Turning now to the figures, FIG. 1 shows a schematic diagram of an apparatus 100 for generating single photons according to an embodiment of the present disclosure. The apparatus 100 includes a single-photon emitter material 112 (also referred to as a quantum emitter material) for generating single photons during use in response to optical or electrical excitation thereof. For example, light from an excitation laser can be transmitted into the cavity to illuminate and optically excite the single-photon emitter material 112 and trigger photons to be emitted by the material 112 at a single-photon emission frequency. An exemplary excitation laser input is shown by a dashed arrow in FIG. 1. The single-photon emission frequency may be a fixed frequency, or the single-photon emitter material 112 may be tunable to emit photons at a specific frequency. The single-photon emitter material 112 may be any material for generating single photons, for example, as a result of an energy level transition. For example, the single-photon emitter material 112 may be a solid material for generating single photons, or a quantum object such as a trapped ion or neutral atom that serves as a quantum bit in a quantum computer, for example. If the single-photon emitter material 112 is a solid material, it may be a low-dimensional material or a film with a thickness of less than 10 micrometers. In this embodiment, the single-photon emitter material 112 is a two-dimensional material consisting of one or several layers of atoms grown in a crystal structure that encompasses one or more fluorescent defects. The two-dimensional single-photon emitter material 112 may be hexagonal boron nitride (hBN), which has a large band gap between the valence band and the conduction band, providing a transition energy of the fluorescent defect that allows single-photon emission at a peak emission wavelength (corresponding to its zero-phonon line) that is resolvable from its phonon side band (PSB). In the case of hBN, the energy levels introduced in the band structure are well isolated, and the large band gap also helps prevent Auger-type non-radiative decay, improving the quantum efficiency of the hBN emitter. The two-dimensional structure of hBN also allows for inherently high extraction efficiency since the emitted photons are not subject to total internal reflection or Fresnel reflection. The single-photon emitter material 112 may be hBN flakes selected to have a peak emission wavelength at or near the desired wavelength.The single-photon emitter material 112 may be formed from single or multiple layers of hBN flakes. The single-photon emitter material 112 may be exfoliated hBN flakes, CVD-grown hBN, or solution-based hBN flakes. One advantage of hBN is that it allows for single-photon emission at room temperature in the range of 0-20 degrees Celsius, and thus devices comprising hBN can operate at room temperature.

[0035] In the device 100, the single-photon emitter material 112 is provided within an optically resonant emission cavity 106 (sometimes also referred to as an interface cavity or chemical cavity) formed between the first reflective surface 102 and the emission reflective surface 104, and fluorescent defects in the single-photon emitter material 112 are aligned and optically coupled into the emission cavity 106 when the emission cavity 106 is resonant with the emission wavelength of the fluorescent defects. The single-photon emitter material may be located on the first reflective surface 102 because this location allows for greater control of the cavity for ease of alignment and provides the strongest electric field. The emission cavity 106 may be arranged as a Fabry-Perot cavity with a planar-concave structure, with the first reflecting surface 102 having a flat mirror surface with a very high reflection coefficient in the region of 99.9% at the wavelength of interest, and the emission reflecting surface 104 having a concave mirror surface with a very high reflection coefficient in the region of 99.99% at the wavelength of interest. The single-photon emitter material 112 is located within the emission cavity 106 aligned so that the fluorescent defects are resonant with it when excited, and is held in place by any suitable positioning means, such as adhesion to the emission reflecting surface 104 by van der Waals forces. One advantage of using a two-dimensional material as the single-photon emitter material 112 is that crystals can be easily attached to a surface using van der Waals forces.

[0036] The device 100 is such that when the emission cavity 106 is tuned to a stable resonant frequency matched to the peak emission wavelength of the single-photon emitter material 112, the efficiency of photon generation by the fluorescent defects is increased by the Purcell effect, and the decay lifetime of the fluorescent defects from their excited states is reduced, resulting in enhanced emission of single photons from the cavity for use in various quantum computing, information processing, or communication applications. Furthermore, the device 100 is such that when the emission cavity 106 is tuned to a stable resonant frequency matched to the peak emission wavelength of the single-photon emitter material 112, the phonon sidebands (PSBs) are completely suppressed, causing the single-photon emitter material 112 to couple more energy into single-photon emission. Other non-resonant noise is also completely suppressed. Thus, providing the single-photon emitter material 112 in the emission cavity 106 allows a high level of control of single photons emitted from the cavity with enhanced optical properties, including improved spectral purity.

[0037] To allow the emitter cavity 106 to be actively tuned to stay in resonance with the single-photon emitter material 112 to compensate for various environmental factors such as temperature fluctuations, an actuator 114, such as a piezoelectric transducer, is used to actuate a mechanical structure supporting the emission cavity 106 to finely tune the length of the emission cavity 106. However, the amount of tuning required must be determined. One way to detect the amount of tuning required is to transmit light from a locking laser into the emission cavity 106, and the portion of the light from the locking laser that is output from the emission cavity 106 indicates the amount of tuning required by the actuator 114. However, due to the same cavity performing both locking and emission, the light from the locking laser and the photons emitted from the single-photon emitter material 112 will interfere because the locking laser will drive the cavity mode into single-photon emission, resulting in a frequency overlap between the locking frequency of the locking laser and the single-photon emission frequency. This reduces the output intensity and therefore the effectiveness of the single-photon emitter. To reduce such interference, advanced polarization is required, which becomes expensive and complicated.Another way to detect the amount of tuning required is to use single photons rather than a locked laser to determine the amount of tuning required. However, such a design also reduces the output intensity of the single photon emitter, since the photon intensity is unstable and therefore these photons are wasted.

