Cryogenic attenuation device

Abstract

The invention relates to a device (1) for the cryogenic attenuation of a microwave signal transmission line, the device comprising: - a superconducting track (3) for conducting microwave signals; - at least one entirely normally conductive ground plane (4, 5), with non-zero resistivity, comprising: - a resistive film (6, 7) that is electrically insulated and spaced away from the superconducting track such that the characteristic impedance of the transmission line is at a predetermined value; - an electrically conductive body (8, 9) that is thermally and electrically connected and arranged adjacent to the resistive film such that at least part of the resistive film is located between the superconducting track and the conductive body, the conductive body being thermally connected to a periphery of the device maintained at cryogenic temperature, the thickness of the conductive body being at least two orders of magnitude greater than that of the resistive film, the thermal resistance per unit area thereof being at least ten times lower than that of the resistive film.

Description

Cryogenic attenuation device

[0001] The present invention relates to a cryogenic attenuation device.

[0002] The field of the invention is, but not limited to, that of quantum systems. State of the art

[0003] Cryogenic attenuators are used in quantum systems, particularly quantum computers. These quantum systems are based on superconducting components operating at cryogenic temperatures. To function correctly, a quantum system must, on the one hand, be isolated as much as possible from the external environment so that its quantum properties are not destroyed by decoherence. On the other hand, a quantum system must still be coupled to the external environment to be controlled and measured.

[0004] For example, superconducting qubits, as two-level quantum systems, must be coupled to microwave instrumentation including sources, signal generators, and acquisition boards. This microwave instrumentation is arranged and operates at room temperature, approximately 300 K. Superconducting qubits are placed in a dilution refrigerator at a temperature of approximately 10 mK.

[0005] Microwave instruments emit, at a minimum, incoherent electromagnetic noise composed of thermal photons having a blackbody distribution at 300 K.

[0006] The thermal energy at 300 K of a microwave mode is far greater than the energy of a microwave photon, and microwave modes are therefore populated with room-temperature thermal photons. These thermal photons are thus a considerable source of interference, causing decoherence of superconducting qubits by inducing excitation or phase-shift channels.

[0007] To suppress these thermal photons, cryogenic attenuators are arranged along the microwave lines connecting the quantum system to the microwave instruments used to measure and control the quantum system.

[0008] Known attenuators rely on the use of resistive films. Electron-phonon coupling typically exhibits a temperature dependence (T). 5 and is proportional to the volume. This electron-phonon interaction volume is very usually small (on the order of a few µm). 3). The attenuation obtained is therefore accompanied by the heating of electrons in resistive films by Joule effect, and the electron-phonon interaction is too weak to cool the electrons down to very low temperatures, such as 10 mK.

[0009] This results in an inefficiency in thermalizing thermal photons to the refrigerator temperature, thus limiting the coherence time of quantum systems such as superconducting qubits.

[0010] One aim of the present invention is to provide a cryogenic attenuation device to overcome the aforementioned drawbacks.

[0011] One of the aims of the present invention is to provide a cryogenic attenuation device enabling efficient thermalization of the electromagnetic environment of a quantum system.

[0012] At least one of these goals is achieved with a cryogenic attenuation device for a microwave signal transmission line, the device comprising: a substrate, a superconducting track arranged on the substrate and adapted to conduct microwave signals to be attenuated, at least one fully normally conductive ground plane, of non-zero resistivity, arranged on the substrate, the ground plane comprising: a resistive film, the resistive film being electrically insulated from the superconducting track, the resistive film being spaced from the superconducting track so that the characteristic impedance of the resulting transmission line is at a predetermined value, an electrically conductive body arranged adjacent to the resistive film so as to be thermally and electrically connected to the resistive film and such that at least a portion of the resistive film lies between the superconducting track and the conductive body,the conductive body being thermally connected to a peripheral part of the device maintained at a cryogenic temperature, the thickness of the electrically conductive body being at least two orders of magnitude greater than that of the resistive film, and the thermal resistance per unit area of ​​the conductive body being at least ten times lower than that of the resistive film.

