Integrated photonic vertical coupler

Abstract

To provide a system and a method for integrated photonic vertical couplers.SOLUTION: A device includes a first waveguide having first photons and second photons propagating in the inside; and a second waveguide having a second connection part adjacent to a first connection part of the first waveguide. The first and second photons propagate in an orthogonal mode; and the physical relation between the first and second waveguides along the length of the second connection part causes the adiabatic transfer of the first and second photons to another orthogonal mode of the second waveguide in a place different from the second connection part.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the benefit of U.S. Provisional Patent Application No. 62 / 924,058, filed Oct. 21, 2019, entitled "INTEGRATED PHOTONICS SOURCE AND DETECTOR OF ENTANGLED PHOTONS", which is hereby incorporated by reference in its entirety.

Background Art

[0002] Networks of synchronized atomic clocks are frequently used to distribute accurate time over distances. For example, global navigation satellite systems (GNSS) such as the global position system (GPS), GLONASS, BeiDou, and Galileo are composed of satellites with synchronized atomic clocks and provide the distribution of international time. Often, satellites are equipped with hardware to facilitate the synchronization of clocks on separate satellites. Synchronization hardware with reduced size and weight and capable of high - precision timing alignment enables the synchronization of atomic clocks on smaller satellites.

Summary of the Invention

[0003] Systems and methods for an integrated photonics vertical coupler are provided herein. In certain embodiments, the device has a first waveguide having a first photon and a second photon propagating therein, and the first photon and the second photon propagate in orthogonal modes. Further, the device includes a second waveguide having a second coupling portion proximate to a first coupling portion of the first waveguide, and the physical relationship between the first waveguide and the second waveguide along the length of the second coupling portion causes adiabatic transfer of the first photon and the second photon into separate orthogonal modes of the second waveguide at different locations of the second coupling portion.

Brief Description of the Drawings

[0004] It should be understood that the drawings illustrate only some embodiments and should not be considered as limiting the scope. Exemplary embodiments will be described with further specificity and detail using the accompanying drawings.

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[0015] According to convention, the various features described are not drawn to scale, but are drawn to emphasize specific features relevant to the exemplary embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0016] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments. However, it should be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made.

[0017] Systems and methods for an integrated photonic source and a detector for entangled photons are provided herein. In certain embodiments, hardware is described herein that uses the quantum interference of time-entangled photons to enable methods for precise and reliable synchronization of optical atomic clocks. For example, optical atomic clocks on orbiting satellites can be precisely and reliably synchronized. Embodiments described herein deployed across a constellation of LEO / MEO satellites can include real-time computational interferometry to increase sensitivity to weak signals, as well as an active beam to form radar / imaging for improved stealth by reducing both signal leakage and time-on-target, enabling an improved form of signal intelligence based on the coherent synthesis of distributed radio or optical apertures.

[0018] In addition, the clock synchronization schemes described herein can use chip-scale, ultra-high flux sources and interferometers for time-energy entangled two photons, with reduced size, weight, and power, a high pair generation rate of entangled photon pairs, and a high flux pair to background ratio. Also, for improved size, weight, and power reduction, and improved deployability of small satellite platforms, the devices described herein can be integrated onto a chip. Specifically, both the photon source and the interference detector can be integrated onto the chip.

[0019] In certain embodiments, entangled photons can be generated via spontaneous parametric down-conversion of pump photons, also known as degenerate difference frequency generation. Typically, the above methods for photon generation can produce entangled photons with polarizations orthogonal to each other. Typically, free space optics is used to separate the entangled photons and convert them to the same polarization state for use within a clock synchronization scheme. The embodiments described herein provide a chip-scale photonic integrated circuit with on-chip waveguide photonics for separating entangled photons and converting the separated photons to the same polarization state.

[0020] In some embodiments, the chip-scale photonic integrated circuit can generate and interfere time-entangled photons. The chip-scale photonic integrated circuit can combine the non-linear properties of a periodically poled potassium titanyl phosphate (ppKTP ) waveguide or a waveguide made of a material similar to ppKTP, with the low transmission loss, high confinement, and filtering capabilities of a silicon nitride waveguide or other waveguides made of materials similar to silicon nitride, to realize the optical functions for generating and interfering photons on a hybrid optical waveguide platform. A chip-scale approach using a combination of waveguides made from different materials allows for improvements over conventional types of light sources based on fibers and free space optics.