[0038] To provide a more stable single-photon emitter with increased output intensity, as shown in FIG. 1, the apparatus 100 of the present disclosure includes a locking cavity 110 that is mechanically coupled to the emission cavity 106. The use of the locking cavity 110 to sense a tuning signal can be used to control an actuator 114 such that the locking cavity 110 and the mechanically coupled emission cavity 106 are maintained to have a constant length and resonant frequency. The apparatus can be configured such that by tuning the locking cavity 110, the emission cavity 106 can be tuned to resonate with the peak emission wavelength of the single-photon emitter material 112. Thus, by actuating a mechanical structure supporting both the emission cavity 106 and the locking cavity 110, the actuator 114 can tune the length of the locking cavity 110 to lock the locking cavity 110 to the locking laser, which in turn tunes and stabilizes the emission cavity 106 at a single-photon emission wavelength, which is the wavelength of the single photons output from the emission cavity 106 by the single-photon emitter material 112. The emission of single photons from the emitter cavity 106 is indicated by the solid arrow in FIG.

[0039] The locking cavity 110 is formed between the first reflecting surface 102 and the locking reflecting surface 108. Thus, the locking cavity 110 and the emission cavity 106 share the first reflecting surface 102. The locking cavity 110 may also be arranged as a Fabry-Perot cavity with a planar-concave structure, with the locking reflecting surface 108 formed to have a concave mirror surface with a very high reflection coefficient in the region of 99.9% at the wavelength of interest.

[0040] The addition of the locking cavity 110 allows tuning of the emission cavity 106. The actuator 114 is controlled to maintain the fundamental mode of the locking cavity in resonance with a stable locking laser using an appropriate locking technique. As shown by the dotted arrow in FIG. 1, the locking cavity 110 can receive light from the locking laser, and the portion of the light from the locking laser that is output, e.g., transmitted or reflected, from the locking cavity 110 indicates whether the locking cavity is in resonance with the locking laser. The wavelength of the locking laser may be different from the single-photon emission wavelength. If the locking cavity is not in resonance with the locking laser, the portion of the light from the locking laser that is output from the locking cavity 110 can indicate a difference between the frequency of the locking laser and the resonant frequency of the locking cavity. The actuator 114 adjusts the mechanical structure supporting the locking cavity 110 based on the portion of the light output from the locking cavity 110 to tune the length of the locking cavity to move the locking cavity into resonance with the locking laser to lock the locking cavity to the locking laser. For example, the actuator 114 can actuate the first reflective surface 102 and / or the locking reflective surface 108 to move them towards or away from each other to adjust the spacing between them. By actuating the mechanical structure in this manner, the emission cavity 106 is also tuned and stabilized at the single-photon emission wavelength because the two cavities 106, 110 are supported by the same mechanical structure. Thus, in addition to maintaining the fundamental mode of the locking cavity 110 in resonance with the locking frequency of the locked laser, the fundamental mode of the emitter cavity 106 is maintained in resonance with the single-photon emission frequency of the single-photon emitter material 112. Although the actuator 114 can directly control the locking reflective surface 108 and can only control the emission reflective surface 104 by a mechanical coupling between the locking reflective surface 108 and the locking reflective surface 108, the actuator is shown in FIG. 1 as being connected to both reflective surfaces to show that the actuator 114 controls both reflective surfaces.

[0041] The emission reflective surface 104 and the locking reflective surface 108 are mechanically coupled such that slight dimensional changes in the cavity, such as changes in cavity length that occur in the emission cavity 106, also occur in the locking cavity 110. Moreover, due to the mechanical coupling, tuning the geometry of the locking cavity 110 to cancel these slight changes so that the locking cavity 110 is resonant with the locking laser to lock the locking cavity 110 to the locking laser also tunes and stabilizes the emission cavity 106 at the single photon emission wavelength. The locking cavity 110 may be laterally spaced apart and substantially axially aligned with the emission cavity 106 such that the cavities are substantially parallel, and thus adjusting the length of the locking cavity 110 adjusts the length of the emission cavity 106 by the same amount.

[0042] Thus, the addition of the locking cavity 110, which is mechanically coupled to the emission cavity 106, effectively tunes the emission cavity 106 to remain in resonance with the single-photon emitter material 112 while reducing interference between the light from the locking laser and the photons emitted from the single-photon emitter material 112. Thus, the emitter cavity 106 can be actively tuned to maintain the single-photon emission output without reducing the power of the single-photon emission output as a result of significant interference. The addition of the locking cavity 110 to provide a dual-cavity locking protocol stabilizes the frequency of the photons emitted from the emission cavity 106, overcoming the instabilities of the emission cavity 106 and the complications associated with transmitting light from the locking laser to the emission cavity, as explained above. Moreover, because the locking laser and the locking cavity 110 are decoupled from the emission cavity 106, the locking laser can be shared across multiple devices, which facilitates multi-device frequency matching. The locking laser may be further locked to a global reference laser to facilitate multi-device synchronization. If a pump laser is used, this may also be shared across multiple devices.

[0043] An initial tuning of the lock cavity 110, and consequently the emitter cavity 106, may be performed during fabrication, where a larger bulk movement of the spacing between the reflective surfaces of the cavities can be achieved during assembly, so that during use, the emitter cavity 106 and the lock cavity 110 can be tuned and fixed in place to bring the emitter cavity 106 into resonance with the peak emission wavelength of the single-photon emitter material 112 by tuning the lock cavity 110 to the lock laser wavelength. Finer tuning of the device 100 using the actuator 114 may then be performed, for example, during testing, also referred to as alignment, where the laser is already locked but drifting, and the tuning performed at this stage is typically fine tuning, and may include tilting the lock cavity 110 (axial alignment) in addition to lateral alignment.

