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Physical Review X

Random-Access Quantum Memory Using Chirped Pulse Phase Encoding

James O’Sullivan, Oscar W. Kennedy, Kamanasish Debnath, Joseph Alexander, Christoph W. Zollitsch, Mantas Šimėnas, Akel Hashim, Christopher N. Thomas, S. Withington, Irfan A. Siddiqi, Klaus Mølmer, John J. L. Morton

Published Nov 7, 2022• 19 citations
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/physrevx.12.041014

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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.
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 photonic quantum memory that can store multiple qubits and later recall any randomly chosen subset is essential for large-scale quantum networking and computing. This capability, similar to random-access memory in classical computers, has been difficult to achieve due to the lack of a versatile random-access mechanism and the limited multi-mode storage capacity of existing quantum memory protocols. One promising route uses photon-echo protocols, which apply optical rephasing pulses to selectively read out frequency-multiplexed photonic qubits within a broadened optical transition. Here we advance a photon-echo-based quantum memory protocol using linearly swept adiabatic rephasing pulses, which imprint phase shifts to suppress unwanted echoes. Using the optical transitions of a 171Yb3+:Y2SiO5 crystal, we demonstrate storage and retrieval of multiple complex spectro-temporal photonic modes and achieve random-access memory across eight distinct spectral modes. We show enhanced mode-mapping versatility while substantially lowering the required rephasing pulse intensity, providing a more efficient and reliable approach for high-fidelity qubit storage and retrieval. Scalable quantum networks require memories that store photonic qubits while allowing selective access to individual states, a capability not currently available. A chirped-pulse phase-encoding approach provides random access to spectro-temporal-multiplexed information, enabling storage and retrieval across eight spectral modes with lower rephasing pulse intensity.