Encapsulated microwave-to-optics transduction device

Abstract

Some embodiments in the present disclosure relate to a photon transduction device for transducing between microwave and optical frequencies including a superconducting element, and means for preventing optical frequency radiation from being absorbed by the superconducting element.

Description

[0001] Encapsulated Microwave-to-optics Transduction Device

[0002] TECHNICAL FIELD

[0003] Embodiments of the present invention relate to the field of Microwave-to-optics transduction devices. This includes the reduction of unwanted interactions between sub-domains of the devices or between device elements and the outside environment.

[0004] BACKGROUND

[0005] Devices that can translate photons between microwave and optical frequencies are a very promising route to scale the size of quantum computers, both by enabling the use of optical channels to manipulate and measure microwave-frequency qubits, as well as realising roomtemperature long-range interconnects between separate quantum processing devices.

[0006] The devices that can perform few- and single-photon level frequency conversion require the combination of superconducting and optical devices, such as resonators, as well as potentially intermediate modes, for instance nanomechanical resonators. These intermediate modes help separate optical fields spatially from superconducting elements. In particular, such transduction devices may coherently route quantum information between microwave-based qubits and telecom-based quantum network infrastructure, typically requiring efficient and low- noise coupling elements with minimum crosstalk between the telecom and microwave components beyond the designed interaction points. The comprised optical devices may for example include nanophotonic elements such as waveguides and couplers. One drawback of the combination of structures is the potential for light-induced microwave noise in the superconducting elements stemming from unwanted scattering of optical fields at device interfaces and at manufacturing imperfections. As such, mitigation and prevention of scattered optical photons reaching superconducting elements is paramount for creating efficient microwave-to-optical quantum transducers.

[0007] While recent progress in micro-electromechanical systems (MEMS) led to widespread implementation of encapsulation concepts for electro-optical devices, the challenge of isolating electro-optomechanical quantum devices from ambient and intra-device noise sources is still an open research question. This challenge is exasperated by the sensitivity of low-noise quantum transduction to changes in the device environment or material composition, for example through aging. Thus, efficient noise-isolation and consistent device performance in general may be desirable. This may be achieved by a combination of light-tight encapsulation and hermetic sealing processes.

[0008] SUMMARY

[0009] Embodiments in the present disclosure relate to a photon transduction device for transducing between microwave and optical frequencies including a superconducting element, and means for preventing optical frequency radiation from being absorbed by the superconducting element.

[0010] BRIEF DESCRIPTION OF DRAWINGS

[0011] An understanding of the nature and advantages of various embodiments may be realized by reference to the following figures.

[0012] Figs. 1A-B represent the transducing device by a series of coupled modes, themselves coupled to external microwave or optical waveguides via a fiber coupler and a bond pad and coaxial connector;

[0013] Figs. 2A-D illustrate a photonic crystal light barrier added to the device between the optical and the microwave resonator; in particular, Figs. 2C-D illustrate additional realizations to encapsulate and hermetically seal the transducing device via an inclusion of the whole die into the hermetic seal, as well as utilizing individual dies for separate functionality and a common hermetic seal;

[0014] Figs. 3A-B illustrate a light-tight encapsulation surrounding a microwave-frequency resonator, wherein the microwave resonator is encapsulated with a deposition of a light-tight material;

[0015] Figs. 4A-B illustrate a light-tight encapsulation surrounding a microwave-frequency resonator, wherein the microwave resonator part is protected with a floating capping;

[0016] Figs. 5A-B illustrate a light-tight encapsulation surrounding an optical resonator;

[0017] Figs. 6A-B illustrate a hermetic seal that is realised around the combined optical, mechanical and microwave resonators. The hermetic seal allows for protection of the MEMS devices from external environments by preserving a controlled atmosphere. The devices can be electrically and optically accessed from outside of the hermetic seal;

[0018] Figs. 7A-B illustrate a hermetic seal that is realised around the combined optical, mechanical and microwave resonators. The hermetic seal allows for protection of the MEMS devices from external environments by preserving a controlled atmosphere. The devices can be electrically and optically accessed from outside of the hermetic seal and the superconductor material is fully included in the hermetic seal;

[0019] Figs. 8A-B illustrate a photonic crystal for transducer devices. The structures are suspended and made of different layers of materials with different dielectric coefficients and refractive indexes.

