Photon pair source for quantum applications

Abstract

Aspects of the present disclosure relate to a photon pair source based on an external cavity laser comprising a gain element and a planar lightwave circuit containing surface waveguide based mirrors and a ring resonator enabling four wave mixing, where the surface waveguide mirrors and the ring resonator reside within the gain cavity of the laser itself. Thus, a photon pair source according to the present disclosure can have: (1) a larger free spectral range of the entire laser cavity to enable generation of a single wavelength to enable single mode operation without additional stabilization; and (2) low laser noise, thereby enabling detection and use of the generated photon pairs.

Description

[0001] Statement regarding the related case

[0002] This case asserts priority to U.S. Provisional Patent Application No. 62 / 012,844, filed April 20, 2020 (Attorney’s Case No.: 142-041PR1), which is incorporated herein by reference.

[0003] If there is any linguistic discrepancy or inconsistency between this application and one or more of the cases incorporated by reference that could affect the interpretation of the claims in this application, then the claims in this application shall be interpreted as consistent with the language in this application. Technical Field

[0004] This invention relates to light sources based on quantum processing and integrated optical devices. Background Technology

[0005] The search seeks to utilize sources of relevant photon pairs for quantum applications such as quantum computing, quantum communication, quantum sensing, quantum imaging, and the like. However, for these to be feasible, such quantum processing systems must be more compact than those demonstrated to date. Unfortunately, many available photon pair sources are considered unsuitable for use in these systems because they typically require a relatively large size.

[0006] Therefore, systems based on integrated optics have recently emerged, which promise to serve as integrated photon pair sources. For example, examples of such methods are disclosed by Y. Jestin et al. in U.S. Patent Publication No. 2018 / 0048110, which is incorporated herein by reference. These systems incorporate a single nonlinear resonant element within the laser cavity of an external cavity laser and create pulses by mode-locking the laser's cavity modes.

[0007] Although the benefits of integration are well understood, the methods published to date still have several potential problems, such as limited free spectral range (FSR) and the need to include optical isolators in the gain cavity, which can negatively impact the utility of such systems, since optical isolators are notoriously difficult to integrate into integrated optical systems.

[0008] Furthermore, the need to establish the mold lock using an external laser leads to undesirable system complexity and operational challenges.

[0009] Therefore, there is a need for an integrated method to create photon pairs without incurring some of the costs and drawbacks of existing technologies. Summary of the Invention

[0010] This disclosure relates to a photon pair source based on an external cavity laser configuration. Advantages over the prior art are achieved by employing a photonic integrated circuit comprising an external cavity laser having a linear laser cavity containing a plurality of series optically coupled waveguide resonators, wherein one of the waveguide resonators is configured to generate photon pairs via four-wave mixing. The laser is contained within the photonic integrated circuit, which includes a gain element and a planar optical wave circuit, both mounted on the same substrate. The gain element includes a first mirror, and the planar optical wave circuit includes a second mirror residing within the laser cavity itself.

[0011] Therefore, embodiments of the present disclosure are given at least three significant advantages over prior art single-ring resonator-based photon pair sources, including: (1) the laser cavity has a large free spectral range, thereby enabling the generation of a single wavelength within the gain spectrum of the gain element to achieve single-mode operation without additional stabilization, such as locking to an external laser; (2) suppressing laser noise (e.g., amplified spontaneous emission, etc.) to a sufficiently low level to enable the detection and use of the generated photon pairs; and (3) eliminating optical isolators within the gain cavity by avoiding the use of a ring laser cavity.

[0012] According to an illustrative embodiment of this disclosure, a photonic integrated circuit is configured to provide photon pairs for an external cavity laser, wherein the external cavity laser is wavelength tunable and has a large FSR. The photonic integrated circuit includes a planar optical wave circuit and a gain element optically coupled to the planar optical wave circuit. The planar optical wave circuit includes a tunable, Vernier-type, waveguide resonator-based mirror located within the laser cavity itself and configured to serve as one mirror of the laser cavity, while a high-reflectivity coating formed on one surface of the gain element serves as another mirror of the laser cavity. Photon pairs are selectively generated via four-wave mixing in a closed-loop waveguide within the Vernier mirror arrangement.

