Photonic circuit for single photon detection

Abstract

Photonic Circuit for Single-Photon Detection. The invention relates to a photonic circuit (Circ) for single-photon detection operating at room temperature and integrated on a silicon substrate (Sub). In particular, the invention relates to a photonic circuit (Circ) for the detection of single photons at wavelengths greater than 1 µm at room temperature. More specifically, the invention relates to a compact photonic circuit (Circ) for single-photon detection comprising silicon waveguides (G1, G2, G3, G4, G5). Figure for the abstract: Fig. 1

Description

Title of the invention: Photonic circuit for the detection of single photons Technical field

[0001] The invention relates to a photonic circuit for the detection of single photons operating at room temperature and being integrated on a silicon substrate. In particular, the invention relates to a photonic circuit for the detection of single photons at wavelengths greater than 1 pm at room temperature. More particularly, the invention relates to a compact photonic circuit for the detection of single photons comprising silicon waveguides. STATE OF THE ART

[0002] Single photon detectors are currently used in a wide variety of fields, including various biological applications (time-correlated photon counting, fluorescence measurement, single molecule detection, environmental analyses), laser rangefinders, quantum cryptography, CMOS testing, and will be required in the future for quantum computing. These applications sometimes require complex circuits with multiple single photon detectors.

[0003] Furthermore, the detectors used in the state of the art require bulky assembly elements such as liquid helium cryostats. Indeed, efficient single photon detectors exist for the telecommunications wavelength (i.e. greater than or equal to 1 pm) but operate at cryogenic temperatures.

[0004] There are single photon detectors that are efficient at room temperature, such as silicon avalanche photodiodes, but these single photon detectors operate in the visible wavelengths.

[0005] Thus, there are currently no compact and integrated single photon detectors operating efficiently at room temperature for telecommunications wavelengths.

[0006] For example, the publication by Ni Yao et. al, Vol. 28, No. 17 / 17 August 2020 / Optics Express 25123 discloses a system for detecting single photons at telecommunication wavelengths operating at room temperature. However, the system is a free-space propagation system comprising large independent elements such as bulk periodically polarized lithium niobate (so-called "bulk"). In addition, the independent elements are connected by optical fibers making it impossible to make the system compact. This system is therefore not suitable for use in quantum computers, for example.

[0007] Indeed, the integration of single photon detectors on an integrated photonic platform has only been demonstrated for superconducting detectors requiring cooling to cryogenic temperatures. SUMMARY

[0008] In order to overcome the aforementioned drawbacks of systems for the detection of single photons, the invention proposes a photonic circuit for the detection of single photons having a telecommunication wavelength (i.e. at wavelengths greater than 1 pm) and operating at room temperature. The photonic circuit operates with a waveguide system and is integrated on a silicon substrate.

[0009] To this end, the invention has as its first object a photonic circuit for the detection of single photons, the photonic circuit being integrated on a silicon substrate, the photonic circuit comprising: - a single photon input configured to receive a first light signal comprising a single photon having a first wavelength greater than or equal to 1 pm, the single photon input comprising a first silicon waveguide; - a laser source emitting a second laser light signal having a second wavelength greater than or equal to 1 pm comprising a second silicon waveguide; - a combiner coupled to the first and second waveguides and configured to generate a third light signal superimposing the first light signal and the second light signal, the combiner comprising at least one third silicon waveguide; - a wavelength converter configured to generate a fourth light signal having a visible wavelength, by non-linear effect in a fourth waveguide made of a first material; - an avalanche photodiode configured to detect the fourth light signal and comprising a fifth silicon waveguide; - a first coupler configured to couple the third and fourth waveguides; and - a second coupler configured to couple the fourth and fifth waveguides.

[0010] The photonic circuit according to the invention thus makes it possible to use a silicon waveguide both to guide the single photon having a wavelength greater than or equal to 1 pm and for the detection of the photon in the visible.

