"josephson junction and cryogenic apparatus for generating a frequency comb signal"

Abstract

Josephson junction (20) having a planar surface defined in a plane (X,Y) and comprising: - a substrate (30); - two superconductive electrodes (26), placed on the substrate (30), and having a width, defined in such plane (X,Y) along a first direction (X), comprised in the range 1-100μm, and - a layer of non-superconductive material (28), placed on the substrate (30), interposed between the electrodes (26) along a second direction (Y) perpendicular to the first direction (X) and having a length, defined along said second direction (Y), comprised between 1 nm and 5000nm; the superconductive electrodes (26) having a thickness of about 100nm, the layer of non-superconductive material (28) having a thickness between 1 nm and 50nm, said thickness being defined in a third direction perpendicular to the plane (X,Y), each superconductive electrode (26) being configured to overlap a respective end of the layer of non-superconductive material (28) along said third direction, for a predetermined length defined along said second direction (Y), thus inducing superconductivity in the non-superconductive material (28) by proximity effect, the Josephson junction (20) being configured, when placed in a cryogenic environment, to generate a train of voltage pulses in response to a time-dependent magnetic flux induced by a magnetic field applied perpendicularly to the planar surface.

Description

[0001] DESCRIPTION

[0002] “Josephson junction and cryogenic apparatus for generating a frequency comb signal”

[0003] The present invention relates to a Josephson junction and a cryogenic apparatus for generating a frequency comb signal.

[0004] The generation of frequency combs in a cryogenic environment in the microwave regime has already been demonstrated in several works using a combination of resonant cavities and nonlinear elements as constituent elements, such as, for example, R. P. Erickson et aL, “Frequency comb generation in superconducting resonators”, Phys Rev Lett 113, 187002 (2014), S.-P. Wang et aL, “Controllable microwave frequency comb generator in a tunable superconducting coplanar-waveguide resonator , Chin. Phys. B, 30, 4, 048501 (2021), X. Hau et aL, “Superconducting cavity electromechanism: the realization of an acoustic frequency comb at microwave frequencies”, Phys Rev Lett 129, 107701 , (2022), J. Shin et aL, “On-hip microwave frequency comb in a superconducting nanoelectromechanical device”, Nano Letters, (2022) or C.-G. Wang et aL,“ Integrated and DC-powered superconducting microcomb”, Nat Commun 15, 4009 (2024).

[0005] The devices proposed in the literature are linked to the physics of the cavity, which limits the geometric dimensions above the millimeter. This fact makes the frequency comb generators already developed difficult to scale and, crucially, incompatible with the need to generate thousands of coherent modes at different frequencies.

[0006] The presence of a cavity also reduces flexibility regarding the choice of frequencies that can be generated. The cavity-carrying mode is chosen at the manufacturing stage.

[0007] There is also one example of a frequency comb generated cryogenically on-chip, not via a resonant cavity disclosed in A. Babenko et aL, "Characterization of a Josephson Junction Comb Generator," 2020 IEEE / MTT-S International Microwave Symposium (IMS), Los Angeles, CA,USA, 2020, pp. 936-939. This device is based on a long chain of Josephson junctions (more than 1000) that requires a combination of a DC and an AC signal in the GHz regime to generate a frequency comb.

[0008] Although this architecture does not require resonant structures, it has the same space limitation as resonator geometries.

[0009] Extended tunnel junctions to be used as comb generators are also known for such kinds of applications, as displayed in Solinas et aL, J. Appl. Phys. 118, 113901 (2015). Extended tunnel junctions offer a more compact solution than long chains of Josephson junctions and do not need to be placed inside a resonator.

[0010] Figure 1 shows a side view of an extended tunnel junction 2 to which an in-plane magnetic field 4 is applied, i.e., parallel to the plane where two superconducting electrodes 6a, 6b mostly overlap.

[0011] The junction 2 comprises a thin insulating layer 8 forming the tunnel barrier, which separates the two superconducting electrodes 6a, 6b. Near the junction, the bottom electrode 6b is placed on a substrate 10, while the top electrode 6a lies on top of the insulating layer 8.

[0012] With this configuration, the magnetic field must be induced within the insulating layer 8, which is typically less than 2nm thick and has an average area of less than 10pm x 10pm.

