Hybrid guided-wave and free-space system for broadband integrated light delivery

Abstract

In one example, an apparatus includes a planar lens comprising a slab waveguide for guiding an optical mode, the slab waveguide having a first end where the optical mode is received as an input from a strip waveguide and a second end, a facet formed in the second end of the slab waveguide, and a reflector positioned at an exit of the facet and configured to redirect the optical mode to a point in free space when the optical mode exits the facet.

Description

HYBRID GUIDED-WAVE AND FREE-SPACE SYSTEM FOR BROADBAND INTEGRATED LIGHT DELIVERYCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States Provisional Patent Application Serial No. 63 / 472,325, filed June 11 , 2023, which is herein incorporated by reference in its entirety.REFERENCE TO GOVERNMENT FUNDING

[0002] This invention was made with Government support under contract number 2016245, awarded by the National Science Foundation. The Government has certain rights in the invention.BACKGROUND

[0003] Photonic integrated circuits (PICs) are utilized in many fields due to their compact size, phase stability, and ability to be mass produced in semiconductor foundries at low cost. For instance, PIC-enabled waveguide-to-free-space beam delivery has been deployed in ion trap quantum computing, atomic clocks, optical tweezers, and other applications. Grating couplers are commonly used in conjunction with the PICs, since through careful design, the grating couplers can generate diffraction-limited focused spots into free space from a waveguide input.SUMMARY OF THE DISCLOSURE

[0004] In one example, an apparatus includes a planar lens comprising a slab waveguide for guiding an optical mode, the slab waveguide having a first end where the optical mode is received as an input from a strip waveguide and a second end, a facet formed in the second end of the slab waveguide, and a reflector positioned at an exit of the facet and configured to redirect the optical mode to a point in free space when the optical mode exits the facet.

[0005] In another example, an apparatus includes a strip waveguide to receive an optical mode, a planar lens coupled to the strip waveguide at a junction, the planar lens including a slab waveguide for expanding the optical mode as the optical mode propagates from the junction toward a curved facet formed in theslab waveguide, wherein the slab waveguide is wider than the strip waveguide, but the slab waveguide has a thickness that is equal to a thickness of the strip waveguide, the curved facet, wherein the curved facet is configured to collimate the optical mode in a single direction, and a metal coating layer formed over the slab waveguide, wherein the metal coating layer includes an electrode to trap ion quantum bits, and an asymmetric freeform reflector positioned at an exit of the curved facet and configured to focus the optical mode into a diffraction limited spot free space when the optical mode exits the curved facet, wherein the reflector is arranged in a 4f optical configuration with the waveguide and the planar lens.

[0006] In another examples, a method includes expanding an optical mode propagating in a first direction through a photonic integrated circuit to produce an expanded optical mode, collimating the expanded optical mode to produce a collimated optical mode, and redirecting the collimated optical mode to propagate in a second direction that is orthogonal to the first direction, so that the collimated optical mode is focused into a diffraction limited spot in a free space above the photonic integrated circuit.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The teaching of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0008] FIG. 1 A illustrates a cross-sectional view of one example of a quantum photonic integrated circuit, according to examples of the present disclosure;

[0009] FIG. 1 B illustrates a top view of the quantum photonic integrated circuit of FIG. 1A;

[0010] FIG. 2 illustrates a chart showing examples of simulated optical profiles in a planar lens of the present disclosure for 375, 455, 650, and 866 nanometer (nm) wavelengths;

[0011] FIG. 3 illustrates a graph plotting an example transmission efficiency of the planar lens of the present disclosure for wavelengths from 375 to 866 nm;

[0012] FIG. 4 illustrates a plurality of graphs the far field angles of an examplebeam emitted from the facet of the planar lens of FIGs. 1A and 1B;

[0013] FIG. 5 illustrates an example virtual plane bounded by optical fields emitted from the planar lens and by the diffraction limited spot of FIGs. 1A and 1B;

