All optical single photon detector

Abstract

A detector system and corresponding method are described, capable of single photon detection. The system comprising: an optical cavity configured for capturing photons to be detected, the optical cavity comprises a selected nonlinear material within a region of the cavity; an optical arrangement configured for transmitting a probe beam through the optical cavity to obtain a modulated probe beam, and for interfering the modulated probe beam with a reference beam to obtain an interference signal; and at least one detector configured for detecting the interference signal. Variation in the interference signal is indicative of presence and / or quantity of one or more photons within the optical cavity.

Description

[0001] ALL OPTICAL SINGLE PHOTON DETECTOR

[0002] TECHNOLOGICAL FIELD

[0003] The present disclosure is directed at single photon detectors and specifically relates to an all-optical single photon detector that may be operable at room temperatures and can be configured to be integrated in chip- scale devices.

[0004] BACKGROUND ART

[0005] References considered to be relevant as background to the presently disclosed subject matter are listed below:

[0006] Andrew M. C. Dawes et al., “All-Optical Switching in Rubidium Vapor” Science 308, 672-674 (2005).

[0007] Wataru Yoshiki and Takasumi Tanabe, “All-optical switching using Kerr effect in a silica toroid microcavity” Opt. Express 22, 24332-24341 (2014). Qi Feng, et al., “Enhanced optical Kerr nonlinearity of graphene / Si hybrid waveguide” Appl. Phys. Lett. 114, 071104 (2019).

[0008] Hyeongrak Choi, et al., “Self-Similar Nanocavity Design with Ultrasmall Mode Volume for Single-Photon Nonlinearities” Phys. Rev. Lett. 118, 223605 (2017).

[0009] Jin-Yue Su, et al. “Ultrafast all-optical switching in a silicon-polymer compound slotted photonic crystal nanobeam cavity” Opt. Rev. 30, 33-40 (2023).

[0010] Paul Seidler, et al. "Slotted photonic crystal nanobeam cavity with an ultrahigh quality factor-to-mode volume ratio," Opt. Express 21, 32468- 32483 (2013).

[0011] Goktug I§ikl ar, et al. “On the trade-off between mode volume and quality factor in dielectric nanocavities optimized for Purcell enhancement,” Opt. Express 30, 47304-47314 (2022). T. Nakamura et al “Improvement in the quality factors for photonic crystal nanocavities via visualization of the leaky components,” Opt. Express 24, 9541-9549 (2016).

[0012] Yun-Feng Xiao, et al. “Quantum non-demolition measurement of photon number via optical Kerr effect in an ultra-high-Q microtoroid cavity”, Opt. Express 16, 21462-21475 (2008).

[0013] Ruppert, L., et al. “High-precision multiparameter estimation of mechanical force by quantum optomechanics”, Sci Rep 12, 16022 (2022).

[0014] ChaoXia Zhang, et al. “Small-tilt measurement based on weak-value- amplification with balanced homodyne detection”, Appl. Phys. Lett. 122, 031107 (2023).

[0015] Likun Zhou et al., “Double -port homodyne detection in a squeezed- state interferometry with a binary-outcome data processing”, Commun. Theor. Phys. 74, 125104 (2022).

[0016] Hui Liao, et al. “Precision vibration measurement using differential phase- modulated homodyne interferometry”, Optics and Lasers in Engineering 169, 107695 (2023).

[0017] Maximilian Protte, et al. “Low-noise balanced homodyne detection with superconducting nanowire single -photon detectors”, Optica Quantum 2, 1-6 (2024).

[0018] Hai He, et al., “Single-photon source with sub-MHz linewidth for cesium- based quantum information processing. Front. Phys. 18, 61303 (2023).

[0019] L. Bremer et al., “Cesium- Vapor-Based Delay of Single Photons Emitted by Deterministically Fabricated Quantum Dot Microlenses”, Adv. Quantum Technol. 3, 1900071 (2020).

[0020] Hristina Georgieva et al., “Radiometric characterization of a triggered narrow-bandwidth single-photon source and its use for the calibration of silicon single-photon avalanche detectors”, Metrologia 57, 055001 (2020).

[0021] Sosnicki, F., et al., “Interface between picosecond and nanosecond quantum light pulses”, Nat. Photon. 17, 761-766 (2023).

[0022] S. Signorini, et al., “A silicon source of heralded single photons at 2 / / m”, APL Photonics 6, 126103 (2021). Zhang, Lin, et al., “Nonlinear Group IV photonics based on silicon and germanium: from near- infrared to mid-infrared”, Nanophotonics 3, 247-268 (2014).

[0023] Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

[0024] BACKGROUND

[0025] Single photon detection is a highly sensitive technique enabling detection and counting of individual photons, the smallest units of light. The ability to measure single photons has become increasingly important in various fields such as quantum optics, quantum computing, and biological imaging. The fundamental principle behind single photon detection is based on measuring extremely weak light signals where traditional photodetectors, like photodiodes or CCD cameras, are often inadequate.

[0026] Conventional single photon detection methods employ several technologies, with the most common being photomultiplier tubes (PMTs), avalanche photodiodes (APDs), and superconducting nanowire single -photon detectors (SNSPDs). PMTs were one of the earliest technologies used for single photon detection due to their high sensitivity in the ultraviolet to visible light spectrum. APDs have become a popular alternative due to their compact size, faster response time, and the ability to detect photons in the near-infrared spectrum. However, they are too large for chip-scale devices. In recent years, SNSPDs have emerged as a state-of-the-art technology, offering higher detection efficiencies, lower dark count rates, and improved timing resolution. While this technique can provide small devices that can be integrated into chip-scale devices, it requires cryogenic temperatures to operate.

[0027] Single-photon detectors are widely used in applications such as fluorescence microscopy, quantum cryptography, and astrophysics. For example, in quantum cryptography, single photon detectors are required for ensuring the security of communication protocols by detecting the presence of eavesdroppers. Similarly, in biological imaging, single photon detection enhances the ability to observe molecular- level processes with higher precision and less photodamage. Furthermore, single photon detectors are highly needed in the emerging field of quantum computing and especially in photonic -based quantum computing.

[0028] Kerr materials have been used in all-optical switching, an operation that allows control of an optical beam using another optical beam. This can be done using nonlinear interaction between the beams, which may be assisted by a nonlinear medium through which the beams interact. The nonlinear nature of all-optical switching may require relatively high intensities, and efforts are dedicated to enable all-optical switching techniques that can be controlled by small optical energy, with an ultimate goal to use such switches in classical applications such as data centers, as well as for quantum applications such as in quantum communication and computing.

[0029] GENERAL DESCRIPTION

[0030] As indicated above, the conventional single-photon detectors relate to one of two main categories: bulky arrangement such as APDs and PMTs or detector configurations that require cryogenic conditions such as SNSPDs. Furthermore, many of the above listed detectors having single -photon detection capabilities suffer from a down-time issue, i.e. require certain time to recover after detection of a photon. This fact limits detection rate and the ability to detect a few photons one immediately after the other. Accordingly, there is a need in the art for a detector system capable of detecting single photons, while operating in room temperature conditions, and while eliminating the large size required for PMTs and APDs, and with short down times.

