Cryogenic attenuation device

Abstract

The invention relates to a cryogenic attenuation device (1) for a microwave signal transmission line, comprising: a superconducting track (3) adapted to conduct microwave signals, at least one ground plane (4, 5), comprising: a resistive film (6, 7) electrically isolated from the superconducting track and spaced from it so that the characteristic impedance of the resulting transmission line is at a predetermined value, an electrically conductive body (8, 9) thermally and electrically connected and arranged adjacent to the resistive film so 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 periphery 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,the thermal resistance per unit area of ​​the conductive body being at least ten times lower than that of the resistive film. Figure for abbreviation: [Fig. 1 ],

Description

Title of the invention: Cryogenic attenuation device technical field

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

[0002] The field of the invention is, without limitation, that of quantum systems. State of the art

[0003] Cryogenic attenuators are used in the field of quantum systems, and in particular 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 the phenomenon of decoherence. On the other hand, a quantum system must still be coupled to the external environment in order to be controlled and measured.

[0004] By way of example, superconducting qubits, as two-level quantum systems, must be coupled to microwave instrumentation comprising sources, signal generators, or acquisition cards. These microwave instruments are arranged and operate at room temperature, i.e., a temperature of approximately 300 K. The 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 much greater than the energy of a microwave photon, and microwave modes are therefore populated with thermal photons at room temperature. 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 are based on the use of resistive films. Electron-phonon coupling typically has a temperature dependence in T5 and is proportional to the volume. This electron-phonon interaction volume is very usually small (on the order of a few pm3). The attenuation obtained This is 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. Description of the invention

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

[0011] In particular, one objective 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 ground plan arranged on the substrate, the ground plan including: • a resistive film, the resistive film being electrically isolated 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 less than that of the resistive film.

[0013] The cryogenic attenuation device according to the invention makes it possible to efficiently attenuate a microwave signal and thermalize the thermal photons emitted by the electromagnetic environment of a quantum system. When a microwave signal propagates along the superconducting track, an electromagnetic field The resistive film penetrates the resistive film. The resistive film then heats up due to the Joule effect. Thanks to the construction of the ground plane using a resistive film and a conductive body, it is possible to efficiently dissipate heat through the electrical and thermal conduction of "hot" electrons. This heat is carried away from the superconducting track and the resistive film. The hot electrons generated in the resistive film are thus carried 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 dissipated from the conductive body by thermal conduction and by phonon bath dissipation.

[0014] Indeed, thanks to the fact that 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 than that of the resistive film, 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 of the electron-phonon coupling at T5 is not limiting. The conductive body is effectively thermalized at the peripheral part of the device, which is maintained at a cryogenic temperature.

[0015] The microwave signal guided in the superconducting track is progressively attenuated during its propagation, the power dissipated being greater at the input of the attenuation device than at the output.

[0016] 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.

[0017] The term "peripheral part of the device" means the part of the device that is in contact with the environment external to the device. The peripheral part may include, for example, a metal casing or enclosure, in particular made of copper or gold-plated copper, containing the components of the device, a printed circuit board of which the device is a part, conductive wires or braids, etc.

[0018] The superconducting track and the ground plane can be arranged so as to be spaced at a constant distance.

[0019] The superconducting track and the ground plane can also be separated by a variable distance along the length of the track. In this case, the width of the superconducting track is also variable.

[0020] The relationship between the width of the superconducting track and its distance from the ground plane will be detailed below.

[0021] In this document, the terms "resistive film" and "conductive body" mean respectively "resistive metal film" and "conductive metal body" or "electrically conductive body".

[0022] The resistive film is in particular made of resistive metal, that is to say, exhibiting a high electrical surface resistivity compared to the conductive body.

[0023] The electrically conductive body is made of a metal exhibiting very good thermal conductivity relative to other materials at these low temperatures.

[0024] The thermal resistance per unit area of ​​the conductive body is at least 10 times lower than that of the resistive film.

[0025] 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.

[0026] The 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.

[0027] 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).

[0028] The conducting body is made of a material whose electrical resistivity is as low as possible, without these materials being superconductors, which would greatly limit the thermal conductivity.

