DEVICE WITH A GROUP OF JOSEPHSONIC INTERFACES WITH HOMOGENEOUS POWER DISTRIBUTION

Description

[0001] The present invention relates to a superconducting device, in particular an electromagnetic field detector or an amplifier.

[0002] The present invention falls within the field of high critical temperature superconducting electronic circuits.

[0003] Superconducting circuits based on Josephson junctions have a very low noise level, and are, particularly as electromagnetic field detectors, very sensitive.

[0004] To increase their performance, networks containing a large number of Josephson junctions are generally used. These networks have a one-dimensional (series or parallel) or two-dimensional structure. Three-dimensional structures have even been proposed.

[0005] One-dimensional series-type structures have a length limited by the total impedance of the device, which must remain low to have good output matching (and therefore a wide bandwidth).

[0006] One solution to this problem is to put elements in parallel, thus obtaining one-dimensional parallel-type networks or two-dimensional networks.

[0007] In both cases, the width (dimension perpendicular to the length) is limited because the bias currents are essentially concentrated on the two edges of the network.

[0008] The situation is somewhat comparable to a radio frequency (RF) current distribution in a metallic conductor, but here this distribution is controlled by the Meissner effect. Beyond a certain characteristic dimension, the junctions located at the center of the network do not receive enough bias current to actively participate in the device's response.

[0009] There are even situations where these junctions destroy the response of junctions located on the edges of the network.

[0010] In the context of the development of such networks in low critical temperature superconducting technology, this problem is solved by employing a network of resistors and a superconducting ground plane.

[0011] However, high-temperature superconductor technology does not allow for the integration of resistors, and multi-layer superconducting structures are difficult to achieve.

[0012] Superconducting devices are also known from an article by D. Crété et al. entitled "Effect of self-induced flux in parallel arrays of Josephson arrays", an article by J. Schuler et al. entitled "Comparison of Josephson vortex flow transistors with different gate line configuration" and from document US 6 690 192 B1.

[0013] Therefore, there is a need for a superconducting device made by a circuit based on relatively large Josephson junctions exhibiting a more homogeneous current distribution.

[0014] To this end, the description describes a superconducting device, in particular an electromagnetic field detector, antenna or amplifier, the device comprising a network of parallel Josephson junctions, a first plane and a width being defined for the network, at least one conductive strip contained in a second plane, the second plane facing the first plane, the conductive strip or strips being made of gold, aluminum, copper, palladium, an alloy of the aforementioned elements, Ta2N or cuprate, the conductive strip or strips being preferably connected to form a conductive grid.The superconducting device includes a power supply unit for supplying the Josephson junctions of the Josephson junction network and each conductive strip with currents of opposite signs, the power supply unit supplying the Josephson junctions of the Josephson junction network and each conductive strip with currents having an intensity chosen so that the total of the currents flowing in each conductive strip is equal to the current flowing in the Josephson junction network to within 10%, the first and second planes being arranged so that each conductive strip is at a distance from a Josephson junction less than or equal to 40% of the width defined for the network.

[0015] It thus appears that the superconducting device makes it possible to distribute the current almost uniformly in a circuit of large dimensions transverse to the direction of the biasing current.

[0016] In specific embodiments, the superconducting device exhibits one or more of the following characteristics, taken individually or in all technically possible combinations: The conductive strip(s) are arranged so that each is at a distance from a Josephson junction less than or equal to 5% of the defined lattice width. The Josephson junctions comprise a superconducting material with a critical temperature above 40 Kelvin. The Josephson junction array is positioned on a first substrate and the conductive strips are positioned on a second substrate, with the two substrates facing each other. The Josephson junction array is positioned on a first substrate and the conductive strips are positioned on a second substrate, with the conductive strips separated from the Josephson junction array by at least one of the substrates. Each conductive strip has a width between 3 micrometers and the defined lattice width. The conductive strip(s) form another Josephson junction array.The device includes a spacer layer inserted between the first and second planes. The spacer layer is made of an insulating material. An average distance between two Josephson junctions of the network is defined; the first and second planes are arranged so that each conductive strip is at a distance from a Josephson junction greater than or equal to the average distance between two Josephson junctions of the network.