[0044] The lock cavity 110 can be optimized for use as a lock cavity using the Pound Drever Hall (PDH) technique. The PDH technique uses phase modulated light from a lock laser that is received in the lock cavity 110. The phase of the portion of the light from the lock laser that is output from the lock cavity 110 is detected, for example by homodyne detection, to determine the error and resonance wavelength of the lock cavity 110 compared to the wavelength of a stable lock laser, and the detected error is used to feed back to the actuator 114 to correct the length of the lock cavity 110 so that it is again in resonance with the lock laser. The PDH technique thus allows the cavity to be locked to the laser. This allows the measurement of slight changes in the length of the lock cavity 110 with very high precision. Any slight change in the length of the lock cavity 110 causes the lock cavity to deviate from resonance with the stable wavelength light from the lock laser, thus causing a change, for example an increase, in the phase error signal detected from the light output from the lock cavity 114 by homodyne detection. By detecting the phase error of the light output from the locking cavity 110, slight changes in the length of the locking cavity 110 can be detected and based on this, the actuator 114 can adjust the length of the locking cavity 110 to compensate for the slight changes so that the locking cavity 110 is resonant with the locking laser, known as locking the locking cavity 110 to the locking laser.

[0045] The direction in which the locking wavelength is out of resonance with the locking cavity 110, and therefore the direction in which the length of the locking cavity 110 is adjusted (whether to increase or decrease the length), is also determined from detection of the light output from the locking cavity 110. In the PDH technique, since the light from the locking laser is phase modulated before entering the locking cavity 110, the direction in which the locking wavelength is out of resonance with the locking cavity 110 can be determined by the light output from the locking cavity 110. This is because it can be determined from the light output from the locking cavity 110 whether the laser frequency is above or below resonance depending on whether the signal from the homodyne detection indicates that the reflected signal is out of phase by a positive amount, which is in front of the phase modulated input locking laser, or a negative amount, which is behind the phase modulated input locking laser. Using the PDH technique is advantageous because it allows the direction of the mismatch, in addition to the size of the mismatch between the laser frequency and the resonance of the locking cavity 110, to be detected and corrected. It is also fairly easy to set up and provides a stable source of an error signal that does not change with intensity fluctuations of the locked laser.The components for implementing the PDH technique are described below in conjunction with FIG.

[0046] In addition to tuning the single-photon output using the cavity locking mechanism provided by the locking cavity 110 and emission cavity 106 of the device 100 of FIG. 1 as described above, the single-photon emitting material 112 can be tuned inside the emission cavity. That is, the zero-phonon line emission frequency of the single-photon emitting material 112 can be electrically tuned by directly stabilizing the optical transitions. Electrical tuning by the Stark effect of the strength of both the cycling transitions and the spin-modified transitions allows for stabilization of the zero-phonon line emission frequency. Electrical tuning of the single-photon emitting material 112 can be achieved by depositing multiple electrodes around the single-photon emitting material and applying a bias between the electrodes to generate an electric field through the material and tune the material. The use of multiple electrodes ensures fine control over the electric field orientation, allowing optimal alignment between the electric field and the emitter dipole orientation, improving tuning capabilities. Utilizing both tuning methods further improves tuning and increases the number of single photons emitted at the single-photon emission frequency.

[0047] FIG. 2A illustrates a first embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. The apparatus 200 illustrated in FIG. 2A is an example of the apparatus 100 of FIG. 1, and therefore like reference numerals apply to like features, a detailed description of which may be provided above in connection with FIG. 1. The apparatus 200 comprises a substrate 214, on which the locking reflective surface 108 and the emitting reflective surface 104 are provided and formed therein by any other suitable means, such as, for example, focused ion beam milling or CO2 laser machining. A locking recess formed in the locking reflective surface 108 forms the locking cavity 110, and an emitting recess formed in the emitting reflective surface 104 forms the emitting cavity 106. Thus, any movement of the substrate 214 by an actuator 114 (not shown) relative to the first reflective surface 102 tunes both the locking cavity 110 and the emitting cavity 106. The locking and emission recesses can have different depths (by different FIB milling amounts) such that the length of the locking cavity 110 is different from the length of the emission cavity 106, allowing the locked laser to have a resonant frequency different from the peak emission frequency of the single-photon emitting material 112 enhanced by the emission cavity 106. Alternatively, the lengths of the locking cavity 110 and the emission cavity 106 can be substantially the same.

[0048] In an exemplary embodiment of the device 200 of FIG. 2A, the single-photon emitting material 112 may have a peak emission wavelength of 575 nm, and the locking frequency of the locked laser may be 561 nm. Moreover, the cavity lengths may be similar. The cavity linewidth is determined by the cavity quality factor Q=Fq, where F is the finesse. The cavity quality factor Q is determined by the cavity length L and the specular reflectivity R. In one example, both cavities have a cavity length of 1-2 μm, resulting in a finesse F of about 500-1000 and a quality factor of about 3000 (q is 5.5). Thus, for the locked cavity, the linewidth may be about 158 ​​GHz. If the locking recesses 110 and the emitting recesses 106 are formed in the same substrate, the recesses, and consequently the cavities, may be arranged as a cluster on the substrate, allowing a suitable locking cavity to be chosen.

