Raman quantum memory demonstrates near-unity performance

Over the past decades, quantum physicists and engineers have developed numerous technologies that harness the principles of quantum mechanics to push the boundaries of classical information science. Among these advances, quantum memories stand out as promising devices for storing and retrieving quantum information encoded in light or other physical carriers.

To be viable for real-world applications, quantum memory must deliver both high efficiency and high fidelity. In other words, they should be able to store and retrieve most of the input quantum information—typically over 90%—and ensure that a recovered state closely matches the original one.

Notably, most previously proposed strategies to develop efficient quantum memories were found to produce undesired random fluctuations (i.e., noise). These fluctuations could in turn degrade quantum information, reducing the system’s fidelity.

The joint team led by Professor Weiping Zhang at Shanghai Jiao Tong University, and Professor Liqing Chen at East China Normal University in China recently introduced a new approach to control of atom-light interactions while quantum information is stored. Using this technique, which is outlined in a paper in Physical Review Letters, they demonstrated a Raman quantum memory that exhibits an efficiency of 94.6%, produces very little noise and can store quantum information with a 98.91% fidelity.

“Quantum memory with near-unity efficiency and fidelity is indispensable for quantum information processing,” Zhang told Phys.org. “Achieving such a performance has long been a central challenge in the field, motivating extensive research efforts and inspiring the published work. The primary objectives of this work were to elucidate the underlying physics and to develop practical approaches for realizing perfect quantum memory.”

A promising mathematically guided technique

The quantum memory developed by Zhang and his colleagues leverages a type of atom-light interaction, known as a far-off resonant Raman scheme. Beyond enabling quantum storage, this scheme also offers a broadband advantage, allowing its memory to store optical signals much faster than that in other schemes.

In their paper, the researchers introduced a precise and robust technique that can be used to adaptively control a quantum memory until it reaches “perfection.” This technique is based on the principle of atom-light spatiotemporal mapping, which is mathematically called the Hankel transform.

“Fundamentally, this work is the first time to uncover the physical mechanism behind the atom-light mapping in the quantum memory,” said Zhang. “Practically, this work makes a breakthrough in developing a new method and promising technique to achieve a benchmark of quantum memory.”

Breaking the limits of earlier quantum memories

So far, the researchers have applied their newly discovered mathematical approach to a Raman quantum memory based on a warm rubidium-87 (⁸⁷Rb) vapor. Their approach was found to break the “efficiency–fidelity trade-off” bottleneck that had so far prevented the realization of ‘perfect’ quantum memories.

This recent effort by Zhang and his colleagues could thus contribute to the realization of increasingly better performing quantum memories. In the future, these memories could open new possibilities for the development of various other quantum technologies, including long-distance quantum communication, quantum computers and distributed quantum sensing systems.

“Our plans for future research include, but are not limited to, studying new physics-driven principles and integrating the memory into quantum repeaters for fault-tolerant quantum computing architectures and quantum networks,” added Zhang.

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