[0013] For example, a tunnel junction 2 having an area in the order of 10_14m2(which is a typical dimension for cryogenic devices applied in the industry), can be used as a microwave comb generator with a field modulation typically in the order of flux quantum <t>o = 2.15x10-15Wb, which implies magnetic fields in the order of 10-1 OOmT.

[0014] However, generating a magnetic field of such intensity with an inplane configuration is a major technological challenge, which places serious restrictions on the practical realization of this architecture. Inducing a magnetic field of 10mT using an on-chip flux line typically requires a current of 10-100 mA, which is an unpractical amount of current to use in a cryogenic environment due to the Joule heating of the systemTherefore, an innovative Josephson junction capable of operating at cryogenic temperatures to generate tones at sub-millimeter distances is needed.

[0015] A Josephson junction having the characteristics defined in independent claim 1 fully achieves these and other objects.

[0016] Preferred embodiments of the invention are specified in the dependent claims, whose subject-matter is to be understood as forming an integral or integrating part of the present description.

[0017] Further characteristics and advantages of the present invention will become apparent from the following description, provided merely by way of a non-limiting example, with reference to the attached drawings in which:

[0018] - Figure 1 shows a side view of an extended tunnel junction of the prior art;

[0019] - Figure 2 shows a top view of an extended junction according to the present invention; and

[0020] - Figure 3 is a side view of the junction of Figure 2.

[0021] Briefly, the present invention is based on the well-known concept of “extended” Josephson junction, i.e., a junction whose some physical dimensions as length and / or width are of the order of typical superconductor lengths, such as the Josephson penetration length or the coherence length (0.01 -1 m). This fact leads to spatially inhomogeneous values of Josephson current density and phase difference. Usually, extended junctions are realized by interposing between two superconductive electrodes another material, not superconductive, in which the superconductivity is induced by proximity with the superconductive materials of larger areas. In the present invention, the junction area is not formed by an overlap of the bottom and the top electrode, but both electrodes are attached to each other face to face.

[0022] Beyond the specific materials that make up the link between the two superconducting contacts, physics governing the transmission of a supercurrent in an extended junction is based on a plurality of transmissionchannels created within the interposed material. When the extended junction is exposed to a magnetic field B, Fraunhofer diffraction phenomena are established between the plurality of transmission channels.

[0023] In the present invention, when, in a cryogenic environment, a magnetic field consisting of a static component B and an oscillating component B(t) is applied, through a magnetic field generation system, to an extended Josephson junction, with both components being directed perpendicularly with respect to the surface of the junction, a voltage signal is obtained.

[0024] The total magnetic field results in a magnetic flux <t> that allows the diffraction between the conductive channels carrying the Josephson current to be adjusted and modifies the interference pattern. When the magnetic flux <t> reaches a value that causes a diffraction node (destructive interference pattern occurs at integers of flux quantum <t>o), the time-varying component <t>(t) causes an abrupt change of the superconducting phase at the ends of the junction. This abrupt phase change causes a voltage pulse at junction electrodes due to Josephson's second law.

[0025] If the magnetic flux <t>(t) oscillates periodically, the generation of voltage pulses will also be periodic. Due to the dualism between time and frequency, a train of pulses evenly spaced in time corresponds to a frequency comb signal with equidistant frequency lines.

[0026] If the magnetic flux <t>(t) is made to oscillate at frequency f, i.e. (|)(t)=si n(2rrft) , then the frequency comb signal will consist of the frequencies 2f, 3f, 4f, ...., nf, where n varies depending on how narrow in time each voltage pulse is.

[0027] Therefore, in the Josephson junction according to the present invention, a voltage signal is generated between the electrodes of an extended Superconducting-Normal-Superconducting (SNS) junction subject to a perpendicular magnetic field oscillating in time B(t), which turns into a magnetic flux <t> (t).The Josephson junction generates therefore a train of voltage pulses in response to a time-dependent magnetic flux <t>(t) induced by a magnetic field applied perpendicularly to the surface of the Josephson junction.

[0028] When the magnetic field applied to the Josephson junction is periodic, a train of voltage pulses evenly spaced in time is generated near diffraction nodes of the critical current-flux relationship, and this series of pulses corresponds, in the frequency domain, to a comb-like structure known as a frequency comb.

