RU2620760C2 - Superconducting quantum grid on the basis of skif structures - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: superconducting quantum lattice based on SKIF-structure comprises two coupled differential serial chain SKIF-structures consisting of parallel-connected Josephson junctions, a means of specifying the magnetic bias field, connected inductively to each SKIF-structure, a superconducting transformer and a DC supply current reference means and voltage measurement.

EFFECT: improving the magnetic signal conversion output signal in response and linearity of the voltage through the use of multielement Josephson structures consisting of two consecutive differentially connected SKIF chain structures, design, operating conditions and specific characteristics of which are selected in the manner described herein.

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Description

Technical field

The invention relates to cryogenic radio electronics, including active broadband devices, and can be used as a detector for receiving and amplifying electromagnetic signals in the frequency range from units of hertz to 10 GHz.

State of the art

Devices with Josephson contacts have long been used to detect the magnetic component of microwave (microwave) electromagnetic signals. Particularly actively used for these purposes are systems based on superconducting quantum interferometers (SQUIDs), which have high sensitivity and low noise temperature, are compatible with other superconducting devices. SQUID is a superconducting ring containing one or two Josephson contacts, has matching and measuring devices that can convert the applied magnetic signal into a voltage response.

Various designs and circuitry solutions for microwave superconductors are described. In particular, a schematic of a matched SQUID amplifier with a load for a range up to 0.1 GHz is described (JP S60247311, Noguchi, 12/07/1985). An amplifier / converter using this principle at a frequency of 100 MHz showed a gain of about 20 dB with a noise temperature of 1 ± 0.4 K when using an input device made on a coaxial cable (see US 4,585,999, Hilbert et al. 04/29/1986 ) The principles of matching the input and output circuits of an SQUID amplifier with a load have been studied in detail for a range of up to 0.1 GHz (JP 60247311, Noguchi, on. 07.12.1985). The invention (JP 1245605, Takami et al., September 29, 1989) describes a DC-SQUID magnetic signal amplifier-converter in which a second SQUID is used as an adjustable inductive element of the matching circuit to electrically match the interferometer with a cooled preamplifier. However, both the operating frequency range and the linearity of conversion for amplifiers on a single low-temperature SQUID are insufficient for modern applications.

There are two problems that make it difficult to switch to higher frequencies of signals of 1-10 GHz while maintaining high gain and noise temperature (about 1 ... 3 K) characteristic for superconducting devices. Firstly, SQUID amplifiers are a special type of parametric amplifiers in which the signal power at its frequency F _S is amplified by converting the signal to the frequency F _S + F _J , where F _J is the frequency of the Josephson generation, and then converting down again to signal frequency. Based on the Manly-Row ratios, the power gain, G, of such an amplifier does not exceed:

Therefore, the Josephson SQUID junctions must have a high characteristic voltage - V _C , so that the characteristic frequency of the Josephson generation

where Ф ₀ - magnetic flux quantum, equal to 2.07 × 10 ^-15 VB, was several orders of magnitude higher than the signal frequency.

For SQUIDs based on Nb Josephson junctions, typical V _C values do not exceed 100-200 μV, respectively, F _C values do not exceed 50-100 GHz. Typical values of F _J at the operating point are approximately an order of magnitude smaller than F _C and, as can be seen from formula (1), the gain disappears for signals with a frequency of the order of 10 GHz. Naturally, the noise characteristics of the amplifier are also deteriorating.

, задающему уровень минимального сигнала на выходе устройства:The second problem for microwave amplifiers based on classical SQUIDs with a multi-turn input coil in the frequency range above 0.01 GHz is the nonlinear response to an external magnetic field and the inability to linearize the response using traditional feedback systems at such high frequencies, which leads to a low gain linearity level (about 20 dB). Also, without the use of feedback systems, SQUID amplifiers are characterized by small values of the dynamic range, i.e., the ratio of the maximum output signal Vmax to the noise level at the output of the device in question

setting the level of the minimum signal at the output of the device:

To solve these problems, multi-element Josephson structures can be used, including those consisting of N series SQUIDs connected in series or in parallel. The addition of the responses from the SQUIDs that make up such a chain makes it possible to achieve the required gain level.