[0013] The gain element is an indium phosphide reflective semiconductor optical amplifier having a first side including a high-reflectivity coating and a second side including an anti-reflective coating configured to facilitate optical coupling with the input portion of a planar optical wave circuit. The gain element is disposed on a mounting region formed on a substrate, wherein the mounting region is configured to facilitate good optical coupling between the second side and the input port, and to provide electrical connectivity to the gain element. In some embodiments, one or both of the gain element and the input port of the planar optical wave circuit include a spot size converter to mitigate mode mismatch at their interface and improve alignment tolerances. In some embodiments, residual reflections at the interface between the gain element and the planar optical wave circuit are mitigated by angling one or both of the waveguides on the gain element and the planar optical wave circuit and / or by filling the gap between the gain element and the input port with a refractive index matching material.

[0014] Planar optical waveguides comprise a network of surface waveguides based on integrated optics formed on a substrate. Each surface waveguide includes a pair of silicon nitride waveguides separated by a thin layer of silicon oxide, thereby defining a dual-waveguide configuration. The waveguides of the planar optical waveguide circuit collectively define an input port, which is optically coupled to a pair of output ports via a tunable coupler and a mirror arrangement based on waveguide resonators. The number and size of the closed-loop waveguides in the mirror arrangement are selected to achieve a desired free speed response (FSR) and substantially suppress noise generated in the gain cavity of the external cavity laser.

[0015] In an illustrative embodiment, the mirror arrangement includes five waveguide resonators whose closed-loop elements are ring-shaped (i.e., five "ring resonators"), including a photon generation ring at which several pairs of photons are simultaneously generated. The size of the photon generation ring is selected such that the source output is matched to a standard wavelength used in optical telecommunications (e.g., ITU grids). Each pair of adjacent rings in the mirror arrangement is optically coupled via different bus waveguides, and the bus waveguide adjacent to the photon generation ring transmits the generated photon pairs to the output port.

[0016] In an illustrative embodiment, the laser cavity further includes a tunable coupler configured to split light from the laser cavity into each of an external bus waveguide in which waveguide resonator mirrors are arranged. The inclusion of the tunable coupler enables control of the optical power within the laser cavity, thereby enabling control of the rate of photon generation.

[0017] In some embodiments, the planar optical wave circuit mirror arrangement includes a different number of waveguide resonators. In some embodiments, at least one of the waveguide resonators in the mirror arrangement includes a closed-loop element other than a ring, such as an ellipse, a racetrack, an irregular shape, and the like. It should be noted that any suitable size (e.g., the radius of the ring or the closed-loop length of the different closed-loop elements) can be used for any waveguide resonator in the planar optical wave circuit mirror arrangement.

[0018] In some embodiments, pulses are generated by directly driving gain elements instead of modulating a laser, directly tuning a waveguide resonator, or tuning an external laser, thereby mitigating pulse interference during operation. In some embodiments, the laser is not modulated.

[0019] In some embodiments, the planar optical wave circuit includes a waveguide comprising one or more materials other than silicon nitride and / or silicon oxide.

[0020] In some embodiments, the planar optical wave circuit includes waveguides having waveguide configurations other than dual waveguides (e.g., single waveguide, channel waveguide, ridge waveguide, etc.).

[0021] An embodiment of the present disclosure is a source for providing correlated photon pairs, the source comprising an external cavity laser having a gain cavity, the external cavity laser comprising: a gain element disposed on a substrate, the gain element including a first mirror; and a planar optical wave circuit (PLC) disposed on the substrate, the PLC comprising: (i) a second mirror, wherein the second mirror includes at least one surface waveguide based on integrated optics; and (ii) a first output port and a second output port for providing the photon pairs; wherein the source is characterized in that: the PLC includes (iii) a plurality of waveguide resonators, each including a first waveguide resonator enabling four-wave mixing; the second mirror residing within the gain cavity; the gain cavity being isolator-free; and the first output port and the second output port being optically connected to the first waveguide resonator.