[0011] Advantageously, the use of an avalanche photodiode for the detection of a photon emitted at a wavelength greater than or equal to 1 pm makes it possible to detect photons with telecommunication wavelengths without the need for cryogenic cooling. In fact, the photonic circuit can be used at room temperature.

[0012] Furthermore, the invention makes it possible to produce a single integrated silicon circuit comprising the detector (avalanche photodiode), the guidance and the wavelength combiner.

[0013] The photonic circuit operating at room temperature and being integrated on silicon with a waveguide system, it therefore makes it possible to produce a compact system usable in numerous applications such as quantum computers.

[0014] In one embodiment, the avalanche photodiode comprises a filter configured to filter a residual portion of the second light signal.

[0015] The filter allows the remaining light signal from the laser source to be filtered in order to avoid non-linear effects.

[0016] In one embodiment, the first material comprises a thin layer of lithium niobate or silicon nitride.

[0017] The thin layer of lithium niobate or silicon nitride allows the wavelength to be converted while allowing the circuit to be compact. Indeed, as indicated above, for example in the publication Ni Yao et. al, a bulky non-linear material is used.

[0018] In one embodiment, the third waveguide comprises a tip and the first coupler is formed by said tip inserted into the fourth waveguide.

[0019] In one embodiment, the first coupler is formed by a contact between the third guide and the fourth waveguide, the coupling being carried out by evanescent wave.

[0020] In one embodiment, the fourth waveguide comprises a tip and wherein the second coupler is formed by said tip inserted into the fifth waveguide.

[0021] In one embodiment, the second coupler is formed by a contact between the fourth waveguide and the fifth waveguide.

[0022] In one embodiment, the photonic circuit comprises a third coupler configured to couple the second and third waveguides.

[0023] The couplers make it possible to couple the waveguides while limiting light signal losses as much as possible.

[0024] In one embodiment, the laser source comprises a III-V material.

[0025] The second subject of the invention is a chip comprising one or more photonic circuits according to the first subject, the photonic circuit(s) being on the same silicon substrate and being arranged so as to form a Qbit.

[0026] Advantageously, the chip comprising the photonic circuits makes it possible to produce a complete quantum circuit on silicon which can be used for example in quantum computers.

[0027] Thus, one of the aims of the invention is to obtain a photonic circuit for the detection of single photons having telecommunications wavelengths without requiring cryogenic cooling. In addition, one of the aims is to obtain a compact, silicon-integrated system that can be used in quantum computers.

[0028] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered. Brief description of the drawings

[0029] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the figures among which:

[0030] [Fig-1] [Fig. 1] illustrates a photonic circuit for the detection of single photons according to the invention;

[0031] [Fig.2a] [Fig.2a] illustrates an example of a photonic circuit for the detection of single photons according to the invention;

[0032] [Fig.2b] [Fig.2b] illustrates an example of a photonic circuit for the detection of single photons according to the invention;

[0033] [Fig.3a] [Fig.3a] illustrates an example of a laser source according to the invention, seen in section;

[0034] [Fig.3b] [Fig.3b] illustrates an example of a coupled combiner according to the invention, seen in section;

[0035] [Fig.3c] [Fig.3c] illustrates an example of a wavelength converter according to the invention, seen in section;

[0036] [Fig.3d] [Fig.3d] illustrates an example of an avalanche photodiode according to the invention, seen in section;

[0037] [Fig.4a] [Fig.4a] illustrates an example of a first coupler according to the invention; and

[0038] [Fig.4b] [Fig.4b] illustrates an example of a second coupler according to the invention.

[0039] [Fig.5] [Fig.5] illustrates an example of a chip according to the invention. DETAILED DESCRIPTION

[0040] The invention relates to a Cire photonic circuit for the detection of single photons. [Fig.l] illustrates an example of a Cire photonic circuit for the detection of single photons. The Cire photonic circuit is integrated on a silicon Sub substrate.

