Diamond layer on photonic circuit

Abstract

A photonic integrated circuit (PIC) and a single crystal diamond layer disposed on the PIC. The single crystal diamond substrate includes at least one spin defect and has a maximum surface having a surface area of at least 250000 [mu] m2. The PIC includes at least one alignment structure, and the single crystal diamond layer includes a corresponding at least one diamond alignment structure. The PIC also includes at least one photonic structure optically coupled to the at least one spin defect.

Description

Technical Field

[0001] The present invention relates to a photonic circuit having a diamond layer, and a method of forming a photonic circuit having a diamond layer. Background Art

[0002] Spin defects in synthetic diamond materials, in particular quantum spin defects and / or optically active defects, have been proposed for use in a variety of sensing, detection and quantum processing applications, including: magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing (QIP) devices such as for quantum computing.

[0003] The problem with quantum materials is that single-photon emission from quantum spin defects in such materials can be very weak. For example, even at low temperatures, NV in diamond - The defect also exhibits broad spectral emission associated with a Debye-Waller factor of the order of 0.05. Single-photon emission in the zero phonon line (ZPL) is extremely weak, typically on the order of a few thousand photons per second. Such count rates may not be sufficient to implement advanced QIP protocols based on coupling between spin states and optical transitions within reasonable data acquisition times.

[0004] In addition to the problem of weak emission, the high refractive index of diamond material means that few photons can be collected within a small solid angle due to total internal reflection.Thus, for applications including quantum information processing, there is a need to increase light collection from quantum spin defects in diamond material.

[0005] To create efficient photonic circuits, spin defects in diamond must then be precisely optically coupled to waveguides in a photonic integrated circuit (PIC) on which the diamond is mounted.

[0006] One limitation to wider adoption of diamond for quantum technologies is the difficulty in fabricating photonic circuits, because large areas of single-crystal diamond are not easily accessible as thin films of a few hundred nanometers for use in PICs.

[0007] Photonic circuits typically include a handle layer, a cladding layer, and a waveguide or active layer. The waveguide layer is usually formed of silicon nitride, gallium phosphide, or aluminum nitride because these materials have good performance in materials such as NV. - The required transparency at the typical emission wavelengths of diamond defects such as SiV centers is required. Diamond can be made of spin defects at certain locations, and these defects must be matched to the corresponding optical waveguides in the PIC. This is difficult to achieve for large-area single-crystalline diamond layers.

[0008] It is desirable to achieve low-loss interconnects between different spin defects (<0.1 dB / cm is possible). Additional functionality of developed photonic circuits can be exploited, such as splitters, interferometers, delay lines, and resonators. Low-loss waveguides and other photonic structures are difficult to fabricate in single-crystal diamond due to the difficulty of microfabrication caused by the strong chemical inertness of single-crystal diamond.

[0009] Furthermore, materials such as SiN have high nonlinear coefficients, allowing for photon pair generation and photon frequency conversion, which can be used for the implementation of quantum computing and quantum communication protocols in addition to the operations supported by diamond defect spin qubits.

[0010] Integration of diamond layers on large-area substrates facilitates the integration of more components due to improved compatibility with standard cleanroom microfabrication techniques. An example is the fabrication of microwave waveguides for the control of diamond defect spins, or the co-integration of other components such as single-photon detectors. Summary of the invention

[0011] The object is to provide a photonic circuit having a large area single crystal diamond layer containing spin defects that are precisely optically coupled to waveguides and other structures in a waveguide layer in a photonic integrated circuit.

[0012] According to a first aspect, a PIC (eg, including a waveguide layer and a cladding layer) and a single crystal diamond layer disposed on the PIC are provided. The single crystal diamond substrate includes at least one spin defect and has a surface area of ​​at least 250,000 μm 2 The PIC comprises at least one alignment structure, and the single crystal diamond layer comprises at least one corresponding diamond alignment structure. The PIC further comprises at least one photonic structure optically coupled to the at least one spin defect.

[0013] Alternatively, the spin defect is selected from any of the following: a negatively charged nitrogen-vacancy center, a silicon vacancy center, a germanium vacancy center, a tin vacancy center, a nickel vacancy NE4, and a nickel vacancy nitrogen defect NE8.