[0017] Features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which: there figure 1 is a schematic perspective view of an early example of a magnetic field detector incorporating superconductors, the figure 2 is a schematic cross-sectional representation of the detector of the figure 1 , there figure 3 is a schematic cross-sectional representation of a second example detector, the figure 4 is a schematic cross-sectional representation of a third example of a detector, the figure 5 is a schematic cross-sectional representation of a fourth example of a detector, the figure 6 is a schematic cross-sectional representation of a fifth detector example, and the figure 7 is a schematic cross-sectional representation of a sixth detector example.

[0018] A magnetic field detector 10 is shown on the figure 1 .

[0019] The detector 10 comprises a network 12 positioned on a first substrate 14, conductive strips 16 forming an assembly 18 positioned on a second substrate 20 and a power supply unit 22.

[0020] The network 12 and the first substrate 14 form a first chip 24 while the set 18 of the conductive bands 16 and the second substrate 20 form a second chip 26.

[0021] Network 12 is a network of Josephson 28 junctions.

[0022] A Josephson 28 junction is an assembly formed by two superconductors coupled by a barrier, the barrier being formed, for example, by a layer of an insulating material or by a layer of a non-superconducting material.

[0023] Following the example of the figure 1 , Josephson 28 junctions incorporate a superconducting material with a critical temperature above 40 Kelvins.

[0024] In the representation of the figure 2 , it appears that network 12 has eight Josephson 28 junctions, this number being chosen for illustrative purposes.

[0025] The eight Josephson junctions 28 are visible through the transparency of the second substrate 20.

[0026] The Josephson junctions 28 are connected in parallel by branches extending along a longitudinal direction X.

[0027] Furthermore, the branches are connected together by connection portions 32 extending along a first transverse direction Y.

[0028] The longitudinal direction X and the first transverse direction Y allow us to define a plane for the network of 12 Josephson junctions 28, called first plane P1.

[0029] A direction perpendicular to the first plane P1 can also be defined, this direction being called the second transverse direction Z.

[0030] An LR width is also defined for network 12.

[0031] The width LR is the dimension of the network 12 along the first transverse direction Y.

[0032] More specifically, the LR width is the greatest distance between two Josephson 28 junctions, the distance being measured along the first transverse Y direction.

[0033] Therefore, the width LR of the network 12 is the sum of all the widths LJ of the Josephson junctions 28 and of all the inter-junction spaces Josephson 28.

[0034] For example, if the superconducting material is made with YBa 2 Cu 3 O 7-x material (with x varying between 0 and 1), the first substrate 14 is made with a material chosen in a non-limiting way from sapphire, MgO, or NdGaO 3.

[0035] Each conductive band 16 extends mainly along the longitudinal direction X.

[0036] In addition, each conductive band 16 has a dimension along the first longitudinal direction Y, called width LB, which is typically greater than 3 micrometers.

[0037] The 16 conductive bands are parallel to each other.

[0038] The conductive bands 16 are arranged according to a second plane P2.

[0039] In this case, the second plane P2 is parallel to the first plane P1.

[0040] According to the example described, the 16 bands are made of gold.

[0041] Alternatively, band 16 is made of aluminium, copper, or palladium.

[0042] Alloys of the aforementioned elements can also be considered.

[0043] A width LE is also defined for the assembly 18 formed by the conductive strips 16.

[0044] Similar to the case of network 12, the width LE of set 18 is the dimension of set 18 along the first transverse direction Y.

[0045] More precisely, the LE width of the assembly 18 is the greatest distance between two conductive bands 16, the distance being measured along the first transverse direction Y. More precisely, taking into account the width of the extreme conductive bands 16, the LE width of the assembly 18 is the greatest distance between two points of two conductive bands 16.

[0046] In this example, the width LR of network 12 and the width LE of assembly 18 are equal to each other within 5%.

[0047] It should be noted that it is conceivable that the assembly 18 may have a single band 16, so that in this case, the width of the band 16 will be equal to the width LE of the assembly 18.

[0048] This means that, in all embodiments, the band or each band 16 has a width between 3 micrometers (µm) and the width LE of the assembly 18.

[0049] Regarding the second substrate 20, a wide variety of materials can be used, including low-loss substrates in the radio frequency range such as silicon. The materials mentioned for the first substrate 14 can also be considered.

[0050] The chips are held in place by gluing. In particular, a nonadecane or apiezon N adhesive can be considered for this purpose.

