Nanophotonic rare-earth quantum memory with optically controlled retrieval

Researchers have built a quantum memory device that is a millionth times smaller than previsous devices. It is small enough to install on a chip.

This crystal is the first quantum memory device of its kind that could fit on a chip alongside nano-sized instruments for detecting and sending signals written in quantum bits. The device iis only about 10 micrometers long and 0.7 micrometers wide — about as wide as a bacterial cell.

The secret to this storage device’s tiny size is its shape, which resembles a Toblerone chocolate bar: a triangular prism with notches etched along the top.

Photons that enter the crystal at one end bounce back and forth between these “mirrors” a few thousand times before they can escape, which increases their likelihood of getting absorbed by an atom along the way. So the crystal’s serrations helped it capture a decent number of photons despite its small number of atoms.

Zhong’s team tested their crystal’s quantum storage capabilities by using an optical fiber to inject it with one bunch of photons, and then another. The quantum bit of information carried on each photon was 0 if it belonged to the earlier pulse, 1 if it belonged to the later pulse, or 0 and 1 simultaneously if (thanks to the bizarre rules of quantum mechanics) it belonged to both the earlier and later pulse.

The ridges cut into a new device’s crystal (seen here in a scanning electron microscope image) collectively act as a pair of mirrors. Photons enter the crystal at one end and bounce back and forth thousands of times between these “mirrors” before escaping. This allows the sawtooth memory device to catch more photons despite its tiny size.

Science – Nanophotonic rare-earth quantum memory with optically controlled retrieval

Optical quantum memories are essential elements in quantum networks for long distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of its readout time. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables over 95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the network nodes.