[0041] The photonic circuit Cire comprises a single photon input Pho configured to receive a first light signal Fl comprising a single photon having a first wavelength XI greater than or equal to 1 pm, the single photon source Pho comprising a first silicon waveguide G1. The single photon input Pho is connected to a source emitting the single photons (not shown in [Fig.l]). The single photon source may be integrated on the silicon substrate Sub or be a source external to the photonic circuit Cire. When the single photon source is external to the photonic circuit Cire, a coupler may be used to couple the single photons with the single photon input Pho. A first example is a grating-type coupler and a second example is a cone coupler having a width decreasing according to the direction of propagation (called a “taper”).The first wavelength XI is chosen to be greater than or equal to 1 pm because at this wavelength silicon is transparent. According to one embodiment the first wavelength XI is of the order of 1550 nm.

[0042] The Cire photonic circuit also comprises a laser source P emitting a second laser light signal F2 having a second wavelength X2 greater than or equal to 1 pm comprising a second silicon waveguide G2. As illustrated in [Fig.3a]) representing the laser source P in section, according to one embodiment the laser source P comprises an IILV material. In this case the IIL V material is included in the main part of the laser, i.e. the gain section, which is composed of a superposition of an IILV heterostructure (including for example quantum wells) and the second silicon waveguide G2 to support a hybrid optical mode. The laser cavity is closed by photonic structures typically defined in the second waveguide G2, such as gratings, rings or Sagnac loops.

[0043] According to one embodiment, the laser source P is distributed feedback (DFB). In this case, the grating is etched in the silicon waveguide of the gain section. According to one embodiment as illustrated in [Fig.2a] representing an example of a Cire photonic circuit, the second light signal F2 generated by the laser source P is produced in a hybrid waveguide (IILV material on silicon) and is transferred to the second silicon waveguide G2 using a cone coupler C3 having an increasing width according to the propagation direction (called "taper") in the silicon and / or the IILV material. For example, the second waveguide G2 forms a tip and the coupler C3 forms a tip inserted in the hybrid waveguide. As illustrated in [Fig.3a]), according to one embodiment, the second waveguide G2 is integrated in a planar layer of silicon dioxide placed on the silicon substrate Sub.The second wavelength X2 is chosen to be greater than or . equal to 1 pm because at this wavelength, silicon is transparent. According to one embodiment, the second wavelength / .2 is of the order of 1310 nm.

[0044] The photonic circuit Cire also comprises a combiner Comb coupled to the first and second waveguides Gl, G2 and configured to generate a third light signal F3 superimposing the first light signal Fl and the second light signal F2, the combiner Comb comprising at least one third silicon waveguide G3. In particular, the first light signal Fl and the second light signal F2 enter the input of the combiner Comb. The combiner Comb generates the third light signal F3 comprising the single photon at the first wavelength X1 and the light signal from the laser source P at the second wavelength X2. As illustrated in [Fig.3b]) representing the combiner Comb, according to one embodiment the third waveguide G3 is integrated in a planar layer of silicon dioxide placed on the silicon substrate Sub.In order to superimpose the first light signal F1 and the second light signal F2, according to one embodiment the coupled combiner Comb comprises an asymmetric Mach-Zehnder interferometer defined in the third waveguide G3. According to another embodiment the coupled combiner Comb comprises a wavelength multiplexer. In another example, the coupled combiner Comb comprises a coupler.

[0045] The photonic circuit Cire also comprises a wavelength converter Conv configured to generate a fourth light signal F4 having a visible wavelength Xvis, by non-linear effect in a fourth waveguide G4 made of a first material. The wavelength converter Conv receives the third light signal F3 comprising the first light signal Fl and the second light signal F2, each having a wavelength greater than 1 pm. The third light signal F3 passes through the first material.

[0046] According to a first embodiment, the first material comprises a thin layer of lithium niobate or silicon nitride. In a preferred embodiment, the thin layer of lithium niobate is periodically polarized (“periodically poled”). The converter converts, by non-linear effect, the third light signal F3 into a fourth light signal F4 having a visible wavelength Xvis. The use of a thin layer of lithium niobate or silicon nitride allows the production of a Cire photonic circuit with greater compactness than circuits using periodically polarized bulk lithium niobates (called “bulk”) typically used as free-space wavelength converters.