[0020] DETAILED DESCRIPTION

[0021] In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

[0022] It is understood that the features of the various exemplary embodiments and / or aspects described herein may be combined with each other, unless specifically noted otherwise.

[0023] For purposes of the description hereinafter, the terms “end,” “upper,” “lower,” “right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” “lateral,” “longitudinal,” and derivatives thereof shall relate to the disclosed subject matter as it is oriented in the drawing figures. However, it is to be understood that the disclosed subject matter may assume various alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments or aspects of the disclosed subject matter. Hence, specific dimensions and other physical characteristics related to the embodiments or aspects disclosed herein are not to be considered as limiting unless otherwise indicated. No aspect, component, element, structure, act, step, function, instruction, and / or the like used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” are intended to include one or more items and may be used interchangeably with “one or more” and “at least one.” Furthermore, as used herein, the term “set” is intended to include one or more items (e.g., related items, unrelated items, a combination of related and unrelated items, and / or the like) and may be used interchangeably with “one or more” or “at least one.” Where only one item is intended, the term “one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based at least partially on” unless explicitly stated otherwise.

[0024] The following describes an optoelectronic interface device (transducer) and methods for fabrication. The device may in particular be configured for the transduction of quantum information, with the fabrication methods entailing light-tight and hermetically sealed enclosures thereof.

[0025] Achieving efficient, low-noise transduction between microwave and optical photons is vital for scalable quantum interconnects. The absence of permissible signal amplification during quantum-state transfer between different modalities demands ultra-low-noise, single-photon- level conversion. Thus an insulation of the transducers from environmental influences is desirable to reduce cross-talk induced, for example, by stray light or thermal noise as well as device degradation upon cycling between ambient and cryogenic temperatures. Additionally, operation in cryogenic environments is used for the preservation of microwave-based quantum information and demands high thermal conductivity of the quantum chips to the cold bath. As such, the provision for scalable integration of quantum transducers to match the scaling paradigms of quantum computing and networking roadmaps calls for the use of integrated photonic and electromechanical concepts for both, encapsulation and packaging of quantum transducers via hermetic sealing and light-tight enclosures with accessible microwave and optical interfaces.

[0026] The requirement for high-strength inter-mode interaction, as well as the specific material combinations that efficient and ultra-low noise frequency conversion demands, necessitate unique approaches to preventing unwanted cross-talk between various physical fields, detrimental effects on the sub-systems on or off the chip, as well as changes in the performance over time. This may be realized, for example, by encapsulating the devices and I or finding the right coupling strategy between different systems. Specifically, for high-strength inter-mode interaction between the microwave and optical domain in devices comprising superconducting elements, hermetic sealing may limit these detrimental effects. Generally, superconducting materials and in particular superconducting quantum bits (qubits) are highly sensitive to optical fields. It is therefore in general advisable to separate optical fields from superconducting circuits whenever possible. For quantum computers it is therefore advisable to separate the microwave-to-optics transducer from the superconducting qubits. This may be realized by separating the chips on which they are fabricated, as well as implementing encapsulations and enclosures for each or some of the components. It is therefore also critical to realize an electrical connection to the qubit or off-chip microwave system. Regarding the high sensitivity of the employed superconducting elements to optical fields, quasiparticles can be created upon interaction with scattered optical light, which may induce detrimental decoherence in the transported quantum signals.

[0027] For a transducing device, the microwave signal interfaces with the superconducting qubits, which may be on a different chip and potentially in a different enclosure than the transducer chip. The microwave mode may be coupled, for example, through a microwave cable to another chip. This may be realized through inductive or capacitive free-space coupling or through a wire-bond. At least part of the microwave signal should be freely accessible to such a cable or free-space coupler and therefore may not be included in the enclosure.

[0028] In addition, even if the qubit chip is separated from the transducer chip, several superconducting components might still be present on the transducer chip, such as non-linear elements, including but not limited to Josephson-junctions or combinations of Josephson junctions (SQUIDs), qubits, amplifiers, non-reciprocal devices, or tunable couplers, among others. These superconducting elements may also be protected from optical fields, either through physical distance or through barriers which protect the various parts from one another. Therefore, a transduction device may contain means which prevent optical fields from reaching sensitive components on and off chip, while still having microwave connections to other chips through, for example, cables as well as optical fibers to route light on and off the transducer chip.