[0014] FIG. 6 illustrates a plurality of example optical field profiles calculated by finite difference time domain simulation at a plane above the reflector 108 of FIGs. 1 A and 1 B for various wavelengths;

[0015] FIG. 7 illustrates a plurality of example far-field distributions in the X-Y plane at 100 micrometers above the photonic integrated circuit of FIGs. 1A and 1B (the focal plane) for 375, 455, 650, and 866 nm wavelengths for one of the input waveguides; and

[0016] FIG. 8 illustrates a flow chart of one example of a method for broadband integrated light delivery, according to examples of the present disclosure.

[0017] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.DETAILED DESCRIPTION

[0018] The present disclosure provides examples of a hybrid apparatus and method for guided-wave and free-space broadband integrated light delivery. For instance, examples of the present disclosure provide a hybrid guided-wave and free-space optical system for tightly-focused, multiple-wavelength control of multiple trapped ion quantum bits (also referred to as “qubits,” or basic units of quantum data).

[0019] As discussed above, photonic integrated circuits (PICs) are utilized in many fields due to their compact size, phase stability, and ability to be mass produced in semiconductor foundries at low cost. For instance, PIC-enabled waveguide-to-free-space beam delivery has been deployed in ion trap quantum computing, atomic clocks, optical tweezers, and other applications. Grating couplers are commonly used in conjunction with the PICs, since through careful design, the grating couplers can generate diffraction-limited focused spots into free space from a waveguide input. However, grating couplers may suffer fromdrawbacks including narrow optical bandwidth, limited efficiency, sensitivity to light polarization, and sensitivity of the emission angle to fabrication variation.

[0020] Quantum systems require stable delivery of multiple wavelengths (often spanning the near ultraviolet (NUV), visible, and near infrared (NIR) spectrums) to multiple locations which may reside tens to hundreds of micrometers above the PIC. To meet these conditions, multiple grating couplers (which typically operate in a single wavelength) are typically required for each unit cell of a quantum system. The greater the number of grating couplers that must be fabricated for the quantum system, the greater the influence of fabrication variance on the performance of the quantum system.

[0021] Examples of the present disclosure employ a 4f optical configuration to provide low-loss broadband light delivery from in-plane waveguides to out-ofplane free space beams. In one example, a single asymmetric freeform reflector is used in place of one or more grating couplers to focus a collimated optical mode exiting a waveguide into a diffraction limited spot in free space. Examples of the present disclosure may be used to control qubits in a trapped ion quantum computer. In further examples, however, examples of the present disclosure may be extended to support other integrated beam delivery applications. Examples of the present disclosure simplify the challenges of free-space beam alignment, stabilization, and scalability issues. These and other advantages of the present disclosure are discussed in further detail below in connection with FIGs. 1-8.

[0022] FIG. 1 A illustrates a cross-sectional view of one example of a quantum photonic integrated circuit (PIC) 100, according to examples of the present disclosure. FIG. 1 B illustrates a top view of the quantum photonic integrated circuit 100 of FIG. 1A. As illustrated in FIGs. 1A and 1 B, the PIC 100 generally comprises a substrate 102 upon which a plurality of optical elements is arranged. In one example, the plurality of optical elements includes a plurality of (i.e., at least two) input waveguides 104i-104n(hereinafter individually referred to as an “input waveguide 104” or collectively referred to as “input waveguides 104”), a planar lens 106 (e.g., a planar waveguide lens), and a reflector 108.

[0023] In one example, the substrate 102 may comprise a silicon (Si) silicon substrate. However, in other examples, the substrate 102 may be formed of othersemiconductor materials, such as silicon carbide (SiC), polycrystalline silicon, germanium (Ge), gallium (Ga), indium (In), gallium nitride (GaN), tin (Sn), zinc (Zn), or the like.