[0022] According to another embodiment of this disclosure, a method for generating correlated photon pairs is provided, the method comprising: exciting light energy in a laser cavity of an external cavity laser including a gain element and a planar optical wave circuit (PLC), the gain element and the PLC being disposed on a substrate, wherein the laser cavity includes a first mirror and a second mirror, wherein the gain element includes the first mirror, and the PLC includes: (i) the second mirror, wherein the second mirror includes at least one surface waveguide based on an integrated optics device; (ii) a first waveguide resonator enabling four-wave mixing; (iii) a first output port and a second output port; and (iv) a first output... The method comprises: a first output port and a second output port for providing the photon pair; generating at least one correlated photon pair in the first waveguide resonator; and transmitting the at least one correlated photon pair to at least one of the first output port and the second output port; wherein the method is further characterized by: providing the external cavity laser such that the second mirror is in the laser cavity and the gain cavity is isolator-free; providing the PLC such that it includes a plurality of waveguide resonators including a first waveguide resonator capable of four-wave mixing; and providing the PLC such that the first output port and the second output port are directly optically coupled to the first waveguide resonator. Attached Figure Description

[0023] Figure 1 A schematic diagram depicting an illustrative embodiment of a photon pair source according to the present disclosure.

[0024] Figure 2 The operation of a method for generating correlated photon pairs according to an illustrative embodiment is described.

[0025] Figure 3 A schematic diagram depicting a more detailed view of a PLC 106 according to an illustrative embodiment.

[0026] Figure 4 A schematic diagram depicting a cross-sectional view of a surface waveguide based on integrated optics according to an illustrative embodiment.

[0027] Figure 5 A schematic diagram depicting a detailed view of an alternative PLC according to this disclosure. Detailed Implementation

[0028] The following terms are defined for use in this specification and are included in the appended claims:

[0029] • Optical coupling is defined as being in the same optical path, allowing light to propagate between two optical elements. Optical coupling elements can be arranged such that: (1) light propagates directly from one to the other without any intermediate elements (e.g., a laser coupled to an optical fiber without a lens, mirror, or other medium); (2) light propagates from one to the other via intermediate elements (e.g., a laser and an optical fiber configured such that the output signal from the laser propagates through a lens to the input surface of the optical fiber); or (3) light propagates from one to the other via multiple intermediate elements (a laser and an optical fiber configured such that the output signal from the laser propagates through a lens to a redirecting mirror that redirects the output signal to the input surface of the optical fiber).

[0030] • Optical connections are defined as direct optical coupling without any intermediary elements (e.g., a laser docked to an optical fiber without lenses, mirrors or other media, a bus waveguide positioned close to a waveguide resonator so that light can be directly and vanished between them, etc.).

[0031] Figure 1 A schematic diagram depicting an illustrative embodiment of a photon pair source according to the present disclosure is shown. Source 100 is a photonic integrated circuit including an external cavity laser 102, the external cavity laser including a gain element 104 and a planar optical wave circuit 106 integrated on a substrate 108.

[0032] Figure 2 The operation of a method for generating correlated photon pairs according to an illustrative embodiment is described. Method 200 begins with operation 201, wherein light energy is excited within the laser cavity of an external cavity laser 102 (hereinafter referred to as "laser 102").

[0033] Gain element 104 is an InP reflective semiconductor optical amplifier (RSOA) having a gain-enabled waveguide comprising a first surface F1 and a second surface F2. The first surface F1 includes a highly reflective coating (e.g., metal, Bragg mirror, etc.) such that the surface and the coating together define mirror M1. The second surface F2 includes an anti-reflective coating suitable for reducing reflections at the operating wavelength of source 100. In some embodiments, gain element 104 comprises a material other than indium phosphide.

[0034] The planar optical wave circuit (PLC) 106 is a surface waveguide network based on integrated optics, which includes an input port 110, a resonant element 112, output ports 114A and 114B, and a mirror M2.

[0035] Gain element 104 and PLC 106 are arranged such that surface F2 is optically coupled to input port 110, thereby defining laser cavity LC1, which includes mirrors M1 and M2. An aspect of this disclosure is that the elements contained in laser cavity LC1 are arranged in series (i.e., laser cavity LC1 is a "linear cavity"). Because laser cavity LC1 is a linear cavity, it does not require optical isolators, thereby reducing the cost and complexity of source 100.

