PERSISTENT CURRENT SWITCH

Abstract

A persistent current switch is disclosed for controlling, e.g. initiating, a persistent current in a superconductor loop. The persistent current switch comprises a piece of superconductor material that is part of the superconductor loop. Further, the persistent current switch comprises an illumination system that is configured to direct light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material. The illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material.

Claims

1. A persistent current switch for controlling, a persistent current in a superconductor loop, the persistent current switch comprising: a piece of superconductor material that is part of the superconductor loop, and an illumination system that is configured to direct light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material.

2. The persistent current switch according to claim 1, wherein the illumination system comprises a light source for generating the light.

3. The persistent current switch according to claim 1, wherein the illumination system comprises a light guiding system to direct the light onto the piece of superconductor material.

4. The persistent current switch according to claim 1, wherein the piece of superconductor material and/or the illumination system and/or the light is and/or are configured such that the light impinging on the piece of superconductor material prevents the piece of superconductor material from adopting a superconducting state.

5. The persistent current switch according to claim 4, wherein the piece of superconductor material and/or the illumination system and/or the light is and/or are configured such that the light impinging on the piece of superconductor material breaks Cooper pairs that are present in the piece of superconductor material.

6. The persistent current switch according to claim 1, wherein the light impinging on the piece of superconductor material has a power of less than 1 W.

7. The persistent current switch according to claim 1, further comprising a control system for controlling the persistent current switch, the control system being configured to perform steps of: causing the illumination system to direct light onto the piece of superconductor material so that the piece of superconductor material does not adopt a superconducting state, and causing the illumination system to not direct light onto the piece of superconductor material, so that the piece of superconductor material adopts the superconducting state.

8. The persistent current switch according to claim 1, wherein the persistent current switch is configured to operate at temperatures below 50 Kelvin.

9. The persistent current switch according to claim 1, wherein the superconductor loop and/or the piece of superconductor material comprises niobium, Nb, and/or niobium nitride, NbN, and/or niobium titanium nitride, NbTiN.

10. A chip having integrated thereon a persistent current switch for controlling a persistent current in a superconductor loop, the persistent current switch comprising: a piece of superconductor material that is part of the superconductor loop, and an illumination system that is configured to direct light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material.

11. A quantum computing system comprising the persistent current switch according to claim 1.

12. The quantum computing system according to claim 11, further comprising a DC power source for energizing the superconductor loop.

13. A method for initiating a persistent current in a superconductor loop comprising a piece of superconductor material, the method comprising cooling the superconductor loop to below a critical temperature, and causing an illumination system to direct light onto the piece of superconductor material so that the piece of superconductor material does not adopt a superconducting state, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material, and energizing the superconductor loop comprising causing an electrical current through at least part of the superconductor loop, and while the electrical current is flowing through at least part of the superconductor loop, causing the illumination system to not direct light onto the piece of superconductor material so that the piece of superconductor material adopts the superconductive state so that the superconductor loop becomes superconductive and conducts the persistent current.

14. The method according to claim 13, wherein the illumination system comprises a light source for generating the light and wherein the step of causing the illumination system to not direct light onto the piece of superconductor material comprises switching off the light source.

15. The method according to claim 13, wherein causing the illumination system to not direct light onto the piece of superconductor material comprises controlling a light guiding system to not direct light onto the piece of superconductor material.

16. The persistent current switch according to claim 2, wherein the light source is a laser.

17. The persistent current switch according to claim 4, wherein the light impinging on the piece of superconductor material has a power of less than 1 W.

18. The persistent current switch according to claim 8, wherein the persistent current switch is configured to operate at temperatures below 10 Kelvin.

19. The chip according to claim 10, wherein the light impinging on the piece of superconductor material has a power of less than 1 W.

20. The chip according to claim 10, wherein the persistent current switch is configured to operate at temperatures below 50 Kelvin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Aspects of the invention will be explained in greater detail by reference to exemplary embodiments shown in the drawings, in which:

[0066] FIGS. 1A and 1B schematically illustrates a persistent current switch and its operation according to an embodiment,

[0067] FIGS. 2A and 2B illustrate a system according to an embodiment comprising a plurality of persistent current switches;

[0068] FIG. 3A presents a three-dimensional view of a persistent current switch according to an embodiment;

[0069] FIG. 3B presents two more three-dimensional views of the persistent current switch shown in FIG. 3A.

[0070] FIG. 4 illustrates a data processing system according to an embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

[0071] In the figures, identical reference number indicate identical or similar elements.