[0014] The photonic circuit optionally includes a plurality of spin defects and a corresponding plurality of photonic structures, wherein the plurality of photonic structures are configured to connect different spin defects according to a quantum computing protocol.

[0015] Examples of photonic structures include optical waveguides and photonic cavities.

[0016] Alternatively, the single crystal diamond layer has a thickness selected from any of: 50 nm to 50 μm, 100 nm to 10 μm, and 150 nm to 5 μm.

[0017] The PIC alignment structure and the diamond alignment structure optionally include corresponding structures that allow light to pass when the alignment structure is aligned. It will be appreciated that alignment is as close as possible when the amount of light passing is maximised.

[0018] In an alternative embodiment, the PIC alignment structure and the diamond alignment structure comprise corresponding diffraction gratings arranged to diffract light during alignment.

[0019] In an alternative embodiment, the PIC alignment structure and the diamond alignment structure comprise corresponding waveguide couplers in the diamond layer for connecting optical waveguides in the PIC.

[0020] Note that a combination of the two embodiments described above may be used; a diffraction grating may be used to achieve a coarse alignment, and a waveguide coupler may then be used to achieve a more precise alignment.

[0021] As a further option, the PIC alignment structure and the diamond alignment structure comprise alignment marks.

[0022] The PIC alignment structure and the diamond alignment structure are optionally formed using any of implantation, electron beam lithography, and inductively coupled plasma reactive ion etching.

[0023] Alternatively, the single crystal diamond layer has a maximum surface area selected from any of the following: at least 500,000 μm 2 , 1mm 2 , 4mm 2 , 25mm 2 and 100mm 2 As the area of ​​the diamond layer increases, alignment becomes more difficult and critical.

[0024] Alternatively, multiple single crystal diamond layers are arranged on the PIC.

[0025] According to a second aspect, there is provided a method of forming a photonic circuit. The method comprises providing a PIC and arranging a single crystal diamond layer on the PIC by aligning diamond alignment structures in the diamond layer with corresponding PIC alignment structures in the PIC. The single crystal diamond layer has a surface area of ​​at least 250,000 μm 2 and comprising at least one spin defect that is optically coupled to an optical waveguide in the PIC after alignment.

[0026] The spin defect is optionally selected from any of the following: a negatively charged nitrogen-vacancy center, a silicon vacancy center, a germanium vacancy center, a tin vacancy center, a nickel vacancy NE4, and a nickel vacancy nitrogen defect NE8.

[0027] Alternatively, the diamond layer comprises a plurality of spin defects and the PIC comprises a corresponding plurality of photonic structures, wherein the plurality of photonic structures are configured to connect different spin defects according to a quantum computing protocol.

[0028] Alternative examples of photonic structures include any of optical waveguides and photonic cavities.

[0029] Alternatively, the PIC alignment structure and the diamond alignment structure comprise corresponding structures that allow light to pass when the alignment structure is aligned, and the aligning step comprises passing light through the alignment structure. In this step, the closest alignment is achieved when the intensity of the light passing through is maximized.

[0030] Alternatively, the PIC alignment structure and the diamond alignment structure comprise corresponding diffraction gratings, and the aligning step comprises passing light through the diffraction gratings.

[0031] Alternatively, the PIC alignment structure and the diamond alignment structure comprise corresponding waveguide couplers in the diamond layer for connecting waveguides in the PIC, and the step of aligning comprises passing light through the waveguide couplers.

[0032] Alternatively, the PIC alignment structure and the diamond alignment structure include alignment marks, and aligning includes aligning corresponding alignment marks.

[0033] Alternatively, and prior to aligning the diamond alignment structure in the diamond layer with the corresponding PIC alignment structure in the PIC, the PIC alignment structure and the diamond alignment structure are formed using any of implantation, electron beam lithography, and inductively coupled plasma reactive ion etching.

[0034] In an alternative embodiment, multiple single crystal diamond layers are disposed on the PIC.

[0035] Alternatively, and before arranging the diamond layer on the PIC, the method includes forming a spin defect in the diamond layer, mapping the location of the spin defect in the diamond layer, and forming a photonic structure in the PIC to correspond to the location of the spin defect, such that after arranging the diamond layer on the PIC, the spin defect and the photonic structure are optically coupled.

