1000 times longer storage time! Oxford team achieves ion trap 'quantum memory'
Researchers at the University of Oxford have recently created a quantum memory within a captured ion quantum network node. Their unique storage design, described in a paper in Physical Review Letters, proved to be very robust, meaning it can store information for long periods of time while network activity continues.
Alice, a quantum network node at the University of Oxford, objectively collects individual photons entangled with strontium ions captured inside a vacuum chamber.
"We are building a quantum computer network that uses captured ions to store and process quantum information," says Peter Drmota, one of the researchers who conducted the study. "To connect quantum processing devices, we use a single photon emitted from a single atomic ion and exploit the quantum entanglement between that ion and the photon."
Trapped ions are charged atomic particles confined in space using electromagnetic fields and are a common platform for implementing quantum computing. On the other hand, photons (i.e., particles of light) are commonly used to transmit quantum information between long-distance nodes. drmota and his colleagues have been exploring the possibility of combining captured ions with photons to create more powerful quantum technologies.
"So far, we have achieved a reliable way to connect strontium ions and photons and use it to generate high-quality remote entanglement between two distant network nodes," Drmota said. "On the other hand, high-fidelity quantum logic and persistent memory have been developed for calcium ions. In this experiment, we combine these capabilities for the first time and show that it is possible to store this entanglement in strontium ions and photons, and then in nearby calcium ions."
Integrating quantum memory into network nodes is a challenging task because the criteria that need to be met for such systems to work properly are higher than those required to create stand-alone quantum processors. Most notably, the storage developed needs to be robust to concurrent network activity.
A view inside the vacuum chamber, where the team traps strontium and calcium ions with electric fields and lasers.
The team integrated long-lived processing qubits (processing qubits) into the hybrid trapped ion quantum network nodes. Ion-photon entanglement is first generated by the network quantum bit.88Sr+ is transferred to 43Ca+ with a fidelity of 0.977(7) and mapped to the robust processing qubit.
The team then entangles the network quantum bit with the second photon without affecting the processing quantum bit. The team performs quantum state tomography to show that the fidelity of ion-photon entanglement decays ~70 by a factor of 1 on the processing quantum bit. Dynamic decoupling further extends the storage duration; the team measured ion-photon entanglement fidelity of 0.81 after 10 seconds (4).
"One of the sources of technical error we face in capturing ion quantum bits is the phase difference due to magnetic field noise," Drmota said. "Nonetheless, 43Ca+ has a magnetic field-insensitive lepton property that eliminates this error, thereby improving their coherence time. While 88Sr+ is well suited to produce photons for networking, it is sensitive to magnetic field noise."
Although 88Sr+ is known to be sensitive to magnetic field noise, the researchers were able to maintain the entanglement between the ions and photons longer by transferring quantum information from strontium to calcium in the system. Specifically, they were able to maintain this entanglement for more than 10 seconds, which is at least 1,000 times longer than they have observed between bare strontium ions and photons.
"In addition, strontium ions can be reused to generate more entangled photons, and we demonstrate that this process does not affect the fidelity of the entanglement between memory and previous photons, thus achieving robustness to network activity." Drmota said.
(a) Overview of the device. (b) 43Ca+ and 88Sr+ hierarchies
Using their design, a single quantum computing node can be loaded with a given number of processing quantum bits (i.e., calcium), while network quantum bits (i.e., strontium) can be used to create quantum links between distant modules.
Ultimately, such promising quantum memories could pave the way for the creation of scalable quantum computing systems, as the use of small modules that can process quantum information and interconnect them with other modules could obviate the need for large, complex ion traps.
[1]https://phys.org/news/2023-03-robust-quantum-memory-trapped-ion-network.html#google_vignette
[2] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.090803