[0031] Generally, chip scale single-photon detectors are needed in various fields, such as in quantum communication and computing. However, the required cryogenic temperatures form a huge operational barrier. Consequently, there is a need in the art for a chip scale single-photon detector that is operable at room temperature.

[0032] The present disclosure provides a detector system utilizing an optical cavity comprising a selected nonlinear material within the cavity, and a probe beam arrangement configured for detecting variations in the effective optical path of a probe beam passing through the optical cavity. The detector system utilizes detection of a phase shift applied to the probe beam after passage through the optical cavity, to determine presence of a photon and / or the number of photons trapped in the cavity. Generally, to provide measurable nonlinear effects resulting from a single photon, the optical cavity is configured with an ultra-small mode volume and a high-quality factor Q. This results in one or more photons that are generally trapped for a relatively long duration in a resonant mode of the optical cavity and generate high electric field due to the photon’s location within the ultra-small volume of the cavity mode.

[0033] To provide detection of one or more photons within the optical cavity, the present disclosure utilizes a probe beam, generally characterized by a wavelength associated with a lossy mode of the optical cavity, transmitted through a region of the optical cavity. Nonlinear (e.g., optical Kerr effect) properties of the material of the cavity, affect the effective optical path of the probe beam and its phase, providing a modified probe beam. Detection of a phase variation of the modified probe beam provides a clear and quantitative indication of the presence of a photon and / or a number of photons trapped within the optical cavity.

[0034] Generally, detection of a phase variation of the modified probe beam can be provided using an interference signal. The interference signal may be a result of an interference between the modified probe beam and a reference beam. Variation in the interference signal indicates a phase shift applied to the modified probe beam, indicating and quantifying the presence of one or more photons of selected wavelength / frequency within the optical cavity. In some examples, the interference signal may be measured using homodyne detection techniques enabling detection of very small phase shifts applied to the modified probe beam. Additional detection techniques, such as heterodyne detection, or other detection techniques may also be used to enable measurement of small phase shifts of the modified probe beam.

[0035] The technique of the present disclosure relates to a highly promising direction associated with an optical cavity utilizing a nonlinear medium within the cavity. While nonlinear properties typically appear in response to high intensity, the back-and-forth path of even a single photon within the optical cavity results in focusing the photon energy within the cavity. As a result, given a sufficiently small mode volume within the cavity, a single photon within the cavity can generate an electric field sufficient to create a measurable change of the index of refraction of the medium. Typically, the technique of the present disclosure utilizes selected nonlinear materials that exhibit optical Kerr effect associated with the / 3’ parameter. Previous suggestions to utilize Kerr materials for single photon detection have not materialized for various reasons, which are addressed in the present disclosure. They include using sizable resonators that inhibit large arrays of detectors integrated into a single chip, and which suffer from a large mode volume suppressing the electric field intensity caused by a single photon and consequently suppressing the Kerr effect and the observable signal. Previous suggestions also used resonators that are near impossible to fabricate. For example, fabricating a high-Q ring (toroid) resonator from a strong Kerr material that is typically crystalline is hard as a very low edge roughness is required, and this is a hard task when working with a crystalline material. Finally, some suggestions utilizing a toroid suffer from complex fabrication and weak signal as the coupling to these toroids is done by the (exponentially sensitive) tunneling of light through a gap between the toroid and a straight waveguide.

[0036] Thus, according to a broad aspect, the present disclosure provides a single photon detector system comprising: an optical cavity configured for capturing photons to be detected, the optical cavity comprises a selected nonlinear material within a region of the cavity; an optical arrangement configured for transmitting a probe beam through the optical cavity to obtain a modulated probe beam, and for interfering the modulated probe beam with a reference beam to obtain an interference signal; and at least one detector configured for detecting the interference signal, wherein variation in the interference signal is indicative of the one or more photons within the optical cavity.

[0037] Typically, changes in the interference signal, such as a change in the signal magnitude, may be a result of a change of phase of the modulated probe beam. This is associated with amplification of the photons to be detected by the optical cavity, and interaction with the selected nonlinear material placed within the optical cavity. The optical cavity is generally configured to be in resonance with wavelength of the photons to be detected.

[0038] According to some embodiment, the optical cavity is characterized by a mode volume below 0.01k3, where k is a wavelength of the one or more photons to be detected.

[0039] According to some embodiment, the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected. According to some embodiment, the optical cavity is formed by a photonic crystal cavity within a waveguide section.

[0040] According to some embodiment, the probe beam propagates in a direction intersecting with a direction of propagation of the photons to be detected, wherein an intersection between the probe beam and the photons to be detected is located within the optical cavity.

[0041] According to some embodiment, the optical cavity is wavelength selective, the probe beam and photons to be detected propagate through a common waveguide comprising the optical cavity.

[0042] According to some embodiment, the optical cavity comprises wavelength selective reflecting elements, selected to allow transmission of a wavelength of the probe beam.

[0043] According to some embodiment, a wavelength of the probe beam is selected to be within wavelength giving rise to a lossy mode of the optical cavity. The probed beam may utilize continuous wave (CW) or pulses having length selected in accordance with lifetime of a photon within the optical cavity.

[0044] According to some embodiment, the single -photon detector system is operable at room temperature. Further, in some embodiments the single-photon detector system may be configured as a system on a chip.

[0045] According to some embodiment, a power of the probe beam is selected to be up to lOpW.

[0046] According to some embodiment, the detection of the interference signal is indicative of a phase shift of the modified probe beam, and wherein the phase shift of the modified probe beam is indicative of a presence and a number of photons trapped within the optical cavity.

[0047] According to some embodiment, the single-photon detector system is configured in a micrometer scale.

[0048] According to some embodiment, the optical cavity is a linear optical cavity.

[0049] According to a further broad aspect, the present disclosure provides a detector array comprising a plurality of single-photon detectors, each configured as described above. The detector array, and the single -photon detectors thereof may be embedded within a photonic circuit. According to yet a further broad aspect, the present disclosure provides a detector system comprising:

[0050] (a) a first waveguide section positioned for accepting photons to be detected;

[0051] (b) an optical cavity within the first waveguide section, the optical cavity comprising a selected nonlinear material having selected nonlinear properties located to overlap a mode volume of a resonant mode of the optical cavity;

[0052] (c) a probe beam arrangement comprising at least one probe waveguide section configured for directing a probe beam through the optical cavity, and collecting a modified probe beam exiting the optical cavity;

[0053] (d) a detector arrangement configured for detecting an interference signal, formed by interference of the modified probe beam with a reference beam; wherein presence of one of more photons within the optical cavity vary refractive index of the selected nonlinear material, thereby affecting a phase of the modified probe beam with respect to the reference beam.