[0036] The gain element 104 is mounted top-down on a mounting region 116 of the substrate 108. The mounting region 116 also includes a contact pad (not shown) configured to enable electrical connectivity with electrical contacts formed on the top surface of the gain element.

[0037] Mounting region 116 is a region of substrate 108 etched to a certain depth based on the distance between the top surface of gain element 104 and its gain-enabling waveguide. In some embodiments, a vertical alignment stop is included as part of at least one of gain element 104 and substrate 108 to facilitate vertical alignment of face F2 and input port 110.

[0038] When the gain element 104 is mounted top-down on the mounting area 116, vertical alignment between the input port 110 and the surface F2 can be passively achieved, significantly reducing the cost and complexity associated with the hybrid integration of the gain element 104 and the PLC 106. In some embodiments, one or both of the gain element 104 and the input port 110 include a spot size converter to facilitate good optical coupling between the input port 110 and the surface F2. In some embodiments, the waveguide tilt on each side and / or the air gap between the gain element 104 and the PLC 106 is filled with a material, such as a thermo- or UV-cured epoxy resin, to match the effective refractive index of the input port 110 (or the gain element 104) to reduce residual reflections at the interface between them. In some embodiments, the gain element 104 is actively aligned with the input port 110.

[0039] In some embodiments, the gain element 104 includes an integrated modulator for direct modulation.

[0040] Figure 3A schematic diagram depicting a more detailed view of a PLC 106 according to an illustrative embodiment. A network of surface waveguides based on integrated optics in the PLC 106 is configured to define an input waveguide 302, a modulator 306, a resonant element 112, and output ports 114A and 114B.

[0041] The input waveguide 302 is a silicon nitride-based double waveguide, the end face of which defines the input port 110.

[0042] Figure 4 A schematic diagram depicting a cross-sectional view of a surface waveguide based on an integrated optics device according to an illustrative embodiment. Input waveguide 302 is a silicon nitride-based dual-strand waveguide comprising a lower cladding 402, a core 404, and an upper cladding 406 disposed on a substrate 108. Input waveguide 302 represents each of the waveguides contained within the PLC 106.

[0043] Each of the lower cladding 402 and the upper cladding 406 is a conventional cladding suitable for substantially confining light energy within the core 404. In the depicted example, each of cladding 402 and 406 comprises silicon dioxide.

[0044] Core 404 is a multilayer core comprising a lower core layer 408, a central core layer 410, and an upper core layer 412. Each of the lower core layer 408 and the upper core layer 412 comprises stoichiometric silicon nitride (i.e., Si3N4), and the central core layer 410 comprises stoichiometric silicon dioxide. The width of core 404 and the thicknesses of the lower, central, and upper layers of the waveguide structure are chosen to facilitate low-loss, single-mode propagation of light with wavelengths within the telecommunications C-band. In the depicted example, core 404 has a width of 1.2 micrometers, the lower core layer 408 and the upper core layer 412 each have a thickness of approximately 170 nm, and the central core layer 410 has a thickness of approximately 500 nm; however, any practical values ​​may be used for any of these dimensions. In some embodiments, the lower core layer 408 and the upper core layer 412 have different thicknesses, thereby forming an asymmetric double-strand waveguide structure. Roeloffzen et al. in “Low-Loss Si3N4 TriPleX” TM Examples of waveguides according to this disclosure are disclosed in Low-loss Si3N4 TriPleX™ optical waveguides: Technology and applications overview (IEEE J.Sel. Top. Quantum Electron 24(4), 1 to 21 (2018)) and U.S. Patent No. 7,146,087, each of which is incorporated herein by reference.

[0045] The use of silicon nitride-based surface waveguides provides specific advantages to embodiments according to this disclosure, such as the ability to achieve small ring diameters and / or high waveguide densities; however, any suitable surface waveguide comprising any suitable material may be used in the PLC 106 without departing from the scope of this disclosure. Furthermore, silicon nitride-based waveguides can be designed to operate anywhere in a wide wavelength range extending from about 400 nm to about 2400 nm. Therefore, such waveguides can be used with any of a wide range of gain elements—virtually any gain element that can operate within this wavelength range.