[0072] FIGS. 1A and 1B schematically illustrates a persistent current switch 2 and its operation according to an embodiment. FIG. 1A illustrates the persistent current switch in a state which may be referred to as the energizing state and FIG. 1B illustrates the persistent current switch in a state which may be referred to as the persistent state.

[0073] In both depicted states, the superconductor loop 4 is at a temperature below a critical temperature, which may be understood to be the temperature below which the superconductor material forming the loop 4 becomes superconductive. To this end, the superconductor loop 4 is typically cooled by means of a cooling system 6, e.g. a cryostat.

[0074] When the persistent current switch 2 is being energized (as schematically depicted in FIG. 1A) an electrical current I_charge is caused through at least part of the superconductor loop. In FIG. 1A, this electrical current I_charge flows through superconductor loop segments 5a, 5b, 5c, yet not through superconductor loop segment 5d. Further, in the energizing state, an illumination system 8, which may comprise a light source 10 and which may comprise a light guiding system 12, directs light 11 onto a piece of superconductor material so that the piece of superconductor material does not adopt the superconducting state. In FIG. 1A, the illuminated piece of superconductor material sits in segment 5d of superconductor loop 4. The incident light 11 may be understood to increase the resistance of the piece of superconductor material, as indicated by resistance R_S in FIG. 1, so that the piece of superconductor material does not adopt the superconductive state. As a result, not the entire superconductor loop 4 is in a superconductive state.

[0075] The illumination system 8 and the piece of superconductor material may be understood to form a superconductor (SC) photonic switch because upon the piece of superconductor material absorbing photons of the light 11, it can transition from the superconductive state to the normal resistive state.

[0076] As described in the microscopic Bardeen-Cooper-Schrieffer (BCS) theory, electrons of a superconductor material condense into Cooper pairs at low temperature. The formation of Cooper pairs requires an attraction potential to overcome the Coulomb repulsion between electrons. In the SC material, this attraction potential arises from the lattice mediated electron-electron interaction below a sufficiently low temperature, known as the critical temperature.

[0077] When the SC material is at temperatures below the critical temperature, the vibrational energy of the material lattice is not enough to disrupt the Cooper pairs. Thus, to break these Cooper pairs at low temperature, sufficient energy must be supplied externally to the SC material for overcoming the energy gap and bringing the collective ensemble into an excited state.

[0078] Photons which have higher energy than the superconducting gap of the SC material can be used as an external stimulus for breaking the Cooper pairs, leading to a transition between superconductive state to normal resistive state.

[0079] Superconductor devices are fast and ultra-sensitive to optical excitation because of their quantum nature and low-noise, cryogenic operation environment. The energy gap of SC materials is typically two to three orders of magnitude smaller compared to the energy gaps in semiconductors. Thus, a superconductor device can efficiently absorb and/or respond to photons with much lower energy.

[0080] If the superconductor material forming the superconductor loop 4 is for example NbTiN (SC energy gap 5 meV), then any light from UV to mid-infrared range could be efficiently used to prevent the piece of superconductor material from adopting the superconductive state. However, visible light may also be used, e.g. green light having a wavelength of approximately 532 nm, 2.33 eV), as this light is associated with an energy much higher than the SC energy gap for NbTiN. Such green light may also be used to excite diamond defect centres (spin qubits). In such case, green light would be integrated or coupled to the system anyway.

[0081] Regarding the optical power needed to prevent the piece of superconductor to adopt the superconductive state, depending on the device geometry a switching between superconductive state and normal resistive state can be efficiently triggered with even a single photon. In an embodiment, the light has an optical power in the sub-microwatt range, which should be sufficient to prevent the piece of superconductor material to adopt a superconductive state.

[0082] In principle, the superconductor loop 4 and the piece of superconductor material in this loop can comprise, e.g. consist of, any superconductor material and a large spectrum of wavelengths can be used for the illumination light to control whether or not the piece of superconductor material adopts the superconductive state, as long as the energy gap of the piece of superconductor material is lower than the photon energy of the light that is used.

[0083] In an embodiment, the piece of superconductor material is a type-II superconductor material. In such embodiment, the rest of the superconductor loop may of course also consist of this same type-II superconductor material. As known, type-II SC materials have two temperature-dependent critical magnetic fields. Between the two critical values, the magnetic field can partially penetrate the SC material by forming isolated points called vortices. These vortices aid the photon mediated state transition of the SC material, and hence type-II SC materials can be suitably used as superconductor material for the piece of superconductor material that is going to be illuminated by the light and for the superconductor loop.

