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arXiv:2609.00772

Superconducting Flux Memory for Cryogenic Applications

Tony X. Zhou, John McFarland, Aruna N. Ramanayaka, Brian Sears, Colin Stack, Aref Fouladi, Robert Smith, Sambarta Rakshit, Zachary A. Stegen, Keith D. Hillaire, Moe Khalil, Robert M. Young, David G. Ferguson, Anthony J. Przybysz, John X. Pryzbysz, Mark Covington, Gregory R Boyd, Jeremy Clark, Aaron Pesetski

quant-phcond-mat.mes-hallcond-mat.supr-conphysics.ins-detSubmitted Sep 1, 2026• Announced Sep 2, 2026• 0 citations• Revised Sep 3, 2026 (v2)

v1 · Sep 1, 2026 v2 · Sep 3, 2026 v3 · Sep 8, 2026

Abstract

We report the development of flux memory for use with superconducting circuits. This technology stores persistent currents in superconducting loops on-chip to be used to provide flux biasing for superconducting circuits, like qubits. We developed three types of flux memory and draw comparisons among them for circuit design. We demonstrate the utility of flux memory by using an in-situ flux detector and characterize each approach and further demonstrate that once flux is set in a memory cell, benchtop DC control sources can be powered off, leaving the on-chip flux bias in place. We propose that flux memory can be arranged in a two-dimensional configuration to multiplex control signals and reduce how line counts scale (N^2 devices -> 2N control lines), and our experimental results pave the path to the proposed scalability. We demonstrate the use of flux memory to flux bias a transmon qubit and show the tunability of the qubit state to a target frequency which remained stable on-chip for 20 hours.

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We report the development of flux memory for use with superconducting circuits. This technology stores persistent currents in superconducting loops on-chip to be used to provide flux biasing for superconducting circuits, like qubits. We developed three types of flux memory and draw comparisons among them for circuit design. We demonstrate the utility of flux memory by using an in-situ flux detector and characterize each approach and further demonstrate that once flux is set in a memory cell, benchtop DC control sources can be powered off, leaving the on-chip flux bias in place. We propose that flux memory can be arranged in a two-dimensional configuration to multiplex control signals and reduce how line counts scale (N^2 devices -> 2N control lines), and our experimental results pave the path to the proposed scalability. We demonstrate the use of flux memory to flux bias a transmon qubit and show the tunability of the qubit state to a target frequency which remained stable on-chip for 20 hours.

Flux-tunable superconducting qubits offer the potential for quantum information processing and simulation. However, flux crosstalk complicates achieving precise qubit frequency control, as characterizing and compensating for it becomes more challenging when scaling up circuits. In this study, we propose a design featuring a dual flux line for flux-tunable superconducting qubits, consisting of a pair of parallel current lines directing the current back to the enclosure. We conduct experimental comparisons of the DC flux crosstalk between the dual flux line design and the traditional one-way approach within a single chip. We observe a significant reduction in the DC flux crosstalk when utilizing the dual flux line, as supported by electromagnetic field simulations. Our design introduces an approach to scaling up flux-tunable superconducting circuits while maintaining a modest on-chip footprint.

Abstract Broadband and low-loss superconducting switches can facilitate large-scale quantum information processors and cryogenic detectors by dynamically reconfiguring the connectivity of their circuits. The time dependent connectivity is enabled by the nonlinearity of lossless Josephson junctions, which are often wired into superconducting loops to be controlled by magnetic flux. However, this approach needs a power-consuming constant flux bias and dynamic flux actuation, both of which are hard to isolate from other switches or flux sensitive elements, limiting their integration density. Here, we design and characterize a microwave switch that implements a persistent current bias and direct current actuation to reduce static power consumption, actuation energy and potential crosstalk to other devices. We show that persistent current associated with tens of flux quanta is stable and long-lived, reducing the need for on-the-fly tuning. We further demonstrate that our switch has desirable performance for superconducting-circuit-based quantum information processing, including an off mode with more than 20 dB isolation comparable to commercial ferrite isolators, power handling larger than 100 pW sufficient for resonator readout tones and amplifier pumps, and modulation bandwidth broader than 600 MHz useful for multiplexing schemes.

Broadband and low-loss superconducting switches can facilitate large-scale quantum information processors and cryogenic detectors by dynamically reconfiguring the connectivity of their circuits. The time dependent connectivity is enabled by the nonlinearity of lossless Josephson junctions, which are often wired into superconducting loops to be controlled by magnetic flux. However, this approach needs a power-consuming constant flux bias and dynamic flux actuation, both of which are hard to isolate from other switches or flux sensitive elements, limiting their integration density. Here, we design and characterize a microwave switch that implements a persistent current bias and direct current actuation to reduce static power consumption, actuation energy and potential crosstalk to other devices. We show that persistent current associated with tens of flux quanta is stable and long-lived, reducing the need for on-the-fly tuning. We further demonstrate that our switch has desirable performance for superconducting-circuit-based quantum information processing, including an off mode with more than 20 dB isolation comparable to commercial ferrite isolators, power handling larger than 100 pW sufficient for resonator readout tones and amplifier pumps, and modulation bandwidth broader than 600 MHz useful for multiplexing schemes.

Over the past few decades, superconducting circuits have been used to realize various novel electronic devices such as quantum bits, SQUIDs, parametric amplifiers, etc. One domain, however, where superconducting circuits fall short is information storage. Superconducting memories are based on the quantization of magnetic flux in superconducting loops. Standard implementations store information as magnetic flux quanta in a superconducting loop interrupted by two Josephson junctions (i.e., a SQUID). However, due to the large inductance required, the size of the SQUID loop cannot be scaled below several micrometers, resulting in low-density memory chips. Here, we propose a scalable memory consisting of a voltage-biased superconducting ring threaded by a half-quantum flux bias. By numerically solving the time-dependent Ginzburg-Landau equations, we show that applying a time-dependent bias voltage in the microwave range constitutes a writing mechanism to change the number of stored flux quanta within the ring. Since the proposed device does not require a large loop inductance, it can be scaled down, enabling a high-density memory technology.

We have realized an on-chip persistent flux bias for our flux based qubit by adding a switchable aluminum shunt to the niobium flux bias coils. This inductive shunt can create a persistent current loop by trapping the bias flux at the superconducting transition of the Al. When the Al is superconducting, the noise spectral density from the flux bias and the damping due to the flux bias should both be reduced by a factor of 100, since the inductance ratio,Lsh/Lp, is 0.1. This reduces one of the sources of decoherence for the qubit. To permit changes to the persistent current in the superconducting loops at mK temperatures, a local heater has been coupled to the Al shunt. This makes it possible to briefly drive the shunt to its normal state to change to flux without heating the entire device.