[0013] The cryogenic attenuation device according to the invention effectively attenuates a microwave signal and thermalizes the thermal photons emitted by the electromagnetic environment of a quantum system. When a microwave signal propagates along the superconducting track, an electromagnetic field penetrates the resistive film. The resistive film then heats up due to the Joule effect. By constructing the ground plane from a resistive film and a fully normally conductive body (i.e., one with non-zero resistivity), it is possible to efficiently dissipate heat through the electrical and thermal conduction of "hot" electrons, with this heat being carried away from the superconducting track and the resistive film.The hot electrons generated in the resistive film are thus transferred to the conductive body and thermalized to the temperature of the peripheral part of the device, which is maintained at a cryogenic temperature, before potentially returning to the resistive film. Heat is removed from the conductive body by thermal conduction and by dissipation in the phonon bath.

[0014] Indeed, because the thickness of the conductive body is at least two orders of magnitude greater than that of the resistive film—that is, at least 100 times greater—the electron-phonon interaction volume is multiplied by at least two orders of magnitude compared to state-of-the-art attenuators. Consequently, the temperature dependence in T 5 Electron-phonon coupling is not limiting. The conductive body is well thermalized at the peripheral part of the device, which is maintained at a cryogenic temperature.

[0015] The area where heat dissipation occurs is within a normally conductive material, thus enabling this dissipation. The very thick conducting body is located far from the heat dissipation area. The ground plane(s) do not contain any non-normally conductive materials, such as superconductors.

[0016] The microwave signal guided in the superconducting track is progressively attenuated during its propagation, with the power dissipated being greater at the input of the attenuation device than at the output.

[0017] Thanks to the device according to the invention, the decoherence induced by thermal photons in quantum systems can be made negligible or at least greatly reduced.

[0018] The term "peripheral part of the device" refers to the part of the device that is in contact with the environment outside the device. The peripheral part may include, for example, a metal casing or enclosure, particularly made of copper or gold-plated copper, containing the device components, a printed circuit board of which the device is a part, conductive wires or braids, etc.

[0019] The superconducting track and the ground plane can be arranged so that they are spaced at a constant distance apart.

[0020] The superconducting track and the ground plane can also be spaced a variable distance apart along the track's length. In this case, the width of the superconducting track is also variable.

[0021] The relationship between the width of the superconducting track and its distance from the ground plane will be detailed below.

[0022] In this document, the terms "resistive film" and "conductive body" mean respectively "resistive metal film" and "conductive metal body" or "electrically conductive body".

[0023] The resistive film is notably made of resistive metal, that is to say, exhibiting a high electrical surface resistivity compared to the conductive body.

[0024] The electrically conductive body is made of a metal exhibiting very good thermal conductivity relative to other materials at these low temperatures.

[0025] The thermal resistance per unit area of ​​the conductive body is at least 10 times lower than that of the resistive film.

[0026] The resistive film is a "thin" film, that is to say, a film whose thickness is on the order of 100 or 1000 times less than that of the conductive body.

[0027] Microwave signals carried by the superconducting track can have a frequency, for example, between about 1 GHz and 40 GHz depending on the applications envisaged.

[0028] The resistive film is spaced from the superconducting track so that the characteristic impedance of the resulting transmission line is at a desired value (typically 50 Ohm or 75 Ohm).

[0029] The conductive body is made of a material whose electrical resistivity is as low as possible, without these materials being superconductors, which would greatly limit thermal conductivity.

[0030] The conductive body is thermally connected to a peripheral part of the device, for example a copper enclosure, either directly or via heat-conducting components. The cryogenic temperature at which the peripheral part of the device is maintained is less than 200 mK, specifically on the order of 10 mK.

[0031] In the device according to the invention, the superconducting track, the resistive film and the conductive body are arranged in a coplanar manner.

[0032] In one example, the superconducting track is arranged in a microstrip shape on a substrate. The resistive film is on the back side of the substrate, and the conductive body covers it.

[0033] In another example, the superconducting track is sandwiched between two dielectric materials, each coated with a resistive film, which is itself coated with a conductor. This configuration is called a "lotline" in English terminology.

[0034] According to a preferred embodiment, the device according to the invention comprises a first and a second ground plane arranged respectively on either side of the superconducting track on the substrate.

[0035] The superconducting track and the ground planes are arranged so that they are spaced at a constant or variable distance, respectively.