[0029] For example, one or more of the following conditions may be met. In particular, one or more of the following conditions may be met for scalable, efficient and ultra-low-noise transduction:

[0030] 1 , The resonance frequencies of the interacting modes may be protected from the environment against environment-induced drift, while preserving the quality factor of their resonances.

[0031] 2, If any intermediate mechanical modes are used, they may be free to oscillate, requiring freestanding or suspended structures. 3, The superconducting parts of the device, as well as other superconducting components off chip may be protected from scattered optical photons, owing to the deleterious effects of optical-frequency radiation on superconducting films.

[0032] 4, The optical resonator as well as any coupler to an optical waveguide, such as a fiber, may be enclosed in order to prevent any effect of stray scattered optical frequency light from interfering with any other sensitive pieces of superconducting material inside the cryogenic environment.

[0033] 5, Owing to the high transduction efficiency required to control and measure as well as to network quantum processors, the interfaces with off-chip optical and microwave frequency waveguides (for instance optical fibers and coaxial cables) may remain efficient.

[0034] 6, Finally, the scaling of these transduction devices to many channels may necessitate an integrated chip-scale approach to encapsulation and interfacing with other chips, such as a superconducting qubit chip. This approach may be implemented on wafer-scale integration platforms.

[0035] Depending on specific applications, one or more of these six conditions may be met.

[0036] Regarding the requirement of superconducting elements inside cryogenic environments for single-photon-level microwave signals, the transduction device may be thermally connected to a cold bath via links with high thermal conductivity without introducing added noise to the participating modes, specifically to the microwave elements.

[0037] The present disclosure provides:

[0038] • Methods for creating the encapsulations / optical blocking;

[0039] • Materials that may be used for encapsulating the devices;

[0040] • Coupling methods between transducers and qubits.

[0041] One possible configuration is shown in Figure 1A. In this figure the device is represented by a series of coupled modes (Optical, Mechanical and Microwave), themselves coupled to external optical and microwave waveguides via a fiber coupler and a bond-pad and coaxial connector, respectively. In this realisation, a microwave-optical transducer is realised through the use of a mechanical intermediary mode. This transducer may also be realized through direct electro-optical coupling (non-linear optics) or other intermediate modes, such as rare- earth ions or magnons. A light-tight encapsulation surrounds a microwave-frequency resonator, with leads extending out from it that interface with the mechanical mode through piezoelectric coupling. Further, a hermetic seal may be realised around the combined optical, mechanical and microwave resonators. The fiber coupler and the bond-pad allow for opticaland microwave-frequency photons respectively to be brought on / off the device die. Finally, the whole package may be included within a light-tight container to prevent scattering of optical light from reaching other devices. In particular, the light-tight encapsulation may preserve quantum information transduced in the photon transduction device. In one exemplary implementation, the light-tight encapsulation may restrict the light-induced microwave noise within the superconducting components of the device to below 0.1 photons of input-referred added noise. The attainable lower limit of added noise may be caused by the non-zero optical transmittance of the encapsulation material stack rooted in physically feasible material responses.

[0042] In particular, in Figure 1 B, the device is represented by a series of coupled modes (Optical 107, Mechanical 108 and Microwave 109), themselves coupled to external optical 112 and microwave 113 waveguides via a fiber coupler 106 and a bond-pad 110 and coaxial connector 111 , respectively. As in Fig. 1A, a microwave-optical transducer is realised through the use of a mechanical intermediary mode. A light-tight encapsulation 104 surrounds a microwavefrequency resonator 109, with leads extending out from it that interface with the mechanical mode through piezoelectric coupling. Further, a hermetic seal 103 may be realised around the combined optical, mechanical and microwave resonators. The fiber coupler 106 and the bondpad 110 allow for optical- and microwave-frequency photons respectively to be brought on / off the device die. Finally, the whole package may be included within a light-tight container 101 to prevent scattering of optical light from reaching other devices. Here, the microwave connector 111 and optical fiber feed-through 105 may realise the microwave and optical interface, while the hermetic seal and substrate keep the thermal connection to the cold bath.

