Record-smashing photon storage a boon for quantum communication

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Quantum computing just took yet another leap forward. (Unsplash/Ben Wicks)

A team of physicists has extended the time that light in the form of photons can be stored and retrieved from one minute to one hour, creating a technological leap that will likely contribute to the development of highly encrypted quantum communication.

Publishing their results April 22 in Nature Communications, researchers at the University of Science and Technology of China preserved photons in a crystalline structure placed within a strong magnetic field, putting safeguards in place to minimize disturbances. The new storage technique also preserved useful information about the photons' initial state, which had previously been achieved for only one second of storage.

One of the paper's lead authors envisions a setup like the one he helped develop being transported long distances over train, satellite or other means to deliver quantum-encrypted information, with the hourlong storage expanding the physical range of transmission.

The experiment is fundamentally based on a core tenet of quantum mechanics: All objects have wave-like properties, and they act like waves in some conditions and particles in others. A photon, the particle form of light, is considered to be in a state of "quantum coherence" when it behaves the least like a wave. When in a coherent state, photons can be used to store information by tracking their attributes, such as their phase. Coherent photons are useful for quantum communication, an experimental technology that encrypts messages using particles in quantum states.

But even small disruptions can make coherent states fall apart, and maintaining coherence for a long period of time has proved challenging. The longest period over which coherence has been maintained is about six hours, accomplished with photons in 2015. But the record for storing photons is only about one minute, demonstrated in rubidium atoms and a crystal of yttrium orthosilicate doped with praseodymium ions.

"If the quantum memory has a very long lifetime, say hours or days, then we can transport the quantum memory to realize the quantum communication," said Zong-Quan Zhou, an associate professor at the University of Science and Technology of China and a co-lead author of the new paper.

Zhou and his co-authors demonstrated a 60-fold improvement in storage time by maintaining coherent photons for more than an hour. A confirmation of the experiment's success found it had a fidelity of 96.4%, a strong performance that "indicates a promising application as a faithful quantum memory" in the future, they said.

To accomplish the long-term photon storage, a laser was used to add photons to a europium ion-doped yttrium orthosilicate crystal, which absorbed the photons and stored the energy in the form of excited electrons; the crystal structure stops decoherence due to the motion of its atoms. A control pulse from another laser made molecules transfer the excitation from the crystal's electrons to its spin, the form in which the energy is stored the longest.

At the end of the experiment, another control pulse was used to revert the energy into excited electrons, and the photons were emitted and retrieved, with some maintaining their coherence.

Two main approaches made the storage possible: an atomic frequency comb and a zero-first-order-Zeeman, or ZEFOZ, magnetic field. The atomic frequency comb, made from the ions of the rare-earth metal europium dispersed in the crystal, helps store and emit the photons and is the leading method of storing photons using long-term spin excitation. The previous longest time over which atomic frequency combs had stored electrons was 0.53 seconds, a period extended by more than 6,000 times by the recent study.

The ZEFOZ magnetic field was tuned to a strength that minimizes the frequency at which the crystal atoms changed their spin, greatly extending the device's ability to store photons. The storage time was boosted through a process known as dynamical decoupling, in which energy pulses were used to cancel out the system's intersection with the environment, protecting the coherence and increasing storage time.

The long storage time accomplished by this experiment leads to "a promising future for large-scale quantum communication," the physicists wrote in the paper. The longer a message can be reliably stored, the farther it can be transmitted by physically moving the storage unit, whether on "the train, on the truck or the helicopter," Zhou said.

Importantly, according to Zhou, the one-hour period for which coherence was preserved is longer than the time it takes for some satellites to orbit halfway around the Earth. A satellite carrying this data-storage technology could therefore transfer a signal between any two places on the planet before coherence is lost, he said.

In 2017, Chinese scientists and engineers demonstrated this concept by beaming photons up to Micius, the first quantum-communication satellite, which transported the photons to a ground-based receiver station 1,203 kilometers, or about 748 miles, away. Rather than being transported and re-emitted to carry information as envisioned by Zhou, the photons were each quantum-entangled — or intrinsically connected — with another photon at the starting station, a different pathway that could be eventually used to communicate over long distances. 

For the task of transporting photons carrying quantum information, the strategy provides an alternative to using optical-fiber cables, which span between continents underground but can absorb or otherwise lose photons over long distances. 

But the storage technology still has room for improvement. Its storage efficiency after five minutes, for instance, is only 0.035%, or about one success for every 2,860 attempts, and Zhou said it drops to about 0.005% — one in 20,000 attempts — at 60 minutes. For practical use in quantum technologies, the efficiency needs to be improved to 10% or 15%, according to the physics professor.

He and his colleagues plan to improve their device in part by reducing the noise that obfuscates its signals. Some of the noise stems from the protective dynamical decoupling pulses, the researchers wrote, and using stronger ZEFOZ magnetic fields would reduce the need for the pulses, in addition to boosting storage time.  

"To realize some applications in the quantum information science, the key challenge is the signal-to-noise ratio and the efficiency for single-photon storage," Zhou said.

The study, "One-hour coherent optical storage in an atomic frequency comb memory," published April 22 in Nature Communications, was authored by Yu Ma, You-Zhi Ma, Zong-Quan Zhou, Chuan-Feng Li and Guang-Can Guo, University of Science and Technology of China.

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