[0036] Advantageously, the surface resistivity of the film metal and the geometric parameters of the superconducting track and the resistive film are adapted to generate a microwave signal attenuation of between 3 and 60 dB, for a microwave signal frequency between 1 GHz and 40 GHz.

[0037] Geometric parameters include the width of the superconducting track and resistive film, as well as the total length of the superconducting track.

[0038] Depending on the embodiment, the superconducting track can have a width between 1 and 100 µm.

[0039] The value of the width of the superconducting track can be chosen in particular according to the total length of the track.

[0040] The width of the superconducting track also affects the distance between the track and the resistive film(s), in order to obtain a specific characteristic impedance, for example 50 Ohms or 75 Ohms. The wider the superconducting track, the further the resistive film(s) are located from the track.

[0041] The thickness of the resistive film ranges from 5 to 50 nm, depending on the resistivity of the metal used. The thinner the film, the higher its resistivity, and therefore its greater surface resistance.

[0042] The width of the resistive film can be, for example, from 10 to 70 µm, depending on the thickness chosen.

[0043] Depending on the embodiment, the resistive film can be made of copper, silver, chrome, nickel or gold.

[0044] These metals are very good conductors of electric current.

[0045] Depending on the embodiment, the thickness of each conductive body is between 1 µm and 100 µm. Their width is between 100 µm and 10 mm.

[0046] These conductive materials can include, for example, copper, gold, or silver.

[0047] According to one embodiment, the ground plane and the superconducting track can be straight and parallel.

[0048] Alternatively, the superconducting track can be curved.

[0049] The superconducting track can notably form a meander in the middle of a ground plane, this ground plane forming a groove or a recess for the location of the track.

[0050] In this case, the length of the superconducting track can be increased without increasing the length of the device.

[0051] According to one embodiment, the length of the device is on the order of a centimeter.

[0052] The device is therefore very compact.

[0053] According to another aspect of the same invention, it is proposed to use a cryogenic attenuation device according to the invention in a control and / or measurement system of a quantum system.

[0054] The device according to the invention can notably be used to prepare coherent states of microwave radiation, that is to say, pure in the sense of quantum physics. This coherent radiation has a wide range of applications in classical and quantum detection, quantum communication, and for the search for dark matter, for example.

[0055] Other areas of application include quantum information, quantum computers, and quantum simulation.

[0056] The cryogenic attenuation device according to the invention can also be used in various fields requiring precise control of an electromagnetic environment at extremely low temperatures. These fields include high-precision scientific instrumentation, cryogenic radio astronomy, and certain advanced medical imaging techniques, where a noise-free cryogenic and electromagnetic environment is essential.

[0057] Furthermore, the device according to the invention can be used to improve high-precision frequency standards in metrology, or integrated into satellite communication systems that require highly sensitive cryogenic receivers.

[0058] Other applications include cryogenic instrumentation for spectroscopy, millimeter astronomy, and particle physics experiments. Description of the figures and methods of implementation

[0059] Other advantages and features will become apparent from the detailed description of non-limiting examples and the accompanying drawings in which: 1a is a schematic representation of a cryogenic attenuation device according to one embodiment of the invention, 2a is another schematic representation of a cryogenic attenuation device according to the embodiment of 1, and 3a is a schematic representation of a cryogenic attenuation device according to another embodiment of the invention.

[0060] It is understood that the embodiments described below are by no means exhaustive. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0061] In particular, all the variants and embodiments described can be combined with each other if there are no technical obstacles to this combination.

[0062] In the figures, elements common to several figures retain the same reference.

[0063] Figures 1 and 2 are schematic representations of a cryogenic attenuation device according to an embodiment of the invention. In Figure 1, the device is shown in cross-section, and in Figure 2, the device is shown in a top view.

[0064] The cryogenic attenuation device 1 comprises a substrate 2 on which a superconducting track 3 is deposited. The superconducting track 3 is arranged on a central part of the substrate 2. The track 3 is adapted to conduct a microwave signal to be attenuated.

[0065] Device 1, as shown in Figures 1 and 2, also includes two ground planes 4, 5. These first and second ground planes 4, 5 are arranged respectively on either side of the superconducting track 3 on the substrate 2.

