Research team synchronizes single photons using an atomic quantum memory

The efficient synchronization of unique and independently created photons (i.e., light particles) has long been a problem in quantum physics. Understanding this would have significant ramifications for the processing of quantum information that depends on interactions between many photons.

At ambient temperature, an atomic quantum memory was used by Weizmann Institute of Science researchers to successfully synchronize a single, independently produced photon. Their article, which was released in Physical Review Letters, may pave the way for further research into multi-photon states and their application to quantum information processing.

"The project idea came about several years ago, when our group and the group of Ian Walmsley demonstrated an atomic quantum memory with an inverted atomic-level scheme compared to the typical memories—the ladder memory, named fast ladder memory (FLAME)," said Omri Davidson, one of the researchers who conducted the study, to Phys.org. These memories are quick and devoid of noise, making them effective for synchronizing single photons.

Multi-photon states must be successfully created in order for photonic quantum computing and other quantum information protocols to work. Since the majority of quantum sources used in research up to this point are probabilistic, they are unsuitable for reliably producing multi-photon states.

Davidson and his coworkers have investigated the prospect of creating these states using an atomic quantum memory, which are systems that can store photons' quantum states while preserving the quantum information they convey. They assumed that their atomic quantum memory would be able to hold photons produced through probabilistic processes and release them when needed to produce a multi-photon state.

The goal of the current study was to show single photon synchronization for the first time utilizing an independent room-temperature atomic quantum memory, according to Davidson. "To do this, we rebuilt the memory with a number of enhancements and created a single-photon source that produces photons that can effectively communicate with the memory. The real photon synchronization, which connected the photon source and memory modules with the appropriate experiment control circuits, was finally ready for demonstration.

The researchers' quantum memory FLAME, which was created as part of earlier research, is based on an inverted atomic-level structure known as a ladder scheme. FLAME is both fast and noise-free compared to traditional ground-state memories, which are often sluggish and susceptible to noise, but can only retain data for a limited amount of time. They hoped that it would enable them to produce multi-photon quantum states since speed and absence of noise are necessary characteristics for the synchronization of single photons.

The little wavelength mismatch of the signal and control light-field transitions is the second benefit of the particular ladder scheme used with rubidium atoms, according to Davidson. "Due to the reduced two-photon Doppler broadening, this permits a reasonably extended memory lifetime compared to other ladder methods with a bigger wavelength mismatch. The photons may effectively couple with the memory because we produced them using the same atomic-level structure as our memory.

The success of the team's experiment was largely due to the FLAME memory scheme's several benefits, which allowed them to quickly synchronize individual photons. They achieved a throughput of more than 1,000 synchronized photon pairs per second using their atomic quantum memory to store and retrieve single photons with an end-to-end efficiency of e2e=25% and final antibunching of g(2)h=0.023.

The amount of photon antibunching, or G (2) h, indicates how "single" the individual photons are. In contrast to classical light, which has g(2)h=1, perfect single photons have g(2)h=0. Due to the memory's noise-free functioning, the researchers' synchronized photons continue to be virtually flawless single-photons at g(2)h=0.023.

"We were able to synchronize photons that are compatible with atomic systems at high rate," stated Davidson. "Many photonic quantum information protocols, such as a deterministic two-qubit entangling gate, depend on photons that may interact with atoms. Previous examples of photon synchronization either employed broadband photons that are incompatible with atomic systems or extremely slow photons that are compatible with atomic systems.

In comparison to earlier demonstrations employing photons that are compatible with atomic systems, the photon synchronization rate that Davidson and his colleagues achieved in their tests is more than 1,000 times better. Their finding opens up new research directions for the investigation of atom-multiphoton interactions, including so-called deterministic two-photon entangling gates. It could have important ramifications for the development of quantum information processing and quantum optics systems in the future.

We are investigating two study avenues right now," Davidson continued. The first is to create robust photon-photon interactions with rubidium atoms in a synchronization-like environment. We will be able to show a deterministic entangling gate between the synchronized single-photons if we are successful in our endeavor.

These gates are a crucial part of photonic quantum computation because they allow for a reduction in the resource overhead compared to the methods currently being used (called linear-optic quantum computation). The scalability of these systems is limited since, up to this point, only cold atom setups, not hot atoms, have been used to show these gates.

The FLAME memory will also be improved in Davidson and his coworkers' upcoming research so that it can store photonic qubits as opposed to only single photons in one polarization state, or a photon in a quantum superposition of two polarization states. They could eventually be able to use photons for quantum calculations thanks to this.