[0049] FIG. 2B illustrates a second embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. The apparatus 250 illustrated in FIG. 2B is an example of the apparatus 100 of FIG. 1, and therefore like reference numerals apply to like features, and a detailed description of which may be provided above in connection with FIG. 1. The apparatus 250 comprises a substrate 254, a locking reflective surface 108 is provided on the substrate 254, and a locking recess formed in the locking reflective surface 108 forms a locking cavity 110. In addition to the substrate 254, the apparatus 250 comprises a further substrate 264 supported by and mechanically coupled thereto. The emitting reflective surface 104 is provided on the further substrate 264, and an emitting recess formed in the emitting reflective surface 104 forms an emitting cavity 106. The further substrate 264 is mechanically bonded to the substrate 254 and is located between the substrate 254 and the first reflective surface 102. For example, the further substrate 264 can be deposited on the substrate 254 during fabrication. Alternatively, the features provided by the substrate 254 and the further substrate 264 may be provided integrally by etching a step in the body to create a height difference relative to the first reflective surface 102 .

[0050] 2B embodiment are mechanically coupled, the locking reflective surface 108 and the emitting reflective surface 104 are also mechanically coupled. Thus, any movement of the substrate 254 by the actuator 114 (not shown) relative to the first reflective surface 102 also moves the further substrate 264, thus tuning both the locking cavity 110 and the emitting cavity 106 together. Moreover, any movement of the further substrate 264 relative to the first reflective surface 102 by the actuator 114 causes this.

[0051] In the apparatus 250 of Figure 2B, the single photon emission output may have a wavelength greater than 560 nm and the locking frequency of the locked laser may be 633 nm. The locking cavity may have a much longer length than the emission cavity to provide a more efficient and simple implementation of the PDH technique.

[0052] To effectively implement PDH, the phase modulation frequency must be substantially larger than the laser linewidth and the cavity linewidth. In the apparatus 200 of FIG. 2A, an exemplary linewidth of the lock cavity was about 158 ​​GHz, but such a high linewidth requires very high frequency phase modulation in the hundreds of GHz, or even THz range, which is too difficult to achieve using standard phase modulation electronics. Thus, with the configuration shown in FIG. 2B, the lock cavity linewidth can be reduced to a level where a lower phase modulation frequency in the GHz range can be used to effectively implement the PDH cavity locking scheme. As mentioned above, the cavity linewidth is determined by the cavity quality factor Q=Fq, which is determined by the cavity length L and the mirror reflectivity R. The mirror reflectivity R is determined to be 99.9% for other reasons, as shown below, and therefore the lock cavity length L can be increased to reduce the lock cavity linewidth. By providing the locking reflective surface 108 and the emitting reflective surface 104 on substrates that are at different heights relative to the first reflective surface 102, the length of the locking cavity 110 can be made longer than the length of the emitting cavity 106. This allows the length of the locking cavity 110 to be increased without having to increase the length of the emitting cavity 106, allowing the locking cavity linewidth to be reduced, and as a result, PDH can be implemented using standard phase modulation electronics.

[0053] As mentioned in relation to FIG. 1, the first reflective surface 102 has a reflectivity coefficient in the 99.9% range at the wavelength of interest. It is advantageous for the emission cavity 106 to have an asymmetric reflectivity setup that biases the emission of single photons towards the first reflective surface 102 so that most photons are transmitted through the plane mirror. To provide such a setup, the emission reflective surface 104 may have a reflectivity coefficient in the 99.99% range different from the reflectivity coefficient of the first reflective surface 102 at the wavelength of interest. For ease of setup, it is advantageous for the lock cavity 110 to have a symmetric reflectivity setup, so that the lock reflective surface 108 can have the same reflectivity in the 99.9% range as the first reflective surface 102 at the wavelength of interest. Providing the lock reflective surface 108 and the emission reflective surface 104 on different substrates, as shown in FIG. 2B, allows the surfaces to have different reflectivities and thus allows differential mirroring. The different reflectivities may be provided by different coatings on the surfaces of the substrates.

[0054] In an exemplary embodiment of the device 250 of FIG. 2B, as described above in connection with FIG. 2A, the emission cavity may still have a length of 1-2 μm, but the lock cavity may have a larger length of 500 μm. A cavity length L of 500 μm results in q being 1580 (using the formula q=2nL / λ), which, if both mirrors have 99.9% reflectivity, results in a finesse of about 3000 and a quality factor of 7.9×10^5. The lock cavity linewidth Δν is therefore 600 MHz (using the formula Δν=ν / Q). Since the phase modulation needs to be substantially larger than the linewidth, this requires only a few GHz for the phase modulation, which can be easily implemented using an electro-optic modulator (EOM) with, for example, lithium niobate or MgO:LN type crystals. Moreover, for q=1,782, the free spectral range is 0.3 nm, and thus approximately 300 GHz, which provides a high density of fundamental modes, which is beneficial in finding modes to lock on. Thus, the device 250 of FIG. 2B is advantageous because it offers greater dimensional flexibility, such as allowing for larger locking cavity lengths, and thus tuning of the device is simpler and less complex to implement due to the less complex phase modulation required to implement PDH.

[0055] The recesses in Figures 2A and 2B are shown as recesses in a substrate formed, for example, by a focused ion beam, as discussed in connection with Figure 7, although other recesses are envisioned.

[0056] 3 shows a schematic diagram of one embodiment of a system 320 for use in generating single photons according to aspects of the present disclosure. The system 320 includes one or more devices 300 for generating single photons. The device 300 is an example of the device 100 of FIG. 1. The device 300 may be the device 200 of FIG. 2A or the device 250 of FIG. 2B. The system 320 further comprises an optical system 322 for transmitting light into a lock cavity of the device. Examples of the system 320 are provided in FIG. 4 and FIG. 5.