[0029] The conductive body is thermally connected to a peripheral part of the device, for example a copper casing, either directly or via heat-conducting parts. The cryogenic temperature at which the peripheral part of the device is maintained is less than 200 mK, in particular on the order of 10 mK.

[0030] In the device according to the invention, the superconducting track, the resistive film and the conductive body are arranged in a coplanar manner.

[0031] According to one embodiment, the superconducting track is arranged in the form of a microstrip on a substrate. The resistive film is on the back side of the substrate and the conductive body covers it.

[0032] According to another example, the superconducting track is sandwiched between two dielectric materials, each of which is covered with a resistive film, itself covered with a conductor. This configuration is called a slotline in English terminology.

[0033] 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.

[0034] The superconducting track and the ground planes are arranged so as to be spaced at a constant or variable distance, respectively.

[0035] Advantageously, the surface resistivity of the film metal and the geometric parameters of the superconducting track and the resistive film are adapted to generate microwave signal attenuation of between 3 and 60 dB, for a microwave signal frequency between 1 GHz and 40 GHz.

[0036] The geometric parameters include the width of the superconducting track and the resistive film, as well as the total length of the superconducting track.

[0037] According to embodiments, the superconducting track can have a width between 1 and 100 pm.

[0038] The value of the width of the superconducting track can in particular be chosen according to the total length of the track.

[0039] The width of the superconducting track also affects the distance between the track and the resistive film(s), in order to obtain a particular characteristic impedance, for example 50 Ohm or 75 Ohm. The wider the superconducting track, the further the resistive film(s) are from the track.

[0040] The thickness of the resistive film is between 5 and 50 nm, depending on the resistivity of the metal used. The thinner the film, the higher its resistivity, and therefore the greater its surface resistance.

[0041] The width of the resistive film can be, for example, from 10 to 70 pm, depending on the thickness chosen.

[0042] According to embodiments, the resistive film can be made of copper, silver, chrome, nickel or gold.

[0043] These metals are very good conductors of electric current.

[0044] According to some embodiments, the thickness of each conductive body is between 1 µm and 100 µm. Their width is between 100 µm and 10 mm.

[0045] These conductive materials may include, for example, copper, gold or silver.

[0046] According to one embodiment, the ground plane and the superconducting track can be straight and parallel.

[0047] Alternatively, the superconducting track can be curvilinear.

[0048] The superconducting track can in particular 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.

[0049] In this case, the length of the superconducting track can be increased without increasing that of the device.

[0050] According to one embodiment, the length of the device is on the order of a centimeter.

[0051] The device is therefore very compact.

[0052] 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.

[0053] The device according to the invention can in particular be used to prepare coherent states of microwave radiation, that is to say, pure within the meaning of the 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.

[0054] Other areas of application include quantum information, quantum computers and quantum simulation.

[0055] 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.

[0056] Furthermore, the device according to the invention can be used to improve high-precision frequency standards in metrology, or can be integrated into satellite communication systems that require highly sensitive cryogenic receivers.

[0057] Other applications include, in particular, cryogenic instrumentation for spectroscopy, millimeter astronomy, and particle physics experiments. Description of the figures and methods of realization

[0058] Other advantages and features will become apparent upon examination of the detailed description of non-limiting examples and the accompanying drawings, in which: - [Fig. 1] [Fig. 1] is a schematic representation of a cryogenic attenuation device according to one embodiment of the invention, - [Fig.2] [Fig.2] is another schematic representation of a device cryogenic attenuation according to the embodiment of [Fig. 1], and - [Fig.3] [Fig.3] is a schematic representation of a cryogenic attenuation device according to another embodiment of the invention.

[0059] It is understood that the embodiments described below are by no means limiting. 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.

[0060] In particular, all the variants and all the embodiments described are combinable with each other if nothing prevents this combination from a technical point of view.

[0061] In the figures, the elements common to several figures retain the same reference.

[0062] Figures 1 and 2 are schematic representations of a cryogenic attenuation device according to an embodiment of the invention. In [Fig. 1], the device is shown in cross-section, and in [Fig. 2], the device is shown in a top view.

