Quantum Physics

A reliable quantum memory that stores data in a trapped-ion quantum network.

A quantum memory was recently developed by Oxford University researchers within a trapped-ion quantum network node. Their one-of-a-kind memory design, which was first described in a paper published in Physical Review Letters, was found to be extremely durable, allowing it to keep information stored for a considerable amount of time despite ongoing network activity.

Peter Drmota, one of the study’s authors, told, “We are building a network of quantum computers, which use trapped ions to store and process quantum information.” We use single photons emitted by a single atomic ion and quantum entanglement between this ion and the photons to connect quantum processing devices.”

Charged atomic particles that are constrained in space by electromagnetic fields and used as a platform for quantum computations are known as trapped ions. On the other hand, most of the time, photons—the particles of light—are used to send quantum information between nodes that are far apart. In order to develop more potent quantum technologies, Drmota and his colleagues have been investigating the possibility of combining photons and trapped ions.

“This means that while a network link is formed, the quantum information contained in the memory must not degrade. This necessitates high isolation between the memory and the network, but there must also be a quick and dependable system that connects the memory to the network when necessary.”

Peter Drmota, one of the researchers who carried out the study, 

Dr. Drmota stated, “Up to this point, we have implemented a reliable method of interfacing strontium ions and photons, and used this to generate high-quality remote entanglement between two distant network nodes.” Calcium ions, on the other hand, have long-lasting memories and high-fidelity quantum logic. We demonstrate that it is possible to create high-quality entanglement between a strontium ion and a photon and then store this entanglement in a nearby calcium ion by combining these capabilities for the first time in this experiment.

It is difficult to incorporate a quantum memory into a network node because the requirements for its operation are higher than those for the development of a standalone quantum processor. The developed memory would especially need to be resistant to concurrent network activity.

According to Drmota’s explanation, “this means that the quantum information stored in the memory must not degrade while a network link is established.” This necessitates extreme isolation of the memory from the network, but it also necessitates a quick and dependable mechanism for coupling the memory to the network when required.”

Credits: David Nadlinger.

View inside the vacuum chamber where electric fields and lasers are used to trap calcium and strontium ions.

Drmota and his colleagues used two distinct atomic species, strontium and calcium, to create their quantum memory. This allowed them to minimize crosstalk while also establishing a network link. They were also able to use in-sequence cooling and detect errors in real time thanks to the minimal crosstalk in this mixed-species architecture. Mixed-species entanglement gates provided the memory and network needed for the missing link.

“Dephasing due to magnetic field noise is one of the technical error sources that we face with trapped-ion qubits,” Drmota stated. However, calcium-43 has transitions that are not affected by magnetic fields, thereby eliminating this error and increasing their coherence time. Despite its excellent suitability for producing photons for networking, strontium-88 is sensitive to magnetic field noise.

By transferring quantum information from strontium to calcium in the system, the researchers were able to preserve entanglement between their memory ion and a photon for a longer period of time, despite the fact that strontium-88 is known to be sensitive to magnetic field noise. In particular, they were able to maintain this entanglement for more than 10 s, which is at least 1000 times longer than what they observed between a photon and a bare strontium ion.

“Furthermore, the strontium ion can be reused to generate further entangled photons, and we show that this process does not affect the fidelity of entanglement between the memory and the previous photon, thereby achieving robustness to network activity,” Drmota stated. “Furthermore, the strontium ion can be reused to generate further entangled photons.” Notably, we were able to combine the complexity of several difficult techniques that had been independently developed over many years in various setups into a single experiment.

The quantum memory developed by Drmota and his colleagues was found to be extremely robust in the initial tests, maintaining entanglement between a trapped ion and photon for at least 10s. The group’s exhibition of this quantum memory could be a significant achievement on the continuous mission to acknowledge disseminated quantum data handling.

Individual quantum computational nodes can be equipped with a predetermined number of processing qubits (calcium), while the network qubit (strontium) can be utilized to establish quantum links between modules located far away. By connecting small modules that can process quantum information to other modules and avoiding the need for large, intricate ion traps, this promising quantum memory may eventually pave the way for the development of scalable quantum computing systems.

Drmota added, “The robust quantum memory is key for new developments in quantum communications, metrology, and time keeping.” It could be used in quantum repeaters for private (blind) quantum computation. The long entanglement storage durations achieved in our experiments, for instance, will result in an order-of-magnitude increase in frequency comparison precision between distant clocks in the emerging field of entangled atomic clocks.

More information: P. Drmota et al, Robust Quantum Memory in a Trapped-Ion Quantum Network Node, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.090803.

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