Coherent single-photon source

Abstract

The present invention relates to a coherent single-photon source (100) comprising a nanodiamond (120) having a quantum emitter (140), wherein the nanodiamond (120) and the quantum emitter (140) are designed and configured such that the quantum emitter (140) is coherent and emits indistinguishable photons (160). The present invention further relates to a system (300) comprising a first coherent single-photon source (101) and a second coherent single-photon source (102), wherein the first coherent single-photon source (101) and the second coherent single-photon source (102) are designed and configured such that a photon (160) emitted from a first quantum emitter (151) of a first nanodiamond (121) of the first coherent single-photon source (101) is indistinguishable from a photon (160) emitted from a second quantum emitter (152) of a second nanodiamond (122) of the second coherent single-photon source (102).

Description

[Technical Field]

[0001] The present invention relates to a coherent single-photon source, a system comprising two such single-photon sources, and the use of such coherent single-photon sources or the system for generating quantum entanglement.

[0002] In emerging fields of quantum technology, quantum entanglement distributions are crucial for establishing, for example, long-range quantum state transitions and quantum networks. One possible source of dispersed quantum entanglement is two-photon interference, commonly known as Hong-Ou-Mandel (HOM) interference. The necessary prerequisite is a single-photon source that produces indistinguishable photons. Two-photon interference can be generated from different single-photon sources, such as atomic vapor in bulk diamond, quantum dots, molecules, bonded atom cavity systems, and negatively charged nitrogen vacancies (NV). - This has been demonstrated using the center.

[0003] In the prior art, coherent single-photon sources are known. In the prior art, methods for manufacturing nanodiamonds are known. SiV - It is known that nanodiamonds with a central core can be produced alternatively by chemical vapor deposition (CVD).

[0004] An object of the present invention is to improve upon the prior art. In particular, an object of the present invention is to provide an alternative coherent single-photon source that generates indistinguishable photons. Another object of the present invention is to provide the above single-photon source in miniaturized form. Yet another object of the present invention is to provide the above single-photon source that can highly control the coupling strength of photons emitted into a cavity. Another object of the present invention is to provide at least one building block for a quantum network. Yet another object of the present invention is to increase the operating bandwidth of a quantum node that can be realized using the above coherent single-photon source. Yet another object of the present invention is to provide a miniaturized coherent single-photon source that can be nano-manipulated. Yet another object of the present invention is to provide a tool that can realize quantum key distribution, quantum entanglement exchange between remote nanodiamonds, and realize a quantum repeater. Yet another object of the present invention is to provide a means that enables the formation of remote quantum entanglement between remote quantum emitters in remote nanodiamonds. Yet another object of the present invention is to provide a building block including a quantum emitter that can be scaled up to a large-scale system including a large number of entangled quantum emitters or a large quantum network. Another objective of the present invention is to provide a quantum emitter system that can be coupled with existing photonic materials in order to construct hybrid quantum photonic devices, i.e., devices that combine a quantum emitter system with a well-studied photonic material. Yet another objective of the present invention is to provide a basic construction block that can construct novel collaborative quantum materials.

[0005] The problem of the present invention is solved by a coherent photon source having the features of independent claim 1. Preferred embodiments of the present invention are disclosed in each dependent claim.

[0006] Nanodiamonds are understood to be diamond particles so small that they can be measured in nanometer units, meaning their size is less than 1000 nm. Particle size is understood to be the maximum range of particles along a given axis when measured in different coordinate systems.

[0007] A quantum emitter can be understood as a quantum system capable of radiated phototransitions. Observing the spontaneous decay of a single-excited quantum emitter suggests the emission of a single photon. If non-radiated decay mechanisms are suppressed, the quantum emitter can essentially function as a 100% efficient single-photon source. A variety of systems provided by nature enable numerous possible experiments and implementations for photonic applications. Quantum emitters can be, for example, single atoms in high-resolution cavities, colloidal nanocrystals of different sizes, color centers in solids, quantum dots, or nanowire quantum dot structures.

[0008] Over the past few decades, single quantum emitters have served as manifold tools for the development of new light sources for nanoelectronic devices, such as lasers, LEDs, and single-photon sources, as well as in the fields of chemistry and life sciences where they function as nanoscopic probes and labels. In particular, in the emerging field of quantum information processing, quantum emitters are being used as single-photon sources or as fixed qubits.

[0009] This application deals with a single-photon source comprising a single nanodiamond having at least one quantum emitter, wherein the nanodiamond and the quantum emitter are designed and configured such that the quantum emitter is coherent and emits indistinguishable photons.

[0010] In this case, where a single particle or single source emits coherent and indistinguishable photons, it is necessary that the photons emitted from the single source are indistinguishable in time with respect to themselves.

[0011] The advantage of the coherent single-photon source according to the present invention is that it provides a compact coherent single-photon source of indistinguishable photons.

[0012] Another advantage of the present invention is that the coherent single-photon source of indistinguishable photons constitutes an alternative solution to coherent single-photon sources of indistinguishable photons known from the prior art.

[0013] Collaborative quantum materials include indiscriminate quantum emitters, such as nanodiamonds or individual SiVs within nanodiamonds. - These quantum emitters can be determined by their centers and can be positioned within a spatial distance of the order of the optical transition wavelength or less than the optical transition wavelength so that they can collectively behave in the form of increased dipole intensity, particularly in the form of superabsorption or superradiation.

[0014] The coherent single-photon source of indiscriminate photons according to the present invention is miniaturized to the nanometer scale, making it nano-manipulable and allowing it to be positioned within a resonator, for example, within the resonator's modes, with almost arbitrary precision, i.e., on the nanometer scale, using atomic force microscope (AFM)-based nano-manipulation. This provides a coherent single-photon source of indiscriminate photons in which the coupling strength of the emitted photons to the cavity modes can be controlled with extremely high precision.

[0015] Another advantage of the present invention is that a coherent single-photon source can be used as a quantum node in a larger quantum network.

[0016] Another advantage of coherent single-photon sources is the possibility of engineering quantum optical fields, particularly path-entangled optical fields, such as NOON states, based on a deterministic scheme rooted in the principle of Hong-Ou-Mandel interference. NOON states, or NOON states, are quantum mechanical many-body entangled states, representing a superposition of N particles in mode a and 0 particles in mode b, or vice versa. Current schemes in the prior art are typically based on probabilistic schemes utilizing parametric downconversion sources (PDCS), which are limited to scaling to large photon-number states, e.g., N>20. Such entangled quantum optical fields can be advantageously used, for example, in quantum sensing of phase estimation, thereby providing sensitivity beyond the classical limit given by the standard quantum limit. The integration potential of nanodiamonds will enable on-chip operation of such sensing devices in the future.

[0017] Another advantage of a coherent single-photon source is that it can be used as a quantum repeater. Quantum repeaters are a crucial component of long-distance quantum communications, overcoming the drawbacks of decoherence and photon loss in long-distance fiber optic communications. Similar to classical repeaters in digital communication technology, repeaters are used to refresh signals between a transmitter and a receiver. However, because amplification or replication in quantum mechanical states is impossible due to the non-cloning principle of signals, quantum repeaters are based on the principle of entanglement exchange and / or entanglement distillation or purification. Many repeater stations exist between the transmitter's Alice station and the receiver's Bob station, and each of these repeater stations can receive, store, and transmit classical and quantum mechanical signals. The storage of quantum mechanical states takes place in quantum memory. In the quantum repeater protocol, states first become entangled in the quantum memory of a neighboring repeater station. Here, each inner repeater station shares at least two entangled states with its neighboring repeater station. These pairs of entangled states allow for entanglement exchange, creating entanglement between the states of neighboring stations. The exchange of entanglement at each station creates entanglement between the first and last repeater stations, which enables the transmission of quantum states from Alice to Bob via quantum teleportation.

[0018] A single nanodiamond may contain multiple quantum emitters, preferably 1 to 100 quantum emitters. A single substrate may contain multiple single nanodiamonds, for example, more than 1000 nanodiamonds. The quality of the manufactured single nanodiamonds depends on the manufacturing method.

[0019] Each nanodiamond is individually addressable. The photoluminescence spectra of various atomic transitions in each quantum emitter can be measured. Individual atomic transitions in each quantum emitter are addressable by resonant photoluminescence excitation (PLE).

[0020] Preferably, in the nanodiamond, only a single quantum emitter is used. Preferably, the single quantum emitter is located inside the nanodiamond.

[0021] Preferably, the single quantum emitter is located at the center of the nanodiamond, which is well protected from any surface influences. At the same time, it is preferably positioned close enough to the surface, i.e., well below the wavelength of light, to enable efficient photocoupling to the outside.