[0021] In some embodiments, materials having non-linear characteristics as well as both low transmission loss, high confinement, and filtering capabilities can be used to implement similar optical functions for generating and interfering photons within an optical waveguide platform based on a single material system such as lithium niobate.

[0022] In certain embodiments, the optical functions for generating and receiving entangled photons are implemented on a single integrated platform, resulting in reduced optical loss, enhanced mode overlap, efficient filtering of photons, increased contrast of the interferometer, and improved mechanical robustness, while reducing size, weight, and power when compared to fiber or free-space-based systems. Additionally, the embodiments described herein enable higher precision time synchronization when used within a system while allowing for use on smaller satellite platforms such as microsatellites.

[0023] FIG. 1 is a diagram illustrating a system 100 for a Hong-Ou-Mandel (HOM) interferometer. As used herein, a chip-scale integrated circuit can be used within the HOM interferometer. As used herein, the HOM interferometer is a device that can generate pump photons 101. System 100 can split the pump photons into two daughter photons 103 (referred to separately herein as photons 103-A and 103-B). For example, the pump photons 101 can be generated by a laser source that generates a laser having a wavelength of 405 nm or other desired wavelength.

[0024] In certain embodiments, the pump photon 101 is split into daughter photons 103 that are guided through an optical structure for recombination. For example, the pump photon 101 is split by the optical structure 105 into daughter photons 103-a and 103-b. The daughter photons 103 can each have a wavelength that is twice the wavelength of the pump photon 101 (i.e., if the pump photon 101 can have a wavelength of 405 nm, the daughter photons 103 can each have a wavelength of 810 nm). Additionally, the system 100 can include guiding optics 107 that guide the daughter photons 103 to a beam splitter 110 where the daughter photons 103 are combined such that the quantum superposition of the daughter photons 103-c and 103-d impinge on the detector 109 for reception. For example, detector 109-a can receive and detect daughter photon 103-a, and detector 109-b can receive and detect daughter photon 103-b, or detector 109-a can receive and detect daughter photon 103-b, and detector 109-b can receive and detect daughter photon 103-a, or detector 109-a can receive and detect both daughter photons 103-a and 103-b, or detector 109-b can receive and detect both daughter photons 103-a and 103-b in the manner of a HOM interferometer.

[0025] In some embodiments, when the detector 109 receives the associated daughter photons 103, the detector 109 can provide a signal to an electronic correlator device 111 that combines the electrical signals of the two detectors 109 for the performance of a HOM interferometer. The electronic correlator device 111 quantitatively determines the degree of temporal correlation of the signals generated by the detectors 109. For example, the electronic correlator 111 can indicate that the coincidence rate of the signals provided by the photodetectors 109 can decrease towards zero when the daughter photons 103 substantially completely overlap in time. This decrease towards a zero rate of coincident detections is the tre ー It is known as the HOM dip exemplified in the graph 113. The dip occurs when the two daughter photons 103 are substantially identical in all characteristics. The HOM dip disappears when the photons 103 become distinguishable, taking into account and in particular including the identity of the flight times of the daughter photons between the source region 105 and the beam splitter 110. In this way, the system 100 is sensitive to the quality of the flight times of the daughter photons 103 between the source region 105 and the beam splitter 110 that are substantially completely equal.

[0026] Figure 2 illustrates different optical paths 201 and 203 on the chip-scale device 200, both of which can generate photons, split photons into daughter photons, provide the daughter photons as outputs (such as in free space or an optical fiber), receive the daughter photons that may have been reflected from a remote mirror or an optical system, provide the received photons to an interferometer to perform HOM interferometry. As shown, Figure 2 illustrates the source path 201 and the interferometer path 203. In the source path 201, the incoming pump photons are split into daughter photons, and the daughter photons can be separated and directed towards different remote platforms. In the interferometer path 203, the daughter photons reflected by the remote platform are received and interfered in the manner of HOM interferometry.

