Abstract Body

Since more than 15 years, Nb/AlOx/Nb-based direct current Superconducting QUantum Interference Device (dc SQUID) current sensors have been developed at PTB [1] for a range of applications, such as biomagnetic measurements in ultra-low field environments, readout of superconducting low-temperature detectors of radiation and particles or noise thermometry at millikelvin temperatures. The sensor circuit concept is based on a second-order gradiometric dc-SQUID combined with a first-order gradiometric double transformer input circuit to couple the signal currents to be measured to the SQUID. This configuration allows in a flexible manner to cover a range of input inductances from about 25 nanohenry to 2 microhenry and reduces the sensitivity of the devices to external magnetic interference. The sensors achieve coupled energy sensitivities below 150 h (Planck's constant) at 4.2 K that reduce to <50 h when operated at 0.3 K. Additional functionalities, e. g., input current limiters, two feedback coil circuits and rf-filters are integrated on-chip. So far, photolithography has been used for the fabrication of the sensors and limits the minimum lateral dimensions of circuit structures to about 2.5 micrometers. Further improvements in their noise performance and functionality requires the dimensions of sensors circuit elements, namely Josephson junction sizes as well as input coil line widths and distances to be reduced to < 1 micrometer. To this end, we are developing a new fabrication process employing electron beam lithography performance. Josephson junctions with lateral lengths as low as 0.7 micrometer and Nb signal input coils with line widths / pitch dimensions as low as 0.15 / 0.3 micrometers can be fabricated this way, a reduction of more than an order of magnitude compared to the photolithography-based patterning. The “fine-pitch” layout of the signal input coils is matched to the existing sensor designs and allows to significantly extend the range of input inductances and increased signal-to-SQUID inductive coupling while improving the overall compactness of the sensors. The presentation discusses design aspects, details of the new sensor fabrication process and results of the characterization of the sub-micrometer circuit elements.

References

[1] D. Drung, C. Aßmann, J. Beyer, A. Kirste, M. Peters, F. Ruede, Th. Schurig, “Highly sensitive and easy-to-use SQUID sensors,” IEEE Trans. Appl. Supercond. vol. 17, no. 2, pp. 699-704, June 2007.

Abstract Body

We report on Hybrid Superconducting Quantum Interference Devices (HSQUIDs) developments and results obtained in collaboration between two laboratories of France and Turkey. HSQUIDs are superconducting devices which combine a Direct Current (DC) SQUID with a digital SQUID through a specific magnetic coupling scheme (see for instance Figure 1). The objective is to preserve as much as possible the ultimate and well-known sensitivity of DC SQUIDs, or analogue SQUIDs based on the Josephson tunneling effect [1], and combine it with a comparator-based [2] digital SQUID [3] relying on the Single-Flux-Quantum (SFQ) technique [4]. This is to allow the device to work under urban environment where strong magnetic pulse can happen at times, by increasing the dynamic range of the analogue SQUID.

We will present the principles of the chosen architectures, with the main properties and results, including underground measurements of the solar eclipse of 10 June 2021. We also measured the thermal noise-induced perturbations inside a closed-cycle GM cooler at 4.2K. A dynamic range of 7720 flux quanta was obtained in shielded environment, with a slew rate about 72 k/s for a sampling frequency of 100 kHz. At last we will present a detailed analysis of the specific pickup loops that were developed for the HSQUIDs.

References

[1] Josephson B 1962 Physics Letters 1(7) 251–3

[2] Myoren H et al 2017 IEEE Trans. Appl. Supercond.27 1-5

[3] Reich T, Febvre P, Ortlepp T, Uhlmann F H, Kunert J, Stolz R and Meyer H-G 2008 J. Appl. Phys. 104 024509

[4] Likharev K K and Semenov V K 1991 IEEE Trans. Appl. Supercond. 1(1) 3–28

Acknowledgment

This work has been partly supported by by the Fonds européen de développement régional (FEDER) in the frame of the LSBB2020 project, the CNES French National Space Agency and by the French-Turkish Partenariat Hubert Curien (PHC) BOSPHORE n°39708XA and TUBITAK under grant 117F266.

Keywords: Josephson junction, SQUID, DC SQUID, digital SQUID, SFQ, RSFQ

Abstract Body

Abrikosov fluxonics [1], a domain of science and engineering at the interface between superconductivity research and nanotechnology, is concerned with the study of properties and dynamics of Abrikosov vortices in nanoengineered superconductors, with particular focus on their confinement, manipulation, and exploitation for emerging functionalities. In this regard, of special interest are the ultimate speed limits for magnetic flux transport via fluxons as well as the quasiparticle relaxation mechanisms underlying these limits and determining the suitability of superconductors for single-photon detectors and magnons generators of the Cherenkov type [2].

In my talk, I will address the issue of vortex velocity enhancement in the context of these two applications. First, nanoengineered superconductors with weak volume pinning and close-to-depairing criticial currents will be introduced as materials supporting vortex velocities exceeding 10 km/s [3]. Herewith, high quality of the edge of the superconducting strip is decisive for the suppression of the flux-flow instability [4]. Next, single edge defects will be presented as tools for the formation of vortex jets [5] and the deduction of the number of vortices and their velocity [6]. Finally, in superconductor-ferromagnet hybrid structures, fast motion of a vortex lattice allows for the emission of spin waves (magnons) via a Cherenkov-type mechanism and the formation of unidirectional spin-wave beams [7]. The emission of spin waves is accompanied by a magnon Shapiro step in the superconductor's current-voltage curve because of the phase-locking of the spin waves with the moving vortex lattice and reduces the dissipation in the superconductor.