[0054] According to some embodiment, a mode volume of the optical cavity with respect to a wavelength of the photons to be detected is below 0.01 Z.3, where X is the wavelength of the one or more photons to be detected, and wherein the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected.

[0055] According to some embodiment, the detector system provides a single photon detection capability.

[0056] According to some embodiment, the first waveguide section and probe waveguide section are overlapping at a region of the optical cavity.

[0057] According to some embodiment, the first waveguide section and probe waveguide section are intersecting at a location of the optical cavity.

[0058] According to some embodiment, a resonant mode of the optical cavity is tuned to the wavelength of the photons to be detected, and wherein a wavelength of the probe beam is tuned to a lossy mode of the optical cavity.

[0059] According to some embodiment, the detector system may be configured in a micrometer scale.

[0060] According to some embodiment, the optical cavity is a linear optical cavity.

[0061] According to yet another broad aspect, the present disclosure provides a method for detection of photons, comprising: (a) providing an optical cavity having a resonant mode associated with a wavelength of the photons to be detected, and comprising a selected nonlinear material located to overlap a mode volume of the optical cavity;

[0062] (b) transmitting a probe beam through the optical cavity to obtain a modified probe beam, the probe beam having a wavelength giving rise to a lossy mode of the optical cavity;

[0063] (c) detecting a phase shift of the modified probe beam with respect to the probe beam; and

[0064] (d) determining a presence and a number of photons to be detected in accordance with the phase shift of the modified probe beam.

[0065] According to some embodiment, the method further comprising using one or more waveguides to direct the photons to be detected to be trapped by the optical cavity.

[0066] According to some embodiment, the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected.

[0067] According to some embodiment, providing an optical cavity comprises providing an optical cavity characterized by an ultra-small mode volume, being below 0.01 Z.3, where X is the wavelength of the one or more photons to be detected.

[0068] According to some embodiment, detecting a phase shift of the modified probe beam comprises generating an interference signal by interfering the modified probe beam with a reference beam and determining variation in intensity of the interference signal.

[0069] According to some embodiment, the method may comprise utilizing a homodyne detection technique.

[0070] According to some embodiment, transmitting a probe beam through the optical cavity comprises transmitting the probe beam through a waveguide intersecting with a path of the photons to be detected at a location of the optical cavity.

[0071] According to some embodiment, transmitting a probe beam through the optical cavity comprises transmitting the probe beam through a common waveguide as the photons to be detected, passing through the optical cavity.

[0072] According to some embodiment, transmitting a probe beam through the optical cavity comprises transmitting a plurality of pulses of the probe beam, where a spacing between the pulses is shorter than the lifetime of a photon to be detected within the optical cavity. According to some embodiment, transmitting a probe beam through the optical cavity comprises transmitting a continuous wave probe beam.

[0073] According to some embodiment, transmitting a probe beam through the optical cavity comprises transmitting the probe beam having intensity not exceeding 10’5W.

[0074] According to some embodiment, said providing and optical cavity comprises providing a photonic crystal forming an optical cavity.

[0075] According to some embodiment, said providing and optical cavity comprises providing a linear optical cavity.

[0076] BRIEF DESCRIPTION OF THE DRAWINGS

[0077] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0078] Fig. 1 schematically illustrates a detector system according to some embodiments of the present disclosure;

[0079] Fig. 2 schematically illustrates another example of a detector system according to some embodiments of the present disclosure;

[0080] Fig. 3 illustrates a waveguide section including an optical cavity according to some embodiments of the present disclosure;

[0081] Figs. 4A and 4B show COMSOL simulations exemplify a resonant mode (Fig. 4A) and a lossy mode (Fig. 4B) of an optical cavity;

[0082] Fig. 5 exemplify selected materials with their respective transparency windows, with some of them exhibiting Kerr nonlinearity; and

[0083] Fig. 6 shows calculated ratio between nonlinear effect induced by a single photon trapped within the optical cavity and by a probe beam passing through the optical cavity.

[0084] DETAILED DESCRIPTION OF EMBODIMENTS

[0085] Reference is made to Fig. 1, schematically illustrating an optical detector system 100 according to some embodiments of the present disclosure. Optical detector system 100 includes an optical cavity 120, with mirrors or reflecting devices 122 and 124, configured for capturing photons to be detected 50, and an optical arrangement 105 configured for detection of presence of a single photon and / or a number of photons within the optical cavity. The optical arrangement 105 is configured for transmitting a probe beam 112 through the optical cavity 120, receiving a modified probe beam 114 and detecting a phase shift in the modified probe beam 114. Phase shift applied to the modified probe beam provides a quantitative measure of the presence of a single photon and / or a number of photons 50 within the optical cavity 120.

[0086] In some embodiments of the present disclosure, the optical arrangement 105 includes a light source 110 (e.g., laser source, or input light through a waveguide) and a detector unit 130. Additionally, the optical arrangement may include a waveguide 111 for transmitting the probe beam 112 toward the optical cavity 120 and for receiving a modified beam 114 after passing through the optical cavity 120. The optical arrangement 105 may also include a reference beam 116 (e.g. transmitted via a reference waveguide that is not shown here) and a beam combiner 140 positioned to interfere the reference beam 116 and the modified probe beam 114 to generate an interference signal 118.

[0087] The optical cavity 120 includes a selected nonlinear material 125, located to overlap the position of the optical mode of the cavity 120. As indicated above, the optical arrangement 105 is configured to transmit a probe beam 112 through the optical cavity 120 such that the probe beam 112 passes through the selected nonlinear material 125 of the optical cavity 120, while not being reflected by reflecting devices 122 and 124. Presence of one or more photons 50 within the optical cavity 120 results in high intensity electric field, which in combination with the nonlinear properties of the selected nonlinear material 125 affects passage of the probe beam 112 and varies phase of the modified probe beam 114 with respect to the probe beam 112.

[0088] The detector unit 130 is configured to detect phase variations of the modified probe beam 114. In some embodiments, the detector unit 130 may receive the interference signal 118 and determine variations in the interference signal 118, indicating data on phase variations to the modified probe beam 114. The variations in the interference signal 118 are associated with a phase variation of the modified probe beam 114 and are therefore indicative of presence of one or more photons within the optical cavity 120, and also be indicative of a number of photons within the cavity 120. The detector unit 130 may be configured to provide homodyne detection of the interference signal 118. To this end the detector unit 130 may include one or more light detectors, beam splitter / combiners and a local oscillator, which are not specifically shown here. Additional detection techniques, such as heterodyne detection, or other detection techniques may also be used to enable measurement of small phase shifts of the modified probe beam.