[0027] In certain embodiments, the chip-scale device 200 is a non-linear optical of degenerate spontaneous parametric down conversion (dSPDC) Taking advantage of the effect, pump photon 205 is split into two "twin" daughter photons 209 and 211, which are "born" at approximately the same instant (e.g., within <100 femtoseconds of each other). This simultaneity is implemented by quantum mechanics and can be utilized to synchronize separated atomic clocks. To synchronize separated atomic clocks (i.e., when different atomic clocks are located on different satellites), the synchronization is achieved by projecting daughter photons 209 and 211 from chip-scale device 200, reflecting some of photons 209 and 211 from each of the satellites, and providing them for recombination within Hong-Ou-Mandel (HOM) interferometer 215. The purely quantum mechanical interference "dip" in the coincidence rate is observed only when the paths are substantially exactly equal, as explained above with respect to FIG. 1. The arrival times of some of the entangled quantum photons from each satellite can be compared with the classical channel, enabling the controller to synchronize the clocks with high precision (i.e., potentially with femtosecond precision).

[0028] In some embodiments, chip-scale device 200 is a chip-scale photonic integrated circuit that generates and interferes time-entangled photons. Chip-scale device 200 can include optical functions and components on a hybrid optical waveguide platform that combines the non-linear properties of a ppKTP waveguide (or other waveguides made from materials with similar properties) with the high confinement and filtering capabilities of a silicon nitride waveguide. This combination enables miniaturization, efficiency improvement, and robustness while increasing the available flux of twin photons 209 and 211.

[0029] In some embodiments, chip-scale device 200 can include optical functions and components on a single optical waveguide material platform, such as lithium niobate, that has both non-linear properties and low transmission loss, high confinement, and filtering capabilities.

[0030] In certain embodiments, the chip-scale device may generate pump photons 205 and generate daughter photons 206a and 206b in a photon generation waveguide from the pump photons 205 in the source path 201 by dSPDC. Each of the twin photons 206a and 206b may occupy a different waveguide mode of either transverse electric (TE) or transverse magnetic (TM). The vertical coupler (VC) region may adiabatically draw the daughter photons 206a and 206b from the photon generation waveguide into a photon conditioning waveguide patterned on top of the photon generation waveguide. Additionally, TM and TE photons may be separated by two diffractive waveguide mode splitters (MS). The TE photons may then pass through a bandpass filter (BPF) to eliminate background photons through a second MS and then leave the chip 200 as radiated photons 211. On the other hand, the original TM photons may be converted to the TE mode by a diffractive mode converter (MC), which may also reverse the propagation direction of the photons. This (here TE-polarized) photon may pass through its own bandpass filter and leave the chip 200 as radiated photons 209. The various functions implemented on the chip may be implemented by photon conditioning waveguides (made of silicon nitride or other similar materials in some embodiments), and the waveguide structure is patterned in a film deposited on top of the substrate containing the photon generation waveguide.

[0031] ​In an additional embodiment, the interferometer path 203, twin photons 209 and 211 can be reflected or sent back from a remote satellite or other remote system and reconnected within the photonics component waveguides on the chip-scale device 200 to complete the HOM interferometer 215. (In some embodiments, the photons can also have their polarizations rotated 90 degrees by conventional wave plates). Twin photons 209 and 211 can enter the same waveguide from which they were emitted earlier, but due to their rotated polarizations here, they can couple to the orthogonal waveguide mode (i.e., TM). Each photon can then interact with a diffraction mode splitter (MS) that can reverse the propagation direction of the photon in the waveguide, sending photons 209 and 211 to a 50 / 50 waveguide coupler. The output port of the interferometer can be directed to a photon detector 212, such as a single-photon avalanche photodetector (SP- APD), where photons 209 and 211 can be detected. The detected signal output of photon detector 212 can be directed to an electronic correlator 215 that can determine the degree of coincidence of the arrival times of the signals, thus completing the HOM interferometer 216.

[0032] Figures 3A - 3C illustrate the propagation of two photons generated by a photon generation waveguide into a photon vertical connection waveguide and through a photon adjustment waveguide network. As described above, the photon generation waveguide generates two photons having orthogonal waveguide modes, with one mode propagating in the TM mode and the other in the TE mode. Depending on the mode of the photons, the photons propagate along different paths through the waveguide network, such that the waveguide network provides two photons propagating in the TE mode from the chip and receives two photons returning onto the chip propagating in the TM mode. Figure 3A illustrates the path of the original photon in the TE mode of the photon vertical connection waveguide 302. Figure 3B illustrates the path of the original photon in the TM mode of the photon vertical connection waveguide 302. Figure 3C illustrates the path of the photon through the photon adjustment waveguide network 304 received from an external device.