[0046] Now return to Figure 3 In some embodiments, input port 110 includes a spot size converter configured to substantially match the mode field size and / or shape to the optical mode of gain element 104. Spot size converters suitable for use according to this disclosure are disclosed in U.S. Patents Nos. 8,718,432 and 9,268,089, each of which is incorporated herein by reference. In some embodiments, input port 110 is a vertical grating coupler.

[0047] Input waveguide 302 optically couples input port 110 to splitter 304. In the depicted example, splitter 304 is a conventional tunable coupler; however, in some embodiments, splitter 304 incorporates different suitable splitting elements (e.g., y-splitter, directional coupler, etc.). Therefore, the optical signal received at the input port is substantially equally distributed into external waveguides 308A and 308B. It should be noted that embodiments employing a tunable coupler for splitter 304 offer particular advantages because this arrangement allows for precise control of the optical power within the laser cavity, which in turn allows for control of the photon pair generation rate.

[0048] The input waveguide 302 includes a modulator 306, which is a phase modulator configured to modulate the phase of light received at the splitter 304. The inclusion of modulator 306 enables the establishment of a desired time slot in which photon pairs can be generated. In some embodiments, modulator 306 is included in gain element 104, rather than in PLC 106.

[0049] In the depicted example, modulator 306 is a conventional thermo-optical phase modulator including a heater disposed on input waveguide 302. In some embodiments, modulator 306 is a stress-optical phase modulator including a piezoelectric material operatively coupled to input waveguide 302. Phase modulators suitable for use according to this disclosure are described in U.S. Patent Nos. 9,221,074 and 9,764,352, each of which is incorporated herein by reference. In some embodiments, modulator 306 differs from a phase modulator, such as an amplitude modulator, an electroabsorption modulator, and the like.

[0050] Resonant element 112 includes ring resonators RR1 to RR5, external waveguides 308A and 308B, bus waveguides 310A to 310D, and output ports 114A and 114B. External waveguides 308A and 308B, ring resonators RR1 to RR5, and bus waveguides 310A to 310D are arranged to collectively define a Vernier-type ring resonator-based waveguide mirror that serves as a mirror M2 for laser cavity LC1.

[0051] Bus waveguides 310A to 310D (collectively referred to as bus waveguide 310) and ring resonators RR1 to RR5 are optically coupled in series between outer waveguides 308A and 308B (collectively referred to as outer waveguide 308), wherein each pair of adjacent ring resonators is evanescently coupled via different ones in bus waveguide 310.

[0052] Each of the ring resonators RR1 to RR5 is a closed-loop waveguide, the radius of which is chosen to produce resonance conditions suitable for the function of mirror M2. Furthermore, each of the ring resonators RR1 to RR5 can typically be tuned via thermal or stress-optical tuning mechanisms.

[0053] Generally, the ring resonators RR1 to RR5 have diameters ranging from about 20 to 100 micrometers, which allows each ring resonator to have a free spectral range ranging from about 4 nanometers (100 micrometers in diameter) to about 10 nanometers (20 micrometers in diameter). However, it should be noted that any suitable radius can be used for any of the ring resonators. In some embodiments, at least one of the ring resonators RR1 to RR5 has a closed loop shape other than a ring, such as an ellipse, an irregular shape, a racetrack-shaped closed loop, and the like. As will be apparent to those skilled in the art, a racetrack-shaped closed loop has a rounded shape that includes straight segments along at least one direction (i.e., approximately rectangles with "rounded" corners).

[0054] Ring resonators RR1 to RR5 are optically coupled via bus waveguides 310A to 310D to utilize the Vinnier principle and increase the total FSR of mirror M2. In some embodiments, ring resonators RR1 to RR5 have different diameters. In some embodiments, ring resonators RR1 to RR5 have the same diameter, thereby realizing a higher-order filter with an increased signal-to-noise ratio.