[0051] Alternatively, the two chips are held together by a mechanical assembly. The power supply unit 22 is usually a current generator.

[0052] The power supply unit 22 is suitable for supplying the Josephson junctions 28 and the conductive strips 16 with currents of opposite signs.

[0053] According to the example described, this means that the current flows in a first direction along the longitudinal direction X and in a second direction along the longitudinal direction X, the second direction being opposite to the first direction.

[0054] In addition, the power supply unit 22 is suitable for generating currents that also satisfy an amplitude condition, namely that the total of the currents flowing in the or each conductive strip 16 is equal to the current flowing in the network 12 of Josephson junctions 28 to within 10%.

[0055] The electrical contacts 34 between the different chips 24 and 26 are schematically represented by ellipses. The two contacts 34 represented by the ellipses in the background are connected to each other by a wire 36, while the two contacts 34 represented by the ellipses in the foreground are the connection points of the power supply unit 22.

[0056] The connecting wires 36 between the two chips 24 and 26 and those of the current leads 38 can be made of gold.

[0057] The first and second planes are arranged so that each conductive strip 16 is at a distance from a Josephson junction 28 less than or equal to 40% of the width defined for the network 12.

[0058] This means that if network 12 is already present, the different conductive bands 16 will be arranged (relative to said network 12) to comply with the aforementioned distance condition.

[0059] Several arrangements can be considered.

[0060] In the example described and as visible on the figure 2 , the conductive strips 16 are spaced from the network 12 of Josephson junctions 28 by at least one of the substrates.

[0061] Put another way, the conductive bands 16 are spaced from the network 12 by a space comprising the second substrate 20.

[0062] Such an arrangement corresponds to a so-called "piggy back" arrangement, which means "mounted on the back" or "stacked" and corresponds to a configuration one on top of the other.

[0063] In such cases, typical widths are on the order of a few hundred micrometers. Also, depending on the case, the width ranges from 0.1 millimeter (mm) to 2 mm.

[0064] Another example is offered to the figure 3 .

[0065] In this example, the network 12 and the conductive strips 16 face each other.

[0066] Such a configuration corresponds to a so-called "flip-chip" arrangement, which literally means "flipped chip".

[0067] In such a case, typical widths are on the order of a few tens of micrometers, specifically between 20 µm and 2 mm.

[0068] Alternatively, by calling the network of Josephson junctions 28 "first network 12", the conductive strips 16 form a second network of Josephson junctions.

[0069] Two examples are presented to figures 4 And 5 where at least one of the 2 chips contains a Josephson junction network.

[0070] In each case, for illustrative purposes, the two Josephson junction networks have the same number of Josephson junctions.

[0071] There figure 4 illustrates the case of a piggy-back configuration in which the second network of Josephson junctions is spaced from the first network of Josephson junctions 28 by the second substrate 20.

[0072] Conversely, the figure 5 illustrates the case of a flip-chip configuration in which the second network of Josephson junctions is opposite the first network of 12 Josephson junctions 28.

[0073] In these embodiments, band 16 is made of a superconducting material such as cuprate.

[0074] According to another example, band 16 is made of Ta 2 N.

[0075] According to another variant, the 16 conductive strips form a grid as very schematically represented on the figures 6 And 7 .

[0076] THE figures 6 And 7 correspond respectively to the cases of a piggy-back configuration and a flip-chip configuration.

[0077] In each case, the number of conductive bands 16 is equal to 5.

[0078] Therefore, it appears that the conductive strips 16 need not be aligned with the Josephson junctions 28 of the network 12 to achieve the current homogenization effect. Only the aforementioned distance condition needs to be considered.

[0079] It can be noted that the homogenizing effect reappears as soon as d 12 <d z < 40%.L R , en notant d 12 la distance moyenne entre deux jonctions Josephson 28 du réseau 12, et dz la distance entre les deux plans P1 et P2.

[0080] In most cases, the distance between any two adjacent Josephson 28 junctions in the 12 network is the same for any pair of adjacent Josephson 28 junctions, but when this is not the case, the average distance is, for example, the arithmetic mean of the distances between the Josephson 28 junctions in the 12 network.

[0081] Furthermore, each of the 16 strips of the grid can be adapted to control the current flowing through it and to control the magnetic field at the level of the network 12 of Josephson junctions 28. This could allow a second-order correction of edge effects or even generate a current distribution that is not necessarily uniform.