To increase the linearity of the SQUID-based voltage-stream conversion systems in the invention KR 100774615 (Jin-Mok Kim et al., 2007.11.12, patent analogue US 7453263), a special feedback scheme is proposed containing an additional Josephson junction, however, the proposed method for increasing linearity SQUID-based amplifier significantly limits the maximum operating frequency, since the operating frequency of the feedback circuit is small.

In the inventions US 2005231196 (Tarutani Y. et al., 10.20.2005) and US 7095227 (Tarutani Y. et al., 08.22.2006) amplifiers based on a sequential SQUID chain, which is a simple superconducting quantum lattice (SCR), are described. Such amplifiers allow using a direct current source as a power source and occupy a small area. However, such a device allows to achieve only a relatively small linear gain of the signal (not more than 20 dB).

Amplifiers-drivers for the GHz range (up to tens of GHz) based on chains of SQUIDs separated into pairs isolated from the ground were proposed to reduce spurious capacitances in the system (US 6486756, Tarutani, 11.26.2002), however, such amplifiers are suitable only for digital applications and cannot be used to amplify an analog signal. An example of a signal amplifier of fast single-quantum (SQF) logic, which converts them at the output into voltage pulses with a value sufficient to use semiconductor electronics, is an invention (US 6917216, Herr Quentin, 10/14/2004), which uses the separation and re-reflection of the output SQF pulse to obtain sufficient values of the output voltage pulses. But this offer is again designed exclusively for use in digital devices.

To increase the dynamic range, it was proposed to switch to amplifiers containing a large number of interconnected SQUIDs. So, a sequential chain of regularly located identical two-contact SQUIDs can be used as the main element for amplifying a digital or analog signal, and also as a magnetic field detector. An irregular chain of SQUIDs having arbitrary areas and orientations of interferometers is the so-called superconducting quantum interference filters (SKIF), which can also be used in the construction of microwave amplifiers. A specific feature of SKIF follows from the irregularity of its structure: it allows one to obtain one central maximum of the voltage-field response at zero external magnetic field.

So, in the invention (DE 3936686, Hoenig, 05/08/1991) an amplifier based on a SQUID matrix cooled by liquid nitrogen is described in the thin-film version, however, it is indicated that it can operate up to frequencies of 100 MHz. The invention (US 6005380, Hubbell Stephen, 12/21/1999) describes an amplifier for an antenna using magnetically coupled SQUIDs that form a multi-element matrix. However, such a device is designed to operate in a significantly narrower frequency range and is characterized by a relatively low limit operating frequency of 516 MHz.

To create a current amplifier with large output currents, it was proposed to use an alternative option for creating SCR - a set of parallel-connected DC SQUIDs (JP 2003209299, Morooka et al., July 25, 2003). This amplifier has high values of the output current, which allows it to be used as an input circuit of systems with direct digitization of the signal, however, the proposed device is not designed to operate in the gigahertz frequency range.

Invention CN 101841077B (Y. Bian et al., 05.22.2013) describes a tunable high temperature superconducting (HTSC) filter and a method for manufacturing it. The proposed filter operates in the high-frequency range and, if necessary, can be characterized by a wide passband. However, the problem of realizing superconducting electronic devices for broadband receiving systems with direct digitization of signals based on the present invention has not yet been solved.

The variable-area SQUID chain (SKIF) was proposed to be used as a highly sensitive magnetometer (WO 01/25805, Schopohl et al., April 12, 2001; US 7369093, Oppenlander et al., May 6, 2008), however, the described structures do not provide the necessary linear signal amplification . They do not provide sufficient linearity in the conversion of the input magnetic signal to the output voltage and heterogeneous parallel contact chains (US 7369093), which do not allow anything else to achieve significant values of the output signal.