[0047] According to a second embodiment, as illustrated in [Fig.2b], the wavelength converter Conv comprises a ring resonator Res. Combining the converter which converts by non-linear effect thanks to, for example the thin layer of silicon nitride, with the Res ring resonator, allows to lower the power required for wavelength conversion.

[0048] As illustrated in [Fig.3c]) representing the wavelength converter Conv seen in section, according to one embodiment the fourth waveguide G4 in first material is integrated in a planar layer of silicon dioxide placed on the silicon substrate Sub.

[0049] The photonic circuit Cire also comprises a first coupler Cl configured to couple the third waveguide G3 with the fourth waveguide G4. According to a first embodiment illustrated in [Fig.4a], the first coupler Cl is a cone coupler having a width decreasing according to the direction of propagation (called "taper"). In this case, the third waveguide G3 comprises a tip and in which the first coupler Cl is formed by said tip inserted into the fourth waveguide G4. According to a second embodiment illustrated in [Fig.4b], the first coupler Cl is a proximity (distance less than a micron) or contact between the third waveguide G3 and the fourth waveguide G4. For example, the third waveguide G3 is placed on the fourth waveguide.This proximity or contact coupling forming the first coupler Cl couples the third and fourth waveguides G3, G4 by evanescence, and thus allows the third light signal F3 to propagate in the wavelength converter Conv. In another example, the third waveguide G3 is interrupted and placed behind in contact with the input of the fourth waveguide G4, (called "butt-coupling").

[0050] The Cire photonic circuit also comprises an avalanche photodiode APD configured to detect the fourth light signal and comprising a fifth silicon waveguide G5. According to an embodiment illustrated in [Fig.3d]) representing the avalanche photodiode APD seen in section, the fifth waveguide G54 made of first material is integrated in a planar layer of silicon dioxide placed on the silicon substrate Sub. According to one embodiment, the avalanche photodiode APD comprises a PIN junction defined in the fifth silicon waveguide G5. The fifth silicon waveguide G5 may be doped. According to one embodiment, the avalanche photodiode APD operates in Geiger mode so that the absorption of the fourth light signal F4 triggers a significant electrical pulse. The fourth light signal F4, resulting from the transformation of the light signal F1 comprising a single photon, is thus detected by the avalanche photodiode APD.

[0051] The photonic circuit Cire also comprises a second coupler C2 configured to couple the fourth waveguide G4 with the fifth waveguide G5. It is noted that the examples of Figures 4a and 4b illustrating the first coupler C1 can also represent the second coupler C2. In particular, according to one embodiment embodiment the second coupler C2 is a cone coupler having a width decreasing according to the direction of propagation (called "taper"). In one example, the fourth waveguide G4 comprises a tip and in which the second coupler C2 is formed by said tip inserted the fifth waveguide G5. . According to another embodiment the second coupler C2 is formed by a contact between the fourth waveguide G4 and the fifth waveguide G5. For example, the third waveguide G3 can be placed on the fourth waveguide. The second coupler C2 makes it possible to couple the fourth and fifth waveguides G4, G5 by evanescence and thus to propagate the fourth light signal F4 in the avalanche photodiode APD.

[0052] In an example, as illustrated in [Fig.2a] representing an example of a Cire photonic circuit for the detection of single photons according to the invention, according to one embodiment the avalanche photodiode APD comprises a filter F configured to filter a residual part of the second light signal F4. In principle, the wavelength conversion performed by the wavelength converter Conv is optimized for certain wavelengths and the avalanche photodiode APD is only capable of detecting high-energy single photons. However, if the remaining light signal from the laser source P has a high energy, it could generate in the avalanche photodiode other nonlinear effects such as two-photon absorption, and generate a dark current. For this reason, the filter F is used to filter the remaining light signal from the laser source P. According to one embodiment the filter F comprises for example coupled ring resonators..