[0029] In an embodiment, the signal so generated is conveyed to a waveguide connected to one of the superconducting electrodes and in turn to an array of resonators, each with a specific characteristic frequency and all coupled to a same signal line, thus allowing “interrogating” the resonators by the frequency comb signal (“multiplexed dispersive-readout’).

[0030] Alternatively, the signal generated by the Josephson junction is sent to an antenna to be transmitted.

[0031] The Josephson junction of the present invention is therefore arranged to be included in a cryogenic apparatus for generating a frequency comb signal, together with the magnetic field generation system above discussed and a transmission line connected to one of the electrodes, wherein the transmission line is configured to convey the generated frequency comb signal to an output (i.e., the resonators or the antenna).

[0032] Figure 2 shows a top view of an extended superconducting-normal-superconducting junction 20 according to the present invention, wherein a magnetic field is applied in a direction 24 entering the plane.

[0033] Junction 20 has a planar surface defined in a plane X,Y and comprises two superconductive electrodes 26 having a width (along a first direction X in Figure 2) comprised in the range 1-100pm, which are separated by a layer of non-superconductive material 28.

[0034] The length (along a second direction Y perpendicular to the first direction X) of the non-superconductive material 28 ranges between 1 nm and 5000nm. The non-superconductive material is a material in whichsuperconductivity is not native but can be induced by proximity with the superconducting electrodes 26.

[0035] The separation between the two electrodes 26, i.e., the length (along the second direction Y in Figure 2) of the layer of non-superconductive material 28, must be of the order of a few coherence lengths of the superconductor material, typically between 1nm and 5000nm as above cited.

[0036] The junction 20 is “planar in the sense that it is much more extended in width and length than in thickness (defined along a third direction Z perpendicular to the plane X,Y, as shown in Figure 3). The width X and the length Y are a few times longer than the thickness Z (for example 2-10 times longer).

[0037] The superconductive electrodes 26 and the layer of non-superconductive material 28 are placed on a substrate 30.

[0038] The planar geometry of junction 20 allows the magnetic field to be applied in the direction 24 perpendicular to the plane of the junction 20 itself, the junction 20 having an area of 10-10m2given by the exposed area of the normal material 28. This allows lowering the required magnetic field to about 20pT.

[0039] This order of magnitude of magnetic field perpendicular to the plane of junction 20 is easily achieved by placing next to the extended junction a flux line, i.e., a closed-loop coplanar waveguide where a current flow is imposed (the magnetic field generation system above disclosed). These are routinely made using lithographic techniques and thin film deposition.

[0040] Figure 3 is a side view of junction 20 of Figure 2, wherein the magnetic field is in the direction 24 from top to bottom.

[0041] The superconductive electrodes 26 have a thickness along the third direction Z of about 10Onm, while the active layer (i.e., where the conduction takes place) of non-superconductive material 28 has a thickness between 1 nm and 50nm. This large difference in thickness is necessary to effectivelypromote the proximity effect from the superconductive electrodes 26 to the layer of non-superconductive material 28.

[0042] For this purpose, an overlap is also created, along the third direction Z, between the superconductive electrodes 26 and the layer of non-superconductive material 28, to increase the interaction area between the respective types of materials.

[0043] In particular, each superconductive electrode 26 overlaps a respective end of the layer of non-superconductive material 28 for a predetermined length L along the second direction Y. L should not be less than few coherence lengths of the superconducting material, for example not less than 1 nm.

[0044] As a result, in order for junction 20 to be planar, the superconductive electrodes 26 have a length (in the direction Y) at least greater than such predetermined length L.

[0045] Many materials can be used for the junction 20.

[0046] For example, for the superconductive electrodes 26, all type I and type II superconductors can be used.

[0047] For the layer of non-superconductive material 28, many materials with high electron diffusivity can be used. Examples include metals (copper, silver, gold, aluminum and mixtures thereof), crystalline semiconductors (indium arsenide, germanium, bismuth arsenide, indium antimonide and mixtures thereof), and two-dimensional materials (graphene, h-Bn).