A known superconducting broadband microwave amplifier containing a series of two-contact SQUIDs connected to input and output superconducting lines is associated with means for setting the operating current and magnetic field modes (JPH 10028021, Takeda et al., January 27, 1998).

A structure with three Josephson junctions is described (US 8179133, Kornev et al., 05/15/2012; US 8933695, Kornev et al., January 13, 2015). In this element, in parallel with the main inductance of the two-contact SQUID, it is proposed to include the third Josephson junction, which is always in the superconducting state and plays the role of nonlinear inductance. Connecting the Josephson junction parallel to the inductance of SQUID forms a double SQUID, hereinafter referred to as bi-SQUID. Such a cell allows achieving a sufficiently high linearity of the conversion of the input signal to the output voltage of the amplifier, however, in this case, the influence of stray inductances and capacitances makes it difficult to obtain a highly linear conversion of the magnetic signal into a voltage response. A similar device intended for implementation on the basis of high-temperature superconductors is described in patent RU 2544275 dated 03/20/2015, but the problem of implementing the required superconducting electronic devices for broadband receiving systems with direct digitization of signals using such a cell has not yet been solved.

The most important non-patent publications devoted to the presentation of new theoretical approaches to the analysis of limit characteristics include the following studies:

- a description of approaches to increase the gain and reduce noise temperature in microwave SQUID amplifiers (Archana Kamal, John Clarke, Michel Devoret. Gain, directionality and noise in microwave SQUID amplifiers: Input-output approach. Physical Review B - 2012, vol. 86 144510);

- Proposal of breakthrough solutions for creating superconducting quantum gratings (V. Kornev, I. Soloviev, A. Sharafiev, N. Klenov and O. Mukhanov. Active Electrically Small Antenna Based on Superconducting Quantum Array. IEEE Transactions on Applied Superconductivity - 2013, vol. 23 (3), 1800405);

- a description of the methods for coupling HTSC SQUID amplifiers with superconducting thin-film filters (Kalabukhov AS, Tarasov M.A., Stepantsov E.A., Gevorgian S., Deleniv A., Ivanov ZG, Snigirev OV, Vendik OG, Mukhanov OA A high- Tc L-band SQUID amplifier combined with superconductive thin-film filters. Physica C: Superconductivity and its Applications - 2002, vol. 368, p. 171-175);

- study of possible approaches to creating compact two-dimensional structures based on highly linear bi-SQUIDs (S. Berggren, G. Prokopenko, P. Longhini, A. Palacios, OA Mukhanov, A. Leese de Escobar, BJ Taylor, MC de Andrade, M. Nisenoff, RL Fagaly, T. Wong, E. Cho, E. Wong, V. In. Development of 2D Bi-SQUID Arrays with High Linearity. IEEE Transactions on Applied Superconductivity - 2013, vol. 23 (3), 1400208).

Closest to the claimed technical solution is the design of a microwave amplifier based on sequential chains of DC SQUIDs, the areas of which are selected so as to provide increased gain and high linearity in the frequency band 1-10 GHz (RU 2353051, Kornev et al., 06.06. 2007). However, the implementation of the required chains of SQUIDs with a given accuracy of parameters today seems to be an extremely difficult technical task, which limits the linearity of the conversion of a magnetic signal into a voltage response achievable in a real experiment at a level of 50 dB.

Disclosure of invention

The inventive superconducting quantum lattice based on SKIF structures contains two connected differentially sequential chains (1 and 2) of SKIF structures (3), including parallel connected Josephson contacts (4) and connecting inductances; means for setting the bias magnetic field (5), connected to a superconducting transformer (6), connected inductively to each SKIF structure; means for setting a direct current supply and voltage measurement (7).

The device can be characterized in that the number of Josephson junctions connected in parallel in each SKIF structure is at least 30 units.