[0043] In a second implementation shown in Figure 2A, a photonic crystal 114 may be added to the device between the optical and the microwave resonator, represented by the filled markings above and below the mechanical mode. In this region of the transducer the signal is being carried by a propagating mechanical vibration. This photonic crystal, for example, formed by a regular geometry either etched into the device, deposited on top of it or both, realises a wavelength-dependent reflector, preventing photons scattered from the optical resonator from reaching the microwave resonator and being absorbed in the superconducting material. In Figure 2B, which is based on Fig. 1 B (described in detail above) the device the photonic crystal 114 according to the second implementation may be added to the device between the optical and the microwave resonator, as in Fig. 2A. In particular, the scattered photons might originate from fabrication imperfections. In further implementations, based on the exemplary implementation of Fig. 2B shown in Figure 2C and 2D other encapsulations of hermetic and light-tight seal may be applied. For example, the device die may be fully encapsulated by a hermetic 103 and light-tight seal 101 , as shown in Fig. 2C. Or device die may be split into separate dies including the optomechanical 102 and microwave 115 components, as shown exemplarily in Fig. 2D.

[0044] Methods of on-chip enclosures for quantum transduction devices

[0045] Light-tight encapsulation surrounding a microwave-frequency resonator

[0046] The microwave resonator may be encapsulated with a deposition of a light-tight material (Fig. 3A.a). Initial state is already processed SOI substrate (Fig. 3A.b) with a superconductor deposited on it (Fig. 3A.c). If required, the light-tight layer (Fig. 3A.d) may then be protected by a resistant layer (Fig. 3.e) that will protect the full structure during the release of the device (Fig. 3.f).

[0047] In particular, in one implementation, the microwave resonator 301 forming part of the quantum transducer may be encapsulated with a deposition of a light-tight material 302 as shown exemplarily in Fig. 3B.a). Initial state may be an already processed silicon-on-insulator (SOI) substrate as shown exemplarily in Fig. 3B.b) with a superconductor 307 deposited on it as shown exemplarily in Fig. 3B.c). If required, the light-tight layer 308 (exemplarily depicted in Fig. 3B.d)) may then be protected by a resistant layer 309 (exemplarily depicted in Fig. 3B.e)) that may protect the full structure during the release of the device as shown exemplarily in Fig. 3B.f). In particular, nanostructures 303 are fabricated into the silicon device layer 304 on top of a buried oxide layer 305 on a silicon substrate 306 to avoid leakage of the microwave mode, while the superconductor may be structured to form a meandering microwave resonator. The resistant layer 309 may specifically be chemically resistant against under-etching 310 with, for instance, hydrofluoric acid.

[0048] The microwave resonator part may be protected with a floating capping. Initial state is superconductor on substrate (Fig. 4A.a)). The first step consists in depositing and patterning a sacrificial layer to protect the area to be capped (Fig. 4A.b)). Then the capping layer is deposited on the top (Fig. 4A.c)) and patterned to create a series of holes in order to access to the sacrificial layer (Fig. 4A.d)). Following that, the sacrificial layer is removed (Fig. 4A.e)). The capping layer floats around the microwave resonators to protect it. A sealing layer can be deposited to encapsulate the microwave resonator fully (Fig. 4A.f)), for instance, by plasma- enhanced chemical vapor deposition.

[0049] In particular, in some instances, the microwave resonator part may be protected with a floating capping layer. The initial state is a superconducting layer 401 on a substrate (exemplarily shown in Fig. 4B.a)). The first fabrication step consists of depositing and patterning a sacrificial layer 406 on the superconducting material to protect the area to be capped (exemplarily depicted in Fig. 4B.b)). Then the capping layer 407 is deposited on top (exemplarily shown in Fig. 4B.c)) and patterned to create a series of holes 408 in order to access to the sacrificial layer (exemplarily shown in Fig. 4B.d)). Following that, the sacrificial layer is removed 409 (exemplarily depicted in Fig. 4B.e)). The capping layer subsequently floats around the microwave resonators to protect it. A sealing layer 410 can be deposited to encapsulate the microwave resonator fully (exemplarily shown in Fig. 4B.f)), for instance, by plasma-enhanced chemical vapor deposition. In particular, the substrate may be a SOI chip formed by a silicon device layer 403 on top of a buried oxide layer 404 on a silicon substrate 405. In some instances, the silicon device layer is removed 402 around the superconducting resonator to avoid leakage of the microwave mode, while the sacrificial layer 406 can be removed via selective etching.