[0033] In a particular embodiment illustrated in FIG. 3A, photons in the TE mode of the photon vertical connection waveguide 302 passing through the photon adjustment waveguide network 304 pass through the mode splitter 303 without diffraction. The photons then pass through a bandpass filter 305 that filters out and excludes fluorescence, as well as stray pump light coupled from the photon generation waveguide 301. The photons then pass through the mode splitter 307 without diffraction and are emitted through the output port 321.

[0034] In a particular embodiment illustrated in FIG. 3B, photons in the TM mode of the photon vertical connection waveguide 302 passing through the photon adjustment waveguide network 304 are diffracted by the mode splitter 303. The photons are further diffracted by the mode splitter 309, and thereon, the photons enter the mode converter 311. The mode converter 311 diffracts the photons again but converts the photons from the TM mode to the TE mode. Here, when the photons are in the TE mode, the photons are not diffracted by the mode splitter 309. The photons then pass through a bandpass filter 313 that filters out and excludes fluorescence, as well as stray pump light coupled from the photon generation waveguide. The photons then pass through the mode splitter 315 without diffraction and are emitted through the output port 319.

[0035] In an additional embodiment illustrated in FIG. 3C, two daughter photons emitted from the photon conditioning waveguide network 304 can be sent back from another optical device such that they are re-coupled to the photon conditioning network 304 in the TM mode in waveguides 319 and 321. The two received photons in the TM mode can each propagate within the waveguide to mode splitters 315 and 307. Both mode splitters 315 and 307 diffract the received photons. The photons are then interfered with each other via a 50 / 50 coupler 317 before being output on ports 323 and 325 for subsequent detection by a photon detector. In the above-described embodiment, the TM mode from the photon vertical coupling waveguide 302 is converted by the photon conditioning waveguide network 304 to the TE mode for transmission from the chip-scale device, while the light received back into the device for subsequent interference detection is in the TM mode. However, in another embodiment, the TE mode from the photon vertical coupling waveguide is converted by the photon conditioning waveguide network 304 to the TM mode for transmission from the chip-scale device, while the light received back into the device for subsequent interference detection is in the TE mode.

[0036] FIG. 4 illustrates different photonics components within a chip-scale device 400. For example, the chip-scale device 400 includes a photon generation waveguide 401, a photon vertical coupling waveguide 427, and a photon conditioning waveguide network (similar to the photon conditioning waveguide network 304 of FIGS. 3A - 3C), the photon conditioning waveguide network including mode splitters 403, 409, 407, and 415, a mode converter 411, bandpass filters 413 and 405, input / output waveguides 419, 421, 423, and 425, and a 50 / 50 coupler 417. Possible embodiments of the vertical coupler 427, mode splitters 403, 409, 407, and 415, mode converter 411, and bandpass filters are described in more detail below.

[0037] FIG. 5 is a side view illustrating the operation of a vertical coupler. In order to efficiently couple photons from the photon generation waveguide 501 into the photon vertical connection waveguide 503, a stacked waveguide is formed. Further, the relatively thin photon vertical connection waveguide 503 related to the width of the photon generation waveguide 501 has little perturbation to the shape of the weak confinement mode in the photon generation waveguide 501. As discussed herein, the photon vertical connection waveguide 503 gradually widens across the entire overlap portion of the stacked waveguide. For example, the photon vertical connection waveguide 503 can spread from 100 nm to 200 nm over a distance of ~500 microns. The gradual spread of the photon vertical connection waveguide 503 adiabatically draws photons from the photon generation waveguide 501 into a much more tightly confined waveguide mode. In addition, the transmission maintains the polarization modes of the propagating photons (i.e., TE→TE and TM→TM) having essentially zero-mode cross-coupling. Photons propagating in such different modes can be coupled from the photon generation waveguide 501 at different locations. Therefore, since different modes can be coupled from the photon generation waveguide 501 at different locations, the vertical coupler may be implemented in other applications such as a mode splitter.