[0084] As already indicated above, the piece of superconductor material and/or the superconductor loop may comprises and/or consist of NbTiN. It has been found that the Ti atoms in the NbTiN compound act as a nitrogen getter and improve the quality of crystals, e.g. as compared to NbN, by preventing the formation of vacancies. In addition, NbTiN growth process is relatively undemanding in terms of thermal budget, e.g. compared to the case of NbN. To illustrate, for NbN, the formation of the SC structure of the B1 stoichiometry takes place efficiently for temperatures above 1350 C. Such high temperatures in an intermediate process step can be incompatible with other steps/materials and not desired in an integrated system. NbTiN, on the other hand, can be deposited on substrates at room temperature and even with a large lattice mismatch from the growth, it can maintain good superconducting properties. RF loss, kinetic inductance etc. are also relatively low for NbTiN thin film devices.

[0085] Also, (polycrystalline) NbTiN has a relatively high critical temperature (approximately 10 K), e.g. as compared to amorphous superconductor materials. Thus, NbTiN based persistent current switches can be operated at 2.5 K in simple, cheap, and reliable cryostats rather than requiring dilution refrigerators for sub-kelvin temperatures. Also, NbTiN has sufficiently high critical current densities which enables suitable operating currents for the persistent current switches disclosed herein.

[0086] At some point in time, while the electrical current I_charge is flowing through at least part of the superconductor loop 4, the illumination system 8 is caused to not direct light onto the piece of superconductor material so that the piece of superconductor material adopts a superconductive state so that the superconductor loop 4 becomes superconductive. This results in a persistent current I_persist through the loop 4 as depicted in FIG. 1B. Note that in the persistent state, no voltage needs to be applied to the loop.

[0087] It should be appreciated that the illumination system 8 is configured such that the light 11 impinging on the piece of superconductor material substantially does not heat the piece of superconductor material. When the piece of superconductor material is biased closed to the critical current and absorbs photons of the light 11, the creation, multiplication, and diffusion of quasiparticles and phonons take place. The quasiparticle cloud causes a local reduction of the superconducting order parameter, re-distribution of the current density, and lowering of the effective critical current density. This instability of the superconducting state leads to the formation of a local non-superconducting state in the piece of superconductor material.

[0088] Once a resistive region is formed in the piece of superconductor material, it experiences internal Joule heating. This causes the resistive region to grow and the resistance can increase to a value on the order of several kilo-ohms. Now, the superconductor loop 4 is preferably designed in such a way that, apart from piece of superconductor material, the rest of the loop remains in the SC state and acts as a parallel current path. Thus, as soon as the joule heating and the local resistance in the illuminated piece of superconductor material starts to grow, the bias current starts to get diverted to a greater extent into the other section of the loop. This effect also reduces the heat generation in the piece of superconductor material. In fact, this heat generation may be considered negligible. The light 11 incident on the piece of superconductor material may be embodied as light pulses Such light pulses are preferably very short (nano/microseconds) and preferably have low power (sub-microwatt), so that any heat contribution (phonon generation) from the light pulses are also negligible. Once the light 11 is no longer directed at the piece of superconductor material, the superconductive state is restored. This causes the entire superconductor loop 4 to be superconductive again and a persistent current starts I_persist to flow in the loop 4.

[0089] PECVD grown silicon nitride (SiN) may be used for making the waveguides. PECVD SiNs have a refractive index around 1.9-2 and negligible extinction coefficient at 532 nm. Hence, SiN is transparent in the visible range of light, PECVD SiN has a waveguide loss of 0.1-2 dB/cm at 532 nm, CMOS process compatible and a prominent workhorse of the state-of-the-art photonic integrated circuits (PICs). Highly efficient PIC components including waveguides, couplers, splitters as well as photonic/optomechanic crystals and cavities have been demonstrated in the visible range with SiN. So, implementing SiN waveguides to route the excitation light is relatively straight forward. A SiN waveguide of width 800 nm and thickness of 200 nm may be used, which efficiently confines and guides light of wavelengths <650 nm. For coupling the waveguide to the piece of superconductor material, we deposit and pattern the SC material on top of or buried inside the waveguide, similar to the design and implementation of waveguide integrated superconducting nanowire single-photon detectors (SNSPDs).

[0090] FIGS. 2A and 2B schematically illustrates system, e.g. a chip, comprising a plurality of persistent current switches as described herein (two are shown). Again, FIG. 2A illustrates the energizing state and FIG. 2B the persistent state. By suitably applying a voltage on line 14 and 16A, 16B, 16C and 16D, and by separately controlling illumination systems 8a and 8b, each superconductor loop may be energized separately.