[0053] It is noted that the Cire photonic circuit described above comprises several planar layers stacked on top of each other, as illustrated in Figures 3a-d. Indeed, the Cire photonic circuit comprises at least one planar silicon layer (the substrate Sub) and a layer in which the waveguides G2-G5 are integrated, for example a planar silicon dioxide layer.

[0054] In order to maximize detection efficiency, the converted single photon must have an energy corresponding to the maximum detection efficiency of the avalanche photodiode.

[0055] According to one example, the maximum detection efficiency of the avalanche photodiode is of the order of 1.8 eV (700 nm). With the sum frequency generation, in order to detect a single photon having an energy of 0.8 eV (corresponding to a wavelength of 1550 nm), it is possible to use a laser source P of 0.95 eV (corresponding to a wavelength of 1310 nm):

[0056] 0.8 eV (1550 nm) + 0.95 eV (1310 nm) = 1.75 eV (708 nm).

[0057] Similarly, it is possible to detect a single photon having a wavelength of 1310 nm with a P laser source having a wavelength of 1550 nm.

[0058] In the case where it is desired to minimize dark counts or eliminate the need for a rejection filter of the laser source P, according to one embodiment a laser source P is used with a longer wavelength so that the energy of two photons is not sufficient to reach the band gap of the silicon. For example, the laser source P has a wavelength of 2200 nm and the single photon has a wavelength of 1310 nm:

[0059] 0.56 eV (2200 nm) + 0.95 eV (1310 nm) = 1.51 eV (822 nm)

[0060] It is also possible to use four-wave mixing to convert the frequency of the photon. In this case, the converted photon has an energy equal to:

[0061] 2 X Epump- Ephoton

[0062] where Epump is the energy of the laser source P and Ephoton is the energy of the single photon.

[0063] For example, if the energy of the laser source P is 1.08 eV (for a wavelength of 1150 nm) and the single photon has an energy of 0.8 eV (corresponding to a wavelength of 1550 nm), the energy of the converted photon is 1.36 eV (corresponding to a wavelength of 912 nm).

[0064] As illustrated in [Fig.5], the invention also relates to a chip Ch comprising one or more Cire photonic circuits according to the examples described above. The Cire photonic circuit(s) being on the same silicon substrate and being arranged so as to form a Qbit. The chip comprising the photonic circuits makes it possible to produce a complete quantum circuit on silicon and can be used for example in quantum computers. For example, the chip can be used as a processor of a quantum computer.

[0065] The Cire photonic circuit can be obtained using different manufacturing processes.

[0066] , According to a first variant, the manufacturing method comprises the steps following: - a thin layer of lithium niobate is deposited on an oxide layer on a silicon substrate. - The fourth G4 waveguide is etched and embedded in planarized silicon oxide. A layer of silicon on insulator is reverse-bonded to a layer of planarized silicon oxide. - The silicon substrate and the oxide layer are removed. The silicon waveguides G1, G2, G3, G5 are etched (including doping for APD) and integrated into a planarized silicon oxide layer. - An IILV heteroepitaxy wafer is bonded upside down onto the planarized silicon oxide layer. - The IILV substrate is removed and the IILV waveguides are etched.

[0067] According to a second variant, the manufacturing process comprises the following steps:

[0068] - the silicon waveguides are etched on a silicon-on-insulator wafer (y including doping for ODA).

[0069] -The silicon waveguides G1, G2, G3, G5 are integrated in planarized silicon oxide.

[0070] -The thin layer of lithium niobate is deposited on a silicon substrate which is bonded reverse side onto the etched silicon on insulator.

[0071] - The silicon substrate and the oxide layer are removed.

[0072] - The fourth lithium niobate G4 waveguide is etched and integrated into planarized silicon oxide.

[0073] - The resulting stack is returned onto a silicon oxide substrate.