[0038] In a further embodiment, the materials used to generate the photon generation waveguide and the materials used to generate the photon vertical connection waveguide can have a large difference between their respective refractive indices. For example, when KTP is used for the photon generation waveguide, the photon vertical connection waveguide can be fabricated using a high-concentration silicon nitride film.

[0039] FIG. 6 is a diagram illustrating a particular embodiment of a mode splitter found in the chip-scale device 200. In particular, FIG. 6 shows an isometric view 600 of the mode splitter, a detailed isometric view 610 of a portion of the mode splitter, and a frequency response graph 620 of the connection of different modes within the mode splitter. FIG. 6 shows an isometric view 600 of the mode splitter, a detailed isometric view 610 of a portion of the mode splitter, and a frequency response graph 620 of the connection of different modes within the mode splitter.

[0040] In certain embodiments, as shown in the isometric view 600, the mode splitter may include a single input port 603. Through the input port, the mode splitter may receive two photons as input 601 propagating in different orthogonal modes in the waveguide. For example, one photon may be propagating in the TE mode and another photon may be propagating in the TM mode. The mode splitter may pass one of the photons received at the input port 603 to the output port 607 as the output photon 609. For example, the mode splitter may pass the TE mode photon received at the input port 603 directly to the output port 607. Additionally, the mode splitter may diffract one of the propagating photons such that one of the propagating photons is coupled to the waveguide in the opposite direction and passes to the output port 613 as output 611. For example, the TM mode may be diffracted by the coupling portion 605 of the mode splitter and passed to the output port 613.

[0041] In some embodiments, as shown in the detailed isometric view 610 of the coupling portion 605 of the mode splitter, the mode splitter may include a chirped diffraction grating assisted counter-directional mode coupler to split two orthogonally polarized photons into different paths. As shown, the detailed isometric view 610 illustrates the waveguide structure and the graph 620 shows the result of the calculation of its spectral response. As shown, the coupling portion is composed of two closely spaced waveguides 621 and 625. The waveguide 621 may be further patterned with a modulated sidewall 623, thus forming a waveguide diffraction grating having a large overlap for TM-to-TM transition from one waveguide to the other. The effect of the modulation is to couple the TM mode from the forward direction in the waveguide 625 to the reverse direction in the waveguide 621, whereas the TE mode passes through the mode splitter in the forward direction and remains in the waveguide 625. Additionally, the frequency of the modulated sidewall 623 may vary along the length of the mode splitter to enable the desired frequency response of the mode splitter.

[0042] FIG. 7 is a diagram illustrating a particular embodiment of a mode converter found in chip scale device 200. In particular, FIG. 7 shows an isometric view 700 of a mode splitter, a detailed isometric view 710 of the conversion portion of the mode splitter, and a frequency response graph 720 of the mode conversion within the mode converter.

[0043] In a particular embodiment, as shown in isometric view 700, the mode converter may include a single port 703. Through port 703, the mode converter may receive photons as input 701 propagating in a particular mode within the waveguide. For example, the photons received through port 703 may propagate in the TM mode. The mode converter may convert the mode from one mode to an orthogonal mode within conversion portion 705, and the mode converter may convert the photons to an orthogonal propagation mode and output them through port 703 as output. For example, when the photons received at port 703 are in the TM mode, the photons output through port 703 may be in the TE mode.

[0044] In some embodiments, as shown in detailed isometric view 710 of conversion portion 705 of the mode converter, to create all waveguide paths similar to those considered for two photons, the chip scale device may use a single waveguide diffraction grating structure designed with asymmetrically modulated sidewalls 709 and 711 to reverse the in-waveguide polarization of TM photons. For example, the modulation of the sidewalls may be out of phase with each other such that the cross-section of the waveguide is constant along the length of the modulation. This asymmetric modulation forms a cross-coupling between the forward TM mode and the reverse TE mode. As shown in graph 720, mode conversion occurs only within the stop band of the diffraction grating. To control the stop band of the diffraction grating, the length of conversion portion 705 may vary with the modulation frequency of the modulated sidewalls 709 and 711. For example, the frequency of the modulated sidewalls may either decrease or increase along the length of the conversion portion of the mode converter.

[0045] FIG. 8 is a diagram illustrating a particular embodiment of a bandpass filter found in chip scale device 200. Specifically, FIG. 8 shows an isometric view 800 of the bandpass filter, a detailed isometric view 810 of the filtering portion of the bandpass filter, and a frequency response graph 820 of the filtering of photons by the bandpass filter.