[0050] In some instances, it is beneficial to include a sacrificial material consisting of SiC>2 or another material, which may be removed before, during or after an undercut release of the transduction device. The sacrificial material may have good (etching) selectivity compared to the cap and the other materials constituting the devices. This sacrificial material may be in some instances formed by resists etched by oxygen plasma.

[0051] In some instances, the capping layer may be formed by materials that block optical, such as telecom wavelength, radiation. Combinations of materials that cover this range may include metals. In this way the capping layer may prevent optical photons from reaching the microwave resonator via absorption.

[0052] In some instances, the capping layer or sealing layer may be formed by combination of materials that block specific parts of the optical spectrum via reflection.

[0053] In some instances, the capping layer or sealing layer may be formed by materials or combination of materials that enhance the thermal connection of the microwave resonator to the environment via the device die without introducing additional thermal noise.

[0054] The dimensions of the cap may be set so that the capping layer does not introduce significant additional losses to the microwave resonator. an optical resonator

[0055] The light-tight encapsulation may also protect the optical resonator. An example fabrication flow to achieve this is shown in Figure 5A. The initial state is device with a microwave resonator and an optical resonator on substrate (Fig. 5A.a)). The first step consists in depositing and patterning a sacrificial layer to protect the area to be capped (Fig. 5A.b)). Then the capping layer is deposited on the top (Fig. 5A.c)) and patterned to create a series of holes in order to access to the sacrificial layer (Fig. 5A.d)). Following that, the sacrificial layer is removed (Fig. 5A.e)) and the suspension of the device can happen at the same time. The capping layer floats around the optical resonator to protect it. A sealing layer may be deposited to encapsulate the optical resonator fully (Fig. 5A.f).

[0056] In particular, the example fabrication flow according to Fig. 5A is shown in detail also in Figure 5B. The initial state is a device with a microwave resonator 501 and an optical resonator 502 structured on a SOI substrate 503-505 (Fig. 5B.a)). The first fabrication step consists in depositing and patterning a sacrificial layer 506 to protect the area to be capped (Fig. 5B.b)). Then the capping layer 507 is deposited on top (Fig. 5B.c)) and patterned to create a series of holes 508 in order to access the sacrificial layer (Fig. 5B.d)). Following that, the sacrificial layer is removed 509 (Fig. 5B.e)), where in some realizations the suspension of the device can happen at the same time. The capping layer subsequently floats around the optical resonator to protect it. A sealing layer 510 may be deposited to encapsulate the optical resonator fully (Fig. 5B.f)).

[0057] Hermetic seal around optical, mechanical and microwave resonators

[0058] A hermetic seal allows for protection of the MEMS devices from external environments by preserving a controlled atmosphere. Moreover, protecting the devices with chip capping encapsulation may enable dicing of the die post release. This may be realised by bonding two chips one to another: the device chip and the capping chip. The superconducting parts of the device chip that will end up under the junction between the two chips are protected or buried (Fig. 6A.a)). Either one chip or both chips have a sealing ring with a bonding material and the device chip is released (Fig. 6A.b)). The capping chip has a recess to not damage the active area of the device chip (Fig. 6A.c)). As a specific example of the process, the bonding technique may include alloy bonding with pressure applied on Cu / Sn vias at 250 °Cs (Fig. 6A.d)). The capping chip may be thinned with DRIE (Fig. 6A.e)). The devices may be electrically and optically accessed from outside of the hermetic seal (Fig. 6A.f)) The electrical port may pass through the support structures for the capping layer, and through adiabatic modification and the optical waveguide may pass under the support structure for the capping layer, and then be coupled to an optical fiber outside of the cap (Fig. 6A.f)).