[0082] All the configurations presented concern cases where the second substrate 20 is at the top while the first substrate 14 is at the bottom.

[0083] The reverse is also possible and the same remarks as before apply.

[0084] Furthermore, it should be noted that these configurations also make it possible to bring the conductive strips 16 closer to the Josephson junctions 28. Thus, according to a particular case, the conductive strips 16 are arranged so that each is at a distance from a Josephson junction 28 less than or equal to 5% of the width defined for the network 12.

[0085] Such a relative arrangement of the conductive strips 16 and the network 12 of Josephson junctions 28 (distance less than 40% of the width of the network 12) makes it possible to obtain a more homogeneous distribution of bias current over the whole of the network 12.

[0086] To fully understand this effect, it is worth noting that there is an analogy between radio frequency signal transmission lines and superconducting bands: the skin effect and the Meissner effect both induce very similar non-uniform current distributions.

[0087] It is observed in the case of radio frequency transmission lines that the microstrip structure (more often called microstrip) is the structure which ensures the most homogeneous distribution of the radio frequency current over the width of the line thanks to the presence of the ground plane.

[0088] This analogy allows us to predict that a structure formed of two superimposed bands, in mutual influence, presents a more homogeneous current distribution.

[0089] More precisely, just as the electric field is uniform between the two electrodes of a parallel-plate capacitor (due to the condition of zero electric field inside the metallic electrodes), the magnetic field will be uniform between the two superconducting strips thanks to the Meissner effect, which dictates that the magnetic field is zero inside the superconductors. According to Maxwell's equations, the current density flowing across the surface of the superconductors is therefore uniform.

[0090] The applicant's experiments have shown that this effect is even capable of controlling the current distribution in a superconducting band when the bands are traversed by equal but opposite intensities.

[0091] The applicant then observed that when the distance d separating the two bands is small compared to their width, the current distribution in a lower band, which would be significantly wider than the upper band, is almost zero at a great distance from the upper band (along the Y axis).

[0092] Furthermore, the current distribution in the lower band is essentially constant in the central region. This principle is used in the various detector implementations.

[0093] If the upper band is not superconducting, but just conductive, the applicant's experiments have shown that the result is essentially the same for the lower band (which remains superconducting).

[0094] Thus, because the aforementioned distance condition is respected, the proposed detector 10 allows for greater homogeneity of the bias currents for a large network 12 of Josephson 28 junctions, and therefore an improved response.

[0095] In fact, without the bands, the response of a detector 10 reaches a maximum as a function of the width of the grating 12, before collapsing. With the bands 16, it is possible that the response of the detector 10 remains proportional to the width of the grating 12 of Josephson junctions 28 well beyond 20 µm, which corresponds to the optimal width observed in prior art solutions.

[0096] This implies that the sensitivity gain for the detector response 10 for an array 12 of 28 Josephson junctions with a width of 100 µm will be approximately 5 times greater. Since the gain is proportional to the width of the arrays used, it will be approximately 15 times greater for an array 12 of 28 Josephson junctions with a width of 300 µm.

[0097] This opens the way to very large networks, which will only be limited by the wavelength of the signals (on the order of a meter in ultra-high frequency).

[0098] Furthermore, the proposed detector 10 is simple in that it does not involve the use of multi-layer superconducting technology or a current distribution resistor network. This simplicity allows the detector 10 to be used even with the constraints of a circuit including Josephson junctions 28 with high-temperature superconducting materials.

[0099] In addition, since the conductive strips 16 and the network 12 can be arranged on different substrates, it is possible to use flip-chip or piggy-back type assemblies.

[0100] The flip-chip designs corresponding to figures 3 , 5 And 7 This may involve the use of an insulating layer to prevent the two opposing circuits (network 12 and assembly 18 of conductive strips 16) from coming into electrical contact. Such an insulating layer will then be sandwiched between the two chips.

[0101] The thickness of this layer is not critical; depending on the technology used for the Josephson junction, there may be additional or different constraints. For example, with irradiated barrier Josephson junctions, the most critical constraint is not to heat the high-temperature superconducting circuit above 80°C for more than 10 minutes. A resin of the same type as for photolithography operations can be used. A cold deposition of SiO₂ (where x can vary between 1 and 2) is also suitable.