[0036] As an alternative, the method comprises forming a photonic structure in the PIC, disposing a diamond layer on the PIC, and forming a spin defect in the diamond layer such that the spin defect and the photonic structure are optically coupled. Note that the steps of disposing a diamond layer on the PIC and forming a spin defect in the diamond layer may be in any order. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0038] Figure 1 schematically illustrates a side cross-sectional view of an exemplary PIC;

[0039] Figure 2 schematically illustrates a side cross-sectional view of an exemplary PIC and corresponding diamond layer;

[0040] Figure 3 schematically illustrates a side cross-sectional view of an exemplary PIC and corresponding diamond layer, wherein the diamond layer is aligned with the PIC;

[0041] Figure 4 schematically illustrates a plan view of an exemplary photonic circuit including a PIC and an aligned corresponding diamond layer;

[0042] Figure 5 schematically illustrates a side cross-sectional view of another exemplary PIC and corresponding diamond layer, wherein the diamond layer is aligned with the PIC;

[0043] Figure 6 schematically illustrates a side cross-sectional view of another exemplary PIC and corresponding diamond layer, wherein the diamond layer is aligned with the PIC;

[0044] Figure 7 is a flow chart showing exemplary steps for forming a photonic circuit;

[0045] Figure 8 is a flow chart illustrating further exemplary steps for forming a photonic circuit; and

[0046] Fig. 9 is a flow chart showing alternative exemplary steps for forming a photonic circuit. DETAILED DESCRIPTION

[0047] In order to accurately align the large area single crystal diamond layer with the corresponding PIC to form a heterogeneous integrated photonic circuit, the PIC is provided with at least one PIC alignment structure and the diamond layer is provided with a corresponding at least one diamond alignment structure.

[0048] To illustrate the process and structure, in the first example, refer to Figure 1 to Figure 3 , assuming that the alignment structure is a diffraction grating.

[0049] Figure 1 The schematic diagram illustrates a side cross-sectional view of a PIC 1 provided with a first alignment optical waveguide 2. The first alignment optical waveguide 2 is located near the surface of the PIC 1 and terminates in a first PIC diffraction grating 3. A second alignment optical waveguide 4 is also provided near the surface of the PIC 1. The second alignment optical waveguide 4 terminates in a second PIC diffraction grating 5. The first diffraction grating 3 and the second diffraction grating 5 are not optically coupled to each other, so if light (at Figure 1 If an optical waveguide 2 (shown as a black arrow in FIG. 1 ) passes through the first alignment optical waveguide 2, it will not pass through the second alignment optical waveguide 4 subsequently.

[0050] Now go to Figure 2 A diamond layer 6 provided with a diamond diffraction grating 7 is provided. When the diamond layer is placed on the surface of the PIC 1, but not aligned with the diffraction gratings 3, 5, 7, light passing through the first alignment optical waveguide 2 will still not subsequently pass through the second alignment optical waveguide 4 because the light is scattered out of plane. When the diamond layer 6 is close (but not completely) aligned with the PIC 1, the alignment signal (depending on the design) is partially reflected to the same waveguide or an adjacent waveguide. Using a multi-axis micromanipulator, the position of the diamond layer 6 relative to the PIC 1 is optimized by monitoring the optical signal, and the optimal position is reached when the reflected signal is maximized. Multiple gratings can be used to improve the alignment accuracy.

[0051] like Figure 3 As shown in , when the diffraction gratings 3, 5, 7 are aligned, the light passing through the first alignment optical waveguide 2 is then diffracted by the first PIC diffraction grating 3 so that it is guided into the diamond diffraction grating 7. The light is then diffracted from the diamond diffraction grating 7 so that it is guided into the second PIC diffraction grating 5. The light is then diffracted from the second PIC diffraction grating 5 into the second alignment optical waveguide 4. When light is detected from the second alignment optical waveguide 4, then the diamond layer 6 is correctly aligned with the PIC 1.

[0052] It will be appreciated that a plurality of corresponding diffraction gratings at different angles may be used to ensure that the diamond layer 6 is aligned in two dimensions on the PIC 1 .

[0053] For example, diffraction gratings can be realized using techniques such as rigorous coupled wave analysis simulations and fabrication techniques including photolithography and inductively coupled plasma reactive ion etching (ICP-RIE).