[0059] As mentioned above, a hermetic seal may allow for the protection of MEMS devices from external environments by preserving a controlled atmosphere. This may be realised by bonding two chips one to another, as shown in detail in Fig. 6B: the device chip 603-605 and the capping chip 608. The superconducting parts 601 of the device chip that will end up under the junction between the two chips are protected or buried (Fig. 6B.a)). Either one chip or both chips may have a sealing ring 607 with a bonding material 606, 609, while the device chip may contain released nanostructures 610 (Fig. 6B.b)). The capping chip may have a recess 611 to not damage the active area of the device chip (Fig. 6B.c)). As a specific example of the process, the bonding technique may include alloy bonding with pressure applied on Cu / Sn vias at 250 °C (Fig. 6B.d)). The capping chip may be thinned 613 with DRIE (Fig. 6B.e)). The devices may be electrically and optically accessed from outside of the hermetic seal (Fig. 6B.f)). The electrical port may pass through the support structures for the capping layer. The optical waveguide may pass under the support structure for the capping layer through adiabatic modification, and then be coupled to an optical fiber 615 outside of the cap (Fig. 6B.f)). In particular, the dicing might happen from full-scale wafers to realise individual devices, while the device chip may contain microwave 601 , mechanical and optical 602 resonator elements. To align the bonding bumps or pads 612 on the two chips and bring them into contact, suitable vacuum tools or visco-elastic stamping processes may be used. Furthermore, the electrical port may be connected via microwave connection pads 614.

[0060] The method described in the previous part may include Through Silicon Vias (TSV) through the capping chip if full hermetic encapsulation of the superconductor is wanted. The sealing ring of the device chip may contact the superconductor for electrical access (Fig. 7A.a)). Both chips may have a sealing ring with a bonding material and the device chip is released (Fig. 7A.b)). The capping chip has a recess to not damage the active area of the device chip (Fig. 7A.c)). The bonding technique includes alloy bonding with pressure applied on Cu / Sn vias at 250 °C (Fig. 7A.d)). The capping chip can be thinned with DRIE, and TSV can be created by etching the top of the capping chip and filling it with conductive material (Fig. 7A.e)). The devices may be electrically and optically accessed from outside of the hermetic seal (Fig. 7A.f)).

[0061] In particular, as shown in detail in Fig. 7B the implementation may include Through Silicon Vias (TSV) 708 through the capping chip 709 if full hermetic encapsulation of the superconductor is wanted. The sealing ring 701 of the device chip 705-707 may contact the superconductor 702 for electrical access (Fig. 7B.a)). Both chips may have a sealing ring with a bonding material 701 , 710 and the device chip may have released nanostructures 711 (Fig. 7B.b)). The capping chip may have a recess 712 to not damage the active area of the device chip (Fig. 7B.c)). As above, in Fig. 7A, the bonding technique includes alloy bonding with pressure applied on Cu / Sn vias at 250 °C (Fig. 7B.d)). The capping chip may be thinned 714 with DRIE, and TSVs may be created by etching the top of the capping chip and filling it with conductive material 715 (Fig. 7B.e)). The devices may be electrically 716 and optically 717 accessed from outside of the hermetic seal (Fig. 7B.f)). The methods described above may also be realized in other configurations, whereby the device chip is placed on top of a handle chip in a flip-chip configuration, with the device protected in the cavity between the two as previously described. In this configuration, bump bonds may be used to provide electrical connection between the two chips.

[0062] The methods described in previous parts may also allow for microwave-frequency elements of the device (including, for instance resonators or nonlinear elements) that may be formed on the capping or handle chip, or on another chiplet residing on the same handle chip and electrically contacted to the device via bump bonds and or wire bonds.

[0063] Light-tight isolation of microwave and optical elements via photonic crystals

[0064] A method for blocking radiation from the photonic elements of the device from reaching the microwave components is to use a photonic crystal. These periodic structures block one or many wavelengths of the light. The structures are suspended and made of different layers of materials with different dielectric coefficients and refractive indexes (Fig. 8A.a)). In some instances this patterned structure may be fabricated directly into the dielectric layer that forms the device. Alternatively their fabrication may include a top-down approach on patterned substrate (Fig. 8A.b)), with a deposition of the encapsulating layers (Fig. 8A.c)) followed by patterning and etching (Fig. 8A.d)). If using the SOI platform the buried oxide layer may form the bottom cladding for the photonic crystal. Top cladding may be a polymer or any other material with similar refractive index (Fig. 8A.e)). Releasing may be performed at the end of the fabrication (Fig. 8A.f)). Alternatively, the photonic crystal may be fully suspended.