[0091] FIG. 3A presents a three-dimensional view of a persistent current switch according to an embodiment. FIG. 3B presents two more three-dimensional views of the same persistent current switch 2.

[0092] The persistent current switch may be fabricated as follows. First, a silicon-on-insulator (SOI) wafer may be provided. The Si device layer 30 may then be oxidized so that a buried oxide layer (BOX) layer 32 is formed. Additionally or alternatively, the Si device layer 20 is removed selectively from locations where the superconductor loop and illumination system will be fabricated in order to expose an oxide layer that sits beneath the Si layer. This is convenient if the illumination system and superconductor loop are designed to be made on an oxide layer, e.g. an SiO2 layer. The illumination system and superconductor loop can then be fabricated on the BOX layer 32 and/or be fabricated on the exposed oxide layer beneath Si layer 30. As an example, SiN waveguides and a NbTiN superconductor loop can be suitably made on an oxide layer, e.g. on a SiO2 layer. The Si layer is convenient to have in the chip because it can be used at other locations at the chip for other circuitry, e.g. for complex photonic circuitry. However, for the purposes of the switch disclosed herein, the Si layer 30 is not per se required.

[0093] In the embodiment of FIG. 3A, the superconductor loop consists of a superconductor inductor 34, also referred to a magnetic field generator (MFG) 34, a first arm 37 a connected to the inductor loop 34, a second arm 37b connected to the inductor loop 34, and three pieces of superconductor material 36a, 36b, 36c which connect the first arm 37a to the second arm 37b.

[0094] Then, an approximately 200 nm thick, low stress silicon nitride layer may be deposited on the selectively exposed BOX (buried oxide) layer 32 using the plasma enhanced chemical vapor deposition (PECVD) technique. The deposition process may be carried out at 300 degrees Celsius using a N2/SiH4/NH3 gas mixture in a PECVD reactor. A post deposition anneal may then be performed at 600 degrees Celsius in order to reduce the residual stress of the deposited film. The inverse tapered photonic waveguides 42 and multimode interference (MMI) coupler sections 40 may be patterned with electron beam lithography (RIE) and subsequent reactive ion etching (RIE) of SiN layer in fluorocarbon plasmas.

[0095] The superconductor loop may be fabricated with a combination of direct-write (laser writer) photolithography, SC (NbTiN) layer sputtering and SC lift-off process steps. SC circuit patterns and alignment markers can be transferred to a positive tone photoresist (S1805) with a maskless, direct-write lithography technique using a laserwriter. Then an approximately 200 nm thick, low stress SC (NbTiN) layer may be deposited in a high vacuum chamber onto the patterned photoresist from a 3 inch 99.99% pure NbTi target in an Ar/N2 atmosphere, using a magnetron sputter system. After the NbTiN deposition, a lift-off step may be carried out. In this step, the photoresist and SC materials can be washed away with hot acetone from the non-patterned area leaving behind NbTiN thin films only in the patterned area. Through these steps each layer of SC circuitry is implemented. For multiple layer SC circuit pattern, these process steps are repeated as required. Accurate overlay of multiple layers can be achieved through utilizing alignment markers from the previous patterning processes.

[0096] The SC (NbTiN) strip-lines 36a, 36b, 36c may be deposited on the inverse tapered SiN waveguides 39a, 39b, 39c, 39d. Through the inverse tapered sections, photon streams can be evanescently coupled to the NbTiN strip-lines 36a, 36b, 36c which means that the photons impinge on the NbTiN stip-lines. The absorption of photons from for example a switching laser pulse leads to a transition from superconducting state to normal resistive state in the NbTiN strip-lines 36a. 36b. 36c. In the absence of the laser pulse and at sufficiently low temperature, NbTiN strip-lines 36a, 36b, 36c will be in a superconducting state and form a superconducting loop to allow a persistent current to circulate through the superconductor loop and thus through the arms 37a and 37b, through the pieces of superconductor material 36a, 36b, 36c and through the magnetic field generator 34.

[0097] Finally, a dielectric structure 44 may be patterned and deposited on an inverse tapered, SiN waveguide 42 at the end facet of the chip in order to allow edge coupling of light into the chip (for example from an optical fiber to the waveguide). The edge coupler structure 44 fabricated on the inverse tapered waveguide 42 reduces the mode mismatch between the optical fiber (not shown) and the waveguide and efficiently couples the switching laser pulses into the chip. The inverse tapered waveguide 42, the MMI 40 and the SC (NbTiN) strip-lines 36a, 36b, 36c may be understood to form the illumination system referred to in this disclosure that is configured to direct light onto a piece of superconductor material, that is part of a superconductor loop, for influencing an electrical resistance of the piece of superconductor material.