[0055] At operation 202, at least one correlated photon pair is generated in the waveguide resonator of resonant element 112.

[0056] In the depicted example, the ring resonator RR3 is configured as a photon pair generation ring, the size of which is selected to facilitate four-wave mixing within it to generate photon pairs.

[0057] It should be noted that the inclusion of multiple waveguide resonators, including closed-loop elements configured to enable four-wave mixing in PLC 106, provides significant advantages over photon pair sources known in the prior art according to embodiments of this disclosure. Specifically, since the ring resonator RR3 is contained within the laser cavity of source 100, four-wave mixing results in the generation of photon pairs within the laser cavity itself. Therefore, it is not necessary to tune ring resonators RR1 to RR5 to generate photon pairs, nor is it necessary to lock / stabilize source 100 with an external laser.

[0058] In addition, by appropriately selecting the gain element 104 and the appropriate size of the ring resonator R3, the output of the source 100 can be customized to match any desired wavelength range, such as the standard wavelengths used in optical telecommunications (i.e., the ITU grid).

[0059] At operation 203, the generated coherent photon pairs are transmitted to output ports 114A and 114B.

[0060] It should be noted that the system according to this disclosure may employ quantum states of light generated by four-wave mixing rather than correlated photon pairs, such as squeezed states of light and the like.

[0061] In the depicted example, bus waveguides 310B to 310C extend to the edge of substrate 108 to define output ports 114A and 114B. In some embodiments, output ports 114A and 114B include spot size converters to mitigate coupling losses with external components, such as optical fibers. In some embodiments, at least one of output ports 114A and 114B includes a vertical grating coupler.

[0062] Another aspect of this disclosure is that the light circulates in two directions within the ring RR3, which allows photon pairs to be advantageously provided at each of the two output ports, thereby enabling the generation of twice as many photon pairs as that produced by prior art photon pair sources. Therefore, embodiments according to this disclosure are given significant additional advantages over prior art photon pair sources.

[0063] Figure 5 A schematic diagram depicting a detailed view of an alternative PLC according to this disclosure. The PLC 500 includes an input waveguide 302, a resonant element 502, a waveguide 504, and output ports 114A and 114B.

[0064] PLC 500 is similar to PLC 106; however, in PLC 500, resonant element 502 is used as a series wavelength filter bank instead of a waveguide mirror, and an additional Sagnac loop mirror is included as mirror M2.

[0065] The resonant element 502 is similar to the resonant element 112; however, the resonant element is optically coupled to the gain element 104 only via the external waveguide 308A, while the external waveguide 308B is extended to become the waveguide 504.

[0066] Similar to resonant element 112, resonant element 502 comprises five waveguide resonators, which are closed-loop waveguides shaped into rings, thereby defining ring resonators RR6 to RR10. The shape and size of ring resonators RR6 to RR10 are selected to achieve the desired wavelength filtering functionality of resonant element 502. It should be noted that any actual shape and / or size can be used for any of the waveguide resonators of resonant element 112.

[0067] Similar to the ring resonator RR3 described above, the ring resonator RR8 is configured as a photon pair generation ring, and its size is selected to facilitate four-wave mixing within it to generate photon pairs. Therefore, associated waveguide pairs are provided to output ports 114A and 114B, as described above and regarding... Figure 3 Discussion.

[0068] Waveguide 504 is configured to define the Sagnac loop mirror used as mirror M2. Waveguide 504 also includes an output port 506 thereon that provides the output of laser 102.

[0069] In some embodiments, the laser output is used as a reference signal, a local oscillator (e.g., in quantum computing applications), and the like.

[0070] It should be understood that this disclosure teaches only one example of an illustrative embodiment, and many variations of the invention can be readily designed by those skilled in the art upon reading this disclosure, and the scope of the invention will be determined by the appended claims.