[0057] The optical system 322 may comprise a locking laser for generating light for transmission into the locking cavity of the device 300. The light generated by the locking laser may be of a substantially single wavelength. To reduce interference, the single wavelength of the locking laser may be different from the single photon emission wavelength. The optical system 322 may further comprise an optional guiding means for guiding the light generated by the locking laser into the locking cavity of the device 300. As described in relation to FIG. 1, the device 300 comprising a locking laser and a locking cavity separated from the emission cavity allows the locking laser to be shared across multiple devices 300. Thus, as shown in FIG. 3, the optical system 322 providing the light generated by the locking laser may also be shared across multiple devices 300. Sharing the optical system 322 between multiple devices 300 reduces the size and power consumption of the system 320, providing a compact system for generating single photons. Furthermore, this allows the single photon emitters in each of the multiple devices 300 to be tuned and held stable at a frequency determined by a common stable locking laser.

[0058] FIG. 4A shows a further schematic diagram of another embodiment of a system for generating single photons according to an aspect of the disclosure. The system 420 of FIG. 4A is an example of the system 320 of FIG. 3. The system 420 includes an apparatus 400, which is an example of the apparatus 100 of FIG. 1 and may be the apparatus 200 of FIG. 2A or the apparatus 250 of FIG. 2B. The system 420 may also include additional apparatuses 400 (not shown). The system also includes optics such as a mirror 428 and an objective lens 430. The mirror 428 may be a dichroic or fiber-based mirror. The system 420 is connected to a lock laser 426, which transmits light to the system 420, and the mirror 428 and the objective lens 430 direct the light received by the lock laser 426 into the lock cavity of the apparatus 400. The system 420 is also connected to an excitation laser 424 for generating light for transmission into the emission cavity 106 of the apparatus 400 to optically excite the single-photon emitter material 112. The pump laser 424 transmits light to the system 420, and a mirror 428 and an objective lens 430 direct the light received by the pump laser 424 to the emission cavity 106 of the device 400. The single photons output from the device 400 are also directed by the mirror 428 and the objective lens 430, as indicated by the arrow 432. In the system 420 of FIG. 4A, the light output from the lock laser 426 and the pump laser 424 and the single photons output from the device 400 share the mirror 428 and the objective lens 430. Although the lock laser 426 and the pump laser 424 are shown as separate from the system 420, they may also form part of the system 420. Moreover, one laser may form the lock laser 426 and the pump laser 424. The system 420 may include additional optics, such as polarization optics.

[0059] FIG. 4B shows a further schematic diagram of another embodiment of a system for generating single photons according to an aspect of the disclosure. The system 470 of FIG. 4B comprises the same components as the system 420 of FIG. 4A, but the components are arranged differently. The system 470 of FIG. 4B further comprises a number of mirrors 428 and an objective lens 430. This is because in the system 470 of FIG. 4B, the outputs from the locking laser 426 and the pumping laser 424 do not share the mirror 428 and the objective lens 430, and the locking laser 426 is provided from a different location. For example, the locking laser 426 may be provided on the side of the locking recess. Using different objective lenses 430 for different lasers is advantageous because then the optical alignment of the pumping laser 424 and the locking laser 426 can be decoupled and optimized separately. Moreover, there may be further mirrors 428 and objective lenses 430 to receive the single photons output from the device 400, as shown by arrow 432, so that the single photons can be optimized separately. Alternatively, the light output from the excitation laser 424 and the single photons output from the device 400 may share the mirror 428 and the objective lens 430 .

[0060] Figure 5 shows a further schematic diagram of another embodiment of a system for generating single photons according to aspects of the disclosure. System 520 of Figure 5 is an example of system 320 of Figure 3 and system 420 of Figure 4. System 520 includes apparatus 500, which is an example of apparatus 100 of Figure 1 and may be apparatus 200 of Figure 2A or apparatus 250 of Figure 2B. System 520 may also include additional apparatus 500 (not shown). The cavity of apparatus 500 is illustrated to show the path of light transmitted to and received from the cavity.

[0061] The system 520 includes a controller 532, a pump laser 524, and an optical system 540 for transmitting light from the pump laser 524 into the emission cavity 106 of the device 500. The system 520 further includes an optical system 522 for transmitting light from the lock laser into the lock cavity 110 and receiving light from the lock cavity. The optical system 522 includes an RF driver 524, a lock laser 526, a modulator 536, and a coherent detector 526. The lock laser 526 is for generating light for transmission into the lock cavity 110 of the device 500, and the modulator 536, such as an electro-optical modulator or a Pockels cell, modulates the phase or frequency of the light generated by the lock laser 526 before it is transmitted into the lock cavity 110 of the device 500. The phase modulator 536 allows for the use of Pound Drever Hall techniques to lock the lock cavity 110. The modulator 536 may be an electro-optic modulator. A local oscillator, such as an RF driver 534, is coupled to the electro-optic modulator and configured to output a modulation signal for driving the modulator 536 to modulate the phase of the lock laser 526 at a phase modulation frequency substantially greater than the linewidth of the lock cavity. The optimal phase modulation frequency is 1 GHz or higher. FIG. 5 shows that the light from the lock laser 526 is transmitted from the top of the device 500 through the first reflecting surface 102 into the lock cavity 110 of the device 500. However, the lock laser 526 may alternatively be configured such that the light from the lock laser 526 is transmitted from the bottom of the device 500 through the lock reflecting surface 108 into the lock cavity 110 of the device 500.

[0062] When the lock cavity receives the phase modulated light from the lock laser, a portion of the light is reflected or transmitted from the lock cavity 110 based on the difference between the resonance of the lock cavity 110 and the lock frequency of the lock laser 526. This light output from the lock cavity 110 is detected by an optical detector, such as a coherent detector 538, which provides a feedback signal to the controller 532 corresponding to the light output from the lock cavity 110. The feedback signal can be based on the phase of the light output from the lock cavity 110. The coherent detector 538 can generate the feedback signal by mixing the detected optical signal with the modulated signal from the RF driver using homodyne detection to account for the phase difference, which is indicative of the phase error and is used as the feedback signal.