[0054] Now go to Figure 4 , illustrates an exemplary photonic circuit 8 in plan view. The heterogeneous integrated photonic circuit 8 comprises a PIC 1, wherein a diamond layer 6 is arranged on the PIC 1. As described above, alignment is performed using a diffraction grating 7 and alignment optical waveguides 2, 4. In this example, the diamond layer 6 comprises three spin defects 9, which are optically coupled to corresponding waveguides 10 arranged in the PIC. For example, three other spin defects 11 in the diamond layer 6 are optically coupled to corresponding photonic cavities 12.

[0055] It can be seen that two sets of diffraction gratings and alignment optical waveguides are provided, wherein one set of diffraction gratings is arranged at 90° relative to the other set of diffraction gratings to ensure two-dimensional alignment of the diamond layer 6 on the PIC 1 .

[0056] Although diffraction gratings are described in the above examples, other types of optical coupling can also be used, such as optical waveguides with adiabatic couplers. As with the diffraction grating example above, the waveguides are structured in the diamond layer 6 before transfer. They terminate in tapered adiabatic couplers. On PIC 1, an array of waveguides is prepared. When the diamond layer 6 is aligned with PIC 1, the optical signal from one set of aligned waveguides in PIC 1 is coupled to the next set of aligned waveguides in PIC 1.

[0057] To illustrate a second exemplary method of aligning the substrate 1 with the diamond layer, Figure 5 A side cross-sectional view of another exemplary PIC and corresponding diamond layer is schematically illustrated, wherein the diamond layer is aligned with the PIC.

[0058] exist Figure 5 In the example of , the first alignment optical waveguide 2 is located near the surface of the PIC 1 and terminates at the first substrate optical coupler 13. When the diamond layer 6 and the substrate are aligned, the first substrate optical coupler 13 is optically connected to the first diamond optical coupler 14. The first diamond optical coupler 14 is connected to the second diamond optical coupler 16 through the diamond waveguide 15. The second substrate optical coupler 17 is disposed at the surface of the substrate, and when the diamond layer 6 is aligned with the substrate 1, the second substrate optical coupler 17 is optically connected to the second diamond optical coupler 16. When the two sets of corresponding optical couplers are aligned, the light passing through the first alignment optical waveguide 2 can be detected at the second alignment optical waveguide 5. The diamond waveguide 15 can be a ridge waveguide, a rib waveguide, a groove waveguide, or a photonic crystal waveguide prepared in the diamond layer before transfer.

[0059] Figure 5 An advantage of the structure shown in is that the groups of corresponding optical couplers can be offset from each other so that the diamond can be aligned in two dimensions by passing light only through the first alignment optical waveguide 2.

[0060] Figure 6 Shows Figure 5 In this case, the first alignment optical waveguide 2 terminates in a single substrate optical coupler 18 which does not allow light to pass from the first alignment optical waveguide 2 to the second alignment optical waveguide 5 unless the single substrate optical coupler 18 is aligned with a corresponding diamond optical coupler 19 in the diamond layer 6. In this example, in order to two-dimensionally align the diamond layer 6 with the PIC 1, at least one pair of corresponding optical couplers is required.

[0061] The above examples of alignment structures have different advantages and disadvantages, and an optimal setup may involve using a diffraction grating for coarse alignment and a waveguide coupler for finer alignment. It will be appreciated that any other photonic structure that can be used to redirect an optical signal from one waveguide to another in a PIC may be used.

[0062] Apart from Figure 1 to Figure 6 In addition to or instead of the alignment structure shown in Figure 1 to Figure 6 The alignment structure shown in , the alignment structure may be a simple mark that can be aligned visually or by automated means to ensure that the diamond layer 6 is correctly aligned with the PIC 1 .

[0063] It should also be noted that while a large area diamond layer is used, it may be desirable to arrange multiple diamond layers on a single PIC to allow for the formation of larger and more complex photonic circuits.

[0064] There are various ways to form the alignment structures in both the diamond layer 6 and the PIC 1. These include implantation, electron beam lithography and inductively coupled plasma reactive ion etching.