[0022] Preferably, quantum emitters that should not be used within the nanodiamonds can be changed to a dark color. This can be done using plasma treatment, or in particular, by using hydrogen or oxygen termination on the surface of the nanodiamonds.

[0023] According to another embodiment, specific quantum emitters of nanodiamonds can be spectrally selected, meaning that only the atomic transitions of those specific quantum emitters are used, while the atomic transitions of other quantum emitters are spectrally blocked or not used.

[0024] It is preferable that the nanodiamonds are coherent and have a single quantum emitter used to generate indistinguishable photons.

[0025] According to a preferred embodiment, nanodiamonds are selected from a sample of manufactured nanodiamonds. According to another preferred embodiment, the method for manufacturing the nanodiamond sample is high-pressure high-temperature (HPHT) manufacturing or chemical vapor deposition (CVD).

[0026] The advantage of the high-pressure, high-temperature (HPHT) manufacturing method is that the resulting nanodiamonds typically have low strain, which suggests that the non-uniform linewidths are narrower. The advantage of the chemical vapor deposition (CVD) manufacturing method is that the resulting nanodiamonds have higher isotopic purity.

[0027] Nanodiamonds preferably meet at least one of the following criteria, namely: Uniform line width, non-uniform line width, optical properties, addressability of nanodiamonds on a substrate, frequency stability of related atomic transitions, in particular frequency stability of atomic transitions used for generating indistinguishable photons, the resulting frequency stability enabling the emission of time-coherent single photons, indistinguishable SiV within multiple nanodiamonds - Samples of fabricated nanodiamonds are selected according to at least one of the following criteria: high probability of finding the center, brightness, and single-photon purity.

[0028] Nanodiamonds can be produced in a sample. The number of nanodiamonds produced in the produced sample is more than 100, preferably more than 1000.

[0029] A method for selecting nanodiamonds from a fabricated nanodiamond sample, in which each of the fabricated nanodiamonds contains a quantum emitter, may include the following steps: Nanodiamonds are selected from a fabricated nanodiamond sample such that the uniform linewidth of the atomic transition of the nanodiamond quantum emitter is less than twice the Fourier transform limit of the atomic transition, and the difference between the maximum and minimum frequencies of the atomic transition on a timescale greater than one minute is less than twice the Fourier transform limit of the atomic transition. This selection, surprisingly, allows for the emission of coherent and indistinguishable photons from a coherent single-photon source.

[0030] If two nanodiamonds are selected, these two nanodiamonds can be used to generate indistinguishable photons capable of causing two-photon interference.

[0031] Alternatively, a single nanodiamond can be selected such that the uniform linewidth of the atomic transition of the nanodiamond quantum emitter is less than twice the Fourier transform limit of the atomic transition, where the atomic transition is used for the emission of indiscriminate photons. The nanodiamond selected here can be used to generate indiscriminate photons.

[0032] Preferably, the method for selecting one or more diamonds may include the spectrum of atomic transitions used, the intensity and / or distribution of strain within the nanodiamond, and other quality characteristics of the quantum emitter within the nanodiamond.

[0033] According to a preferred embodiment, the quadratic correlation function at zero time delay of emitted light or emitted photons is less than 0.5, preferably less than 0.35. (Quadratic correlation function g) (2) (τ) is well known in the art. Determining whether the properties of the measured light are quantum or classical is a standard measurement known in the art. Here, the quadratic correlation function is the intensity correlation between the two output ports of a beam splitter that uses only one input signal from one quantum emitter. Correlation measurements between the intensities of the two output ports of a beam splitter with two input components are performed to determine the indistinguishability of the photons at the two input ports. Correlation measurements with the two input components are performed to determine the quadratic correlation function. The first input component of the correlation measurement includes light emitted from the quantum emitter, and the second input component of the correlation measurement includes light from the quantum emitter that can be time-delayed by an amount Δt relative to the first input component. Quadratic correlation function g (2) τ from (τ) is the time delay between the first and second output signals of the beam splitter. The quadratic correlation function g at point τ equal to 0. (2) (τ) is 0 for identical photons, i.e., indistinguishable photons, and 0.5 for distinguishable photons. Equation g (2) (τ=0)<0.5 means that the photons of the first input component and the photons of the second input component are indistinguishable.

[0034] The time delay Δt can be, for example, 0 seconds. According to another preferred embodiment, the time delay Δt is greater than a time difference greater than 1 second. This means that the coherence time is greater than the above-mentioned time difference. Advantageously, this achieves an extremely long coherence time that is significantly longer than in the case of equivalent systems realized in the solid state.

[0035] Second-order correlation function g (2) According to another embodiment where noise is present during the measurement of (τ), it is preferable to use the efficiency coefficient η as an alternative measure for determining when the quantum emitter emits indistinguishable photons. The efficiency coefficient η is part of Equation 1 at the end of the detailed description and can be determined by the fitting described with respect to Equation 1. Regarding the emission of indistinguishable photons, the efficiency coefficient η is greater than 0. Preferably, the efficiency coefficient η is greater than 0.3, and more preferably greater than 0.9.

[0036] These indistinguishable photons can advantageously be used for, for example, quantum entanglement distribution, which are important elements for establishing long-distance quantum state transfer and quantum networks.

[0037] According to a preferred embodiment of the present invention, the size of the nanodiamond is smaller than the wavelength of the emitted photon. Here, the relevant wavelength of the emitted photon is preferably the atomic transition used, as described below. The size of the nanodiamond is more preferably less than 100 nm. The size of the nanodiamond is even more preferably less than 30 nm.

[0038] According to a preferred embodiment, the quantum emitter is a color center. According to another preferred embodiment, the quantum emitter is a Group-IV defect center, particularly a Group-IV color center. Group-IV color centers include, for example, the neutral charged silicon vacancy (SiV0) center, the negatively charged germanium vacancy (GeV - ) center, the negatively charged tin vacancy (SnV -) Center, or negatively charged lead vacancies (PbV - ) can be the center. In yet another particularly preferred embodiment, the quantum emitter is a single negatively charged silicon vacancy (SiV - ) is the center. Diamond, in particular, has negatively charged silicon vacancies (SiV - The central group IV color centers exhibit intrinsic spectral stability and a uniform and heterogeneous narrow linear distribution.

[0039] The uniform linewidth of the atomic transitions of a quantum emitter is the individual linewidth of a single specific quantum emitter in a particular nanodiamond. The uniform linewidth depends on the temperature of the color center and the crystal environment, the charging environment, and the interaction between the environment and the color center.

[0040] The non-uniform linewidth of atomic transitions in quantum emitters refers to the width of the distribution of linewidths of multiple quantum emitters within a sample, for example, of nanodiamonds on a common substrate. This non-uniform linewidth of quantum emitter atomic transitions depends on the quality or uniformity of the nanodiamond sample, particularly the strain present in the nanodiamonds. Furthermore, the local environment surrounding the nanocrystals or nanodiamonds can influence, for example, spectral diffusion.

[0041] The uniform linewidth of the atomic transitions used to generate indistinguishable photons is preferably less than twice the Fourier transform limit of such atomic transitions.

[0042] The Fourier transform limit of an atomic transition is given by the Fourier transform of the radiative lifetime of the optical transition, which is given by the spontaneous emission rate in a free-space environment. The Fourier transform limit can be calculated by the equation Δω = 1 / τ. When only radiative relaxation channels are considered, the Fourier transform limit has no temperature dependence.

[0043] SiV -Center 150 is a point defect in the diamond crystal lattice, in which case one silicon atom Si replaces two carbon atoms C, and as shown in Figure 1, the silicon atom Si is located between two adjacent vacancies V.

[0044] SIV - The central atom 150 can be considered an artificial atom. SiV - The electronic level structure of central 150 is shown in Figure 2. Both the ground state 200 and the excited state 210 exhibit level partitioning due to spin-orbit coupling. Therefore, both the ground state 200 and the excited state 210 have doublets separated by ΔGS and ΔES, respectively. The ground state 200 has a lower ground state 202 and an upper ground state 204. The excited state 210 has a lower excited state 212 and an upper excited state 214. These level partitions yield four optically active transitions, which can be observed at extremely low temperatures. As shown in Figure 2, these are referred to as transitions A, B, C, and D. Transition D is between the lower excited state 212 and the upper ground state 204, transition C is between the lower excited state 212 and the lower ground state 202, transition B is between the upper excited state 214 and the upper ground state 204, and transition A is between the upper excited state 214 and the lower ground state 202. The wavelengths of transitions A, B, C, and D are known in the art.