[0063] Based on the feedback signal, the controller 532 determines the amount and direction of the adjustment of the lock cavity and outputs an error signal to the actuator 114 to control the actuator 114 to adjust the length of the lock cavity 110 to lock the lock cavity to the lock laser. The controller 532 may be a feedback controller such as a PID controller. By tuning the lock cavity, the optical system 522 and the actuator 114 also tune the emission cavity when the lock cavity and the emission cavity are mechanically coupled such that any change in the length of the optical cavity also changes the length of the emission cavity. Thus, by locking the lock cavity to the lock laser by removing any slight changes in the length of the lock cavity, these slight changes are also removed from the length of the emission cavity. If the emission cavity was initially resonant with a single photon emission wavelength, removing these slight changes from the length of the emission cavity tunes and stabilizes the emission cavity at the single photon emission wavelength. This provides a single photon emitter with long-term stability.

[0064] The optical system 522 may include a mixer (not shown) to sum the input from the RF driver, which may be phase shifted, and the input from the coherent detector 538, and pass the resulting signal, a feedback signal, to the controller 532 or directly to the actuator 114 to adjust the length of the laser cavity. The resulting signal may be filtered by a low pass filter (not shown) before being passed to the controller 532.

[0065] The pump laser 524 is for generating light for transmission into the emission cavity 106 of the device 500, and the optical system 540 guides the light generated by the pump laser 524 into the emission cavity 106 of the device 500. The pump laser 524 may provide a phase modulated pulse of light having a wavelength of, for example, 532 nm to excite the single-photon emitting material 112. In response to being excited by the light from the pump laser 524, the single-photon emitting material 112 emits a single photon with higher quantum efficiency due to the enhancement by the emitter cavity, a stable wavelength due to the dual cavity locking arrangement, and suppressed noise from phonon sidebands.

[0066] FIG. 6 illustrates a method 600 for generating single photons according to an embodiment of the present disclosure. The method 600 may be performed by the apparatus 100 of FIG. 1, the apparatus 200 of FIG. 2A, or the apparatus 250 of FIG. 2B. The method 600 may be performed during manufacture of the apparatus and / or during use of the apparatus. The method 600 includes receiving 602 light from a locking laser in a locking cavity 110, the locking cavity 110 being formed from a locking recess formed in the first reflective surface 102 and the locking reflective surface 108, and a portion of the light output from the locking cavity 110 indicating whether the locking cavity 110 is in resonance with the locking laser.

[0067] The method further includes tuning 604 a length of the locking cavity 110 based on a portion of the light output from the locking cavity to resonate with the locking laser to lock the locking cavity 110 to the locking laser. The tuning of the length may be performed by an actuator 114. The method may further include receiving light in the emission cavity 106 from an excitation laser to optically stimulate the single-photon emitter material 112 to emit a photon. The method further includes emitting 606 a single photon by the single-photon emitter material 112 located in the emission cavity 106 in response to optical or electrical excitation of the material 112, the emission cavity 106 being formed from an emission recess formed in the first reflective surface 102 and the emission reflective surface 104, the locking reflective surface 108 and the emission reflective surface 104 being mechanically coupled such that the emission cavity 106 is tuned and stabilized at a single-photon emission wavelength by locking the locking cavity 110.

[0068] FIG. 7 illustrates a method 700 of fabricating an apparatus for generating single photons according to an embodiment of the present disclosure. The method 600 may be for fabricating the apparatus 100 of FIG. 1 or the apparatus 250 of FIG. 2B. The method 700 includes providing 702 a lock substrate including a lock reflective surface 108. The lock substrate may be made of any suitable material, for example, the substrate may be a borosilicate glass substrate. The method 700 further includes forming 704 a recess in the lock reflective surface 108 to form a lock cavity 110 with the first reflective surface 102 of the first substrate. The recess may be formed by any suitable technique, such as CO2 laser machining or FIB milling. Using FIB milling to form the recess results in a recess having an accurate and precise curvature with a small radius of curvature. During the milling process, I2 gas may be added to minimize surface roughness. The method 700 further includes providing 706 an emission substrate including an emission reflective surface 104, where the emission substrate and the lock substrate are mechanically coupled. For example, the emitting substrate may be deposited on the lock substrate using a mask. The method 700 further includes forming 708 a recess in the emitting reflective surface 104 to form the emitting cavity 106 with the first reflective surface 102 of the first substrate. This recess may also be formed by FIB milling. Such FIB milling may be performed at a wavelength of λ 3 This provides a very small mode volume of about

[0069] The method 700 further includes providing 710 a single-photon emitter material 112 for emitting single photons located within the emission cavity 106. The single-photon emitter material 112 may be provided at the focal point of the emission cavity 106. The single-photon emitter material 112 may be treated, for example, using plasma, UV ozone exposure, electron beam irradiation, or ion beam irradiation. The plasma treatment may use a plasma source such as oxygen, hydrogen, or methane. In an example where the single-photon emitter material 112 is hBN flakes, these may be treated with oxygen plasma followed by rapid thermal annealing before being provided to the emission cavity 106. The single-photon emitter material 112 may be provided on the first reflective surface 102 or on the emitting reflective surface 104 within the emission cavity 106. The method 700 further includes providing 712 a first substrate including the first reflective surface 102. A spacer may be formed on the emitting substrate, and the first substrate may be formed on the spacer to form the emission cavity 106. For example, the spacer may be deposited, followed by the first substrate.