The device can be characterized in that the number of SCIF structures connected in series in each chain is at least 100 units.

The device may be characterized in that the number of Josephson contacts in each SKIF structure should be the same for each of the chains.

The device can be characterized in that the number of SKIF structures must be the same for each of the chains.

The device can be characterized in that Josephson contacts are made in the form of layered thin-film structures containing: a lower superconducting electrode, an insulator layer deposited on a lower superconducting electrode, an upper superconducting electrode with current leads, deposited on an insulator layer.

The device can be characterized in that niobium, aluminum, or an alloy based on these metals are used as the material of the lower and upper superconducting electrodes, superconducting transformers, superconducting compounds between Josephson junctions in the SKIF structure and between the SKIF structures in the chain.

The device can also be characterized in that alumina Al ₂ O _{3 is} used as the material of the insulator layer.

The device can also be characterized in that the thickness of the lower superconducting electrode and the upper superconducting electrode is 50-500 nm.

The device can also be characterized by the fact that the thickness of the insulator layer is 1-20 nm.

The technical result of the invention is to increase the level of the output signal and the linearity of the conversion of the magnetic signal into a voltage response through the use of multi-element Josephson structures consisting of two differentially connected sequential chains of SKIF structures.

Brief Description of the Drawings

The invention is illustrated by drawings.

In FIG. 1 is a structural diagram of the inventive superconducting quantum lattice based on SKIF structures, where I _M is a current defining a bias magnetic field ± B _DC , Ib is a supply current, B _ext is an external field converted by a superconducting transformer into a field acting on SKIF- Structures B _tr , V _out1,2 - voltage removed from successive chains of SKIF structures.

In FIG. Figure 2 illustrates the principle of constructing an elementary quantum cell of a superconducting quantum lattice consisting of two arms in the form of differentially connected SKIF structures, each of which is a chain of parallel connected Josephson junctions. Here Φ is the magnetic flux created by the magnetic field acting on the SKIF structure; ± δФ is the magnetic flux created by the bias field, I ₁ , I ₂ are the currents flowing into the left and right shoulders of the cell, respectively, I _b is the supply current, I _e is flowing into the load, Re is the load resistance.

In FIG. Figure 3 shows the voltage responses of a superconducting quantum lattice consisting of two SKIF structures, each of which contains 10 Josephson contacts with a normalized value of the coupling inductance l = 0.5: without load at the “critical” bias current Ib = I _C (curve 8); at a bias current of Ib = 1.06I _C (curve 9); loaded at the resistive impedance Re = 10R _N at the same bias current Ib = 1.06I _C (curve 10), where R _N is the normal resistance of the cell shoulder, LA and RA are the left and right shoulders of the unit quantum cell, respectively.

In FIG. 4 - calculated dependences of the linearity of the response function of the voltage of the superconducting quantum lattice on the magnitude of the bias flux with normalized coupling inductances of Josephson contacts l = 0.5 for different amplitudes of the input harmonic signal, which correspond to the voltage amplitude at the cell output of 30%, 50%, 60%, 70% and 80% (boundaries of linearity regions 11, 12, 13, 14 and 15, respectively). It can be seen that for the proposed technical solution, the achievable linearity level of converting the magnetic signal into a voltage response is 60 ... 80 dB, which is much higher than the achievable characteristics of the closest analogue.

The positions in the drawings indicate: 1 and 2 - connected differentially sequential chains of SKIF structures; 3 - SKIF structures consisting of parallel connected Josephson junctions; 4 - Josephson contacts; 5 - means for setting the bias magnetic field; 6 - superconducting transformer; 7 - means of setting a constant current supply and voltage measurement.