[0098] FIG. 3B shows the same persistent current switch from two different angles.

[0099] FIG. 4 depicts a block diagram illustrating a data processing system according to an embodiment.

[0100] As shown in FIG. 4, the data processing system 100 may include at least one processor 102 coupled to memory elements 104 through a system bus 106. As such, the data processing system may store program code within memory elements 104. Further, the processor 102 may execute the program code accessed from the memory elements 104 via a system bus 106. In one aspect, the data processing system may be implemented as a computer that is suitable for storing and/or executing program code. It should be appreciated, however, that the data processing system 100 may be implemented in the form of any system including a processor and a memory that is capable of performing the functions described within this specification.

[0101] The memory elements 104 may include one or more physical memory devices such as, for example, local memory 108 and one or more bulk storage devices 110. The local memory may refer to random access memory or other non-persistent memory device(s) generally used during actual execution of the program code. A bulk storage device may be implemented as a hard drive or other persistent data storage device. The processing system 100 may also include one or more cache memories (not shown) that provide temporary storage of at least some program code in order to reduce the number of times program code must be retrieved from the bulk storage device 110 during execution.

[0102] Input/output (1/O) devices depicted as an input device 112 and an output device 114 optionally can be coupled to the data processing system. Examples of input devices may include, but are not limited to, a keyboard, a pointing device such as a mouse, a touch-sensitive display, or the like. Examples of output devices may include, but are not limited to, a monitor or a display, speakers, the light source referred to herein, the illumination system referred to herein, the light guiding system referred to herein, the cooling system referred to herein, the power source for providing electrical power for energizing the superconductor loop referred to herein, or the like. Input and/or output devices may be coupled to the data processing system either directly or through intervening/O controllers.

[0103] In an embodiment, the input and the output devices may be implemented as a combined input/output device (illustrated in FIG. 4 with a dashed line surrounding the input device 112 and the output device 114). An example of such a combined device is a touch sensitive display, also sometimes referred to as a touch screen display or simply touch screen. In such an embodiment, input to the device may be provided by a movement of a physical object, such as e.g. a stylus or a finger of a user, on or near the touch screen display.

[0104] A network adapter 116 may also be coupled to the data processing system to enable it to become coupled to other systems, computer systems, remote network devices, and/or remote storage devices through intervening private or public networks. The network adapter may comprise a data receiver for receiving data that is transmitted by the systems, devices and/or networks to the data processing system 100, and a data transmitter for transmitting data from the data processing system 100 to the systems, devices and/or networks. Modems, cable modems, and Ethernet cards are examples of different types of network adapter that may be used with the data processing system 100.

[0105] As pictured in FIG. 4, the memory elements 104 may store an application 118. In various embodiments, the application 118 may be stored in the local memory 108, the one or more bulk storage devices 110, or apart from the local memory and the bulk storage devices. It should be appreciated that the data processing system 100 may further execute an operating system (not shown in FIG. 4) that can facilitate execution of the application 118. The application 118, being implemented in the form of executable program code, can be executed by the data processing system 100, e.g., by the processor 102. Responsive to executing the application, the data processing system 100 may be configured to perform one or more operations or method steps described herein.

[0106] In one aspect of the present invention, the data processing system 100 may represent a control system that is configured to control any of the systems and/or subsystems described herein to perform their respective functions. To illustrate, such control system may be configured to control the light source referred to herein, the illumination system referred to herein, the light guiding system referred to herein, the cooling system referred to herein and/or a power source for providing electrical power for energizing the superconductor loop as described herein, by sending appropriate control signals to these systems and/or subsystems.

[0107] Various embodiments of the invention may be implemented as a program product for use with a computer system, where the program(s) of the program product define functions of the embodiments (including the methods described herein). In one embodiment, the program(s) can be contained on a variety of non-transitory computer-readable storage media, where, as used herein, the expression non-transitory computer readable storage media comprises all computer-readable media, with the sole exception being a transitory, propagating signal. In another embodiment, the program(s) can be contained on a variety of transitory computer-readable storage media Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., flash memory, floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. The computer program may be run on the processor 102 described herein.

[0108] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms a, an, and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms comprises and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0109] The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of embodiments of the present invention has been presented for purposes of illustration, but is not intended to be exhaustive or limited to the implementations in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the present invention. The embodiments were chosen and described in order to best explain the principles and some practical applications of the present invention, and to enable others of ordinary skill in the art to understand the present invention for various embodiments with various modifications as are suited to the particular use contemplated.