[0046] In a particular embodiment, as shown in isometric view 800, the bandpass filter may include a single port 803. Through port 803, the bandpass filter may receive photons as input 801 propagating in a particular mode within the waveguide. For example, photons received through input port 803 may propagate in the TE mode. The bandpass filter may filter photons having an undesirable wavelength within filtering portion 805 and provide the filtered photons through output port 807 as output 809.

[0047] In some embodiments, as shown in detailed isometric view 810 of filtering portion 805 of the bandpass filter, a waveguide bandpass filter is implemented to eliminate any background fluorescence photons that may propagate within the waveguide and to eliminate any residual pump photons. As shown, the filter is fabricated from two high reflectivity waveguide diffraction gratings 811 and 813 that appear as a chirp of modulation period along the length of the waveguide, in other words, the modulation of the waveguide diffraction grating varies symmetrically longitudinally along the length of the sidewalls of the filter. Light just outside the passband is diffracted back down below the waveguide, while light at the pump wavelength is completely scattered from the waveguide. In some embodiments, the spectral locations of waveguide diffraction gratings 811 and 813 may vary along the length of filtering portion 805 of the passband.

[0048] FIG. 9 is a method 900 of using a chip-scale device to generate and interfere a pair of correlated photons as described above. The method 900 proceeds to 901 where a pair of photons are generated in a photon generation waveguide. Additionally, the method 900 proceeds to 903 where the pair of photons are coupled to a photon vertical coupling waveguide. Further, the method 900 proceeds to 905 where one of the photons in the pair of photons is converted within a photon conditioning waveguide network such that the photons propagate in the same mode within two different waveguides. In a particular embodiment, the method 900 proceeds to 907 where the photons are provided to one or more external devices. Further, the method 900 proceeds to 909 where the photons are received from one or more external devices. Additionally, the method 900 proceeds to 911 where an interference method is performed on the received photons.

[0049] FIG. 10 is a method 1000 for vertically coupling two photons from a first waveguide to a second waveguide. The method 1000 proceeds to 1001 where a first photon and a second photon are generated in a first waveguide within a first waveguide layer. Further, the first photon and the second photon may be in different modes orthogonal to each other. For example, the first photon may propagate in a TE mode and the second photon may propagate in a TM mode. Additionally, the method 1000 proceeds to 1003 where the first photon is coupled from the first waveguide to the second waveguide at a first location within a coupling portion of the second waveguide. Further, the method 1000 proceeds to 1005 where the second photon is coupled from the first waveguide to the second waveguide at a second location different from the first location within the coupling portion. For example, the first photon and the second photon are coupled to one of the first location and the second location based on the mode of propagation within the first waveguide. Exemplary embodiments

[0050] Example 1 is a device having a first photon and a second photon propagating therein A first waveguide in which a first photon and a second photon propagate in orthogonal modes, and a second waveguide having a second connection portion proximate to the first connection portion of the first waveguide, wherein the physical relationship between the first waveguide and the second waveguide along the length of the second connection portion causes adiabatic transfer of the first photon and the second photon into separate orthogonal modes of the second waveguide at different locations of the second connection portion, including a device.

[0051] Example 2 includes the device described in Example 1, in which adiabatic transfer of the first photon and the second photon into the second waveguide maintains the orthogonal modes of the first photon and the second photon when propagating within the first waveguide.

[0052] Example 3 includes the device described in Example 1 or 2, in which the first photon is in a TE mode and the second photon is in a TM mode.

[0053] Example 4 includes the device described in any one of Examples 1 to 3, in which the first photon is connected to the second connection portion before the second photon.

[0054] Example 5 includes the device described in any one of Examples 1 to 4, in which the first waveguide is formed within a first waveguide layer, the second waveguide is formed within a second waveguide layer, and the first waveguide layer and the second waveguide layer are made of materials having different refractive indices.

[0055] Example 6 includes the device described in Example 5, in which the first waveguide layer is made of periodically poled lithium niobate.

[0056] Example 7 includes the device described in Example 5 or 6, in which the second waveguide layer is made of silicon nitride.