The implementation of the invention

A superconducting quantum lattice based on SKIF structures (see Figs. 1 and 2) is a connected differentially sequential chain (1 and 2) of SKIF structures (3). Each SKIF structure consists of parallel-connected Josephson contacts (4) in the resistive state, switched on differentially and offset mutually opposite to some magnetic flux δФ by means of setting the bias magnetic field (5). The total supply current for each circuit, specified through the means for setting the direct current (7), exceeds the maximum value of the critical current of the circuit and, thus, causes its transition to a resistive state. The superconducting transformer (6) converts the external magnetic field into a signal applied to the SCIF structures, forming a voltage response for each of them, which, when connected in series, forms a chain response.

The inventive circuit is characterized by the fact that with the differential addition of the responses of sequential chains, the resulting conversion of the magnetic signal into a voltage response will have a high linearity.

The chain of connected differentially quantum cells is actually two differentially included sequential chains of right and left shoulders of quantum cells (SKIF structures including parallel connected Josephson junctions). Each of these chains is inductively coupled to a common magnetic flux transformer. All quantum cells must be in the same resistive state, which is achieved by setting the same bias current of the chains Ib, exceeding the critical current of the shoulder of the quantum cell. The same input signal (magnetic flux) is applied to all elements of these chains. The magnetic bias current I _M flows directly on the sides of the common transformer, coinciding in direction with the circular current of the input signal on one side of the transformer and having a direction opposite to the signal current on the other side of the transformer.

In FIG. Figure 3 shows the type of voltage response of a superconducting quantum lattice (i.e., the average voltage on a superconducting quantum lattice, the magnitude of which depends on the magnitude of the magnetic flux generated by the magnetic field acting on the SCIF structure), consisting of two SCIF structures, each of which contains 10 Josephson contacts with the normalized value of the coupling inductance l = 0.5:

no load at the “critical” bias current Ib = I _C (curve 8);

no load at bias current Ib = 1.06I _C (curve 9).

The most optimal value of the bias current is Ib = 1.06I _C , which allows achieving linear response up to 100 ... 105 dB with a magnetic displacement of the cell shoulders of 0.65 ... 0.7 quanta of magnetic flux Ф ₀ .

For the case when the superconducting quantum lattice is loaded on the resistive impedance Re = 10R _N (R _N is the normal resistance of the SKIF structure) at the same bias current Ib = 1.06I _C , the current I ₁ and I ₂ deviate from the real supply chain of the SKIF structures set values. The calculated response for this case is shown in curve 10. A decrease in the supply current makes it possible to significantly compensate for the effect of the load and obtain a response close to the response shown in curve 9 if the ratio between the impedances Re and R _{N is} at least 10.

In FIG. Figure 4 demonstrates how the linearity of the response function of the voltage of a superconducting quantum lattice depends on the magnitude of the bias flux with normalized coupling inductances of Josephson contacts l = 0.5 for different amplitudes of the input harmonic signal that correspond to the amplitude of the voltage at the output of the cell. Given a magnetic bias of 0.7 to achieve the best linearity of 70 ... 80 dB and far superior to the linearity of the transformation in the prototype of the invention, the total response range of the simplest superconducting quantum lattice will be approximately 1.4V _C.

The high linearity (70 ... 80 dB) of the characteristics of active devices based on superconducting quantum gratings is ensured by the high linearity of the response of the quantum cell, which consists of two differentially connected and mutually oppositely opposed magnetic flux δФ (magnitude 0.65 Ф ₀ ... 0.7Ф ₀ ) SKIF structures, and the dynamic range can be significantly increased with an increase in the number K of quantum cells in the lattice. If DR ₁ is the dynamic range of one unit quantum cell, then the dynamic range of the lattice containing K quantum cells can reach DR = DR ₁ K ^1/2 . Changing the relationship between the number of series-connected quantum cells and the number of Josephson junctions in the SCIF structure connected in parallel allows you to change the output impedance of the superconducting quantum lattice, achieving the relation Re> 10R _N. Estimates for the interaction radius between Josephson contacts at the signal (low) frequency allow us to determine the optimal value of N for maximizing the dynamic range as 30 ... 40 units. Estimates for the expected value of the resistive load impedance Re (of the order of 50 Ohms) allow us to establish that the minimum number of series-connected SKIF structures in the K chain is 100 units.