[0045] Negatively charged silicon vacancies (SiV - The core possesses robust intrinsic properties. In bulk diamond, SiV - The center exhibits good optical properties, with a high Debye-Waller factor of approximately 0.7, a narrow, heterogeneous linewidth in the range of several gigahertz, and a uniform linewidth at cryogenic temperatures with a Fourier transform limit. Spectral stability is SiV - Center D 3d This symmetry allows for the achievement of excellent optical properties even in close proximity to the diamond surface. Protecting this high degree of symmetry requires high-quality diamond hosts, especially in low-strain environments. In nanodiamonds produced by chemical vapor deposition (CVD), SiV -Single-photon emission from the center and a sub-gigahertz linewidth photostable quantum emitter have been experimentally demonstrated. Low-strain nanodiamonds are produced using high-pressure, high-temperature (HPHT) manufacturing methods. - This makes it possible to preserve the excellent optical properties of the core even in nanodiamonds.

[0046] Negatively charged silicon vacancies (SiV - In the case of the center, transition C is preferably used to generate indiscriminate photons or indiscriminate light. This is advantageous because the intensity of transition C is greater than that of the other transitions A, B, or D. The natural linewidth of transition C is about 94 MHz at a low temperature of about 4 K, corresponding to the Fourier transform limit described above. The lifetime can be changed as the temperature increases, resulting in an increase in the natural linewidth of less than one order of magnitude at room temperature.

[0047] The linewidth of quantum emitters inside a solid state depends, in particular, on the environment and ambient temperature. (Negatively charged silicon vacancies (SiV)) - The linewidth of the central transition C is preferably less than 300 MHz, and more preferably less than 200 MHz. In a more preferred embodiment, the transitions C are less than 180 MHz and 100 MHz, respectively.

[0048] According to one embodiment, the non-uniform linewidth of atomic transition C is less than 50 GHz, preferably less than 2 GHz. According to a more preferred embodiment, the non-uniform linewidths of atomic transition C are less than 500 MHz and 200 MHz, respectively. Preferably, negatively charged silicon vacancies (SiV - The center has a low-strain environment, which enables the aforementioned heterogeneous linewidth. The advantage of these features is that the overall heterogeneous linewidth of atomic transitions in the quantum emitter is reduced, resulting in a higher yield for finding indistinguishable quantum emitters.

[0049] According to another preferred embodiment, the surface of the nanodiamond is plasma-treated, and in particular hydrogen-terminated. This feature is advantageous because it allows for the removal of negatively charged silicon vacancies (SiV) near the surface of the nanodiamond.- ) Darkens the center, i.e., prevents further resonance. This results in a spectrally more stable quantum emitter with a generally narrower, more heterogeneous linewidth, and optically active SiV in the relevant wavelength range. - The number of centers can be reduced. This makes it possible to achieve a non-uniform overall linewidth that is lower than excited-state resolution. Experimentally, excellent spectral stability under resonant excitation was achieved with a hydrogen-terminated nanodiamond surface.

[0050] According to another preferred embodiment, the coherent single-photon source further includes a substrate on which nanodiamonds are arranged. The substrate may include a photonic device, an open-type resonator, a plasmon device, a plasmon mode channel, and / or a metasurface. A device that is both a photonic and a phononic device is referred to as a phonophotonic device.

[0051] The material for photonic devices or phonophotonic devices may be, for example, Si3N4. This material is advantageously preferred for industrial-related chip technologies, as it allows for future integration into scalable photonic chip technologies. Alternatively, the substrate may be a low-fluorescence material with good thermal conductivity, such as diamond, e.g., electronic-grade diamond plate, quartz plate, sapphire plate, or quartz coverslip. Optical resonators, particularly open Fabry-Perot resonators or fiber-based resonators, and on-chip photonics, especially photonic crystal cavities or bullseye antennas, may utilize such materials.

[0052] A coherent single-photon source including the aforementioned quantum emitter containing a photonic material as a substrate can also be referred to as a hybrid quantum photonics device.

[0053] In another preferred embodiment, the temperature of the photon source containing nanodiamonds is below 20K. This has the advantage of fewer phonons than at higher temperatures, meaning that, for example, the optical transitions of the quantum emitter used can be resolved much better. Both uniform and non-uniform linewidths are not broadened much by electron-phonon interactions, resulting in significantly narrower linewidths. Furthermore, the coherence time of the quantum emitter is increased, which is advantageous for quantum applications. Longer coherence times require fewer quantum repeaters, making quantum computer calculations less prone to errors.

[0054] The coherence time of a quantum emitter determines the possible distance over which quantum communication can be achieved. SiV - Regarding the core, the coherence time at 4K is limited by orbital relaxation in two ground state levels introduced by phonons from an ambient thermal bath with a frequency of approximately 46 GHz. The lifetime between these levels is denoted as T1. There are three different methods to eliminate ground state mixing by phonons. First, by cooling to the millikelvin temperature, suppression of the resulting phonons is achieved. Second, by introducing strain, ground state resolution is increased. Third, SiV - It is also possible to eliminate ground state mixing by phonons by changing the size and / or geometry of the material surrounding the center. As a result, phonon separation and extension of orbital relaxation time can be achieved. Furthermore, it is preferable that the temperature of the photon source be less than 10K. It is even more preferable that the temperature of the photon source be less than 5K. The above advantages increase as the temperature decreases.

[0055] According to another embodiment, nanodiamonds can be positioned and / or oriented using an atomic force microscope (AFM). Such methods are widely used and therefore standardized. This allows for easy and reliable pickup and placement of nanodiamonds.

[0056] In a more preferred embodiment, the coherent single-photon source further includes a filter designed and configured to filter photons emitted from the quantum emitter of the coherent single-photon source. This advantageously filters out discriminable photons, leaving the remaining photons with a higher degree of indiscriminateness, which is the objective of the present invention. A filter can be used to filter the photons. The filter may be, for example, a spectral filter, an etalon, a dichroic mirror, a prism, a grating, or other wavelength-sensitive element. Preferably, the filter is SiV - It has a central wavelength corresponding to the central transition C.

[0057] According to a different embodiment, the coherent photon source further has a cavity designed and configured such that atomic transitions of a quantum emitter are coupled to modes of the cavity.

[0058] The coupling here advantageously achieves the ability for the quantum state of the quantum emitter to be transferred to an electric field or photons inside the cavity. The photons inside the cavity can be coupled to a fiber that can connect to a remote location, which may be hundreds of kilometers away, or to a waveguide on a photonic chip that disperses entanglement between different nodes on the chip, or to a waveguide in on-chip quantum optical field engineering.

[0059] As described above, a quantum emitter, which is part of a coherent single-photon source, can realize a quantum node. Through the coupling described above, different quantum nodes can be connected to form a quantum network.

[0060] Preferably, the cavity is a photonic crystal cavity (PCC), an open Fabry-Perot cavity, a ring resonator, a plasmon ring cavity, a bullseye antenna, or a fiber-based cavity.

[0061] The photonic crystal cavity may be, for example, a one-dimensional photonic crystal cavity.

[0062] The cavity can be implemented as an on-chip resonator. Alternatively, the cavity may be an open resonator, such as a Fabry-Perot resonator or a fiber resonator.

[0063] In the case of a photonic crystal cavity, the cavity can be realized as a waveguide, particularly as a waveguide having two 1D photonic crystal cavity mirrors.

[0064] In a preferred embodiment, the quantum emitter is located at the point of maximum magnetic field value. This maximum value may be a global maximum or a local maximum. Preferably, the quantum emitter is located within the antinodes of the cavity field.

[0065] According to a preferred embodiment, the Parcell coefficients for atomic transitions in the cavity and quantum emitter are greater than 1. The Parcell effect is the shortening of the lifetime of excited states in a quantum system by its environment, such as atoms inside a resonant cavity. The magnitude of the enhancement is given by the Parcell coefficients, which are well known in the art. The Parcell coefficients depend on the vacuum wavelength of the atomic transitions used for the quantum emitter, the refractive index of the cavity material, the quality factor Q, and the mode volume V of the modes used within the cavity. When the Parcell coefficient is greater than 1, most photons are emitted into a single mode in the cavity. Preferably, the atomic transitions used are SiV -The central transition is C. A high Parcell coefficient suggests strong coupling of the cavity field to the quantum emitter. Preferably, the Parcell coefficient is greater than 4. More preferably, the Parcell coefficient is greater than 14. High coupling suggests a high operating bandwidth for the quantum node achievable by a coherent photon source. A high Parcell coefficient increases the operating bandwidth due to shorter lifetimes and higher directivity of photon emission to the cavity mode. Furthermore, at the highest Parcell coefficients, a further boost can be achieved with improved quantum yield, which means suppression of non-radiative channels. In addition, the shortened lifetime due to the Parcell effect expands the Fourier transform limit of the transition here accordingly, and relaxation conditions are obtained for obtaining even more indistinguishable photons.