[0024] In one example, each of the input waveguides 104 may comprise a strip waveguide configured to guide light of a plurality of wavelengths through the PIC 100. In one example, the input waveguides 104 may be directly coupled to the planar lens 106 at slightly different offsets and angles (e.g., so that the input waveguides 104 are not positioned parallel to each other and do not guide respective wavelengths of light along parallel paths).

[0025] In one example, the planar lens 106 comprises a slab waveguide configured to operate as a spherical lens. The planar lens 106 may be positioned to collimate light guided by the input waveguides 104 into a guided optical mode (beam) 110 in the lateral direction (z-direction in FIGs. 1A and 1 B). To this end, the planar lens 106 may comprise a waveguide core 112 through which the light that is collimated from the input waveguides 104 propagates and a cladding layer 114 that surrounds and optically isolates the waveguide core 112. In one example, the waveguide core 112 may comprise aluminum oxide (AI2O3), which is transparent for visible and ultraviolet wavelengths; however, in other examples, the waveguide core 112 may be formed of other materials including polymers (e.g., photoresists, polytetrafluoroethylene, etc.), halides and chalcogenides (e.g., zinc selenide (ZnSe), calcium fluoride (CaF2), sodium bromide (NaBr), etc.), oxides (e.g., silicon dioxide (SiC ), aluminum oxide (AI2O3), titanium dioxide (TiO?), tantalum pentoxide (Ta2Os), etc.), nitrides (e.g., silicon nitride (SisN i), gallium nitride (GaN), etc.), diamond, or semiconductors.

[0026] In one example, the cladding layer 114 is formed from a material or combination of materials having a lower index of refraction than the material from which the waveguide core 112 is formed. For instance, if the waveguide core 112 is formed of aluminum oxide, then the cladding layer 114 may be formed of silicon dioxide. At least a first portion of the cladding layer 114 may directly contact the substrate 102, so that the first portion of the cladding layer 114 is positioned between (i.e., physically separates) the substrate 102 and the waveguide core 112. In another example, a dielectric layer (not shown) formedof, for instance silicon nitride (SisNzi), may be deposited on the substrate 102, so that the dielectric layer is positioned between the substrate 102 and the first portion of the cladding layer 114.

[0027] A second portion of the cladding layer 114 that does not directly contact the substrate 102 may be coated with a metal coating layer 116. In one example, the metal coating layer 116 comprises aluminum (Al); however, in other examples, the metal coating layer 116 may comprise other metallic materials (including silicon nitride, hafnium oxide, and other materials). The metal coating layer 116 provides one or more electrodes for trapping ion qubits and performing quantum operations. In one example, the thickness of the cladding layer 114 provides sufficient separation between the waveguide core 112 and the metal coating layer 116 to minimize optical absorption of the optical mode propagating through the waveguide core 112.

[0028] The optical mode 110 is emitted as a beam to free-space 118 after passing through the planar lens 106, at which point the beam begins to diverge or spread in the vertical direction (y-direction in FIGs. 1A and 1 B). The reflector 108 may be positioned in the path of the propagation of light through the waveguide core 112 of the planar lens 106 (e.g., at an exit of the planar lens 106). In one example, the reflector 108 may comprise a freeform reflector. In a further example, the reflector 108 may include a metal coating.

[0029] The reflector 108 may focus the diverging beam to one or more diffraction-limited spots 120, where each diffraction limited spot 120 may be located at approximately one hundred micrometers above the surface of the substrate 102, as shown in FIG. 1A.

[0030] In one example, the planar lens 106 and the reflector 108 are arranged in 4f configuration. The 4f configuration is an optical relay focal arrangement that typically includes two positive lenses, with the input plane located one focal length (f1 ) in front of the first lens and the output plane located one focal length (f2) after the second lens. When the planar lens 106 and the reflector 108 are arranged in a 4f configuration, light from the different input waveguides 104 will be focused into different diffraction limited spots 120. For a trapped ion quantum system, light from each input waveguide 104 containing multiple wavelengths may befocused on a qubit in an array of trapped ions. Tight focusing ensures low crosstalk between adjacent ion qubits.