[0048] The current through the closed-loop of the flux line induces a magnetic flux threading the area of the non-superconductive material 28. The static component of such flux sets a working (or bias) point of the junction 20, while the time-dependent component is a driving tone (or “pump tone”) with frequency f, which gets multiplied in frequency, thus forming the overall comb spectrum with a frequency content of 2f, 3f, 4f, .. nf. The comb signal is conveyed to an output by a transmission line connected to one of the two electrodes 26, while the other electrode is grounded.The junction 20 above disclosed allows obtaining a cryogenic apparatus capable, for example, to generate comb spectra with multiple harmonics in a wide range spanning from 10MHz to 300GHz.

[0049] As indicated above, junction 20 can be employed in a cryogenic apparatus also including a waveguide and a resonator array. In such an embodiment, the final cryogenic device is remarkably compact, as microwave radiation generation and resonators coexist on the same chip. This advancement makes it possible to thicken many elements, a key feature in the perspective of scalability. Current fabrication technology allows for immediate implementation in industrial and research environments.

[0050] The present invention, therefore, represents, for example, a solution to the scalability problem in the dispersive readout of ensembles of devices in a cryogenic environment, as the scaling of interconnections and control lines with the number of quantum devices is expected to be a central bottleneck in creating large-scale quantum technology. When there is a high number of such devices in ultra-cold environments, they are affected by the problem of “frequency crowding”. The present invention solves this problem by providing an easy and scalable way to obtain thousands of coherent tones generated on-chip and in a cryogenic environment, i.e. at submillimeter distances from the output devices (devices such as the resonator array whose readout is required or antennas to transmit the signal), thus reducing latency, signal distortions and thermal loads.

[0051] Clearly, the principle of the invention remaining the same, the embodiments and the details of production can be varied considerably from what has been described and illustrated purely by way of non-limiting example, without departing from the scope of protection of the present as defined in the attached claims.

Claims

CLAIMS1. Josephson junction (20) having a planar surface defined in a plane (X,Y) and comprising:- a substrate (30);- two superconductive electrodes (26), placed on the substrate (30), and having a width, defined in such plane (X,Y) along a first direction (X), comprised in the range 1-100pm, and- a layer of non-superconductive material (28), placed on the substrate (30), interposed between the electrodes (26) along a second direction (Y) perpendicular to the first direction (X) and having a length, defined along said second direction (Y), comprised between 1nm and 5000nm;the electrodes (26) having a thickness of about 100nm, the layer of non-superconductive material (28) having a thickness between 1nm and 50nm, said thickness being defined in a third direction (Z) perpendicular to the plane (X,Y),each superconductive electrode (26) being configured to overlap a respective end of the layer of non-superconductive material (28) along said third direction (Z), for a predetermined length (L) defined along said second direction (Y), thus inducing superconductivity in the non-superconductive material (28) by proximity effect,the Josephson junction (20) being configured, when placed in a cryogenic environment, to generate a train of voltage pulses in response to a time-dependent magnetic flux induced by a magnetic field applied perpendicularly to the planar surface.

2. The Josephson junction of claim 1 , wherein the Josephson junction (20) is configured to generate a train of voltage pulses evenly spaced in time in response to a periodic magnetic flux, thus obtaining a frequency comb signal with equidistant frequency lines.

3. The Josephson junction (20) of claims 1 or 2, wherein said predetermined length (L) is not less than 1nm.

4. The Josephson junction (20) of any of the above claims, wherein the non-superconductive material (28) comprises any material selected from the group consisting of metals, crystalline semiconductors and two-dimensional materials.

5. The Josephson junction (20) of claim 4, wherein the metal is selected from the group consisting of copper, silver, gold, aluminum and mixtures thereof.

6. The Josephson junction (20) of claim 4, wherein the crystalline semiconductor is selected from the group consisting of indium arsenide, germanium, bismuth arsenide, indium antimonide and mixtures thereof.

7. The Josephson junction (20) of claim 4, wherein the two-dimensional material is selected from the group consisting of graphene and h-Bn.

8. Cryogenic apparatus for generating a frequency comb signal comprising: - a Josephson junction (20) according to any of the claims 2 to 7;- a magnetic field generation system configured to apply a magnetic field perpendicularly to the planar surface, the magnetic field comprising a static component and an oscillating component, wherein the oscillating component induces the periodic magnetic flux; and- a transmission line connected to one of the superconductive electrodes (26), the transmission line being configured to convey the generated frequency comb signal to an output.

Images