Chinese team makes progress in long-lived coherent storage of microwave photons

Recently, Beijing Institute of Quantum Information Science (BIQIS), in collaboration with Beijing Research Center for Computational Science (BRCS) and School of Integrated Circuits (SIC) of Tsinghua University (TU), realized on-demand storage and readout of microwave states based on an acoustic quantum system with a high quality factor, creating a new record for the coherent storage time of optical force, and developing important applications in the areas of microwave coherent states for long-lived storage, on-demand readout and writeout.

On August 11, 2023, the research results were published as "Coherent memory for microwave photons based on long-lived mechanical excitations" in npj Quantum Information.

In recent years, the manipulation of macroscopic mechanical oscillator states by the radiation pressure of light has become one of the popular research fields internationally. Depending on the physical properties of different systems, the mechanical oscillators can be flexibly designed and materials selected to realize the coupling between different systems. Therefore, the cavity optical force system has strong system compatibility and scalability, and has a wide range of applications in many directions of physical quantity measurement and sensing. Through sideband jumps in the cavity field, thermal phonons are transported and rapidly dissipated, and mechanical vibrations can be cooled to their quantum ground state. Quantized mechanical vibrations are also known as phonons, and quantum acoustic systems relying on mechanical vibrators can be coupled with various natural or artificial quantum systems, which provide ideal and efficient quantum coherent interfaces (interfaces) for communication, interconnection, and scaling up between different quantum systems.

Meanwhile, quantum memory is one of the key modules for quantum information processing and the construction of universal quantum networks, such as long-lived memory for fault-tolerant coding and relaying of quantum states over long distances. Cavity-optical force devices with ultra-long-life mechanical modes are ideal candidates for realizing quantum state storage, coherent interconnection of electron-based quantum computation and laser-band communication networks. Optical radiation pressure manipulation means also provide new technical means for quantum state manipulation such as on-demand controllable read-write.

Fig. 1 On-chip superconducting cavity optical force device and schematic diagram of the pulse sequence. (a) Superconducting cavity photoforce device consisting of metallized silicon nitride film and Nb film superconducting circuitry; (b) film vibrational configuration; (c) schematic of the device lumped-parameter model; and (d) storage of states and laminar pulse sequence of states.

Through the characterization of the vibrational energy relaxation and spectral analysis of the mechanical oscillator, the energy relaxation time of the mechanical oscillator is measured to be close to 16 s, the corresponding spectral linewidth is as low as 10.2 mHz, and the quality factor of the mechanical oscillator is as high as 7.3 million. If microwave signals can be stored into this type of mechanical vibrator, the energy storage lifetime of microwave photons will represent the highest level in the international arena, which is one of the driving reasons why this work hopes to construct a quantum memory based on mechanical vibrators.

In order to ensure the phase coherence of the stored microwave states, in this work, the researchers also overcame the key technological bottleneck of low-frequency mechanical vibrational sideband cooling, and successfully cooled the mechanical vibrator to the ground state, which is a prerequisite for carrying out quantum modulation studies based on mechanical vibrators.

Fig. 2 Mechanical memory performance characterization (a) time-domain ringdown test; (b) vibration energy spectrum analysis.

On the basis of realizing the quantum ground state cooling of the mechanical vibrator, the researchers systematically carried out the research work related to the process of capturing, storing, and resurrecting the roaming microwave photons. In the experiment, the mechanical memory was first initialized to the ground state by a constant sideband drive. Subsequently, the signal pulse will be mapped into the mechanical memory device (i.e., the mechanical mode) under the action of a write pulse, and then the write pulse will be turned off and the signal writing will stop. The written signal will undergo energy decoherence in the mechanical memory as well as quantum decoherence due to environmental thermal reservoirs. When further processing of the target microwave pulse signal is required, a readout pulse will be applied to resurrect the signal stored in the mechanical resonant memory. It is worth noting that the write as well as the readout pulses are sideband-driven fields of constant amplitude. The sideband drive field plays multiple roles in the experiment, including oscillator initialization, writing, and readout. To improve the writing efficiency of the signal, the signal pulse was modulated into an exponentially rising envelope to resist the optical damping (attenuation) introduced by the write field.

The quantum efficiency of writing is maximized when the exponential gain rate of the signal matches the rate of attenuation introduced by the write field. The quantum state phase space distribution of the macroscopic mechanical oscillator can be obtained after repeating thousands of reads and writes and processing the signal with orthogonal direction projection. Since thermal noise is random and obeys a Gaussian distribution with zero mean, the variance of the quantum state phase space distribution represents the number of thermal phonons and the mean represents the coherent component of the signal. By characterizing the time-domain variation of the state distribution, the researchers found that the decoherence time of the stored microwave photons in the device was as high as 55.7 ms, as shown in Fig. 3.

Time-domain evolution of thermal phonons and coherent microwave photons in mechanical memory

In order to realize coherent storage of quantum states at the single-photon level, subsequent work will further cool the mechanical resonator to a lower number of thermal phonons.

Experimental results show that the vibronic thermal noise is greatly suppressed when the mechanical resonant mode is cooled to its quantum ground state. The study of quantum memory based on mechanical vibrators is a hot field nowadays, and various international research groups are pursuing more extreme device performance.

This work realizes on-demand storage as well as readout of microwave states, and the energy decoherence time of stored microwave states is nearly 16 seconds, and the quantum decoherence time is up to 55 milliseconds, which creates a new record for the optical force coherence storage time. This technological breakthrough will bring new opportunities for superconducting quantum-coded storage, microwave-photon relay interconnections, fundamental tests of quantum gravity, and even the search for dark matter.