[0031] Another advantage of the PIC 100 is the broadband light delivery. Optical beams with different wavelengths can be focused onto roughly the same diffraction limited spot 120. This is important, since a trapped ion qubit should interact with multiple wavelength laser beams for quantum operations.

[0032] The PIC 100 achieves guided-wave manipulation via the planar lens 106. As discussed above, the planar lens 106 may, in one example, comprise a single spherical lens. However, in other examples, the PIC 100 may be tailored to a desired application, and multiple lenses or materials can be used to minimize aberration and customize the beam shape. In this example, the reflector 108 may comprise a three-dimensional reflector that is arranged to redirect and focus the light out-of-plane onto a trapped ion qubit. The reflector 108 may be designed with different vertical and horizontal radii of curvature to match the asymmetric mode from the planar lens 106, accomplishing diffraction-limited focusing 100 pm above the surface of the substrate 102. Freeform three-dimensional printing with two-photon polymerization may be used to create a highly controllable reflector 108 with an optically smooth surface, which is covered with thin-film aluminum to enable reflection. Due to the refractive and reflective nature of the disclosed PIC 100, the quantum PIC 100 is broadband and has very low optical loss.

[0033] As discussed above, examples of the disclosed PIC 100 rely on 4f optics to image waveguide inputs to free space beam outputs. The first interface in a two-lens 4f system is a waveguide junction 122. An input (strip) waveguide 104 containing a fully guided mode is joined with the slab waveguide of the planar lens 106 at the junction 122. Thus, the junction 122 is located at a first end of the planar lens 106, where the optical mode 110 is received as an input from an input waveguide 104. The optical mode 110 exits a second end of the planar lens 106 as an output.

[0034] As shown in FIG. 1 B, since the slab waveguide is wider than the strip waveguide, the optical mode 110 begins to diverge in the width (z) direction as the optical mode propagates in the x direction through the planar lens 106. The thickness t of the slab waveguide remains constant, so that the optical mode 110remains confined in the vertical (y) direction. The divergence half angle of the strip / slab interface in this case is a function of the effective refractive index of the strip waveguide and the slab waveguide. In one example, the divergence half angle 0 may be calculated according to:where neff, strip represents the effective refractive index of the strip waveguide and neff, slab represents the effective refractive index of the slab waveguide.

[0035] After the waveguide junction 122, the optical mode 110 expands with an angle 0 within the slab waveguide. The size of the optical mode 110 is a function of the length I of the slab waveguide. In one example, a slab waveguide having a length £ =140 pm is used to expand the optical mode 110 to ~80 pm in width to achieve a tightly focused diffraction limited spot 120. For applications where such tight focusing of the diffraction limited spot 120 is not required, a slab waveguide having a shorter length £ and a smaller lens radius could be used.

[0036] After the optical mode 110 expands within the slab waveguide, the optical mode 110 is partially collimated by a facet (curved portion) 124 of the planar lens 106. In the width (z) direction, an end of the slab waveguide is curved to form the facet 124 of the planar lens 106 as shown in FIG. 1 B. In the height (y) direction, the waveguide core 112 and cladding layer 114 of the planar lens 106 may be vertically etched, essentially forming a cylindrical lens where the light is collimated along one direction, but not collimated along the perpendicular direction. The divergence angle 0 along the width (z) direction is dictated by the curvature and effective index of the facet 124 of the planar lens 106, whereas the divergence angle along the height (y) direction is a function of the size of the optical mode 110.