[0063] 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.

[0064] The 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.

[0065] 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.

[0066] The substrate 1 may in particular be made of silicon.

[0067] Each ground plane 4, 5 comprises a resistive film 6, 7 and a conductive body 8, 9.

[0068] The resistive films 6, 7 are respectively electrically isolated from the superconducting track 3. In the present 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.

[0069] 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 touch, respectively, so 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.

[0070] The conductive bodies 8, 9 are cooled to a given cryogenic temperature, for example below 200 mK. This temperature is produced outside the device, the device being thermally connected to this cryogenic environment. This cryogenic environment may consist, for example, of a dilution refrigerator

[0071] 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.

[0072] In practice, a resistive film can cover the substrate over 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 track is located in the middle of the space without resistive film.

[0073] The resistive films 6, 7 are preferably made of copper. They can also be made of silver, chrome, nickel or gold.

[0074] These metals exhibit, in the films, a very high surface resistivity.

[0075] The conductive bodies 8, 9 are preferably made of copper. They can also be made of silver or gold.

[0076] The widths of the resistive films 6, 7 (W1) and of the superconducting track 3, the thickness of the track 3 as well as 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.

[0077] By way of example, the superconducting track 3 can have a width between 1 and 100 pm.

[0078] The value of the width of the resistive film can in particular be chosen according to the total length of track 3 to guarantee a desired total attenuation.

[0079] The width also has an impact on the distance between track 3 and the resistive films 6, 7. The wider the films, the greater the distance between track 3 and the resistive films 6, 7 is to obtain a characteristic impedance of the transmission line.

[0080] The width W1 of the resistive films 6, 7 can be between approximately 10 and 70 pm, in function of the thickness of the films 6, 7.

[0081] The thickness el of each of the resistive films 6, 7 can be between approximately 5 and 50 nm, depending on the surface resistivity of the metal used. The thinner the film, the greater its surface resistivity for the same material.

[0082] 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.

[0083] In the embodiment of Figures 1 and 2, the lengths of the superconducting track 3 and the ground planes 4, 5 are substantially identical.

[0084] The device according to the invention can have a length on the order of a centimeter.

[0085] Figure 3 is a schematic representation of a cryogenic attenuation device according to another embodiment of the invention. In Figure 3, the device 11 is shown in a top view.

[0086] Device 11 according to the embodiment shown in [Fig.3] comprises the same elements as that shown in Figures 1 and 2. It differs from this first embodiment by the shape of the elements.

[0087] In the embodiment of [Fig. 3], the superconducting track 13 can be curvilinear. In particular, the superconducting track 13 forms 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.

[0088] In this case, the length of the superconducting track can be increased in relation to the length of the device 11 and that of the conducting body 18.

[0089] The cryogenic attenuation device according to the present invention can be implemented in a wide variety of applications.

[0090] The cryogenic attenuation device according to the invention can be implemented in a control and / or measurement system of a quantum system.

[0091] The device can in particular be implemented in semiconductor circuits, for the control and / or measurement of semiconductor structures, quantum dots or spin qubits.

[0092] The cryogenic attenuation device according to the invention also makes it possible to attenuate microwave signals while allowing a direct current circuit to be biased without any resistance or heating, which can be useful in the case of multifunction microwave / DC ports.

[0093] 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, as well as certain advanced medical imaging techniques.

[0094] 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.

[0095] Other applications include, in particular, cryogenic instrumentation for spectroscopy, millimeter astronomy, and particle physics experiments.

[0096] 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

Demands

1. 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 ground plane (4, 5) 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).

2. Device (1) according to claim 1, characterized in that the surface resistivity of the resistive film (6, 7) and 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.

3. Device (1) according to any one of the preceding claims, characterized in that the superconducting track (3) has a width between 1 and 100 pm.

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12. Device (1) according to any one of the preceding claims, characterized in that the thickness (el) 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 pm and 100 pm, and / or the width (W2) of the conductive body (8, 9) is between 100 pm 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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