[0066] In the embodiment as shown in Figures 1 and 2, the superconducting track 3 and the ground planes 4, 5 are straight and arranged parallel on the substrate 2. The superconducting track 3 and each ground plane 4, 5 are arranged at a constant distance from each other.

[0067] Substrate 1 can notably be made of silicon.

[0068] Each ground plane 4, 5 comprises a resistive film 6, 7 and a conductive body 8, 9.

[0069] Ground planes 4, 5 are entirely normally conductive, with non-zero resistivity.

[0070] The resistive films 6, 7 are respectively electrically isolated from the superconducting track 3. In this case, the electrical isolation is achieved by the spacing between the films 6, 7 and the superconducting track 3. The distance between the films 6, 7 and the track 3 is chosen so that an electromagnetic field can penetrate the films 6, 7 when a microwave signal is conducted through the track 3.

[0071] The conductive bodies 8, 9 are arranged respectively adjacent to one of the resistive films 6, 7 on the opposite side with respect to the superconducting track 3. A conductive body 8, 9 and a resistive film 6, 7 are in contact, respectively, such that each conductive body 8, 9 is thermally and electrically connected to one of the resistive films 6, 7. The resistive film 6, 7 is closer to the track 3 than the conductive body 8, 9.

[0072] The conductive bodies 8 and 9 are cooled to a given cryogenic temperature, for example, below 200 mK. This temperature is generated outside the device, the device being thermally connected to this cryogenic environment. This cryogenic environment could consist, for example, of a dilution refrigerator.

[0073] In the embodiment as shown in Figures 1 and 2, the superconducting track 3, the resistive films 6, 7 and the conductive bodies 8, 9 are arranged coplanarly on the substrate 2.

[0074] In practice, a resistive film can cover the substrate across the entire area of ​​the ground planes. The conductive materials are deposited on this resistive film in such a way as to leave a narrow strip of resistive film without conductive materials at the edge of each ground plane. The superconducting trace is located in the middle of this strip without resistive film.

[0075] The resistive films 6, 7 are preferably made of copper. They can also be made of silver, chrome, nickel or gold.

[0076] These metals exhibit, in the films, a very high surface resistivity.

[0077] The conductive bodies 8, 9 are preferably made of copper. They can also be made of silver or gold.

[0078] The widths of the resistive films 6, 7 (W1) and of the superconducting track 3, the thickness of the track 3 and the metal of the resistive films 6, 7 are chosen to obtain an attenuation of the microwave signal of between approximately 3 and 60 dB, for microwave signals with a frequency between 1 GHz and 40 GHz.

[0079] As an example, the superconducting track 3 can have a width between 1 and 100 µm.

[0080] The value of the resistive film width can be chosen in particular according to the total length of track 3 to guarantee a desired total attenuation.

[0081] The width also has an impact on the distance between track 3 and resistive films 6, 7. The wider the films, the greater the distance between track 3 and resistive films 6, 7 is to obtain a characteristic impedance of the transmission line.

[0082] The width W1 of the resistive films 6, 7 can be between 10 and approximately 70 µm, depending on the thickness e1 of the films 6, 7.

[0083] The thickness e1 of each of the resistive films 6, 7 can range from approximately 5 to 50 nm, depending on the surface resistivity of the metal used. The thinner the film, the higher its surface resistivity for the same material.

[0084] The thickness e2 of the conductive bodies is preferably between 1 µm and 100 µm. The width W2 of the conductive bodies is preferably between 100 µm and 10 mm.

[0085] In the embodiment of Figures 1 and 2, the lengths of the superconducting track 3 and the ground planes 4, 5 are substantially identical.

[0086] The device according to the invention can have a length on the order of a centimeter.

[0087] Figure 1 is a schematic representation of a cryogenic attenuation device according to another embodiment of the invention. In Figure 1, the device 11 is shown in a top view.

[0088] Device 11 according to the embodiment shown on the includes the same elements as that shown in Figures 1 and 2. It differs from this first embodiment by the shape of the elements.