Claims

1. A source for providing correlated photon pairs, the source comprising an external cavity laser having a gain cavity, the external cavity laser comprising: Gain element, disposed on a substrate, the gain element including a first mirror; and A planar optical wave circuit (PLC) disposed on the substrate, the PLC comprising: (i) a second mirror, wherein the second mirror comprises at least one surface waveguide based on integrated optics; (ii) A first output port and a second output port, which are used to provide the photon pair; as well as (iii) A plurality of waveguide resonators, including a first waveguide resonator having a selected size to facilitate four-wave mixing, the plurality of waveguide resonators being coupled in a Vinyler-type configuration located within the laser cavity; The second mirror resides within the laser cavity; The laser cavity described herein is without an optical isolator; and The first output port and the second output port are optically connected to the first waveguide resonator, and each of the first output port and the second output port is operable to provide one or more photon pairs.

2. The source according to claim 1, further comprising: A first bus waveguide is optically connected to the first waveguide resonator, allowing light to evanescently couple between them, and the first bus waveguide includes the first output port; and A second bus waveguide, which is optically connected to the first waveguide resonator, allows light to evanescently couple between them, and the second bus waveguide includes the second output port.

3. The source according to claim 1, wherein at least one of the plurality of waveguide resonators comprises a closed-loop waveguide element selected from a group consisting of loops, ellipses, and racetracks.

4. The source of claim 1, wherein the second mirror comprises a surface waveguide configured as a Sagnac loop mirror.

5. The source according to claim 1, wherein the second mirror comprises the plurality of waveguide resonators.

6. The source according to claim 5, wherein the waveguide resonators of the plurality of waveguide resonators are optically coupled in series between a first external bus waveguide and a second external bus waveguide, and wherein the PLC further includes a tunable coupler having a first output optically coupled to the first external bus waveguide and a second output optically coupled to the second external bus waveguide.

7. The source of claim 6, wherein the plurality of waveguide resonators are located between the gain element and the second mirror, and wherein each of the plurality of waveguide resonators is configured as a wavelength filter.

8. The source of claim 1, wherein the at least one surface waveguide based on integrated optics has a multilayer core comprising a silicon dioxide layer located between a pair of silicon nitride layers.

9. A method for generating correlated photon pairs, the method comprising: Light energy is excited in the laser cavity of an external cavity laser, which includes a gain element and a planar optical circuit (PLC). The gain element and the PLC are mounted on a substrate. The laser cavity is without an optical isolator and includes a first mirror and a second mirror located within the laser cavity. The gain element includes the first mirror, and the PLC includes: (i) the second mirror, wherein the second mirror comprises at least one surface waveguide based on an integrated optics device; (ii) A plurality of waveguide resonators, including a first waveguide resonator having a selected size to facilitate four-wave mixing, wherein the waveguide resonators of the plurality of waveguide resonators are optically coupled in a Vinyler-type configuration located within the laser cavity; as well as (iii) A first output port and a second output port, each of the first output port and the second output port being (1) directly optically coupled to the first waveguide resonator and (2) operable to provide one or more photon pairs.

10. The method of claim 9, further comprising providing the PLC such that it includes: (iv) A first bus waveguide optically connected to the first waveguide resonator, such that light can evanescently couple between them, the first bus waveguide including the first output port; and (v) A second bus waveguide, which is optically connected to the first waveguide resonator so that light can evanescently couple between them, the second bus waveguide including the second output port.

11. The method of claim 9, wherein the PLC is provided such that the second mirror includes the plurality of waveguide resonators.

12. The method of claim 11, further comprising: The PLC is provided such that the waveguide resonators of the plurality of waveguide resonators are optically coupled in series between a first external bus waveguide and a second external bus waveguide, and the PLC further includes (v) a tunable coupler having a first output optically coupled to the first external bus waveguide and a second output optically coupled to the second external bus waveguide; and The tunable coupler is controlled to control the rate at which photon pairs are generated.

13. The method of claim 9, wherein the PLC is provided such that the second mirror includes a surface waveguide configured as a Sagnac loop mirror.

14. The method of claim 13, wherein the PLC is provided such that the plurality of waveguide resonators are located between the gain element and the second mirror, and each of the plurality of waveguide resonators is configured as a wavelength filter.

15. The method of claim 9, wherein the PLC is provided such that the at least one surface waveguide based on integrated optics has a multilayer core, the multilayer core comprising a silicon dioxide layer located between a pair of silicon nitride layers.

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