[0089] The detector system 100 of the present disclosure may operate as a single-photon detector, or as a photon counting detector, distinguishing between a single photon, two photons, three photons etc. The detector system 100 can be configured as an all-optical detector system and may be configured in a chip-size dimension to be integrated as a single detector or an array of detectors within a selected chip-scale system and can operate at room temperatures. To this end, the optical cavity 120 is configured as a high-quality, high-finesse optical cavity, having an ultra-small mode volume 126, and such that the resonant mode of the optical cavity is tuned to wavelength of the photons to be detected 50. The cavity 120 may be defined by first 122 and second 124 reflectors where the selected nonlinear material 125 is located at least within the cavity 120, preferably overlapping with mode volume 126 of the cavity 120. In some embodiments, the optical cavity is configured with a mode volume 126 of the resonant mode, of no more than 0.01X3, or below 0.005X3, some calculated parameters relate to mode volume of 0.0025X3, or O.OO33Z or as small as 10’5X3, where X is the center wavelength of the detected photons 50.

[0090] The use of ultra-small mode volume 126 within the optical cavity 120 enhances the electric field and localized lifetime of photons 50 within the cavity, enabling measurable nonlinear interaction between the photon(s) within the cavity and the probe beam 112 passing through the cavity via the intermediate Kerr effect. The probe beam 112 may preferably utilize a wavelength associated with a lossy mode of the optical cavity. This means that the probe beam is not trapped within the optical cavity and consequently its effect on the index of refraction of the material is minimal. The presence of a selected nonlinear material 125, typically characterized by a Kerr nonlinearity, defined by the / 3’ parameter of electrical susceptibility of the material 125, varies the phase of the probe beam 112 because the nonlinear material changes its index of refraction in response to one or more photons 50 within the optical cavity 120. Selection of the quality factor and the mode volume of the optical cavity provide for enhancing the electric field of one or more photons within the cavity, thus causing a refractive index variation of the selected nonlinear material 125, which affects the effective optical path of the probe beam 112 passing through the cavity 120 and consequently its phase. The probe beam 112 may be a continuous wave beam, or formed of a sequence of short pulses, where spacing between the pulses is preferably shorter than the lifetime of a photon 50 within the optical cavity 120. As indicated above, the presence of one or more photons within the optical cavity change refractive index of the nonlinear material 125 by a factor of(3)E2(Ph) where E(Ph) is the electric field accumulated by one or more photons within the optical cavity, being increased in accordance with finesse and mode volume of the optical cavity.

[0091] The change in the optical path, and therefore change in the phase, between the modified probe beam 114 and the probe beam 112, can be detected and monitored using a reference beam 116 and by generating an interference signal 118 between the modified probe beam 114 and the reference beam 116. As indicated above, the detector unit 130 may utilize homodyne detection (or heterodyne detection or other suitable detection technique) of the interference signal 118 to provide sensitivity to very small phase changes in the modified probe beam 114, enabling the detection of phase variation of the order of 10’3radians or smaller.

[0092] Generally, the probe beam 112 may be selected to be of a relatively low intensity (e.g. within the microwatts range) and having wavelength outside of linewidth of the optical cavity resonance. This minimizes change in the refractive index of the nonlinear material 125 in response to the probe beam 112 itself and allows the probe beam 112 to be transmitted through the optical cavity.

[0093] In some embodiments, the detector system 100 may be configured as illustrated in Fig. 1, i.e. utilizing an X configuration of any angle between path of the photons to be detected and the probe beam 112. More specifically, the optical cavity 120 is positioned in the path of one or more photons 50 to be detected, intersecting with path of the probe beam 112. The photons 50 may propagate through a waveguide 150 from a photon source and are to be captured by the optical cavity 120 for a duration associated with finesse of the optical cavity, defining the lifetime of the photon within the cavity 120. The optical arrangement transmits a probe beam 112 in a direction intersecting with a direction of propagation of the photons 50 to be detected, such that the intersection is located within the optical cavity 120, at a region of the nonlinear material 125 within the cavity 120. Presence of one or more photons within the cavity changes the refractive index of the nonlinear material 125 in accordance with %(3)E2(Ph) term, where / 3’ is the third order term of electrical susceptibility of the material, and E(Ph) is the electric field accumulated by one or more photons within the optical cavity. The change in refractive index affects the speed of light or the effective optical path, and accordingly the phase of the modified probe beam 114 with respect to its phase in case there is no photon in the optical cavity 120. The phase variation may for example be determined using the interference signal 118, providing a measurable indication to presence of one or more photons within the cavity 120.

[0094] An additional configuration of the detector system 100 is exemplified in Fig. 2. Fig. 2 illustrates the optical cavity 120 positioned along a waveguide 150, providing propagation path to the photons 50 to be detected and also to the probe beam 112. As shown, waveguide section 150 may include a beam combiner 152 upstream of the optical cavity 120, configured to combine the photons 50 emitted from a photon source and the probe beam 112, e.g. provided by the light source 110.

[0095] In this configuration, the optical cavity 120 may preferably be configured to trap the photons to be detected 50, while allowing the probe beam 112 to pass through the cavity 120. Generally, the optical cavity is selected such that the wavelength of photons 50 is tuned to a resonant mode of the optical cavity 120, while the wavelength of the probe beam 112 is tuned to relate to a lossy mode of the cavity. In some examples, the reflecting elements 122 and 124 of the optical cavity 120 may be configured to be wavelength selective reflectors, allowing transmission of wavelength of the probe beam 112.

[0096] Accordingly, one or more photons 50 may be captured in the optical cavity 120 for a respective lifetime and affect the refractive index of the selected nonlinear material 125. This is while the probe beam 112 may pass through the optical cavity 120 and may be affected by the change in refractive index, varying phase of the modified probe beam 114 when one or more photons are trapped in the optical cavity 120, relative to the modified probe beam when no photons are trapped in the optical cavity 120. Furthermore, a phase variation of the modified probe beam 114 may be determined by the detector unit 130 to determine presence of one or more photons within the optical cavity 120.

[0097] Detection of a phase variation may be similar to that in the examples of Fig. 1. For example, the modified probe beam 114 may be mixed with reference beam 116, which propagates in a reference waveguide 160, to form an interference signal 118. The detector unit 130 may be positioned and configured to detect the interference signal 118 and provide output data indicative of presence of one or more photons within the cavity 120, and optionally to provide data on a number of photons within the cavity 120. It should be noted that as beams 112, 114, 116 and 118 include quite a significant photon flux (photon number per second), detection unit 130 may utilize standard available technology which may be miniaturized and operational at room temperature.

[0098] An exemplary configuration of an optical cavity having an ultra-small mode volume is illustrated in Fig. 3 showing an optical cavity 126 formed within a waveguide section 150 including a waveguide core 152 and surrounding / cladding 154. The optical cavity 120 is formed by a photonic crystal structure defined by an arrangement of a plurality of “holes” drilled in the waveguide core 152. The photonic crystal is formed in this example using a one-dimensional photonic crystal. It should however be noted that two-dimensional and even three-dimensional photonic crystals may also be used.