[0066] In the case of quantum nodes where atomic transitions are coupled to a PCC, the number of photons emitted into the cavity mode is one of the limiting factors for the operating bandwidth. The quantum node rate, which characterizes the bandwidth of the input / output channels, is a significant limiting factor for its operating speed. Enhancements to coupled emission using the Purcell effect improve the performance of potential quantum networks. Furthermore, improvements in quantum yield result in efficient connectivity of distant quantum nodes, i.e., higher operating bandwidth, providing access to a wider variety of applicable protocols with, for example, improved security, better fault tolerance, or higher fidelity.

[0067] According to one embodiment, two or more quantum emitters can be arranged within a single nanodiamond. In this case, the two or more quantum emitters are preferably arranged such that the maximum distance between each pair of quantum emitters is smaller than the wavelength of the emitted photon. This advantageously achieves collective effects because at least two nondiscriminable quantum emitters are positioned within a small volume having a diameter smaller than the wavelength of the optical transition. The increase in the dipole intensity of the collective system results in superradiance and / or superabsorption.

[0068] According to another aspect of the present invention, the problem of the present invention is solved by a system comprising a first coherent single-photon source and a second coherent single-photon source. Both the first coherent single-photon source and the second coherent single-photon source may be the coherent single-photon sources described above. The first coherent single-photon source and the second coherent single-photon source are designed and configured such that a photon emitted from the first quantum emitter of the first nanodiamond of the first coherent single-photon source is indistinguishable from a photon emitted from the second quantum emitter of the second nanodiamond of the second coherent single-photon source. The system may be alternatively referred to as an entity.

[0069] For the first or second quantum emitter, the wavelength of the emitted photon can correspond to the atomic transition of the respective quantum emitter.

[0070] The system here advantageously realizes a source capable of emitting mutually indistinguishable photons. Using such a source of indistinguishable photons, two-photon interference, commonly known as Hong-Ow-Mandel (HOM) interference, can be performed, particularly between distant locations. Here, photons from a first quantum emitter collide with the first input port of the beam splitter, and photons from a second quantum emitter collide with the second input port of the beam splitter. The indistinguishability of the photons can be measured by measuring the quadratic correlation function between the two output ports.

[0071] This system advantageously provides tools that can realize quantum key distribution, quantum entanglement exchange between remote nanodiamonds, and quantum repeaters.

[0072] Two-photon interference can also be used to generate dispersed quantum entanglement, which is a crucial element for establishing long-range quantum state transitions and quantum networks. Two-photon interference can also be used for multi-entangled quantum distributions, which are fundamental building blocks for quantum computing, quantum networks, and many other applications.

[0073] We will experimentally demonstrate two-photon interference with this system and explain it with reference to the figures.

[0074] Preferably, the transition frequencies of the first and second quantum emitters are both spectrally stable, preferably stable on timescales larger than one minute, and the Fourier transform is limited.

[0075] According to another preferred embodiment, the system includes two or more coherent single-photon sources. Each of the two or more coherent single-photon sources may be a coherent single-photon source as described above. For each pair of two coherent single-photon sources of these two or more coherent single-photon sources, the photon emitted from the first quantum emitter of the first nanodiamond of the pair of coherent single-photon sources is indistinguishable from the photon emitted from the second quantum emitter of the second nanodiamond of the second coherent single-photon source of the pair of coherent single-photon sources. Preferably, the system comprises three or more coherent single-photon sources.

[0076] In embodiments of the system comprising two or more coherent single-photon sources, it is further preferable that the atomic transitions of each quantum emitter of the two or more coherent single-photon sources are coupled to a single-cavity mode of the common cavity. This advantageously achieves collective effects because at least two indistinguishable quantum emitters are coupled to the same single mode of the cavity or optical resonator. The increase in the dipole intensity of the collective system results in superradiance and / or superabsorption.

[0077] In a more preferred embodiment, both the photons emitted from the first quantum emitter of the first coherent single-photon source and the photons emitted from the second quantum emitter of the second coherent single-photon source are filtered. The photons emitted from the first quantum emitter of the first coherent single-photon source and / or the photons emitted from the second quantum emitter of the second coherent single-photon source can be fed to different optical elements, such as half-wave plates, polarizers or quarter-wave plates, or spectral filters.

[0078] This advantageously achieves filtering out discriminable photons so that the remaining photons possess the higher degree of indiscrimination desired by the present invention. The filters described above can be used to filter the photons.

[0079] According to another preferred embodiment, the first coherent single-photon source may include a first substrate, and the second coherent single-photon source may include a second substrate. This may be the case when the first coherent single-photon source and the second coherent single-photon source are separated from each other. However, according to another embodiment, the first coherent single-photon source and the second coherent single-photon source may also include a common substrate on which both the first coherent single-photon source and the second coherent single-photon source are arranged.

[0080] The advantage here is that the first nanodiamond of the first coherent single-photon source and the second nanodiamond of the second coherent single-photon source are placed on the same substrate, so that the nanodiamonds are exposed to the same or very similar physical environment, such as temperature and vibration. In yet another embodiment, more than two coherent single-photon sources are placed on the same substrate. Preferably, the number of coherent single-photon sources is more than 1000. In this embodiment, it is preferable that nanodiamonds having similar atomic transitions are spectrally selected such that the non-uniform linewidth is smaller than in the case where the nanodiamonds are not spectrally selected.

[0081] The method for preparing nanodiamond samples for the system is the same as the method for coherent single-photon sources.

[0082] A method for selecting two nanodiamonds from a sample of manufactured nanodiamonds, wherein each of the manufactured nanodiamonds contains a quantum emitter, may include the following steps: In other words, the first nanodiamond of the first coherent single-photon source and the second nanodiamond of the second coherent single-photon source are selected from a common sample of nanodiamonds manufactured such that the difference between the first frequency of the atomic transition of the first quantum emitter of the first nanodiamond and the second frequency of the atomic transition of the second quantum emitter of the second nanodiamond is less than twice the Fourier transform limit of the atomic transition, and the first uniform linewidth of the atomic transition of the first quantum emitter of the first nanodiamond and the second uniform linewidth of the atomic transition of the second quantum emitter of the second nanodiamond are both less than twice the Fourier transform limit of the atomic transition, and on a timescale greater than one minute, both the first difference between the maximum and minimum values ​​of the first frequency and the second difference between the maximum and minimum values ​​of the second frequency are less than twice the Fourier transform limit of the atomic transition. Surprisingly, this selection allows the system to emit coherent photons that are indistinguishable from one another.

[0083] Preferably, atomic transitions are used to emit indistinguishable photons.

[0084] According to a preferred embodiment of this system, the quadratic correlation function at zero time delay of emitted light or emitted photons is less than 0.5, preferably less than 0.35. Here, the quadratic correlation function is the two-photon wavefunction. As with the coherent single-photon source, correlation measurements are performed with two input components to determine the quadratic correlation function. The first input component of the correlation measurement includes light or photons emitted from a first quantum emitter, and the second input component of the correlation measurement includes light or photons emitted from a second quantum emitter. The second input component may be time-delayed by a quantity Δt relative to the first component, but preferably there is no time delay. The statements given above regarding the quadratic correlation function for a coherent single-photon source having a single quantum emitter also apply to the quadratic correlation function for a system using two quantum emitters at two input ports. The quadratic correlation function g at point τ equal to 0 2 (τ) is 0 for identical, i.e., indistinguishable photons, and 0.5 for discriminable photons. This is the quadratic correlation function g at zero time delay. 2 Since (0) is less than c0.5, it means that the photons emitted from the first quantum emitter and the photons emitted from the second quantum emitter are indistinguishable. This will be explained in detail by referring to the diagram.

[0085] quadratic correlation function g (2) In another embodiment where noise is present during the measurement of (τ), it is preferable to use an efficiency coefficient η as an alternative means of determining whether the photons emitted from the first quantum emitter and the photons emitted from the second quantum emitter are indistinguishable from each other. The efficiency coefficient η is described in Equation 1 in the final part of the specification and can be determined by fitting as described with respect to Equation 1. For the emission of indistinguishable photons, the efficiency coefficient η is greater than 0. The efficiency coefficient η is preferably greater than 0.3 and more preferably greater than 0.9.

[0086] These photons can be advantageously used for entanglement distributions, for example, for long-range quantum state transitions and quantum networks, which are important elements for establishing such distributions. Preferably, the quadratic correlation function at zero time delay of the emitted light or emitted photons is less than 0.1.