The above sources confirm the validity and relevance of the approach to the implementation of superconducting detectors. Technological applicability: for the implementation of the inventive device, materials used in cryoelectronic technology and known to specialists can be used. As a substrate, on which all elements of the superconducting quantum lattice are located, any standard substrates (silicon, sapphire, etc.) can be used. As a material for the insulator layer - aluminum oxide. As a material for superconducting elements - niobium, aluminum, as well as niobium nitride, vanadium, indium, tin, lead. Typical layer thicknesses for the patented structure are in the range technologically feasible for thin-film electronics.

The overall efficiency of electrically small transformer-type antennas depends on the transformer area, the coefficient of transformation of the flux into quantum cells and the number of cells in the chain, which can be located mainly along the transformer circuit. In this case, the total number of quantum cells increases in proportion to the increase in the perimeter of the transformer. Since the inductance of the transformer (optimal square shape) is proportional to its perimeter (side of the square a ), the magnetic flux applied to one quantum cell increases with increasing side of the square transformer as a . As a result, the steepness of the conversion of the magnetic component of the incident electromagnetic wave into voltage across a sequential chain of quantum cells increases with the side a of the square transformer as a ² . In this case, the dynamic range of such an antenna, increasing in proportion to the square root of the number of quantum cells, will increase with increasing side a as a ^0.5 . Using one of the most developed and widely used technologies for manufacturing superconducting integrated circuits, which is niobium technology with a critical current density of Josephson tunnel structures of 4.5 kA / cm ² , the rules for designing the topology of superconducting structures (http://www.hypres.com/wp- content / uploads / 2010/11 / DesignRules-4.pdf), including the available sizes of the constituent elements, allow us to make the following estimates of the possible number of quantum cells in the antenna. In the case of using a 5 × 5 mm ² chip, a transformer-type antenna can contain up to 100 differential cells, and on a 10 × 10 mm ² chip, up to 200 differential cells.

Claims (10)

1. A superconducting quantum lattice based on SKIF structures, containing two connected differentially sequential chains of SKIF structures, including parallel connected Josephson contacts and connecting inductances; means for setting the bias magnetic field connected to a superconducting transformer inductively coupled to each SKIF structure; means for setting a direct current supply and voltage measurement. 2. The superconducting quantum lattice according to claim 1, characterized in that the number of Josephson junctions connected in parallel in each SKIF structure is at least 30 units. 3. The superconducting quantum lattice according to claim 1, characterized in that the number of SCIF structures connected in series in each chain is at least 100 units. 4. The superconducting quantum lattice according to claim 1, characterized in that the number of Josephson junctions in each SKIF structure is the same for each of the chains. 5. The superconducting quantum lattice according to claim 1, characterized in that the number of SKIF structures is the same for each of the chains. 6. The superconducting quantum lattice according to claim 1, characterized in that the Josephson contacts are made in the form of layered thin-film structures containing: a lower superconducting electrode, an insulator layer deposited on a lower superconducting electrode, an upper superconducting electrode with current leads, deposited on an insulator layer. 7. The superconducting quantum lattice according to claim 6, characterized in that niobium, aluminum or an alloy are used as the material of the lower and upper superconducting electrodes, superconducting transformers, superconducting compounds between the Josephson junctions in the SKIF structure and between the SKIF structures in the chain based on these metals. 8. The superconducting quantum lattice according to claim 6, characterized in that the aluminum oxide Al ₂ O _{3 is} used as the material of the insulator layer. 9. The superconducting quantum lattice according to claim 6, characterized in that the thickness of the lower superconducting electrode and the upper superconducting electrode is 50-500 nm. 10. The superconducting quantum lattice according to claim 6, characterized in that the thickness of the insulator layer is 1-20 nm.

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