[0099] The optical cavity 120 of Fig. 3 may be formed in accordance with the description of Hyeongrak Choi, et al., “Self-Similar Nanocavity Design with Ultrasmall Mode Volume for Single-Photon Nonlinearities” Phys. Rev. Let. 118, 223605 (2017), Jin-Yue Su, et al. “Ultrafast all-optical switching in a silicon-polymer compound slotted photonic crystal nanobeam cavity” Opt. Rev. 30, 33-40 (2023), and Gbktug I§iklar, et al. “On the trade-off between mode volume and quality factor in dielectric nanocavities optimized for Purcell enhancement, ” Opt. Express 30, 47304-47314 (2022) listed above. The optical cavity may be formed of various selected materials including e.g., silicon, silicon- graphene hybrid, Germanium, Arsenic Trisulfide, Arsenic Triselenide, or various other materials exhibiting optical Kerr effect and having certain transparent window within the respective UV, visible and infrared wavelength ranges. Further, various organic and amorphic materials exhibit an optical Kerr effect and may be used as the nonlinear material 125 in some embodiments of the present disclosure.

[0100] In this exemplary configuration, the reflective elements 122 and 124 of the optical cavity 120 are formed of an arrangement of holes drilled / etched in the waveguide core 152. This configuration of the optical cavity enables to define an ultra-small mode volume 126. Additionally, the use of a photonic crystal enables the reflective elements 122 and 124 to be formed as wavelength selective reflective elements, allowing the probe beam 112 to pass through the optical cavity, while the photons to be detected 50 overlap in frequency a resonant mode of the cavity 120.

[0101] This is exemplified in Figs. 4A and 4B showing a COMSOL simulations of the resonant mode 52 in Fig. 4A and a COMSOL simulation of a lossy mode 113 in Fig. 4B within an optical cavity 120. Fig. 4A exemplifies a resonant mode 52 that may be occupied by one or more photons to be detected 50. Fig. 4B exemplifies a lossy mode 113, generally associated with the probe beam 112 passing through the optical cavity 120. As visible in Fig. 4A, the resonant mode 52 occupies a very small region within the optical cavity 120, defined by mode volume Vm126. The lossy mode 113 extends along the entire length of the optical cavity 120, including the reflective elements 122 and 124 formed by an arrangement of holes along the waveguide 150. Intensity of the optical field is listed by grayscale color in a bar beside the figure showing that the resonant mode reaches an intensity of ~7xl08(W / m2) due to amplification of the single photon field. This is while the lossy mode shows intensities that are lower by at least an order of magnitude, being up to 8xl07(W / m2) in response to a relatively weak probe beam intensity as described herein. Accordingly, light components associated with the lossy mode 113 are generally transmitted through the optical cavity 120 and can thus be used to probe the status of the optical cavity, and especially to determine if one or more photons occupy the resonant mode 50.

[0102] For example, some embodiments utilize an optical cavity 120 having length of L~1 pm, within a waveguide section having width of 550 nm and thickness of 220 nm as exemplified in Fig. 3 above. Here, the optical cavity 120 is configured as a photonic crystal utilizing an arrangement of a selected number of air holes (eighteen holes in this example) within a silicon waveguide core 152, having a resonant wavelength of k=l 577.8 nm. The mode volume 126 of the resonant mode of the optical cavity 120 was calculated to be Vm=Ie|E|2dv / max(s|E|2)=0.0033k2. In view of the ultra-small mode volume, the electric field intensity is very high and is calculated to be I=hc2 / ZVm=2.91xlO9W / m2for a single photon in the resonant mode of the optical cavity 120. The quality factor of the optical cavity 120 was calculated to be Q=Re(co) / 2Im(co)~6.35xlO4, and the finesse of the optical cavity was calculated to be F=ZQ / noL=2.88xlO4for this example. The quality factor and finesse parameters indicate a photon lifetime r=27iZ.Q / c=2. I O ns within a resonant mode of the optical cavity 120.

[0103] The high quality-factor (Q) and finesse (F) of the optical cavity 120 provide a relatively long lifetime for photons within the optical cavity 120 and give rise to a high amplitude of the electric field generated by such photons. These effects together with the nonlinear nature of the selected material 125 within the optical cavity 120, enable the detection of a single photon, or counting a number of photons within the optical cavity 120. On the other hand, the high finesse optical cavity results with a relatively narrow frequency linewidth of the optical mode, which in this specific example is calculated to be Av=0.48 GHz. Typically, to provide long lifetime in the cavity and efficient detection, it is preferable that the linewidth of the photon to be detected is narrow with respect to the linewidth of the detection equipment, or in this case, the linewidth of the optical cavity 120 used for detecting of single photons. Sosnicki, F., el al., “Interface between picosecond and nanosecond quantum light pulses”, Nat. Photon. 17, 761-766 (2023) describes a source for single photons having linewidth of 0.494+0.036 GHz around a central wavelength of 1560 nm, which is thus detectable by the detector system utilizing an optical cavity 120 as exemplified in Fig. 3.

[0104] It should be noted that the mode volume, quality factor and finesse of the optical cavity 120 are design parameters that can be selected in accordance with the selected detection scheme. For example, in some configurations utilizing a slot-bridge cavity, a proper design of the optical cavity provides mode volume of Vm=0.0025k2for the resonant mode. When providing an optical cavity with sufficiently small mode volume, the quality factor may be balanced by a selected linewidth range, maintaining the factor Q / Vm to be sufficiently high, to cause a measurable phase shift to the modified probe beam 114.

[0105] An additional parameter to determine the phase shift applied to the modified probe beam 114 by a single photon that gives rise to a resonant mode of the optical cavity 120, relates to Kerr coefficients of the nonlinear material 125. For example, at a wavelength of 1550 nm, the Kerr coefficient of silicon is approximately n2~4.5x l0-18m2 / W. Given the above parameters of the optical cavity 120, the use of silicon as nonlinear material 125 may provide a phase shift of cp=5.8xl0-8radians to the modified probe beam 114 after transmission across the optical cavity 120 (e.g. as exemplified in Fig. 1). Materials with higher nonlinearity may be used, providing an increased phase shift.

[0106] When within an optical cavity having a mode volume Vm, the electro-magnetic energy density of a single photon can be described by u = ha) / . Accordingly, the

[0107] 'vm intensity of the photon can be given by I = where c is the speed of light in vm 'iVm vacuum, h is Planck’s constant, h is h / 2it, co is the photon’s central frequency and the photon's central wavelength. Following the intensity calculation, the change in the refractive index of a material having a Kerr coefficient is An = n2X / = n2- — . This A-Vmin turn provides a phase shift applied to the modified probe beam 114 passing through the Kerr material to be A(p = 8nkL where k=27t / X of the probe beam and L is length of passage through the Kerr material. As indicated above, this calculation indicates a phase shift of (p=5.8xl0-8radians when using Silicon as the nonlinear material.