[0089] In this embodiment, the superconducting track 13 can be curvilinear. The superconducting track 13 forms, in particular, a meander in the middle of a ground plane. The ground plane, and more specifically the conductive body 18, forms a groove or recess in which the superconducting track 13 is arranged, on the substrate 12. At the edge of this groove, on either side of the superconducting track 13, are also resistive films 16, 17, which are part of the ground plane.

[0090] In this case, the length of the superconducting track can be increased relative to the length of the device 11 and that of the conducting body 18.

[0091] The cryogenic attenuation device according to the present invention can be implemented in a wide variety of applications.

[0092] The cryogenic attenuation device according to the invention can be implemented in a control and / or measurement system of a quantum system.

[0093] The device can notably be implemented in semiconductor circuits, for the control and / or measurement of semiconductor structures, quantum dots or spin qubits.

[0094] The cryogenic attenuation device according to the invention also makes it possible to attenuate microwave signals while allowing a DC circuit to be biased without any resistance or heating, which can be useful in the case of multifunction microwave / DC ports.

[0095] The cryogenic attenuation device according to the invention can also be implemented for the precise control of an electromagnetic environment at extremely low temperatures, in fields such as high-precision scientific instrumentation, cryogenic radio astronomy, and certain advanced medical imaging techniques.

[0096] The device according to the invention can also be used to improve high-precision frequency standards in metrology, or be integrated into satellite communication systems.

[0097] Other applications include cryogenic instrumentation for spectroscopy, millimeter astronomy, and particle physics experiments.

[0098] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

Claims

A cryogenic attenuation device (1) for a microwave signal transmission line, the device (1) comprising: a substrate (2), a superconducting track (3) arranged on the substrate (2) and adapted to conduct microwave signals to be attenuated, at least one fully normally conductive ground plane (4, 5) of non-zero resistivity arranged on the substrate (2), each ground plane (4, 5) comprising: a resistive film (6, 7), the resistive film (6, 7) being electrically insulated from the superconducting track (3), the resistive film (6, 7) being spaced from the superconducting track (3) such that the characteristic impedance of the resulting transmission line is at a predetermined value, an electrically conductive body (8, 9) arranged adjacent to the resistive film (6, 7) so as to be thermally and electrically connected to the resistive film (6, 7) and such that at least a portion of the resistive film (6,7) is located between the superconducting track (3) and the conducting body (8, 9), the conducting body (8, 9) being thermally connected to a peripheral part of the device (1) maintained at a cryogenic temperature, the thickness of the electrically conducting body (8, 9) being at least two orders of magnitude greater than that of the resistive film (6, 7), and the thermal resistance per unit area of ​​the conducting body (8, 9) being at least ten times less than that of the resistive film (6, 7). Device (1) according to claim 1, characterized in that the surface resistivity of the resistive film (6, 7) and the geometric parameters of the resistive film (6, 7) are adapted to generate an attenuation of the microwave signal of between 3 and 60 dB, for a microwave signal frequency between 1 GHz and 40 GHz. Device (1) according to any one of the preceding claims, characterized in that the superconducting track (3) has a width between 1 and 100 µm. Device (1) according to any one of the preceding claims, characterized in that the thickness (e1) of the resistive film (6, 7) is between 5 and 50 nm. Device (1) according to any one of the preceding claims, characterized in that the thickness (e2) of the conductive body (8, 9) is between 1 µm and 100 µm, and / or the width (W2) of the conductive body (8, 9) is between 100 µm and 10 mm. Device (1) according to any one of the preceding claims, characterized in that a first and a second ground plane (4, 5) are arranged on either side of the superconducting track (3). Device (1) according to the preceding claim, characterized in that the ground planes (4, 5) and the superconducting track (3) are straight and parallel. Device (1) according to any one of claims 1 to 6, characterized in that the superconducting track (3) is curvilinear. Device (1) according to any one of the preceding claims, characterized in that the length of the device (1) is on the order of a centimeter. Device (1) according to any one of the preceding claims, characterized in that the resistive film (6, 7) is made of copper, silver, chrome, nickel or gold. Device (1) according to any one of the preceding claims, characterized in that the conducting body (8, 9) is made of copper, gold or silver. Use of a cryogenic attenuation device (1) according to any one of the preceding claims in a control and / or measurement system of a quantum system.

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