[0057] Example 8 includes the device described in any one of Examples 1 to 7, in which the first photon and the second photon are generated within the first waveguide.

[0058] Example 9 includes the device according to any one of Examples 1 to 8, wherein the physical relationship includes varying the width of the second waveguide along the length of the second connection portion.

[0059] Example 10 includes the device according to Example 9, wherein the width gradually changes by spreading along the propagation directions of the first photon and the second photon within the second waveguide.

[0060] Example 11 is a device comprising: a first waveguide layer having a first waveguide therein, the first waveguide having a first photon and a second photon propagating therein, the first photon and the second photon propagating in orthogonal modes; a second waveguide layer having a second waveguide therein, the second waveguide having a second connection portion proximate to the first connection portion of the first waveguide, the first photon and the second photon being adiabatically transferred to separate orthogonal modes of the second waveguide; wherein the first waveguide layer and the second waveguide layer are made of materials having different refractive indices.

[0061] Example 12 includes the device according to Example 11, wherein the width of the second waveguide changes along the length of the second connection portion.

[0062] Example 13 includes the device according to Example 11 or 12, wherein the width gradually changes by spreading along the propagation directions of the first photon and the second photon within the second waveguide.

[0063] Example 14 includes the device according to any one of Examples 11 to 13, wherein the first photon and the second photon are connected to the second waveguide at different locations within the second coupling portion.

[0064] Example 15 includes the device according to any one of Examples 10 to 14, wherein the first photon is connected to the second connection portion before the second photon.

[0065] Example 16 includes the device according to any one of Examples 10 to 15, in which the first waveguide layer is made of periodically poled potassium titanyl phosphate.

[0066] Example 17 includes the device according to any one of Examples 10 to 16, in which the second waveguide layer is made of silicon nitride.

[0067] Example 18 is a method including generating a first photon and a second photon in a first waveguide formed in a first waveguide layer, where the first photon is in a first mode and the second photon is in a second mode orthogonal to the first mode; coupling the first photon from the first waveguide to a second waveguide at a first location within a connection portion of the second waveguide, where the connection portion is a section of the second waveguide adjacent to the first waveguide; coupling the second photon from the first waveguide into the second waveguide at a second location separate from the first location within the connection portion, where the first photon and the second photon are coupled to one of the first location and the second location based on whether the propagation in the first waveguide is in the first mode or the second mode.

[0068] Example 19 includes the method according to Example 18, in which the width of the second waveguide varies along the length of the connection portion by spreading along the propagation direction of the first photon and the second photon within the second waveguide.

[0069] Example 20 includes the method according to Example 18 or 19, in which the first waveguide is formed in the first waveguide layer, the second waveguide is formed in the second waveguide layer, and the first waveguide layer and the second waveguide layer are made of materials having different refractive indices.

[0070] Although specific embodiments are illustrated and described in this specification, it will be understood by those skilled in the art that any configuration that is predicted to achieve the same purpose may be used instead of the specific embodiments shown. Therefore, it is clearly intended that the present invention be limited only by the claims and their equivalents.

Claims

1. A device, comprising: a first waveguide (501) having a first photon and a second photon propagating therein, the first photon and the second photon propagating in an orthogonal mode; and a second waveguide (503) having a second coupling portion in close proximity to a first coupling portion of the first waveguide (501), wherein a physical relationship between the first waveguide (501) and the second waveguide (503) along a length of the second coupling portion causes adiabatic transfer of the first photons and the second photons to separate orthogonal modes of the second waveguide (503) at different locations of the second coupling portion.

2. The device of claim 1 , wherein the physical relationship includes varying a width of the second waveguide along a length of the second coupling portion.

3. 1. A method comprising: generating first and second photons in a first waveguide (501) formed in a first waveguide (501) layer, the first photons being in a first mode and the second photons being in a second mode orthogonal to the first mode; coupling the first photon from the first waveguide (501) to a second waveguide (503) at a first location within a coupling portion of the second waveguide (503), the coupling portion being a section of the second waveguide (503) proximate to the first waveguide (501); coupling the second photons from the first waveguide (501) into the second waveguide (503) at a second location within the coupling portion distinct from the first location, wherein the first photons and the second photons are coupled to one of the first location and the second location based on whether propagation within the first waveguide (501) is in the first mode or the second mode.

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