[0108] A more accurate model, presented in Yun-Feng Xiao, et al. “Quantum nondemolition measurement of photon number via optical Kerr effect in an ultra-high-Q microtoroid cavity”, Opt. Express 16, 21462-21475 (2008), may utilize a ratio of the coupling constant between the photons 50 occupying a resonant mode of the optical cavity 120, and the probe beam 112 transmitted through the optical cavity 120, considering its decay constant. Generally, the phase shift can be determined by <p = — where the coupling constant is given by K = isthe center frequency of the photon or photons to be detected and is the frequency of the probe beam 112, and the cavity decay constant is K = — where Qpis the quality factor of the optical cavity 120 with respect to the lossy mode associated with frequency of the probe beam 112. In this calculation, the phase shift takes into account the reduced speed of the probe beam 112 within the nonlinear material 125 of the optical cavity 120. As a result, the phase shift to the modified probe beam 114 is given by: where / 3)is the third order electrical susceptibility of the nonlinear material 125 relating to the Kerr coefficient of the material, and e0is the vacuum permittivity. For silicon, having n2=5xl018m2 / W, using the characteristics of the optical cavity defined above, the phase shift to the modified probe beam 114 in response to a single photon being resonant within the optical cavity 120 is calculated to be ^?=7.39x 10’7radians.

[0109] Utilizing the detector system configuration of the present disclosure, the inventors have calculated the expected phase shift induced by a single photon 50 within the optical cavity 120. In this exemplary calculation, the optical cavity 120 utilizes silicon (Si) as the nonlinear material 125. Additionally, the optical cavity 120 has length selected as L=1 pm and configured with resonant mode at k=l 577.8 nm. The probe beam 112 is selected in this non-limiting example to be within a wavelength range associated with a lossy mode of the optical cavity 120, for example at Z.p= 1228.4 nm. The optical cavity 120 thus provides quality factors being QR=6.4X104for the resonant mode associated with the wavelength the photons to be detected 50 and QL=45 for the lossy mode associated with the wavelength of the probe beam 112. Further, mode volume 126 of the optical cavity 120 was calculated for this example to be Vm=0.003X3for the resonant mode and Vm=0.12X3for the lossy mode.

[0110] In an additional example, the inventors have calculated parameters for an optical cavity 120 using germanium (Ge) as the nonlinear material 125 of the optical cavity 120. Again, the length of the optical cavity 120 is selected to be L=1 pm and the resonant mode is at X=2042.4 nm associated with the photons to be detected 50. The probe beam 112 is selected in this example to be within wavelength range of a lossy mode of the optical cavity 120. In this example the probe beam is selected to have wavelength of Z.p=2301 .2 nm. Quality factor of the resonant mode of the optical cavity 120 was calculated to be QR=1.5X104, and quality factor of the lossy mode associated with the wavelength of the probe beam 112 is QL=66. The mode volume was calculated for this example to be Vm=0.0046X3for the resonant mode and Vm=0.021X3for the lossy mode.

[0111] The above parameters for the optical cavity 120 examples utilize Silicon and Germanium as the nonlinear material 125 used for calculating an expected phase shift to the modified probe beam 114. Table 1 below lists phase shifts as calculated using several techniques and mode volumes including the above-described calculations, i.e. determining the phase shift in accordance with a reduced speed within the medium and determining the phase shift in accordance with the slot-bridge-slot cavity model including a reduced mode volume of the cavity described by Hyeongrak Choi, el al., “Self-Similar Nanocavity Design with Ultrasmall Mode Volume for Single -Photon Nonlinearities” Phys. Rev. Let. 118, 223605 (2017). As indicated above, the nonlinear material used within the optical cavity 120 may be selected in accordance with the material’s Kerr coefficient (or / 3’) and with transparency window of the material and wavelength of the photons to be detected 50. Fig. 5 shows transparent windows for selected materials, including materials that exhibit Kerr effect such as silicon (Si), germanium (Ge), arsenic trisulfide (AS2S3), and arsenic triselenide (As2Se3), and their respective transparent window. Fig. 5 also shows silicon nitride (SisN4) and silicon oxide (SiO2), which are transparent materials that do not exhibit Kerr nonlinearity. The light regions of Fig. 5 indicate that the material is transparent for the respective wavelength, and the dark regions indicate absorption regions. Fig. 5 further shows, using narrow dark bars, bandgap wavelength and half-bandgap wavelength for silicon, germanium, arsenic trisulfide and arsenic triselenide. As indicated above, these materials, as well as many other materials having Kerr nonlinearity and being transparent for the relevant wavelength ranges can be used as nonlinear material 125 within the optical cavity 120 of the present disclosure. Further research and development may uncover additional materials exhibiting high Kerr nonlinearity and may further increase sensitivity of the photon detector according to the present disclosure.

[0112] Table 1

[0113] The expected phase shifts for selected nonlinear materials including Silicon, Graphene- silicon hybrid, and Germanium are listed in Table 1. The phase shifts were calculated using two different models, as described above. The second row of table 1 is based on calculation in Yun-Feng Xiao, et al. “Quantum non-demolition measurement of photon number via optical Kerr effect in an ultra-high-Q microtoroid cavity”, Opt. Express 16, 21462-21475 (2008), providing the quality factor of the optical cavity for the lossy mode associated with wavelength of the probe beam 112, Vmis the mode volume determined for the resonant mode and co is the frequency of the photons of the resonant mode, using the mode volumes the inventors calculated themselves from a rough (not-optimized) simulation, as described above. The third row in table 1 uses the same equation for <p as in the second row, and is based on a slot-bridge-slot configuration having a reduced mode volume in accordance with the description of Hyeongrak Choi, et al., “Self-Similar Nanocavity Design with Ultrasmall Mode Volume for Single-Photon Nonlinearities” Phys. Rev. Lett. 118, 223605 (2017) and utilizes a reduced mode volume of Vm = 7xl0-5X3. Furthermore, while various existing materials exhibit Kerr nonlinearity and provide certain transparency window, development of materials having greater Kerr coefficients and / or larger transparency windows may further improve the sensitivity of the detector system of the present disclosure. It should however be noted that the available materials already provide sufficient sensitivity for detection of single photons within transparency windows of the materials as listed above. For example: (a) Ruppert, L., et al. “High-precision multiparameter estimation of mechanical force by quantum optomechanics”, Sci. Rep. 12, 16022 (2022) describes detection of phase shift of the order of 3.16X 10’4radians; (b) ChaoXia Zhang, et al. “Small-tilt measurement based on weak- value-amplification with balanced homodyne detection”, Appl. Phys. Let. (2023) describes detection of phase shift of the order of 4X 10’9radians; (c) Likun Zhou et al., Commun. Theor. Phys. 74, 125104 (2022) describes detection of phase shift of the order of ~102radians; (d) Hui Liao, et al. “Precision vibration measurement using differential phase-modulated homodyne interferometry”, Optics and Lasers in Engineering 169, 107695 (2023) describes detection of phase shift of the order of 5.6X10’6radians; and (e) Maximilian Protte, et al. “Low-noise balanced homodyne detection with superconducting nanowire single-photon detectors”, Optica Quantum 2, 1-6 (2024) describes detection of phase shift of the order of 1.3x 10’3radians. All these documents are incorporated herein by reference with respect to techniques for determining a phase shift of the optical signal. Additional nonlinear materials, including e.g., organic nonlinear materials, may be used, providing advantages such as chip-scale fabrication simplicity and low costs, according to some embodiments of the present disclosure.