[0087] In one preferred embodiment of the system, the distance from the first nanodiamond of the first coherent single-photon source to the second nanodiamond of the second coherent single-photon source is greater than 1 meter. Preferably, the distance from the first nanodiamond to the second nanodiamond is greater than 100 meters. This distance is more preferably greater than 1 km, and even more preferably greater than 100 km.

[0088] Preferably, the system has a first cavity and a second cavity. A first cavity is designed and configured such that atomic transitions of a first quantum emitter are coupled to modes in the first cavity, and a second cavity is designed and configured such that atomic transitions of a second quantum emitter are coupled to modes in the second cavity, and the first and second cavities are further coupled. Preferably, the first and second cavities are coupled to a fiber or free space. Alternatively, the outputs of the first and second cavities are sent to two input ports of a beam splitter, thereby performing HOM interference measurements.

[0089] The advantage of this feature is that quantum entanglement can be induced between the first and second quantum emitters using two-photon interference. Indiscriminate photons emitted from both the first and second quantum emitters mediate information exchange, for example, via optical fiber or free-space transmission, and are ultimately incident on the two input ports of a beam splitter to perform Hong-Ow-Mandel interference measurements. The system according to this embodiment can be used to form a quantum network. This system has two indiscriminate quantum emitters. However, this system can be extended to have a given number N indiscriminate quantum emitters, where N is an integer greater than 2.

[0090] According to another preferred embodiment of the system, the distance from the first nanodiamond of the first coherent single-photon source to the second nanodiamond of the second coherent single-photon source is smaller than the wavelength of the emitted photon.

[0091] In embodiments where the system comprises two or more coherent single-photon sources, this necessarily means that the two or more coherent single-photon sources have the maximum distance of the wavelength of the emitted photon. This favorably achieves collective effects because at least two non-discriminable quantum emitters are located within a small volume having a diameter smaller than the wavelength of the optical transition. The increase in the dipole intensity of the collective system results in superradiance and / or superabsorption.

[0092] Another configuration involves arranging the first nanodiamond of the first coherent single-photon source and the second nanodiamond of the second coherent single-photon source within a volume smaller than the cube of the wavelength of the emitted photon.

[0093] For the first or second quantum emitter, the wavelength of the emitted photon can correspond to the atomic transition of the respective quantum emitter. Photons from the first and second quantum emitters are indistinguishable.

[0094] This feature favorably achieves the superradiation effect of coherent single-photon sources. Using such systems, cooperative quantum materials can be formed or constructed. Collective states in the Dicke domain can be prepared.

[0095] According to another preferred embodiment, the system further includes a common cavity in which the atomic transitions of a first quantum emitter and a second quantum emitter are coupled to a mode in the common cavity. This favorably achieves collective effects because at least two indistinguishable quantum emitters are coupled to the same single mode in the cavity or optical resonator. The increase in the dipole intensity of the collective system results in superradiance and / or superabsorption.

[0096] According to another preferred embodiment, the system has a cavity, in particular a common cavity, where the first nanodiamond of the first coherent single-photon source and the second nanodiamond of the second coherent single-photon source are located within a single-mode volume of the cavity. The mode is preferably a collective mode. Preferably, the mode volume is less than 100 times, preferably less than 10 times, i.e., less than the cubic wavelength of the emitted photon.

[0097] According to another aspect of the present invention, the problem of the present invention is solved by using nanodiamonds having quantum emitters as a coherent single-photon source. As shown above, a coherent single-photon source includes nanodiamonds having quantum emitters. It has been shown above that such nanodiamonds can be used as a coherent single-photon source.

[0098] According to another aspect of the present invention, the problem of the present invention is solved by using two nanodiamonds, each having a quantum emitter, as a system having a first coherent single-photon source and a second coherent single-photon source.

[0099] According to another preferred embodiment, the above system comprising a first coherent single-photon source and a second coherent single-photon source can alternatively be realized with a single nanodiamond.

[0100] An alternative system consists of nanodiamonds having a first quantum emitter and a second quantum emitter, wherein the distance from the first to the second quantum emitter is smaller than the wavelength of the emitted photon. This favorably achieves collective effects because at least two indistinguishable quantum emitters are located in a small volume with a diameter smaller than the wavelength of the optical transition. The increase in the dipole intensity of the collective system leads to superradiance and / or superabsorption.

[0101] Embodiments of the present invention are shown in the drawings and will be described in more detail in the following description. [Brief explanation of the drawing]

[0102] [Figure 1] This diagram shows silicon atoms (Si) located between two adjacent vacancy points (V). [Figure 2] This figure shows the electronic level structure of the SiV-center 150. [Figure 3] This is a schematic diagram showing a coherent single-photon source according to one embodiment of the present invention. [Figure 4] This figure shows the photoluminescence spectrum of the SiV-center of a nanodiamond coherent single-photon source according to one embodiment of the present invention. [Figure 5] This figure shows a sample of nanodiamonds manufactured on a substrate. [Figure 6] This figure shows a basic experimental setup for realizing a coherent single-photon source according to one embodiment of the present invention. [Figure 7] This diagram shows a pick-and-place procedure for realizing a coherent single-photon source coupled to a photonic crystal cavity. [Figure 8]This diagram shows a pick-and-place procedure for realizing a coherent single-photon source coupled to a photonic crystal cavity. [Figure 9] This diagram shows a pick-and-place procedure for realizing a coherent single-photon source coupled to a photonic crystal cavity. [Figure 10] This figure shows an experimental setup for achieving Hong-Ou-Mandel two-photon interference using a system with two coherent single-photon sources according to one embodiment of the present invention. [Figure 11] This diagram illustrates the basic principle of Hong-Ou-Mandel interference. [Figure 12] This is an overview diagram showing the distribution of transition C in a sample of manufactured nanodiamond. [Figure 13] This is an enlarged view of the distribution in Figure 12, showing several possible transitions suitable for measuring two-photon interference. [Figure 14] This figure shows the photoluminescence spectrum of one of the two SiV-centers in a system according to one embodiment of the present invention. [Figure 15] This figure shows the photoluminescence spectrum of one of the two SiV-centers in a system according to one embodiment of the present invention. [Figure 16] This figure shows the filtered photoluminescence spectrum of one of the two SiV centers in a system according to one embodiment of the present invention. [Figure 17] This figure shows the photoluminescence excitation (PLE) spectra of two SiV centers in a system according to one embodiment of the present invention. [Figure 18] This figure shows the normalized correlation function for one of the two SiV centers in a system according to one embodiment of the present invention. [Figure 19] This figure shows the normalized correlation function for one of the two SiV centers in a system according to one embodiment of the present invention. [Figure 20]This figure shows the measurement results of the quadratic correlation function for two-photon interference measurements. [Figure 21] This figure shows the normalized correlation function for two SiV-centers of a system according to one embodiment of the present invention, in a zoomed-out version of Figure 18. [Figure 22] This figure shows the normalized correlation function for two SiV-centers of a system according to one embodiment of the present invention, in a zoomed-out version of Figure 19. [Figure 23] This figure shows the measurement results of the second-order correlation function for two-photon interference measurements, in a zoomed-out version of Figure 20.

[0103] Figure 3 shows a single negatively charged silicon vacancy (SiV). - A coherent single-photon source 100 is shown, comprising a nanodiamond 120 having a quantum emitter 140 which is the center 150. The nanodiamond 120 is shown schematically. Figure 3 simply shows silicon vacancies (SiV - This indicates that the center 150 is located within the nanodiamond 120. Nanodiamond 120 and silicon vacancies (SiV - ) The center 150 is a silicon vacancy (SiV - The central 150 is designed and configured to emit coherent and indistinguishable photons 160.

[0104] The methods for producing nanodiamond (120) samples are high-pressure high-temperature (HPHT) manufacturing or chemical vapor deposition (CVD).

[0105] SiV used in a series of embodiments - The central 150 is located within the nanodiamond 120, which has an average size of approximately 30 nm. (SiV, Figure 2) - Figure 3 shows the SiV structure of the central 150 atomic levels. -The photoluminescence spectrum of the center 150 is shown in Figure 4. Individual color centers can be excited off-resonant using a continuous-wave 532 nm laser. In Figure 4, the intensity I as a function of emission wavelength λ is shown in relative units. The four peaks here correspond to the four transitions A, B, C, and D. The wavelength difference between transitions B, A and D, C corresponds to ΔGS in Figure 2, and the wavelength difference between transitions C, A and D, B corresponds to ΔES in Figure 2.

[0106] According to this embodiment, the atomic transition used to generate the coherent and indistinguishable photon 160 is SiV - The central transition C is 150. The wavelength of transition C is approximately 737 nm. Therefore, the size of nanodiamond 120 is much smaller than the wavelength of the indistinguishable emitted photon 160.