[0037] To achieve collimation of the optical mode 110, the radius of curvature of the facet 124 of the planar lens 106 may be chosen to satisfy:where f represents the focal length f of the planar lens 106 (i.e., the length of the optical path from the waveguide junction 122 to the lens facet 124), R represents the radius of the lens facet 124. EQN. 2 is derived from the ray optical lensmaker equation, assuming a waveguide-air interface. The radius R was finetuned in finite difference time domain (FDTD) simulation to minimize the numerical aperture along the width direction, maximizing the collimation.

[0038] Given the hybrid guided-wave and free-space nature of the planar lens 106, the optical mode 110 exiting the planar lens 106 is highly elliptical. The light must also be directed out-of-plane, so an off-axis reflecting element is necessary. As such, a standard symmetric reflector cannot accurately manipulate light for these purposes. A more complex, optimized reflector geometry would be needed to achieve the desired diffraction limited results.

[0039] To achieve the desired diffraction limited results of the present disclosure, the exact numerical aperture and beam size of the light exiting the planar lens 106 were captured with three-dimensional FDTD simulation. The light specifications were translated to an industry-standard ray tracing software where the optical source was rebuilt to model the FDTD result. To match the asymmetric optical source, an asymmetric reflector is needed. In one example, the surface of the reflector 108 may be parameterized according to a biconical Zernicke equation as:(EQN. 3) where z is the sag of the reflector 108, r is a radial coordinate of the reflector 108, cr / cvis the curvature of the reflector 108, and kJkvis the conic constant in thex / y direction of the reflector 108. Sixteen polynomial terms were used, to increase the degrees of freedom of the design.

[0040] To achieve diffraction limited focusing, the Modulation Transfer Function (MTF) at a given spatial frequency was optimized using the damped least-squares method. The damped least-squares method minimizes the wavefront differences among pairs of rays according to the targeted spatial frequency. The ray tracing optimization produces an elliptical spot (e.g., diffraction limited spot 120) with sub-micron diameter size in the lateral direction (along the ion chain) and ~5 micron diameter in the transverse direction (perpendicular to the ion chain), which is effective for controlling individual ion qubits in an ion chain with minimum crosstalk. The optimized reflector shape was then imported to the FDTD model for a more accurate simulation of the optical field profiles.

[0041] The PIC 100 can effectively extract guided waves from photonic integrated circuits into free space with desired optical field profiles, allowing for versatile applications such as focusing and beamsteering for optical beams with a broad range of wavelengths. In further examples, the top surface of the PIC 100 can be coated with metal (e.g., aluminum, gold, niobium, or the like), which is highly beneficial for quantum applications where exposed dielectric materials can cause unwanted charging effects. The PIC 100 has the potential to unlock countless applications across many fields where integrated beam delivery is needed.

[0042] It should be noted that the example design illustrated in FIGs. 1A and 1B provides one way in which a quantum PIC of the present disclosure may be arranged. Other designs of the planar waveguide lenses and three dimensional reflectors can be used to improve and enhance the functionality or the optical quality of the focused beams. The hybrid PIC can also be combined with other known waveguide components, such as metal electrodes for trapping and manipulating ion qubits, splitters, combiners, wavelength division multiplexers and demultiplexers, optical modulators, detectors, and optical switches to enhance the functionality of the quantum PIC.

[0043] FIG. 2 illustrates a chart 200 showing examples of simulated optical profiles in a planar lens of the present disclosure (such as planar lens 106 of FIGs. 1 A and 1 B) for 375, 455, 650, and 866 nanometer (nm) wavelengths. The 375, 455, 650, and 866 nm wavelengths may be used to manipulate calcium and barium ions. In examples where other types of ions are to be manipulated, a different set of wavelengths may be used. The broadband nature of the quantum PIC of the present disclosure ensures that the quantum PIC can be used for a wide range of wavelengths and applications.

[0044] FIG. 3 illustrates a graph plotting an example transmission efficiency of the planar lens of the present disclosure for wavelengths from 375 to 866 nm. As shown in FIG. 3, efficiency higher than 92% can be achieved by the planar lens of the present disclosure (such as planar lens 106 of FIGs. 1A and 1 B). Experimental results have further shown that the PIC of the present disclosure can achieve overall optical efficiency of approximately ninety percent for all wavelengths from 375 to 866 nm.