[0114] While the photons 50 can be detected in accordance with their interaction through the nonlinear material 125, the probe beam 112 by itself preferably does not affect the nonlinear material 125, providing a stable zero measurement and selectivity of measurement of photons 50. This can be achieved using selectivity of the optical cavity 120, which may be achieved in several manners. As indicated above, the probe beam 112 may be transmitted in a path intersecting with path of the photons 50, such that the propagation path of the probe beam 112 intersects the axis of the optical cavity 120. Additionally, or alternatively, the wavelength of the probe beam 112 may be selected to relate to a lossy mode of the optical cavity 120 as illustrated in Fig. 4B above. This may be achieved for example, using reflective surfaces 122 and 124 configured and engineered to be wavelength selective, thus providing high reflection with respect to the wavelength of the photons 50, while allowing transmission of the wavelength of probe beam 112.

[0115] Moreover, in some situations utilizing a selected nonlinear material 125 within the optical cavity 120 may still result in nonlinear effects acting on the probe beam 112 in response to the probe beam 112 passing through the nonlinear material 125, i.e. induce a phase shift without the presence of one or more photons 50 within the cavity 120. The present disclosure may utilize two main techniques to eliminate, or at least significantly reduce phase shift applied to the modified probe beam 114 that is not associated with photons to be detected 50 being in the optical cavity 120. One solution relates to selection of wavelength of the probe beam 112 to be significantly shifted from the resonant wavelength of the optical cavity 120 so that it occupies a lossy mode of the optical cavity 120. This eliminates, or at least significantly reduces amplification of nonlinear effects by “back and forth” passage of the probe beam 112 within the optical cavity 120. A second solution utilizes a probe beam which intersects with the optical mode of the cavity in an angle, as described above with reference to Fig. 1. In this configuration, the optical cavity does not affect the probe beam 112 as the path of the probe beam 112 does not go through the reflecting elements of the optical cavity 120. Finally, depending on the quality of the detector 130, the detector system 100 may utilize a relatively low intensity probe beam 112.

[0116] As described above, the probe beam 112 is used to determine presence of photons within the optical cavity by undergoing a phase shift due to nonlinearity of the material within the cavity. Reference is made to Fig. 6 showing a calculated relation between a phase shift applied to the modified probe beam 114 due to presence of a photon within the cavity, which creates a change of index of refraction An, and a phase shift applied to the modified probe beam 114 due to the probe beam 112 acting on the nonlinear material within the cavity Anp=n2’Iprobe.

[0117] As shown in Fig. 6 The lower the intensity of the probe beam 112, the lower the relation Anp / An, indicating that the phase shift resulting from the photons to be detected 50 within the optical cavity 120 affects the phase of the modified probe beam 114 to an extent larger than the phase shift of the modified probe beam 114 as a result of the probe beam 112 passing through the optical cavity 120. For example, while using a probe beam 112 having intensity around 10’3W may result in a comparable phase shift, reducing the intensity of the probe beam 112 to 10’8W reduces the ratio between the phase shifts by over four orders of magnitude, and thus improves sensitivity and specificity of detection.

[0118] The data in Fig. 6 is determined in accordance with an assumption that the intensity of a single photon is given by 2.9 lx 109W / m2, which is calculated for a cavity of 18 holes with a mode volume of Vm=0.0033X3, a wavelength of 1577.8 nm, a cavity length of 1 m and a waveguide cross section of 550 nm x 220 nm. The probe beam intensity was calculated to be the probe beam power (x-axis of Fig. 6) divided by the area of 550 nm x 220 nm.

[0119] Using a more accurate analysis, in accordance with the above, an estimation of the self-phase modulation induced by the probe beam 112 can be given as <pp= 16 - where Kpis the coupling between the photons of the probe beam 112 induced by the nonlinear material 125. Accordingly, the self-induced phase shift applied on the modified i i i i i z5a probe beam 114 is estimated by cpp where the sub-note p relates to the respective parameters associated with the wavelength of the probe beam 112. This enables to determine an expected ratio between self-induced phase shift of the probe beam 112 and phase shift induced to the probe beam by a single photon 50 associated with a resonant mode of the optical cavity 120 as:

[0120] Where subscript ‘p’ indicates parameters of the probe beam 112 and parameters without the subscript relate to the photons to be detected 50, being associated with the resonant mode of the optical cavity 120.

[0121] Using exemplary parameters noted above, the ratio between self-induced phase shift and phase shift associated with detection of a single photon, for a probe beam 112 power of 1 pW or below and using Silicon as the nonlinear material 125, is determined = 0.034. Accordingly, the self-induced phase shift may be around 3% of the phase induced by a single photon 50 exciting a resonant mode of the optical cavity 120.

[0122] Accordingly, the present disclosure provides a photon detector system 100 capable of detection of a single-photon 50, as well as operate as a photon counter by differentiating between detection of a single photon, two photons, three photons, etc.. The detector system 100 utilizes a high finesse optical cavity 120 carrying a selected nonlinear material 125 in a position overlapping with a resonant mode of the optical cavity 120, such that photons 50 occupying the resonant mode of the optical cavity 120 interact with the nonlinear material 125, affecting its refractive index (in the case of Kerr nonlinearity). The system 100 further utilizes a probe beam 112 transmitted through the nonlinear material 125 of the optical cavity 120, and an optical detection arrangement configured to detect phase shifts to the modified probe beam 114 after passing through the nonlinear material 125 of the optical cavity 120. The presence of one or more photons 50 within the optical cavity 120 affects the effective optical path of the probe beam 112 due to change in refractive index of the material 125, providing the modified probe beam 114 with a phase shift that is proportional to the number of photons 50 within the optical cavity 120.

[0123] This configuration enables a miniature chip-scale detection system which may operate at room temperature. It may be integrated with available fabrication technology to enable arrays of such detectors on a single chip. Furthermore, it eliminates down time, which is a typical characteristic of conventional single -photon detectors. This is since the Kerr nonlinearity of the selected material 125 within the optical cavity 120 has a lifetime that is directly associated with duration of presence of the photon 50 within the optical cavity 120. Additionally, if two or more photons 50 are captured within the optical cavity 120, the field intensity accumulates, resulting in a phase shift proportional to the number of photons 50. This enables the detector system 100 of the present disclosure to operate as a high-speed photon detector, as well as a photon counter detecting a number of photons and enabling to determine the number of photons captured in the optical cavity 120 in accordance with phase shift of the modified probe beam.

[0124] The detector system may utilize an arrangement of a selected number of detector systems 100 as described above, enabling detection of photons within a selected spatial distribution and increasing detection probability.