[0107] Nanodiamond 120 is coated onto the diamond substrate 220 to ensure good thermal conductivity (see Figure 5).

[0108] There are several methods for selecting nanodiamonds 120 from a sample of fabricated nanodiamonds 120 for use with a coherent single-photon source 100. According to one embodiment, the nanodiamonds 120 are selected from the silicon vacancies (SiV) of the nanodiamonds 120. - A sample of manufactured nanodiamonds 120 is selected such that the uniform linewidth of the central atomic transition C is less than twice the Fourier transform limit of the atomic transition C, and that the difference between the maximum and minimum frequencies of the atomic transition C is less than twice the Fourier transform limit of the atomic transition C on a timescale greater than one minute.

[0109] Flashing SIV - Nanodiamond 120 or SiV with a center of 150 -Nanodiamonds 120 located in undesirable positions within the center 150 are plasma-treated, and in particular, the surfaces of the relevant nanodiamonds 120 can be hydrogen-terminated. This allows for off-resonance of the associated SiV for the transition C which is preferably used. - This will result in a center value of 150.

[0110] The samples were measured using off-resonance photoluminescence (PL) and primarily single SiV. - This can be investigated by resonance photoluminescence excitation (PLE) measurements, which show a spectral distribution of approximately 50 GHz for the central 150 and transition C. This means that the non-uniform linewidth of transition C is approximately 50 GHz.

[0111] Figure 6 shows a schematic diagram of the experimental setup for realizing a quantum node. Figure 6 shows only the basic elements necessary to illustrate the operation of such a quantum node; however, those skilled in the art will understand that, for reasons of clarity, not all elements are shown in detail in this figure.

[0112] Figure 6 shows SiV - A coherent single-photon source 100 is shown having a cavity 280 designed and configured such that a central atomic transition C is coupled to a mode in the cavity 280. Cavity 280 is realized as a photonic crystal cavity 282. The coherent single-photon source 100 is SjV - It contains nanodiamonds 120 having a center 150, where SjV - The transition C at the center 150 is coupled to the mode in the photonic crystal cavity 282.

[0113] Efficient bonding is SiV - By spatially overlapping the central 150 dipole with the maximum value of the cavity field using AFM-based nanomanipulation, and by using the resonance conditions of the cavity 282 mode and, for example, resonant gas tuning, SiV -This is achieved by overlapping it with the transition frequency of the center 150.

[0114] Photonic crystal cavity 282 and single SiV - The Purcell coefficient for coupling with the central atomic transition C at 150 is greater than 10, and especially greater than 14. The lifetime of atomic transition C is shortened by the Purcell coefficient, which in turn increases the operating bandwidth of the quantum node, resulting in a significant improvement in the efficiency of the quantum node. The shortened lifetime corresponds to an increase in linewidth that has the Fourier transform limit, thereby relaxing the conditions for finding indistinguishable optical transitions.

[0115] Figure 6 shows a quantum node, including the optical infrastructure to the interface, where photons are coupled into optical fiber 354, or to a collimated free-space optical system for long-distance photon exchange.

[0116] The cryostat 320 contains nanodiamonds 120 and a photonic crystal cavity 282. The objective lens 332 of the home-built confocal microscope is located inside the cryostat 320, while all other components of the confocal microscope are located outside the cryostat 320. The cryostat 320 may be a continuous-flow cryostat that can operate at a low temperature, for example, 2.5 K. Here, the temperature of the coherent single-photon source 100 and the nanodiamonds 120 is a local temperature of approximately 3-5 K, close to the temperature of the cryostat's low-temperature fingers. The cryostat 320 may also be a closed-cycle cryostat if there is no liquid helium supply at the quantum node location.

[0117] Laser 340 is used to excite nanodiamonds 120 within cryostat 320. The laser is, for example, SiV - It may be a continuous-wave 532 nm laser capable of off-resonance excitation of the central 150.

[0118] The laser beam 342 emitted from the laser 340 is deflected by the pellicle beam splitter 344 and focused to the objective lens 332 of the confocal microscope using two lenses 346. The optical path can be adjusted so that the laser beam 342 strikes the nanodiamond 120.

[0119] The inner portion of the cryostat 320, which is located within a rectangle with a dashed line, is shown in an enlarged view on the right side of Figure 6.

[0120] The photonic crystal cavity 282 comprises a rod-shaped or bar-shaped first portion, shown on the right side of the diagram in Figure 6, where the bar has a rectangular cross-section and has a plurality of holes 284 arranged along the longitudinal axis of the bar of the photonic crystal cavity 282. Each axis of the holes 284 extends perpendicular to the axis of the bar of the photonic crystal cavity (PCC) 282. The nanodiamond 120 of the single photon source 100 is placed inside one of the holes 284. The nanodiamond 120 is located at the top of the hole 284. The nanodiamond 120 can be placed inside one of the holes 284 by a so-called pick-and-place method, which is described with reference to Figures 7-9.

[0121] Referring to Figure 5, the nanodiamonds 120 coated on the diamond substrate 220 can be picked up by the cantilever 230 of an atomic force microscope (AFM). See Figure 7 for details. The picked-up nanodiamonds 120 adhere to the cantilever 230 of the atomic force microscope and can be moved onto the photonic crystal cavity 282. See Figure 8 for details. Furthermore, they can be placed into specific pores 284 within the photonic crystal cavity 282. See Figure 9 for details.

[0122] The second portion of the photonic crystal cavity 282 shown on the left side of Figure 6 is SiV -It has a waveguide portion 286 that guides the photons 160 emitted from the center 150 into the mode of the photonic crystal cavity 282 toward the mirror portion 288, which focuses the photons 160 toward the objective lens 332 of a household confocal microscope.

[0123] Laser beam 342 is used to analyze SiV within nanodiamond 120. - After the center 150 is excited, SiV - The center 150 becomes an excited state. SiV - Photons 160 emitted from the center 150 toward the mirror section 288 are reflected by the lens section 290 toward the objective lens 332 of the confocal microscope. The optical path 348 of the emitted photon 160 exits the objective lens 332, passes through two lenses 346, goes through the beam splitter 344, is deflected by the galvanoscanner 350, and from the galvanoscanner 350, the optical path 348 further passes through the long-pass filter 352. Finally, the emitted photon 160 is coupled to the fiber 354. The emitted photon 160 can be transmitted via the fiber 354 to a remote location that can be more than 100 km away from the experimental setup in Figure 6. Quantum nodes can then be developed based on different protocols. For example, projection measurements performed by the Hong-Ou-Mandel interferometer can project two setups in a remotely entangled state, which can be a starting point for implementing quantum network technology. In non-classical optical field engineering applications, while most quantum sensors are implemented on a chip, optical setups provide on-chip optical addressing of indistinguishable single-photon sources.

[0124] Figure 10 shows an experimental setup of system 300 comprising a first coherent single-photon source 101 and a second coherent single-photon source 102, where the first coherent single-photon source 101 and the second coherent single-photon source 102 are used to generate the first SiV of the first nanodiamond 121 of the first coherent single-photon source 101. -Photon 160 emitted from center 151 is second SiV of second nanodiamond 122 of second coherent single-photon source 102 - The setup is designed and configured to be indistinguishable from photon 160 emitted from center 152. This indistinguishability can be demonstrated by performing two-photon interference, known as the Hong-Ou-Mandel effect and / or Hong-Ou-Mandel interference, and measuring that the quadratic correlation function at zero time delay of the emitted photon or emitted light is less than 0.5, as described below. Because the setup in Figure 10 is complex, the basic principle of Hong-Ou-Mandel interference will be explained with reference to Figure 11.

[0125] Figure 11 shows a schematic diagram of the Hong-Ou-Mandel interference experiment.

[0126] The first coherent single-photon source 101 is connected to the first nanodiamond 121 and the first SiV - The center 151 is included. The second coherent single-photon source 102 contains the second nanodiamond 122 and the second SiV - Includes center 152.

[0127] First SiV - Center 151 and second SiV - Both centers 152 can be prepared in an excited state using a laser not shown in Figure 11. In this state, the first SiV - Center 151 and second SiV - Both central 152 emit photons 160 at random times. First SiV - Photon 160 emitted from center 151 and the second SiV -If the photon 160 emitted from the center 152 is the same photon 160 within the so-called coalescing time window from two different input ports, and further enters the 50:50 unpolarized beam splitter 367, then the probability amplitudes exiting from the same port will constitutively interfere, while the probability amplitudes exiting from different output ports will destructively interfere. This means that the two photons 160 entering the unpolarized beam splitter 367 will exit the unpolarized beam splitter 367 together as a photon pair through either the first or second exit port. In this case, the possibility of each photon 160 exiting through a different exit port is excluded. Therefore, the quadratic correlation function g by the same photon 160 is... 2 The measurement of (τ) is expected for 160 single but discriminable photons. 2 In contrast to (0)=0.5, dip g 2 This causes antibunching at (0)=0.