[0070] The method 700 further includes tuning 714 the length of the lock cavity 110 based on the cavity length of the lock cavity 110 to tune the length of the emission cavity 106 to a fixed configuration so as to be finely tuned by the actuator during use. Thus, as mentioned in relation to FIG. 1, the actuator 114 can be utilized to coarsely tune the lock cavity 110 during fabrication and can then be used to finely tune the lock cavity 110 during testing / alignment. In one example, the method 700 may further include providing a tunable polymer spacer instead of or in addition to a spacer prior to providing the first substrate 712, the tunable polymer spacer being connected to the actuator 114 and receiving a tuning force from the actuator 114 to compress the polymer and thus change the length of the lock cavity. Providing a tunable polymer spacer allows for tuning the cavity length in situ. An exemplary polymer capable of reversible deformation is polydimethylsiloxane (PDMS). A tunable polymer spacer may be etched around the emission cavity 106 to prevent any effect of the spacer on single-photon emission by the single-photon emitter material 112 .

[0071] FIG. 8A illustrates a third embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. The apparatus 800 illustrated in FIG. 8A is an example of the apparatus 100 of FIG. 1, and therefore like reference numerals apply to like features, a detailed description of which may be provided above in connection with FIG. 1. The structure of the apparatus 800 is similar to that of the second embodiment illustrated in FIG. 2B, with the difference being that the single-photon emitting material 112 is a quantum object, such as a trapped ion or neutral atom in a physical qubit of a quantum computer. Here, the quantum object is suspended in an emitter cavity 106, which is not planar-concave in shape, but is concave-concave, formed between an emitter recess 104a and a complementary emitter recess 104b formed in the first reflecting surface 102. Providing a concave-concave cavity in this manner allows the trapped ion or neutral atom providing the single-photon emitter material 112 to be located in the center of the cavity at the cavity waist, where the enhancement effect is greatest. In this regard, the tuned emitter cavity 106 provides a light-matter interface to enhance single-photon emission from the quantum object after stimulation with an excitation laser (indicated by the dashed arrow) that interacts with the quantum object and causes an optical transition. This facilitates photonic manipulation and control of qubits, for example to aid in implementing quantum gate operations in a quantum computer. In a third embodiment, a locking laser back-illuminates the locking cavity 110 (as indicated by the dotted arrow).

[0072] FIG. 8B illustrates a fourth embodiment of an apparatus for generating single photons according to an aspect of the present disclosure. The apparatus 850 illustrated in FIG. 8B is an example of the apparatus 100 of FIG. 1 and corresponds in particular to the apparatus 800 illustrated in FIG. 8A, and therefore similar reference numerals are applied to similar features, a detailed description of which may be provided above in connection with FIG. 1. In the embodiment of FIG. 8B, the arrangement is similar to that of the embodiment illustrated in FIG. 8A, with the difference being that the excitation laser illuminates the apparatus 850 from the side as indicated by the dashed arrow and enters the emitter cavity 106. This arrangement facilitates interaction with trapped ions or neutral atoms providing the single-photon emitter material 112 in the center of the cavity, removing the excitation laser from the optical path of the single-photon emission.

[0073] Each feature disclosed in this specification (including any accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

[0074] The invention is not limited to the details of any of the preceding embodiments. The invention extends to any novel or any novel combination of features disclosed in this specification (including the accompanying claims, abstract, and drawings), or any novel or any novel combination of steps of any method or process so disclosed. The claims should be construed to encompass not only the preceding embodiments, but also any embodiment that falls within the scope of the claims.

Claims

1. 1. An apparatus for providing a light-matter interface for enhancing a transition that generates a single photon, comprising: a first reflecting surface; a locking recess formed in a locking reflective surface, the first reflective surface and the locking recess forming a locking cavity for receiving light from a locking laser, a portion of the light output from the locking cavity indicating whether the locking cavity is in resonance with the locking laser; and an emission recess formed in an emission reflective surface, the first reflective surface and the emission recess forming an emission cavity for receiving photons from a single-photon emitter material positioned within the emission cavity for emitting a single photon in response to optical or electrical excitation of the material; an actuator for tuning a length of the locking cavity based on the portion of light output from the locking cavity that is resonant with the locking laser to lock the locking cavity to the locking laser; An apparatus wherein the locking and emitting reflective surfaces are mechanically coupled to lock the locking cavity, thereby tuning and stabilizing the emission cavity at a single photon emission wavelength.

2. The apparatus of claim 1 , wherein the portion of light output from the lock cavity is a portion of light reflected and / or transmitted from the lock cavity.

3. 3. The apparatus of claim 1, wherein the locking and emitting reflective surfaces are mechanically supported by the same substrate.

4. 3. The apparatus of claim 1 or 2, wherein the emission cavity receives light from an excitation laser, and the single-photon emitter material emits a single photon in response to the light received from the excitation laser being incident on the material.

5. 3. The apparatus of claim 1, wherein the locking and emitting reflective surfaces are at different distances from the first reflective surface such that a locking cavity has a different length relative to the emitting cavity.

6. 3. The apparatus of claim 1, wherein the locking and emitting reflective surfaces are on different substrates.

7. The apparatus of claim 6 , wherein the substrate comprising the emitting reflective surface is mechanically coupled to the locking reflective surface of the substrate comprising the locking reflective surface.

8. 3. The apparatus of claim 1, wherein the emitter cavity and the locking cavity are laterally spaced apart and substantially axially aligned.

9. 3. The device of claim 1, wherein the ejection recess and the locking recess have different depths.

10. 3. The apparatus of claim 1, wherein the locking cavity and the emission cavity operate in different modes and have different resonant wavelengths.

11. 3. The device of claim 1, wherein the ejection cavity has a length in the range of 0.3 to 10 μm and the locking cavity has a length in the range of 10 to 1000 μm.

12. 3. The apparatus of claim 1, wherein the emission cavity has a linewidth in the range of 10-1000 GHz and the locking cavity has a linewidth in the range of 10-1000 MHz.