[0125] It is to be noted that the various features described in the various examples can be combined according to all possible technical combinations.

[0126] It is to be understood that the invention is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The invention is capable of other examples and of being practiced and carried out in various ways. Hence, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception upon which this disclosure is based can readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the presently disclosed subject matter.

[0127] Those skilled in the art will readily appreciate that various modifications and changes can be applied to the examples of the invention as hereinbefore described without departing from its scope, defined in and by the appended claims.

Claims

CLAIMS:

1. A single photon detector system comprising: an optical cavity configured for capturing photons to be detected, the optical cavity comprises a selected nonlinear material within a region of the cavity; an optical arrangement configured for transmitting a probe beam through the optical cavity to obtain a modulated probe beam, and for interfering the modulated probe beam with a reference beam to obtain an interference signal; and at least one detector configured for detecting the interference signal, wherein variation in the interference signal is indicative of the one or more photons within the optical cavity.

2. The single-photon detector system of claim 1, wherein the optical cavity is characterized by a mode volume below 0.01X3, where is a wavelength of the one or more photons to be detected.

3. The single-photon detector system of claim 1 or 2, wherein the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected.

4. The single-photon detector system of any one of claims 1 to 3, wherein the optical cavity is formed by a photonic crystal cavity within a waveguide section.

5. The single-photon detector system of any one of claims 1 to 4, wherein the probe beam propagates in a direction intersecting with a direction of propagation of the photons to be detected, wherein an intersection between the probe beam and the photons to be detected is located within the optical cavity.

6. The single-photon detector system of any one of claims 1 to 4, wherein the optical cavity is wavelength selective, the probe beam and photons to be detected propagate through a common waveguide comprising the optical cavity.

7. The single-photon detector system of claim 6, wherein the optical cavity comprises wavelength selective reflecting elements, selected to allow transmission of a wavelength of the probe beam.

8. The single-photon detector system of claim 6 or 7, wherein a wavelength of the probe beam is selected to be within wavelength giving rise to a lossy mode of the optical cavity.

9. The single-photon detector system of any one of claims 1 to 8, operable at room temperature.

10. The single-photon detector system of any one of claims 1 to 9, configured as a system on a chip.

11. The single-photon detector system of any one of claims 1 to 10, wherein power of the probe beam is selected to be up to lOpW.

12. The single-photon detector system of any one of claims 1 to 11, wherein detection of the interference signal is indicative of a phase shift of the modified probe beam, and wherein the phase shift of the modified probe beam is indicative of a presence and a number of photons trapped within the optical cavity.

13. The single-photon detector system of any one of claims 1 to 12, configured in a micrometer scale.

14. The single-photon detector system of any one of claims 1 to 13, wherein the optical cavity is a linear optical cavity.

15. A detector array comprising a plurality of single -photon detectors according to any one of claims 1 to 14, embedded within a photonic circuit.

16. A detector system comprising:(a) a first waveguide section positioned for accepting photons to be detected;(b) an optical cavity within the first waveguide section, the optical cavity comprising a selected nonlinear material having selected nonlinear properties located to overlap a mode volume of a resonant mode of the optical cavity;(c) a probe beam arrangement comprising at least one probe waveguide section configured for directing a probe beam through the optical cavity, and collecting a modified probe beam exiting the optical cavity;(d) a detector arrangement configured for detecting an interference signal, formed by interference of the modified probe beam with a reference beam; wherein presence of one of more photons within the optical cavity vary refractive index of the selected nonlinear material, thereby affecting a phase of the modified probe beam with respect to the reference beam.

17. The detector system of claim 16, wherein mode volume of the optical cavity with respect to a wavelength of the photons to be detected is below 0.01X3, where X is the wavelength of the one or more photons to be detected, and wherein the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected.

18. The detector system of claim 16 or 17, having a single photon detection capability.

19. The detector system of any one of claims 16 to 18, wherein the first waveguide section and probe waveguide section are overlapping at a region of the optical cavity.

20. The detector system of any one of claims 16 to 18, wherein the first waveguide section and probe waveguide section are intersecting at a location of the optical cavity.

21. The detector system of any one of claims 16 to 20, wherein a resonant mode of the optical cavity is tuned to the wavelength of the photons to be detected, and wherein a wavelength of the probe beam is tuned to a lossy mode of the optical cavity.

22. The detector system of any one of claims 16 to 21, configured in a micrometer scale.

23. The detector system of any one of claims 16 to 22, wherein the optical cavity is a linear optical cavity.

24. A method for detection of photons, comprising:(a) providing an optical cavity having a resonant mode associated with a wavelength of the photons to be detected, and comprising a selected nonlinear material located to overlap a mode volume of the optical cavity;(b) transmitting a probe beam through the optical cavity to obtain a modified probe beam, the probe beam having a wavelength giving rise to a lossy mode of the optical cavity;(c) detecting a phase shift of the modified probe beam with respect to the probe beam; and(d) determining a presence and a number of photons to be detected in accordance with the phase shift of the modified probe beam.

25. The method of claim 24, further comprising using one or more waveguides to direct the photons to be detected to be trapped by the optical cavity.

26. The method of claim 24, wherein the selected nonlinear material is characterized by an optical Kerr nonlinearity within a wavelength range of the photons to be detected.

27. The method of claim 24 or 25, wherein said providing an optical cavity comprises providing an optical cavity characterized by an ultra- small mode volume, being below 0.01 Z.3, where is a wavelength of the one or more photons to be detected.

28. The method of any one of claims 24 to 26, wherein said detecting a phase shift of the modified probe beam comprises generating an interference signal by interfering the modified probe beam with a reference beam and determining variation in intensity of the interference signal.

29. The method of claim 27, comprising utilizing a homodyne detection technique.

30. The method of any one of claims 24 to 28, wherein said transmitting a probe beam through the optical cavity comprises transmitting the probe beam through a waveguide intersecting with a path of the photons to be detected at a location of the optical cavity.

31. The method of any one of claims 24 to 28, wherein said transmitting a probe beam through the optical cavity comprises transmitting the probe beam through a common waveguide as the photons to be detected, passing through the optical cavity.

32. The method of any one of claims 24 to 30, wherein said transmitting a probe beam through the optical cavity comprises transmitting a plurality of pulses of the probe beam, where a spacing between the pulses is shorter than a lifetime of a photon to be detected within the optical cavity.

33. The method of any one of claims 24 to 30, wherein said transmitting a probe beam through the optical cavity comprises transmitting a continuous wave probe beam.

34. The method of any one of claims 24 to 32, wherein said transmitting a probe beam through the optical cavity comprises transmitting the probe beam having intensity not exceeding 10’5W.

35. The method of any one of claims 24 to 33, wherein said providing and optical cavity comprises providing a photonic crystal forming an optical cavity.

36. The method of any one of claims 24 to 33, wherein said providing and optical cavity comprises providing a linear optical cavity.

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