[0128] The experimental setup shown in Figure 10 is described in detail below.

[0129] The two nanodiamonds 121 and 122 of system 300 are selected from a sample of fabricated nanodiamonds 120 in a similar manner to how a single nanodiamond 120 is selected for a coherent single-photon source 100. According to one embodiment, the first nanodiamond 121 of the first coherent single-photon source 101 and the second nanodiamond 122 of the second coherent single-photon source 102 are the first SiV of the first nanodiamond 121 - The first frequency of the atomic transition C at the center 151 and the second SiV of the second nanodiamond 122 - The difference between the second frequency of atomic transition C at the center 152 and the first SiV of the first nanodiamond 121 is less than twice the Fourier transform limit, and the first SiV of the first nanodiamond 121 is such that - The first uniform line width of the central atomic transition C at 151 and the second SiV of the second nanodiamond 122 -A selection is made from a common sample of manufactured nanodiamonds 120 such that both the second uniform linewidth of the central atomic transition C and the atomic transition C are smaller than twice the Fourier transform limit of the atomic transition C, and that on a timescale larger than one minute, both the first difference between the maximum and minimum values ​​of the first frequency and the second difference between the maximum and minimum values ​​of the second frequency are smaller than twice the Fourier transform limit of the atomic transition C.

[0130] A sample of the fabricated nanodiamond 120 was placed in a continuous flow cryostat 320 and cooled to approximately 4K using liquid helium. The nanodiamond 120 was coated onto a diamond substrate 220 for good thermal conductivity, reaching a local temperature of 5K to 10K.

[0131] Matching SiVs that satisfy the above requirements - The pair, i.e., the first SiV - Center 151 and second SiV - To select or find the center 152, first set the scanning laser frequency to the first SiV - Center 151 or second SiV - The sample can be fixed to the resonance of one of the transition C's central 152 and then scanned laterally. In this way, the SiV resonates with the fixed laser frequency. - Only the center is found.

[0132] Figure 12 shows the SiV of nanodiamond 120 having a specified position P relative to transition C in a sample of fabricated nanodiamond 120. - A bar graph distribution of center 150 against the number N representing how many of these centers exist or occur is shown. The specified position P for transition C is shown in THz. For example, in a sample of fabricated nanodiamond 120, there are 14 SiVs with each frequency for a transition C of approximately 406.82 THz. -There is a center 150. A fitting function 392 is fitted to a bar graph distribution 390. The fitting function 392 is a Gaussian distribution having a standard deviation of about 50 GHz. This means that the fitting function 392 shows a non-uniform distribution with respect to the position P of the transition C at about 50 GHz. In other words, in the sample of the manufactured nanodiamond 120, the non-uniform linewidth for the transition C is about 50 GHz.

[0133] Two nanodiamonds 121, 122 have two SiV - Centers 151, 152 can be selected from the sample of the manufactured nanodiamond 120 such that the above conditions are satisfied.

[0134] In FIG. 13, for each plotted SiV of the nanodiamond 120 - Center 150, for a part of the sample of the manufactured nanodiamond 120, the uniform linewidth LW [MHz] measured for the position P with respect to the transition C is shown. The size of each dot is the same, but the corresponding SiV - Center 150 can easily find two or more dots having an overlap of the corresponding frequency distributions. Here, the frequency distribution can also be referred to as the distribution of the transition C. For each measured dot, the frequency distribution is centered at the position P and has a linewidth LW. The vertical dotted line indicates various SiV having significantly overlapping frequency distributions - Center 150. The first SiV of the first nanodiamond 121 - Center 151 and the second SiV of the second nanodiamond 122 - Center 152, two SiV used as - Center 150 are marked with circles. Two SiV - Centers 151, 152 are used for two-photon interference. Two SiV - Centers 151, 152 are in two remote nanodiamonds 121, 122 separated by about 100 μm.

[0135] The first SiV of the first nanodiamond 121- The spectrum 400 of the center 151 is shown in FIG. 14. The first nanodiamond 121 is the same nanodiamond as in the embodiments of FIGS. 3 and 4 of the coherent single photon source 100. The second SiV - The spectrum 410 of the center 152 of the second nanodiamond 122 is shown in FIG. 15. Atomic transitions A, B, C, and D are shown in both FIGS. 14 and 15. Further, the ground state splitting ΔGS and the excited state splitting ΔES are shown in both FIGS. 14 and 15. The first SiV - The ground state splitting ΔGS of the center 151 differs from the ground state splitting ΔGS of the second SiV - center 152 by only 12 GHz. However, transition C shows good overlap for both the first SiV - center 151 and the second SiV - center 152. Although the ground state splittings of both the first SiV - center 151 and the second SiV - center 152 are different, the fact that transition C overlaps can be explained by different combinations of axial strain and transverse strain of the host crystal.

[0136] Returning to FIG. 10, an experimental setup of the system 300 used to show two-photon interference is shown here.

[0137] Both the first coherent single photon source 101 and the second coherent single photon source 102 are disposed inside a cryostat 320 on a substrate 220. The inner part of the cryostat 320 inside the dashed circle is shown enlarged adjacent thereto. The first nanodiamond 121 and the second nanodiamond 122 are disposed on the substrate 220.

[0138] The first nanodiamond 121 includes the first SiV - center 151, and the second nanodiamond 122 includes the second SiV - center 152. A laser 340 is used to generate between the first SiV - center 151 and the second SiV- The center 152 is excited off-resonance. The beam of laser 340 is first deflected by mirror 356, then passes through half-wave plate 362 and 50:50 polarized beam splitter 358, and is then split into two optical paths, namely the first optical path 359 and the second optical path 360, both of which terminate within cryostat 320. The half-wave plate 362 and 50:50 polarized beam splitter 358 allow the excitation output for the first optical path 359 and the second optical path 360 to be adjusted, thus the first SiV - Emission from center 151 and second SiV - This allows for equilibrium with the emission from center 152.

[0139] In the first optical path 359, upstream of the cryostat 320 are a mirror 356, a dichroic mirror 364, a galvanometer 350, a lens 346, another mirror 356, a knife-edge prism 365, and another lens 346. In the second optical path 360 are a dichroic mirror 364, a galvanometer 350, and a lens 346. The knife-edge prism 365 is used to split the field of view of the confocal setup into two independent channels. The beam of the laser 340 is directed through the first SiV using the first optical path 359. - Focused on the center 151, the second SiV uses the second optical path 360. - The focus is set to the center 152.

[0140] First SiV - Photons 160 emitted from the center 151 can return via the first optical path 359 through the dichroic mirror 364. The optical path downstream of the dichroic mirror 364 is shown as a dashed line. Downstream of the dichroic mirror 364, the emitted photons 160 strike two mirrors 356, pass through the long-pass filter 352, etalon 361 and half-wave plate 362, and then enter the first port of the 50:50 unpolarized beam splitter 367.

[0141] Second SiV -Photon 160 emitted from center 152 can return through the second optical path 360 via the dichroic mirror 364. The optical path downstream of the dichroic mirror 364 is shown as a dashed line. Downstream of the dichroic mirror 364, the emitted photon 160 strikes mirror 356, passes through long-pass filter 352, etalon 361 and half-wave plate 362, and enters the second port of 50:50 unpolarized beam splitter 367. This unpolarized beam splitter 367 is where two-photon interference or Hong-Ou-Mandel interference occurs.

[0142] The dichroic mirror 364, long-pass filter 352, and etalon 361 are used in the first SiV - Center 151 and second SiV - It is used in both the first optical path 359 and the second optical path 360 to filter the photons 160 emitted from the center 152. If further suppression of undesirable signals is required, the second etalon can be used with slightly varied thicknesses for each optical path to enable suppression over a wide wavelength range.

[0143] The long-pass filter 352 is a 740 / 13 band-pass filter. The etalon in the first optical path 359 has a free spectral range (FSR) of 850 GHz and a linewidth of 90 GHz, and the etalon 361 in the second optical path 360 has an FSR of 10 GHz and a linewidth of 1 GHz.

[0144] Two half-wave plates 362 upstream of the 50:50 unpolarized beam splitter 367 are used to adjust the polarization in their respective optical paths 359 and 360.