[0065] The method according to Fig. 8A for blocking radiation stemming from the photonic elements of the device from reaching the microwave components is further depicted in Fig. 8B. The structures are suspended and made of different layers of materials with different dielectric coefficients and refractive indexes (Fig. 8B.a)). In some instances, this patterned structure may be fabricated directly into the dielectric layer that forms the device. Alternatively, their fabrication may include a top-down approach on a patterned substrate 811 (Fig. 8B.b)), with a deposition of the encapsulating layers 809, 810 (Fig. 8B.c)) followed by patterning and etching 813 (Fig. 8B.d)). If using the SOI platform 806-808 the buried oxide layer 807 may form the bottom cladding for the photonic crystal. The top cladding may be a polymer 814 or any other material with similar refractive index to the bottom cladding (Fig. 8B.e)). Releasing 815 may be performed at the end of the fabrication (Fig. 8B.f)). Alternatively, the photonic crystal may be fully suspended. In particular, photonic crystals may be used as optical barrier against unwanted optical radiation scattered off photonic elements. For example, the photonic crystal may be a 2D photonic crystal with a full in-plane bandgap in the desired wavelength range. For example, the photonic crystal may be a 1 D periodic structure. In some instances, the photonic crystal might be non-suspended. In one realization, the photonic crystal 803 may be placed between an optomechanical crystal 805 and a superconducting microwave line 802, with the two connected via a piezoelectric element 804 and isolated from the surrounding device layer via phononic shields 801. The photonic crystal material layers may contain metals, semiconductors or dielectric materials.

[0066] The transducer comprises means for preventing optical frequency radiation from being absorbed by the superconducting element. For example, (parts of) the transducer may be encapsulated in order to both enhance its performance via blocking of optical radiation, as well as maintain its properties against environmental effects (influences), which may result in a degraded or altered performance over time.

[0067] For example, the transducer may feature a mechanical mode as an intermediary. Such a mechanical mode may be (localized) in a mechanical resonator. In these cases, in some instances a hermetic seal may be formed around the mechanical resonator to ensure both the resistance to environmental-induced aging of the device, as well as enabling the free mechanical oscillation required to facilitate efficient transduction. This seal may for example prevent oxidation or other unwanted effects on the performance by enclosing, for example, a low-pressure (vacuum) environment or other forms of inert atmospheres, such as gases.

[0068] In some instances, a photonic crystal patterned into the material or deposited onto the material may prevent scattered optical light from reaching a superconducting element, for example, a superconducting film. Any occurrence of superconducting film throughout this disclosure may be replaced by any other superconducting element, without limiting the present disclosure.

[0069] In some cases, only the optical resonator may be enclosed within the encapsulated region (encapsulation). In some cases, the optical resonator and the mechanical resonator may be enclosed within the encapsulated region. For example, the encapsulation of the optical and mechanical resonators features a thin-film distributed Bragg mirror to reflect scattered infraredlight and prevent it from being absorbed in the superconducting resonator. For example, an additional metal layer may block optical radiation with a wider range of propagation vectors. The Bragg mirror may for example consist of alternating layers of two dielectric materials such as Si, SiN, SiO2, AI2O3, TiO2 or similar materials.

[0070] For example, a combination of a low-loss dielectric and a metal layer or a distributed Bragg reflector may be deposited on top of the superconducting film to inhibit infrared radiation from reaching the superconducting film. For example, such protective layer may also prevent the oxidisation of the superconducting film. For example, a small portion of the superconducting film may extrude from the protective layer to contact a freely suspended piezoelectric interface, in order to exchange excitations with an intermediate mechanical resonator.

[0071] For example , the hermetic seal may only enclose the optical and mechanical resonators. For example, the seal may enclose the whole device, including the optical resonator, any intermediate (e.g. mechanical) resonators and the microwave-frequency resonator. The device may include further components, which may be included in the encapsulation.