[0065] Figure 7 is a flow chart showing the exemplary steps described above. Figure 7 Number in:

[0066] S1. Provide a PIC, the PIC comprising at least one PIC alignment structure, such as a diffraction grating, a waveguide coupler or an alignment mark. In most practical applications, at least two PIC alignment structures are provided.

[0067] S2. Arranging a single crystal diamond layer on the PIC by aligning the diamond alignment structures in the diamond layer with corresponding PIC alignment structures in the PIC. The single crystal diamond layer has a surface area of ​​at least 250,000 μm 2 and includes at least one spin defect. After alignment, the spin defect is optically coupled to an optical waveguide in the PIC. Exemplary spin defects are any of the following: negatively charged nitrogen-vacancy centers, silicon vacancy centers, germanium vacancy centers, and tin vacancy centers. In most cases, multiple spin defects are provided, each of which is optically coupled to a corresponding optical waveguide.

[0068] As mentioned above, a key requirement for precise alignment of the diamond layer 6 with the PIC 1 is to enable the spin defects to optically couple to corresponding optical waveguides or other optical structures such as photonic cavities. To do this, the positions of the spin defects and corresponding optical structures must be controlled to ensure optical coupling after alignment.

[0069] In a first example, the positions of spin defects in the diamond layer 6 are mapped and then optical structures are formed in the PIC 1 based on the known positions of the spin defects. Figure 8 is a flowchart showing exemplary steps of the method, and the following numbers correspond to Figure 8 Number in:

[0070] S3. Preparing a single crystal diamond layer 6 containing spin defects. There are several ways to incorporate spin defects into the diamond layer 6. For example, when diamond is grown in a CVD reactor, spin defects can be added to the diamond lattice. For example, as described in WO2010149775, nitrogen can be added to the process gas during the growth process, resulting in nitrogen being incorporated into the diamond lattice. By subsequent irradiation and annealing, some of the nitrogen can be converted into NV - An alternative method of incorporating spin centers into diamond is by ion implantation, for example as described in WO2015071487. Using this technique, NVs are formed by nitrogen ion implantation and annealing (optionally including a vacancy generating irradiation step before or after the ion implantation). - Ion implantation has many advantages over incorporating nitrogen into the crystal lattice during growth. The first advantage is the - The degree of control of the depth of the center, NV - The center needs to be close to the surface. The second advantage is the ability to control the location of the nitrogen implant on the surface of the diamond so that the nitrogen can be implanted at the desired location close to the desired location of the photonic structure. The third advantage is when low concentrations (less than 10 15 cm -2 ) ion implantation allows more control over the concentration of nitrogen in the diamond. Of course, similar considerations apply to the formation of other types of defects.

[0071] S4. Mapping the positions of spin defects in the diamond layer 6. This mapping can be achieved by characterizing the diamond layer 6 with microphotoluminescence spectroscopy and additionally using autocorrelation measurements to confirm the presence of single defects in the volume of interest.

[0072] S5. Prepare a PIC 1 with photonic structures such as optical waveguides and photonic cavities positioned such that after positioning of the diamond layer 6 on the PIC 1, the spin defects will optically couple to corresponding optical structures in the insulating layer 1. Note that some optical structures may also be formed in the diamond layer 6.

[0073] S6. Position the diamond layer 6 on the PIC 1 using the alignment structure described above.

[0074] In a second example, an optical structure is formed in the PIC 1 and then spin defects are introduced in the diamond in desired locations so that after alignment and positioning of the diamond layer 6 on the PIC 1 the spin defects optically couple to the optical structure in the insulating layer 1 . Fig. 9 is a flowchart showing exemplary steps of the method, and the following numbers correspond to Fig. 9 Number in:

[0075] S7. Prepare the PIC 1 having photonic structures such as optical waveguides and photonic cavities positioned to form a photonic circuit.

[0076] S8. Introducing spin defects into locations in the diamond layer 6 such that they will optically couple to corresponding photonic structures after positioning of the diamond layer 6 on the PIC 1. These may for example be formed using the ion implantation technique described above in step S3.

[0077] S9. Position the diamond layer 6 on the PIC 1 using the alignment structure described above.