[0045] FIG. 4 illustrates a plurality of graphs showing the far field angles of an example beam emitted from the facet 124 of the planar lens 106 of FIGs. 1A and 1 B. The beam has a narrow emission angle in the transverse direction (z direction in FIGs. 1A and 1 B) and a broad angle in the vertical direction (y direction in FIGs. 1A and 1 B).

[0046] FIG. 5 illustrates an example virtual plane 500 bounded by optical fields emitted from the planar lens 106 and by the diffraction limited spot 120 of FIGs. 1 A and 1 B. In one example, the optical fields may be collected and converted in the virtual plane 500 to a numerical aperture and beam size for optimizing the shape of the reflector 108. In one example, the optical fields may be obtained by ray tracing (which may be performed using a software application, such as the Zemax software application). The source from the PIC(s) may be modeled from the numerical aperture and beam size obtained from FDTD to accurately represent the planar lens system.

[0047] FIG. 6 illustrates a plurality of example optical field profiles calculated by finite difference time domain simulation in a horizontal plane above the reflector 108 of FIGs. 1A and 1 B for various wavelengths. In one example, theoptical fields may be used to calculate the far-field profiles at the focal plane where the ion qubits are located.

[0048] FIG. 7 illustrates a plurality of examples far-field distributions in the X-Y plane at 100 micrometers above the photonic integrated circuit 100 of FIGs. 1A and 1 B (the focal plane) for 375, 455, 650, and 866 nm wavelengths for one of the input waveguides 104. As illustrated, the optical beams have a good overlap at the trapped ion locations for all wavelengths.

[0049] FIG. 8 illustrates a flow chart of one example of a method 800 for broadband integrated light delivery, according to examples of the present disclosure. In one example, the method 800 may be performed using the PIC illustrated in FIGs. 1A and 1 B and may be implemented to extract guided waves of light from a photonic integrated circuit into free space (e.g., forthe purposes of focusing, beamsteering, and / or other applications including quantum applications).

[0050] The method 800 may begin in step 802. In step 804, an optical mode propagating in a first direction through a photonic integrated circuit may be expanded to produce an expanded optical mode.

[0051] In one example, the optical mode may contain a plurality of different wavelengths of light. In one example, the optical mode may be provided as an input to the photonic integrated circuit via a strip waveguide. The strip waveguide may be coupled to a slab waveguide at a junction. The slab waveguide may be wider (e.g., in a direction that is orthogonal to the first direction) than the strip waveguide, so that as the optical mode propagates through the junction and into the slab waveguide, the optical mode begins to diverge or widen.

[0052] In step 806, the expanded optical mode may be collimated to produce a collimated optical mode. In one example, collimation of the optical mode may be achieved using a planar lens that is positioned at an end of the slab waveguide, opposite the end of the slab waveguide that is coupled to the strip waveguide. In one example, the planar lens may comprise a cylindrical lens that collimates the optical mode along a single direction.

[0053] In step 808, the collimated optical mode may be redirected to propagate in a second direction that is orthogonal to the first direction, so that the collimatedoptical mode produces a diffraction limited spot in a free space above the photonic integrated circuit. In one example, the collimated optical mode is redirected as the collimated mode exits the planar lens. At this point, the optical mode may have an elliptical shape. In one example, a reflector is positioned in a direction of propagation of the collimated optical mode so that as the collimated optical mode exits the planar lens, the reflector directs the collimated optical mode out-of-plane. Thus, the reflector may comprise an asymmetric reflector.

[0054] The diffraction limited spot produced in free space by the redirected collimated optical mode may comprise an elliptical spot having a sub-micron diameter size in the lateral direction (e.g., along the ion chain) and a ~5 micron diameter in the transverse direction (e.g., perpendicular to the ion chain). As discussed above, this result is useful for controlling individual ion qubits in an ion chain with minimum crosstalk.