[0072] The embodiments and exemplary implementations mentioned above show some non-limiting examples. It is understood that various modifications may be made without departing from the claimed subject matter. For example, modifications may be made to adapt the examples to new systems and scenarios without departing from the central concept described herein.

[0073] Summarizing, some embodiments in the present disclosure relate to a photon transduction device for transducing between microwave and optical frequencies, including a superconducting element, and means for preventing optical frequency radiation from being absorbed by the superconducting element.

[0074] According to a first aspect, a photon transduction device for transducing between microwave and optical frequencies is provided the photon transduction device including a superconducting element, and means for preventing optical frequency radiation from being absorbed by the superconducting element.

[0075] According to a second aspect in addition to the first aspect, the means include a light-tight encapsulation, the light-tight encapsulation surrounding the superconducting element.

[0076] According to a third aspect in addition to any of the first or second aspect, the photon transduction device is further including a optical-frequency element, and wherein the means include a light-tight encapsulation, the light-tight encapsulation surrounding the optical- frequency element.

[0077] According to a fourth aspect in addition to any of the second or third aspect, the light-tight encapsulation includes a Bragg mirror.

[0078] According to a fifth aspect in addition to the fourth aspect, the light-tight encapsulation further includes a metal layer.

[0079] According to a sixth aspect in addition to any of the first to fifth aspect, the photon transduction device is further including a photonic crystal, wherein the photonic crystal is arranged between the optical-frequency element and the superconducting element. According to a seventh aspect in addition to any of the first to sixth aspect, the superconducting element is a microwave-frequency resonator.

[0080] According to an eighth aspect in addition to the seventh aspect, the photon transduction device is further including a capping layer for protecting the microwave-frequency resonator. According to a ninth aspect in addition to any of the eight aspect, the capping layer provides a hermetic seal.

[0081] According to a tenth aspect in addition to any of the first to ninth aspect, the photon transduction device is placed on a handle layer in a flip-chip configuration.

[0082] According to an eleventh aspect in addition to the tenth aspect, a hermetic seal is located between the photon transduction device and the handle layer.

[0083] According to a twelfth aspect in addition to any of the first to eleventh aspect, the photon transduction device is further including means for preventing optical radiation from scattering out from photon transduction device.

[0084] According to a thirteenth aspect in addition to the twelfth aspect, the means for preventing optical radiation from scattering out from photon transduction device include a light-tight encapsulation.

Claims

CLAIMS1. A photon transduction device for transducing between microwave and optical frequencies including a superconducting element, and means for preventing optical frequency radiation from being absorbed by the superconducting element.

2. The photon transduction device according to claim 1 , wherein the means include a lighttight encapsulation, the light-tight encapsulation surrounding the superconducting element.

3. The photon transduction device according to any of the claims 1 or 2, further including an optical-frequency element, and wherein the means include a light-tight encapsulation, the light-tight encapsulation surrounding the optical-frequency element.

4. The photon transduction device according to any of the claims 2 or 3, wherein the lighttight encapsulation preserves quantum information transduced in the photon transduction device.

5. The photon transduction device according to any of the claims 2 to 4, wherein the lighttight encapsulation includes a Bragg mirror.

6. The photon transduction device according to claim 5, wherein the light-tight encapsulation further includes a metal or semiconducting layer.

7. The photon transduction device according to any of the claims 1 to 5, further including a photonic crystal, wherein the photonic crystal is arranged between the optical- frequency element and the superconducting element.

8. The photon transduction device according to any of claims 1 to 7, wherein the superconducting element is a microwave-frequency resonator.

9. The photon transduction device according to claim 8, further including a capping layer for protecting the microwave-frequency resonator.

10. The photon transduction device according to claim 9 wherein the capping layer provides a hermetic seal.11 . The photon transduction device according to any of claims 1 to 10, wherein the photon transduction device is placed on a handle layer in a flip-chip configuration.

12. The photon transduction device according to claim 11 , wherein a hermetic seal is located between the photon transduction device and the handle layer.

13. The photon transduction device according to any of claims 1 to 12 further including means for preventing optical radiation from scattering out from the photon transduction device.

14. The photon transduction device according to claim 13, wherein the means for preventing optical radiation from scattering out from the photon transduction device include a light-tight encapsulation.

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