[0145] Two-photon interference occurs when a photon 160 from the first optical path 359 enters the unpolarized beam splitter 367 simultaneously with an indistinguishable photon 160 from the second optical path 360, i.e., within the coalescing time window. This means that the two indistinguishable photons 160 entering the beam splitter 367 exit the unpolarized beam splitter 367 together as a photon pair, passing through either the first or second exit port.

[0146] Photons 160 exiting the unpolarized beam splitter 367 at either the first or second exit port are focused into two single-mode fibers 369, each of which is coupled to a fiber coupler 366, detected by a single-photon counting module (SPCM) 368, and correlated with a time tagging device 370.

[0147] To increase the probability that two indistinguishable photons 160 reach the unpolarized beam splitter 367, SiV - It is preferable to filter the photons 160 emitted from the centers 151 and 152. Figure 16 shows the first SiV - The filtered emission 420 from the center 151 is shown.

[0148] As shown in Figure 17, the first SiV - Photoluminescence excitation (PLE) measurement of transition C at central 151 and second SiV - The corresponding measurement 432 at the center 152 shows linewidths of 158 MHz and 177 MHz, with a detuning amount Δ of Δ / (2π) = 83 MHz relative to each other. Both measurements 430 and 432 are measured as a function of relative frequency ν, obtained by subtracting the average frequency of the peak values ​​of measurements 430 and 432 from the frequency of the excitation laser. First SiV - Center 151 and second SiV - Off-resonance quadratic correlation measurements were performed on both sides of the central 152, and these were then normalized.

[0149] First SiV -The quadratic correlation function g1 measured for the center 151 (2) (τ) is shown in Figure 18, and the second SiV - The quadratic correlation function g2 measured for the center 152 (2) (τ) is shown in Figure 19. The first SiV - Regarding the center 151, g1 (2) (τ=0) is equal to 0.33, and the second SiV - Regarding the center 152, g2 (2) (τ=0) is equal to 0.35. Thus, the first SiV - Center 151 and second SiV - Single-photon emission was confirmed at both ends of central 152.

[0150] g i (2) (τ) is used to measure the correlation function including background noise, while decorative letter notation

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[0151] To measure two-photon interference, the first SiV - Center 151 and second SiV - Both centers 152 are excited independently of each other in an off-resonance manner, and after the polarization of photons 160 is adapted by the half-wave plate 362, the emitted photons 160 interfere in the 50:50 beam splitter 367. The resulting correlation function is shown in Figure 20. Parallel polarization data 440 is shown as dots, and perpendicular polarization data 442 is shown as triangles. The fitting 441 to parallel polarization data 440 has a dashed curve. The fitting 443 to perpendicular polarization data 442 has a solid curve.

[0152] For both data 440 and data 442, data 440 and 442 are:

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[0153] The signal and noise for each quantum emitter 140 were determined in advance for both SiV - The signal-to-noise ratio of the individual correlation measurements at centers 151 and 152 was fixed. The variable η before the interference term can be interpreted as an efficiency coefficient, where a value of 0 means no two-photon interference and a value of 1 means maximum interference. For parallel polarization, a value of η of 0.61 ± 0.16 was determined. As an additional measure of merit, the coalescence time window (CTW) can be calculated.

[0154] This provides a time window in which coalescence can be generated on the beam splitter 367. Visibility function

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[0155] First SiV - Center 151 and second SiV - To determine the long-term dynamics of center 152, the correlation functions of both emitters are analyzed. A three-level model for correlation functions is fitted to the correlation function of the emitters. First SiV - Center 151 and second SiV - A shelving time of approximately 25 ns is found for both of the central 152 points. Therefore, the correlation function g i (2) (τ) and

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Claims

1. A coherent single-photon source (100), It contains nanodiamonds (120) having quantum emitters (140), The nanodiamond (120) and the quantum emitter (140) are designed and configured such that the quantum emitter (140) emits coherent and indistinguishable photons (160). Coherent single-photon source (100).

2. The coherent single-photon source (100) according to claim 1, wherein the size of the nanodiamond (120) is smaller than the wavelength of the emitted photon (160).

3. The quadratic correlation function of the emitted photon (160) at zero time delay is less than 0.

5. The first input component of the correlation measurement includes a photon (160) emitted from the quantum emitter (140), and the second input component of the correlation measurement includes a photon (160) from the quantum emitter (140) that is time-delayed by a time difference of more than 1 second relative to the first component. A coherent single-photon source (100) according to claim 1 or 2.

4. The quantum emitter (140) is a color center of group IV, particularly a single negatively charged silicon vacancy (SiV). － The center (150) A coherent single-photon source (100) according to any one of claims 1 to 3.

5. The surface of the nanodiamond (120) is hydrogen-terminated. A coherent single-photon source (100) according to any one of claims 1 to 4.

6. The aforementioned coherent single-photon source (100) further The filter (352, 361, 364) is designed and configured to filter the photons (160) emitted from the quantum emitter (140) of the coherent single-photon source (100), A coherent single-photon source (100) according to any one of claims 1 to 5.

7. The nanodiamond (120) is such that the uniform linewidth of the atomic transitions (C, A, B, D) of its quantum emitter (140) is smaller than twice the Fourier transform limit of the atomic transitions (C, A, B, D), and On a timescale larger than one minute, the difference between the maximum and minimum frequencies of the atomic transition (C, A, B, D) is less than twice the Fourier transform limit of the atomic transition (C, A, B, D). Selected from the manufactured nanodiamond (120) samples, A coherent single-photon source (100) according to any one of claims 1 to 6.

8. The aforementioned coherent single-photon source (100) further A cavity (280, 282) having a cavity (280, 282) which is designed and configured such that the atomic transitions (C, A, B, D) of the quantum emitter (140) are coupled to the modes of the cavity (280, 282), A coherent single-photon source (100) according to any one of claims 1 to 7.

9. The coherent single-photon source (100) according to claim 8, wherein the Purcell coefficient for the cavity (282) and the atomic transitions (C, A, B, D) of the quantum emitter (140) are greater than 1.

10. System (300), A first coherent single-photon source (101) according to any one of claims 1 to 9, A second coherent single-photon source (102) according to any one of claims 1 to 9 and Equipped with, The first coherent single-photon source (101) and the second coherent single-photon source (102) are designed and configured such that a photon (160) emitted from the first quantum emitter (151) of the first nanodiamond (121) of the first coherent single-photon source (101) is indistinguishable from a photon (160) emitted from the second quantum emitter (152) of the second nanodiamond (122) of the second coherent single-photon source (102). System (300).

11. The first nanodiamond (121) of the first coherent single-photon source (101) and the second nanodiamond (122) of the second coherent single-photon source (102) are The difference between the first frequency of the atomic transition (C, A, B, D) of the first quantum emitter (151) of the first nanodiamond (121) and the second frequency of the same atomic transition (C, A, B, D) of the second quantum emitter (152) of the second nanodiamond (122) is less than twice the Fourier transform limit of the atomic transition (C, A, B, D), and The first uniform linewidth of the atomic transition (C, A, B, D) of the first quantum emitter (151) of the first nanodiamond (121) and the second uniform linewidth of the atomic transition (C, A, B, D) of the second quantum emitter (152) of the second nanodiamond (122) are both smaller than twice the Fourier transform limit of the atomic transition (C, A, B, D), and On a timescale larger than one minute, both the first difference between the maximum value and the minimum value of the first frequency and the second difference between the maximum value and the minimum value of the second frequency are less than twice the Fourier transform limit of the atomic transition (C, A, B, D). Selected from a common sample of manufactured nanodiamonds (120). The system (300) according to claim 10.

12. The quadratic correlation function of the emitted photon (160) at zero time delay is less than 0.

5. The first input component of the correlation measurement includes a photon (160) emitted from the first quantum emitter (151), and the second input component of the correlation measurement includes a photon (160) emitted from the second quantum emitter (152). The system according to claim 10 or 11.

13. The system (300) according to claim 10, 11, or 12, wherein the distance from the first nanodiamond (120) of the first coherent single-photon source (101) to the second nanodiamond (120) of the second coherent single-photon source (100) is greater than 1 meter.

14. The distance from the first nanodiamond (121) of the first coherent single-photon source (101) to the second nanodiamond (122) of the second coherent single-photon source (102) is smaller than the wavelength of the emitted photon (160). or The atomic transitions (C, A, B, D) of the first quantum emitter (151) and the atomic transitions (C, A, B, D) of the second quantum emitter (152) are coupled to a mode in a common cavity (280, 282). The system (300) according to claim 10, 11, or 12.

15. Use of one nanodiamond (120) having a quantum emitter (140) or two nanodiamonds (120) each having a quantum emitter (140) as a coherent single-photon source (100), or as a system having a first coherent single-photon source (101) and a second coherent single-photon source (102).

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