13. 3. The device of claim 1, wherein the single-photon emitter material is a two-dimensional material having fluorescent defects or quantum dots.

14. 3. The device of claim 1, wherein the single-photon material emits single photons at a temperature in the range of 0 to 20°C.

15. 3. The device of claim 1, wherein the single-photon emitter material comprises hexagonal boron nitride (hBN), trapped ions, or neutral atoms.

16. 3. The device of claim 1, wherein the single photons emitted from the single-photon emitter material have a wavelength of about 400 nm or greater.

17. The apparatus of claim 1 or 2, wherein tuning the length of the lock cavity comprises adjusting the first reflective surface and / or the lock reflective surface.

18. 3. The device according to claim 1, wherein the actuator is a piezo actuator.

19. 3. The apparatus of claim 1, wherein the actuator tunes the length of the locking cavity based on the phase of the portion of light output from the locking cavity.

20. 3. The device of claim 1, wherein the locking cavity and the emission cavity are Fabry-Perot cavities.

21. 3. The apparatus of claim 1, wherein the locking cavity receives light from the locked laser modulated in phase.

22. 1. A system for providing a light-matter interface for enhancing a transition that generates a single photon, comprising: An apparatus according to claim 1 or 2; an optical system for transmitting light into the lock cavity of the device; A system comprising:

23. 23. The system of claim 22, wherein the optical system comprises a locking laser for generating light for transmission into the locking cavity of the device.

24. 24. The system of claim 23, wherein the locked laser produces light having a wavelength different from that of the single photon emission wavelength.

25. 23. The system of claim 22, wherein the optical system further comprises a modulator for modulating the phase of the light produced by the locked laser and transmitting the modulated light into the locked cavity of the device.

26. 26. The system of claim 25, wherein the modulator is an electro-optic modulator, the system further comprising an RF driver coupled to the electro-optic modulator and configured to output a modulation signal for driving the modulator to modulate the phase of the locked laser at a phase modulation frequency substantially greater than a linewidth of the locking cavity.

27. 27. The system of claim 26, wherein the phase modulation frequency is at least 1 GHz.

28. the optical system further comprising a coherent detector for detecting the portion of light output from the locking cavity of the device and mixing the detected light signal with the modulation signal from the RF driver to output a feedback signal based on the detected portion of light indicative of drift of the locking cavity from the locking laser; 26. The system of claim 25, further comprising a controller configured to receive the feedback signal and output an error signal to the actuator of the device based on the feedback signal, the actuator tuning a length of the locking cavity based on the error signal to maintain the locking cavity in resonance with the locking laser.

29. 30. The system of claim 28, wherein the controller outputs the error signal to the actuator of the device based on a phase of the detected portion of light output from the lock cavity.

30. 30. The system of claim 28, wherein the controller is a proportional-integral-derivative PID controller.

31. 23. The system of claim 22, wherein the system is configured to control the actuator to resonantly lock the locking cavity with the locking laser using a Pound-Drever-Hall technique.

32. 23. The system of claim 22, wherein the system is configured to resonantly lock the locking cavity with the locking laser such that a fundamental mode of the emitter cavity is maintained in resonance with the single-photon emission frequency of the single-photon emitter material.

33. 23. The system of claim 22, further comprising a pulsed excitation laser and optics for directing light from the pulsed excitation laser to illuminate the single-photon emitter material in the emitter cavity to trigger emission of single photons at a fixed single-photon emission frequency.

34. 23. The system of claim 22, further comprising at least one additional device for generating single photons as defined in claim 1, wherein the optical system is configured to transmit light from the same locking laser into the locking cavity of each of the devices.

35. 35. The system of claim 34, wherein the optical system is further configured to transmit light from the same pulsed excitation laser into the emission cavity of each of the devices, the photon emitter material of each device emitting a single photon in response to the light received from the excitation laser being incident on the material.

36. 35. The system of claim 34, wherein a plurality of the emitter cavities in the plurality of devices are for emitting single photons, each emitter cavity emitting a single photon at substantially the same fixed single-photon frequency in response to illumination by an excitation laser.

37. 1. A method of manufacturing a device for providing a light-matter interface for enhancing transitions that generate single photons, comprising: providing a lock substrate having a lock reflective surface; forming a recess in the locking reflective surface to form a locking cavity with the first reflective surface of the first substrate; providing an emission substrate having an emission reflective surface, the emission substrate and a locking substrate being mechanically coupled; forming a recess in the emitting reflective surface to form an emitting cavity with the first reflective surface of the first substrate; providing a single-photon emitter material located within the emission cavity for emitting single photons; providing the first substrate having the first reflective surface; tuning the length of the lock cavity based on a cavity length of the lock cavity to tune the length of the ejection cavity to a fixed configuration so that it is finely tuned by an actuator during use; A method comprising:

38. The method of claim 37 , wherein the recesses are formed using a focused ion beam.

39. 1. A method for generating single photons, comprising: receiving light from a locking laser in a locking cavity, the locking cavity being formed from a first reflective surface and a locking recess formed in the locking reflective surface, a portion of the light output from the locking cavity indicating whether the locking cavity is in resonance with the locking laser; tuning a length of the locking cavity based on the portion of light output from the locking cavity to resonate with the locking laser to lock the locking cavity to the locking laser; emitting single photons in response to optical or electrical excitation of said material by a single-photon emitter material located within an emission cavity, said emission cavity being formed from said first reflective surface and an emission recess formed in said emitting reflective surface; Including, The method, wherein the locking and emitting reflective surfaces are mechanically coupled to lock the locking cavity, thereby tuning and stabilizing the emission cavity at a single photon emission wavelength.

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