[0102] When flip-chip technology uses gold beads deposited by ultrasonic welding, a process known as "ball bonding," the size of the beads (several tens of micrometers) is sufficient to ensure adequate separation of the two circuits. Therefore, an additional insulating layer is not required.

[0103] Piggy-back designs do not have this problem.

[0104] The presence of the additional layer does not prevent each of these assemblies from being compatible with the manufacturing technology of high-temperature superconducting materials.

[0105] In the case where the conductive strips 16 form a second network of Josephson junctions, this allows each network to be on a respective substrate, and to gain a factor of 2 in the response of the detector 10.

[0106] It can also be noted that the networks 12 and / or the conductive strips 16 can not only be Josephson junction networks but also two-dimensional networks.

[0107] In the end, the detector 10 that has just been described is a magnetic field detector combining sensitivity, broadband (from continuous to 10 Gigahertz) and dynamic range.

[0108] It can be noted that this homogeneity of current distribution can also be used for other superconducting devices 10.

[0109] Thus, device 10 can detect elements other than a magnetic field. In particular, device 10 also detects electromagnetic waves.

[0110] This means that device 10 is, according to one embodiment, a radio frequency wave receiver.

[0111] In other embodiments, device 10 is a magnetic anomaly detector.

[0112] Alternatively, device 10 is an antenna. The antenna is then small in size and combines sensitivity and wide bandwidth.

[0113] According to another variant, device 10 is an amplifier, this amplifier being a low noise, low power dissipation and wideband amplifier.

Claims

1. A superconducting device (10), notably an electromagnetic field detector (10), antenna, or amplifier, the device (10) comprising: - a network (12) of Josephson junctions (28) in parallel, a first plane and a width (LR) being defined for the network (12), - at least one conductive strip (16) contained in a second plane, the second plane facing the first plane, the or each conductive strip (16) being realized in gold, aluminum, copper, palladium, an alloy of the aforementioned elements, Ta2N, or cuprate, the or each conductive strip (16), preferably, being connected to form a conductive grid, and - a power supply unit (22) able to supply the Josephson junctions (28) of the network (12) of Josephson junctions (28) and the or each conductive strip (16) with currents of opposite signs, the power supply unit (22) supplying the Josephson junctions (28) of the network (12) of Josephson junctions (28) and the or each conductive strip (16) with currents having an intensity chosen so that the total currents flowing in the or each conductive strip (16) are equal to the current flowing in the network (12) of Josephson junctions (28) within 10%, the superconducting device (10) being characterized in that the first plane and the second plane are arranged so that each conductive strip (16) is at a distance from a Josephson junction (28) less than or equal to 40% of the width defined for the network (12).

2. The superconducting device according to claim 1, wherein the conductive strips (16) are arranged to be each at a distance from a Josephson junction (28) less than or equal to 5% of the width (LR) defined for the network (12).

3. The superconducting device according to claim 1 or 2, wherein the Josephson junctions (28) include a superconducting material presenting a critical temperature greater than 40 Kelvin.

4. The superconducting device according to any one of claims 1 to 3, wherein the network (12) of Josephson junctions (28) is positioned on a first substrate (14), and the conductive strips (16) are positioned on a second substrate (20), the two substrates (14, 20) facing each other.

5. The superconducting device according to any one of claims 1 to 3, wherein the network (12) of Josephson junctions (28) is positioned on a first substrate (14) and the conductive strips (16) are positioned on a second substrate (20), the conductive strips (16) being spaced from the network (12) of Josephson junctions (28) by at least one of the substrates (14, 20).

6. The superconducting device according to any one of claims 1 to 5, wherein each conductive strip (16) presents a width (LB) between 3 micrometers and the width defined for the network (12).

7. The superconducting device according to any one of claims 1 to 6, wherein the conductive strip or strips (16) form another network of Josephson junctions.

8. The superconducting device according to any one of claims 1 to 7, wherein the device (10) includes a spacing layer inserted between the first plane and the second plane.

9. The superconducting device according to claim 8, wherein the spacing layer is made of an insulating material.

10. The superconducting device according to any one of claims 1 to 8, wherein an average distance between two Josephson junctions (28) of the network (12) is defined, the first plane and the second plane are arranged so that each conductive strip (16) is at a distance from a Josephson junction (28) greater than or equal to the average distance between two Josephson junctions (28) of the network (12).