[0055] The method 800 may end in step 810.

[0056] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

IN THE CLAIMSWhat is claimed is:

1. An apparatus comprising: a planar lens comprising a slab waveguide for guiding an optical mode, the slab waveguide having a first end where the optical mode is received as an input from a strip waveguide and a second end; a facet formed in the second end of the slab waveguide; and a reflector positioned at an exit of the facet and configured to redirect the optical mode to a point in free space when the optical mode exits the facet.

2. The apparatus of claim 1 , wherein the apparatus comprises a quantum photonic integrated circuit.

3. The apparatus of claim 2, wherein the quantum photonic integrated circuit is configured to trap ion quantum bits.

4. The apparatus of claim 3, further comprising: a metal coating layer deposited over the slab waveguide, wherein the metal coating layer includes an electrode to trap the ion quantum bits.

5. The apparatus of claim 1 , wherein the strip waveguide guides light of a plurality of different wavelengths.

6. The apparatus of claim 1 , wherein the slab waveguide is coupled to the strip waveguide at a junction to expand the optical mode as the optical mode propagates toward the facet.

7. The apparatus of claim 6, wherein the slab waveguide is wider than the strip waveguide, but the slab waveguide has a thickness that is equal to a thickness of the strip waveguide.

8. The apparatus of claim 1 , wherein the facet is configured to form a spherical planar lens.

9. The apparatus of claim 8, wherein the spherical planar lens collimates the optical mode in a single direction.

10. The apparatus of claim 1 , wherein the reflector is arranged in a 4f optical configuration with the planar lens.11 . The apparatus of claim 1 , wherein the reflector comprises an asymmetric freeform reflector.

12. The apparatus of claim 11 , wherein the point in the free space comprises a diffraction limited spot.

13. The apparatus of claim 11 , wherein the asymmetric freeform reflector further comprises a metal coating.

14. An apparatus comprising: a strip waveguide to receive an optical mode; a planar lens coupled to the strip waveguide at a junction, the planar lens comprising: a slab waveguide for expanding the optical mode as the optical mode propagates from the junction toward a curved facet formed in the slab waveguide, wherein the slab waveguide is wider than the strip waveguide, but the slab waveguide has a thickness that is equal to a thickness of the strip waveguide, wherein the curved facet is configured to collimate the optical mode in a single direction; and a metal coating layer formed over the slab waveguide, wherein the metal coating layer includes an electrode to trap ion quantum bits; and an asymmetric freeform reflector positioned at an exit of the curved facet and configured to focus the optical mode into a diffraction limited spot free spacewhen the optical mode exits the curved facet, wherein the asymmetric freeform reflector is arranged in a 4f optical configuration with the planar lens.

15. A method comprising: expanding an optical mode propagating in a first direction through a photonic integrated circuit to produce an expanded optical mode; collimating the expanded optical mode to produce a collimated optical mode; and redirecting the collimated optical mode to propagate in a second direction that is orthogonal to the first direction, so that the collimated optical mode is focused into a diffraction limited spot in a free space above the photonic integrated circuit.

16. The method of claim 15, wherein the optical mode contains a plurality of different wavelengths of light.

17. The method of claim 15, wherein the expanding is performed by providing a junction at which a strip waveguide via which the optical mode is input into the photonic integrated circuit is coupled to a slab waveguide, wherein a width of the slab waveguide is greater than a width of the strip waveguide.

18. The method of claim 17, wherein the slab waveguide is configured to operate as a planar lens.

19. The method of claim 18, wherein the collimating is performed using a curved facet formed in an end of the slab waveguide.

20. The method of claim 19, wherein the redirecting is performed using an asymmetric freeform reflector positioned at an exit of the curved facet.

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