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Physics

Chirping toward a Quantum RAM

Jarryd J. Pla

Published Nov 7, 2022• 1 citation
Publisher

Abstract

A new quantum random-access memory device reads and writes information using a chirped electromagnetic pulse and a superconducting resonator, making it significantly more hardware-efficient than previous devices.
DOI
10.1103/physics.15.168

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A superconducting quantum interference device (SQUID) is used as a memory cell in a superconducting Josephson random access memory (RAM) system. In the Josephson RAM, power is mainly consumed by driver circuits, which drive word and bit lines to select the target memory cell. We investigated the power reduction of READ/WRITE operation of the SQUID memory cell by using a stochastic resonance phenomenon. By using a noise-assisted state transition of the SQUID, the driving current required for the READ/WRITE operation of the memory cell can be reduced. We investigated optimum bias and noise conditions where stochastic resonance efficiently occurs on the basis of analog circuit simulation that takes influences of thermal noises into account. According to the simulation results, the power dissipation required for READ/WRITE operation can be reduced by approximately 20% under the optimum bias and noise conditions when the error rate of READ/WRITE operation is 10-10. We designed a 1-bit rf-SQUID memory cell using the AIST 2.5 kA/cm2Nb standard process 2, and tested the cell by applying a white noise from the room-temperature instrument. The probability that a datum is written to the memory cell and read out correctly was measured under various bias and noise conditions. We have experimentally observed stochastic resonance and obtained the error rate of 10-5 when the power dissipation of READ/WRITE operation is reduced to 80% compared to the conventional Josephson RAM.
A new design concept for the Josephson-junction random access memory (RAM) has been developed. In contrast to previous RAMs based on single flux quantization (SFQ), in our system READ and WRITE operations employ ballistic transfer of SFQ pulses along bit lines (either Josephson transmission lines, or passive superconducting microstrip lines, or their combination). The basic memory cells are the single-junction SQUIDs, connected serially by the bit lines and inductively coupled to word lines. READ and WRITE operations are performed by sending SFQ pulses in appropriate directions along bit lines, and DC currents of appropriate polarity into word lines. This approach allows design of very dense memories with n/spl les/2 Josephson junctions per bit, memory cell area smaller than 80 /spl lambda//sup 2/ (where /spl lambda/ is the minimum feature size), and the critical parameter margin well above /spl plusmn/20%. In this paper we present the general structure of the RSFQ RAM, as well as design and results of testing of the basic memory cell and decoder circuitry using Hypres' 3.5-/spl mu/m, 1-kA/cm/sup 2/ Nb-trilayer technology.>
As in conventional computing, key attributes of quantum memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit or pulsed field gradients. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses in distinct memory modes and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds, while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.