Efficiency over 1000 times! Quantum memory efficiently synchronizes single photons

A long-standing challenge in the field of quantum physics is how to efficiently synchronize individual photons (i.e., light particles) that are independently generated. Achieving this will have critical implications for quantum information processing that relies on interactions between multiple photons.

Recently, researchers at the Weizmann Institute of Science (Israel) demonstrated the synchronization of independently generated individual photons using an atomic quantum memory that operates at room temperature, opening new avenues for the study of multiphoton states and their applications in quantum information processing.

Their paper was published July 18 in Physical Review Letters.

"Single-Photon Synchronization with a Room-Temperature Atomic Quantum Memory."

Weitzman's team's experimental setup, with two clouds of rubidium vapor at its core.

"The idea for this project arose a few years ago when our group and Ian Walmsley's group demonstrated an atomic quantum memory. It uses an inverted atomic-level scheme compared to the typical memory, the 'fast ladder memory (FLAME),'" Omri Davidson, one of the researchers who carried out the study, said. Omri Davidson, one of the researchers who conducted the study, said, "These memories are fast and noise-free, so they are useful for synchronizing single photons."

In contrast to FLAME, the quantum memory used by the researchers, which is fast and noise-free compared to conventional ground-state memories that are typically slow and susceptible to noise, FLAME can only store information for a short period of time.

Optical quantum computing and other quantum information protocols rely on the successful generation of multiphoton states. Since most quantum sources used in research to date are probabilistic, they are not suitable for generating multiphoton states at a reasonable rate.

As part of their research, Davidson and his colleagues have begun to explore the possibility of realizing these states using atomic quantum memories: devices that can store quantum states of photons while retaining the quantum information they carry. They predicted that atomic quantum memories would be able to store probabilistically generated photons and release them on demand to generate multiphoton states.

Davidson said, "The goal of the current research is to demonstrate the synchronization of single photons for the first time using a stand-alone room-temperature atomic quantum memory. To achieve this goal, we must make several improvements to the memory and build a single-photon source capable of generating photons to effectively interface with the memory. Finally, we are ready to demonstrate actual photon synchronization, which connects the photon source and the memory module to the appropriate control electronics for the experiment."

Photon synchronization scheme

Davidson explains, "A second advantage of the specific laddering scheme we employ in rubidium atoms is that there is very little wavelength mismatch between the signal and the wavelengths controlling the optical field transitions. Compared to other ladder schemes with large wavelength mismatches, the memory lifetime is relatively long because of the small two-photon Doppler spread. Finally, we use the same atomic-level structure as the memory to generate the photons, which allows for efficient coupling between the photons and the memory."

The many advantages of the team's FLAME memory solution combined to contribute to the success of their experiment, allowing them to synchronize individual photons at high speeds. Using their atomic quantum memory, they were able to store and retrieve individual photons at a rate of more than 1,000 synchronized photon pairs per second with an end-to-end efficiency of ηe2e = 25% and a final antibinding of g(2)h = 0.023.

The g(2)h, or "photon antibunching", is a measure of how "single" a single photon is. A perfect single photon has g(2)h= 0, while classical light has g(2)h= 1. Thus, at g(2)h= 0.023, the researchers synchronized a photon that was still almost perfectly single, thanks to the memory's noise-free operation.

The team said, "We were able to synchronize photons compatible with atomic systems at high speeds." Atom-compatible photons are important for many photonic quantum information protocols, such as deterministic double-qubit entanglement gates. Previous demonstrations of photon synchronization have either used broadband photons that are incompatible with atomic systems or photons that are compatible with atomic systems but at very low rates.

The photon synchronization rate achieved in this experiment is more than 1000 times higher than previous demonstrations using photons compatible with atomic systems. Their work opens up new avenues for studying interactions between multiphoton states and atoms (e.g., deterministic two-photon entanglement gates).

In the future, this could have important implications for realizing both quantum information processing and quantum optical systems.

Storage of single photons

Photon synchronization

Photon synchronization rate enhancement

"We are currently exploring two research paths. The first is to achieve strong photon-photon interactions with rubidium atoms in a system similar to the synchronization system. Achieving this will allow us to demonstrate deterministic entanglement gates between synchronized single photons." The team adds.

"These gates are an important component of photonic quantum computation because they can reduce the resource overhead compared to currently employed methods (linear optical quantum computation). So far, these gates have only been demonstrated in cold atom setups, not hot atoms, which limits the scalability of these systems."

In upcoming research, the experimental team also plans to further develop their FLAME memory so that it can store optical quantum bits (i.e., photons in a quantum superposition of two polarization states), rather than just individual photons in one polarization state.

Eventually, they will be able to utilize photons for quantum computing.

Reference link:

[1] https://phys.org/news/2023-07-team-synchronizes-photons-atomic-quantum.html

[2]https://physics.aps.org/articles/v16/s96

[3]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.033601