Ultrathin superconducting (SC) films embedded between two ferromagnetic insulators (FI) experience magnetic-exchange coupling, enabling the switching of the superconducting state by controlling the relative FI magnetization alignment. We investigate micrometer-scale devices patterned from such thin-film heterostructures and demonstrate their reproducible operation as nonreciprocal superconducting memory elements. Single-cell writing is realized using current-pulse-induced, heat-assisted magnetic switching. Furthermore, at a lower temperature, symmetry breaking gives the memory elements a marked nonreciprocity, with zero magnetic field superconducting diode efficiencies exceeding ±60%. These results present highly scalable FI/SC/FI hybrid structures as a contender for state-of-the art robust superconducting memories for neuromorphic superconducting computing and new data guiding schemes.
Diode is one of the basic electronic components. It has a nonreciprocal current response, associated with a broken space/time reversal symmetry. Here we demonstrate prototypes of superconducting diodes operational at zero magnetic field. They are based on conventional niobium planar Josephson junctions, in which space/time symmetry is broken by a combination of self-field effect from nonuniform bias and stray fields from a trapped Abrikosov vortex. We demonstrate that nonreciprocity of critical current in such diodes can reach an order of magnitude and rectification efficiency can exceed 70%. Furthermore, we can easily change the diode polarity and switch nonreciprocity on/off by changing the bias configuration and by trapping/removing of a vortex. This facilitates a memory functionality. We argue that such a diode-with-memory can be used for a future generation of in-memory superconducting computers.
The quantum behavior of Josephson junctions is often exploited to produce superconducting devices of outstanding performance. With respect to information technology applications, Josephson junctions can be used in circuits that perform logic operations in picoseconds, which may result in energy-efficient, high-performance cryogenic computers, provided that memory elements can be developed that can be switched between two stable states by integrated superconducting logic circuits. Here, we show that Josephson junctions based on pseudo-spin-valve barriers could enable such memory elements, by unambiguously differentiating the controlled changes in Josephson coupling from size-dependent remanent-field effects. These changes can be very large and even reversed in magnitude, which is strong evidence of the exchange-field effect on the Cooper pair phase. These devices are the first that demonstrate nonvolatile, size-independent switching of the Josephson coupling in magnitude or phase, and they may allow for the first scalable superconducting memory devices.
Cryogenic (cryo) memory devices, designed to operate at/below 4 Kelvin (K) temperature, is a prime enabler of practical quantum computing systems, and superconducting (SC) electronic platforms (Figs. 1(a), (b)) [1]. The state-of-the-art quantum algorithms require many arbitrary rotations which demand a large memory to store program instructions [2]. SC qubits (used in most of the existing quantum computing systems) are highly sensitive to noise and hence, to protect the qubit states from thermal disturbances, they are placed at a few milli-Kelvin (mK) temperature. Furthermore, to preserve the integrity of the quantum states, the SC qubits undergo continuous error correction schemes, requiring extensive memory and bandwidth [2]. Superconducting electronics (SCE) (targeted towards space applications, and high-performance computing) outperforms the conventional CMOS counterparts in terms of speed and energy-efficiency (Fig. 1(c)) [3]. Decades of research efforts have given rise to three major categories (and several sub-variants) of cryo-memories based on SC, non-SC, and hybrid technologies (Fig. 2) [2], [4]–[6]. However, the existing variants suffer from one or more of the following challenges - (i) limited scalability, (ii) process complexity, (iii) bulky peripherals, (iv) array-level interference, (v) volatility, and (vi) speed incompatibility. Therefore, a scalable cryo-memory system remains elusive. To address these existing issues, here, we present a novel cryo-memory system utilizing -(i) the polarization-induced Cooper-pair [7] modulation in a ferroelectric
(FE)superconducting quantum interference device (SQUID) (Fig. 3(a)) [8], and (ii) current controlled
SC↔non−SCswitching in a heater cryotron
(hTron)(Fig. 4) [4]. Discrete prototypes of these devices have been demonstrated recently, but their coupled interactions (which we harness in our work) were never explored before.
Magnetic Josephson junctions are the preferred candidate devices for designing fast and scalable cryogenic memory elements. This is especially the case for rapid single-flux quantum-based superconducting electronics, where the speed mismatch between logic and memory elements have remained a long-standing challenge. In this Letter, we demonstrate a simple tri-layer Josephson memory device using ferromagnetic insulating (FI) GdN-based S/FI/S vertical mesa-type junctions, with reliable nonvolatile memory operation without the need of a shunt resistor at 4.2 K. The characteristic frequency of our devices is approximately 90 GHz, corresponding to an IcRn product of 177 μV. We demonstrate a thorough study of the parameter spaces required for designing these devices and identify the scope for future improvements that can lead to further miniaturization and higher operating speed of these devices.
Efficient, scalable, and magnetic-field-free superconducting diodes are essential for future superconducting electronics; yet, despite significant efforts, such practical devices remain unrealized. The main challenge lies in achieving broad-range in-situ tunability, both for optimization and for achieving transistor-like operation. Here, we study diodes based on four-terminal niobium planar Josephson junctions. We show that the multiterminal structure eliminates the need for an external magnetic field and enables essentially unrestricted in-situ tunability, along with reconfigurability of the diode polarity, leading to new functionality. For example, we demonstrate that such diodes can operate as Gauss neurons via reentrant superconductivity. By deliberately tuning the junction parameters, we obtain effectively infinite nonreciprocity (within experimental resolution) leading to threshold-free ac-current rectification. Such technologically simple, reconfigurable, and broadly tunable diodes could be instrumental for future digital and neuromorphic computing.
We develop a fast magnetic Josephson junction (MJJ)—a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient single flux quantum (SFQ) circuits. We present experimental results for superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs with high characteristic voltage IcRn of >700 μV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ∼30% lower than that of conventional junction co-produced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.