[1] Abrikosov fluxonics in washboard nanolandscapes, O. Dobrovolskiy, Physica C 533, 80–90 (2017)

[2] Cherenkov radiation of spin waves by ultra-fast moving magnetic flux quanta, O. Dobrovolskiy et al. arXiv:2103.10156.

[3] Ultra-fast vortex motion in a direct-write Nb-C superconductor, O. Dobrovolskiy et al. Nat. Commun. 11, 3291, (2020).

[4] Rising speed limits for fluxons via edge quality improvement in wide MoSi thin films, B. Budinska et al., Phys. Rev. Appl. 17, 034072 (2022).

[5] Vortex jets generated by edge defects in current-carrying superconductor thin strips, A. Bezuglyj et al., Phys. Rev. B 105, 214507 (2022).

[6] Vortex counting and velocimetry for slitted superconducting thin strips, V. M. Bevz et al. Phys. Rev. Appl. 19, 034098 (2023).

Abstract Body

Research on quantum computers using superconducting qubits is actively being pursued. Among these, topological quantum computers, which utilize the wave function changes accompanying the braiding operations of Anyons in two-dimensional electron gas, are expected to be highly robust to thermal noise and to enable the realization of large-scale quantum computers with the addition of small-scale quantum error correction circuits.

By using the surface states of three-dimensional topological insulators (3D-TI) that do not require the application of strong magnetic fields for realizing two-dimensional electron gas phase, coexistence with superconducting digital circuits such as single flux quantum (SFQ) logic circuits can be achieved, allowing the classical computing components to be placed in a cryogenic environment.

By depositing superconductors like Nb on the surface of a 3D-TI and realizing a topological superconducting state through the proximity effect, a topological Josephson junction array (TPJJ) can be created. As an Anyon, a Majorana bound state (MBS) bound to a 2-vortex placed in the junction array is used. Previously, it was demonstrated through simulation that braiding operations and CNOT operations of 2-vortices driven by a bipolar current pulse source (BCP) driven by SFQ pulses are possible [1]. In the next phase, an initialization circuit is needed to perform the initial placement of 2-vortices in the topological junction array.

This study reports the results of an investigation into an initialization circuit for the initial placement of 2-vortices using a current application method that combines two bipolar current pulse sources.

References

[1] H. Myoren et al., IEEE Trans. Appl. Supercond.,Vol. 34, No. 3, ArtNo. 1701306, 2024.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 21K18708. This study has been partially supported by the VLSI Design and Education Center (VDEC) at the University of Tokyo, in collaboration with Cadence Design Systems, Inc. Circuits were fabricated in the clean room for analog-digital superconductivity (CRAVITY) of the National Institute of Advanced Industrial Science and Technology (AIST) with the high-speed standard process (HSTP). The AIST-STP2 process and AIST-HSTP process are based on the Nb circuit fabrication process developed by the International Superconductivity Technology Center (ISTEC).

Keywords: Braiding operations, Topological quantum computer, Majorana bound state, SFQ logic, bipolar current pulse generator.

Abstract Body

By combining superconducting single flux quantum (SFQ) digital circuits and complementary metal oxide semiconductor (CMOS) circuits, it is expected to realize super high-performance hybrid devices. A three-terminal superconducting nanowire cryotron, nTron [1], has been developed as an interface to connect SFQ and CMOS circuits. The nTron utilizes the transition from the superconducting state to the normal state of a superconducting nanowire. To understand the basic direct-current and pulse-current operations, we have simulated the three-terminal response of the nTron [2,3]. In the present work, we conduct the numerical simulation for the case where a pulse current from the SFQ circuit is injected to the gate of the nTron.

  We have developed the numerical technique to simulate the operation of the nTron by using the finite element method to solve the time-dependent Ginzburg-Landau (TDGL) equation and heat-diffusion equation [3]. In our simulation, we first apply the finite channel bias current (I_bias) and inject the finite pulse input to the gate, and we investigate the channel voltage (V_ch). Initially, we used a current source model to simulate the pulse input to the gate, but it resulted in unrealistically large voltage at the gate. The gate voltage was greater than , which corresponded to tens of times the signal from SFQ circuits. We, therefore, changed the gate input to a voltage source model, where a pulse wave of the same magnitude as the signal from SFQ circuits was injected to the gate.

  Figure 1(A) shows snapshots of the transition from the superconducting state (blue) to the normal state (red) when the pulse signal (height 0.5 mV, width 5 ps) is applied to the gate. The pulse input causes a vortex to enter from the gate, resulting in the normal transition at the center of the channel. As time goes on, the normal-state area expands and eventually recovers to the superconducting state. Figure 1(B) shows the numerical results of the time evolution of the channel voltage V_ch. When a pulse signal is input at t=0 ps, V_ch is generated and reaches a maximum of V_ch =5.3 mV at t=18 ps. V_ch then decreases and finally returns to a zero-voltage state at t ~ 50 ps.

Figure 1. (A) Distribution of normalized super-electron density . (B) Output characteristic for the pulse input operation of the nTron.

This work was supported by JSPS KAKENHI, Grant Numbers JP20K05314 and JST SPRING, Grant Number JPMJSP2151.

[1] A. N. McCaughan and K. K. Berggren, Nano Lett. 14, 5748 (2014).

[2] N. Yasukawa et al., The 36th International Symposium on Superconductivity (ISS 2023) ED-3-4.

[3] N. Yasukawa et al., Supercond. Sci. Technol. 37, 065013 (2024).

Keywords: superconducting nanowire cryotron, three-terminal device, TDGL simulation