[0078] Note that steps S8 and S9 may be reversed; the spin defects may be formed in the diamond layer 6 before positioning of the diamond layer 6 on the PIC 1, or alternatively, may be formed in the diamond layer 6 after positioning of the diamond layer 6 on the PIC 1. Note however that, although the ion implantation may be performed after the diamond layer 6 is already on the PIC 1, and thus does not require any precise alignment between the diamond layer 6 and the PIC 1, but only alignment between the ion beam and the target, the other defect creation steps may be incompatible with the photonic platform used. After the implantation, an annealing step at high temperature (typically greater than 1000°C) is required to recover most of the damage caused by the implantation and to diffuse the vacancies to form ion-vacancy centres that act as spin defects. The annealing temperature may be above the thermal budget of the photonic platform, which requires the annealing step to be performed before positioning the diamond layer 6 on the PIC 1 target.

[0079] An exemplary process flow can be summarized as follows:

[0080] The diamond substrate is appropriately processed to the desired surface roughness, flatness, parallelism, strain and sub-surface damage.

[0081] The diamond substrate is implanted with photoions to form a damaged layer for subsequent electrochemical stripping.

[0082] The diamond substrate is overgrown in a microwave plasma CVD reactor to form a high purity diamond layer 6 .

[0083] Isolated defects such as N, Si, Ge or Sn are implanted, which can be subsequently processed to form spin defects.

[0084] The diamond layer 6 is annealed to create spin defects and convert the damaged layer into graphite for subsequent electrochemical exfoliation.

[0085] Alignment structures are produced in the diamond layer 6 by photolithography or electron beam lithography and ICP-RIE etching.

[0086] The positions of the spin defects in the diamond layer 6 are mapped using the alignment structure as a reference.

[0087] Additional photonic structures are optionally fabricated in the diamond layer to create any waveguides and photonic cavities required for the operation of the photonic chip.

[0088] Create the layout of target PIC 1 and prepare the PIC.

[0089] Electrochemical stripping of the diamond layer 6 is performed to remove the diamond substrate at the damaged layer.

[0090] The diamond layer 6 is aligned to the PIC 1 using transfer and micromanipulation while monitoring the optical signal on the alignment waveguide in the PIC 1 .

[0091] Further preparation steps are performed, such as the deposition of cladding layers, the creation of strip microwave waveguides for spin defect control, or the heterogeneous integration of other components such as single-photon detectors or lasers or control electronics.

[0092] The above process allows the integration of large (>500 μm in lateral dimensions) diamond layer 6 films with high thickness uniformity (intrinsic to the implantation and lift-off technique) onto PIC 1 to form photonic circuits. The lateral dimensions are mainly limited by the size of the starting single crystal substrate. In addition, the preparation using implantation and lift-off provides higher thickness uniformity over large areas compared to mechanical polishing or ICP-RIE etching. Thickness uniformity is a key factor in this application as it guarantees uniform optical properties such as mode volume, effective refractive index and resonant frequency over the entire area of ​​the film and between different films, making the photonic platform robust to bulk preparation.

[0093] As diamond layers are used over large surface areas, alignment becomes more difficult and more critical. The use of alignment structures as described above allows alignment of large diamond layers having a maximum linear dimension greater than 500 μm.

[0094] While the invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A heterogeneous integrated photonic circuit, comprising: Photonic integrated circuits; A single crystal diamond layer is disposed on the photonic integrated circuit, the single crystal diamond substrate comprises at least one spin defect, the single crystal diamond has a surface area of ​​at least 250,000 μm 2 the maximum surface area; wherein the photonic integrated circuit comprises at least one photonic integrated circuit alignment structure, and the single crystal diamond layer comprises a corresponding at least one diamond alignment structure; and The photonic integrated circuit also includes at least one photonic structure optically coupled to the at least one spin defect.

2. A photonic circuit according to claim 1, wherein the spin defect is selected from any of the following: a negatively charged nitrogen-vacancy center, a silicon vacancy center, a germanium vacancy center, a tin vacancy center, a nickel vacancy NE4 and a nickel vacancy nitrogen defect NE8.

3. A photonic circuit according to any one of claims 1 or 2, comprising a plurality of spin defects and a corresponding plurality of photonic structures, wherein the plurality of photonic structures are configured to connect different spin defects according to a quantum computing protocol.

4. The photonic circuit according to any one of claims 1 to 3, wherein the photonic structure is selected from any of an optical waveguide and a photonic cavity.