[0074] - The silicon wafer and planarized silicon oxide are removed.

[0075] - A III-V heteroepitaxy wafer is glued onto the stack.

[0076] - The III-V wafer is removed and the III-V waveguide is etched.

[0077] According to one embodiment, different stacks can be manufactured and deposited by transfer printing.

[0078] , According to one embodiment, the silicon oxide is replaced by benzocyclobutene.

[0079] Advantageously, the Cire photonic circuit presented above thus makes it possible to use a silicon waveguide both to guide the single photon having a wavelength greater than or equal to 1 pm and for the detection of the photon in the visible. In addition, the photonic circuit makes it possible to produce a single silicon integrated circuit comprising the detector (avalanche photodiode), the guidance and the wavelength combiner. Thus, the photonic circuit according to the invention can be integrated on a chip and the single photons are detected on a silicon platform.

[0080] This advantageously makes it possible to obtain a complete quantum circuit on silicon which can be used for example in quantum computers. According to one embodiment, several photonic circuits according to the invention are combined on the same silicon chip.

[0081] The photonic circuit operating at room temperature and being integrated on silicon with a set of waveguides, it therefore makes it possible to obtain a compact system usable in numerous applications such as quantum computers.

[0082] Although the invention has been illustrated and described in detail using a preferred embodiment, the invention is not limited to the disclosed examples. Other variations may be deduced by those skilled in the art without departing from the scope of protection of the claimed invention.

Claims

Claims

1. Photonic circuit (Cire) for detecting single photons, the photonic circuit being integrated on a silicon substrate (Sub), the photonic circuit comprising: - a single photon input (Pho) configured to receive a first light signal (Fl) comprising a single photon having a first wavelength (XI) greater than or equal to 1 pm, the single photon input comprising a first silicon waveguide (Gl); - a laser source (P) configured to emit a second laser light signal (F2) having a second wavelength (X2) greater than or equal to 1 pm and comprising a second silicon waveguide (G2); - a combiner (Comb) coupled to the first and second waveguides and configured to generate a third light signal (F3) superimposing the first light signal and the second light signal, the combiner comprising at least one third silicon waveguide (G3);- a wavelength converter (Conv) configured to generate a fourth light signal (F4) having a visible wavelength (Xvis), by non-linear effect in a fourth waveguide (G4) made of a first material; - an avalanche photodiode (APD) configured to detect the fourth light signal and comprising a fifth waveguide (G5) made of silicon; - a first coupler (Cl) configured to couple the third and the fourth waveguides; and - a second coupler (C2) configured to couple the fourth and the fifth waveguides.;

2. Photonic circuit according to claim 1, wherein the avalanche photodiode comprises a filter (F) configured to filter a residual portion of the second light signal.

3. A photonic circuit according to claim 1 or claim 2, wherein the first material comprises a thin layer of lithium niobate or silicon nitride.

4. Photonic circuit according to one of the preceding claims, in which the third waveguide comprises a tip and in which the first coupler is formed by said tip inserted into the fourth waveguide.

5. Photonic circuit according to one of claims 1 to 3, in which the first coupler is formed by a contact between the third guide and the fourth waveguide, the coupling being carried out by evanescent wave.

6. Photonic circuit according to one of the preceding claims, in which the fourth waveguide comprises a tip and in which the second coupler is formed by said tip inserted into the fifth waveguide.

7. Photonic circuit according to one of claims 1 to 5, in which the second coupler is formed by a contact between the fourth waveguide and the fifth waveguide.

8. Photonic circuit according to one of the preceding claims, the photonic circuit comprising a third coupler (C3) configured to couple the laser source with the second waveguide.

9. Photonic circuit according to one of the preceding claims, in which the laser source comprises a III-V material.

10. Chip (Ch) comprising one or more photonic circuits (Cire) according to one of claims 1 to 9, the photonic circuit(s) being on the same silicon substrate and being arranged so as to form a Qbit.

Images