5. The photonic circuit according to any one of claims 1 to 4, wherein the single crystal diamond layer has a thickness selected from any of the following: 50 nm to 50 μm, 100 nm to 10 μm, and 150 nm to 5 μm.

6. The photonic circuit of any one of claims 1 to 5, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding structures that allow light to pass when the alignment structures are aligned.

7. The photonic circuit of claim 6, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding diffraction gratings arranged to diffract light during alignment.

8. The photonic circuit of claim 6 or 7, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding waveguide couplers in the diamond layer for connecting optical waveguides in the photonic integrated circuit.

9. The photonic circuit of any one of claims 1 to 8, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise alignment marks.

10. The photonic circuit of any one of claims 6 to 9, wherein the photonic integrated circuit alignment structure and the diamond alignment structure are formed using any of: implantation, electron beam lithography, and inductively coupled plasma reactive ion etching.

11. A photonic circuit according to any one of claims 1 to 10, wherein the single crystal diamond layer has a maximum surface area selected from any of the following: at least 500,000 μm 2 , 1mm 2 , 4mm 2 , 25mm 2 and 100mm 2 .

12. The photonic circuit according to any one of claims 1 to 11, wherein a plurality of single crystal diamond layers are arranged on the photonic integrated circuit.

13. A method of forming a photonic circuit, the method comprising: Providing photonic integrated circuits; disposing a single crystal diamond layer on the photonic integrated circuit by aligning diamond alignment structures in the diamond layer with corresponding photonic integrated circuit alignment structures in the PIC; wherein the single crystal diamond layer has a surface area of ​​at least 250,000 μm 2 and comprising at least one spin defect optically coupled to an optical waveguide in the photonic integrated circuit.

14. The method according to claim 13, wherein the spin defect is selected from any of the following: a negatively charged nitrogen-vacancy center, a silicon vacancy center, a germanium vacancy center, a tin vacancy center, a nickel vacancy NE4, and a nickel vacancy nitrogen defect NE8.

15. The method of any one of claims 13 or 14, wherein the diamond layer comprises a plurality of spin defects, and the photonic integrated circuit comprises a corresponding plurality of photonic structures, wherein the plurality of photonic structures are configured to connect different spin defects according to a quantum computing protocol.

16. The method according to any one of claims 13 to 15, wherein the photonic structure is selected from any of an optical waveguide and a photonic cavity.

17. The method of any one of claims 13 to 16, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding structures that allow light to pass when the alignment structure is aligned, and the aligning step comprises passing light through the alignment structure.

18. A method according to any of claims 17, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding diffraction gratings, and the step of aligning comprises passing light through the diffraction gratings.

19. The method of claim 17 or 18, wherein the photonic integrated circuit alignment structure and the diamond alignment structure comprise corresponding waveguide couplers in the diamond layer for connecting waveguides in the photonic integrated circuit, and the aligning step comprises passing light through the waveguide couplers.

20. The method of any one of claims 13 to 19, wherein the photonic integrated circuit alignment structure and the diamond alignment structure include alignment marks, and aligning includes aligning corresponding alignment marks.

21. The method of any one of claims 17 to 20, further comprising forming any photonic integrated circuit alignment structure and diamond alignment structure using any of implantation, electron beam lithography, and inductively coupled plasma reactive ion etching before aligning the diamond alignment structure in the diamond layer with a corresponding photonic integrated circuit alignment structure in the photonic integrated circuit.

22. The method of any one of claims 13 to 21, further comprising arranging a plurality of single crystal diamond layers on the photonic integrated circuit.

23. The method of any one of claims 13 to 22, comprising, before disposing the diamond layer on the photonic integrated circuit: forming spin defects in the diamond layer; mapping locations of spin defects in the diamond layer; and A photonic structure is formed in the photonic integrated circuit to correspond to the location of the spin defect, such that after the diamond layer is disposed on the photonic integrated circuit, the spin defect and the photonic structure are optically coupled.

24. The method according to any one of claims 13 to 22, comprising: forming a photonic structure in the photonic integrated circuit; disposing the diamond layer on the photonic integrated circuit; as well as A spin defect is formed in the diamond layer such that the spin defect and the photonic structure are optically coupled.

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