Quantum Memory and Repeaters

What is Quantum Memory?

Practical quantum computing may require networking a number of smaller quantum-computing systems, thus requiring transport of quantum entangled particles. Such transport may use “quantum memories” for long distance communication, similar to the use of repeaters for classical communication. It may also use sophisticated machine learning methods and error correction techniques for longer distance communication without quantum repeaters.

Quantum memories are devices that provide quantum computers an analog to the memory used in binary digital devices. Quantum memories store the quantum state of a photon or other entangled particle without destroying the quantum information of that particle. The quantum memory should be able to release an entangled particle with the same quantum state as the stored particle for data retrieval. Whereas digital binary memory stores two states, 0 or 1, a quantum memory can store several states in a quantum superposition. In other words, quantum memories store qubits for later retrieval. Current quantum memories use photons (light) where the quantum state of a photon is mapped onto a group of atoms and can later be restored as a photon with the same quantum state.

Quantum memories are limited by similar coherence issues that limit the use of quantum computing. They require coherent matter systems so the quantum information stored in the memory isn’t lost due to decoherence. These coherent matter systems could be a very cool gas or a solid-state system.

It is also our belief the any practical device should be based on solid-state materials. Rare-earth-ion (RE) doped crystals are highly interesting matter systems for quantum memories, owing to their unique optical and spin coherence properties at low temperatures (around 4 K). To cool down the RE crystals we use closed-cycle coolers that do not require any cooling liquids.

 Quantum memories will be key components in future quantum networks, such as quantum repeaters which can provide a solution for long-distance quantum communication beyond the limit of 500 km using today's technology. To store photons in RE-doped crystals we use the Atomic Frequency Comb technique.

The AFC technique can store quantum information in highly time-multiplexed fashion, which is key to building quantum repeaters for future quantum networks.

In addition to future applications, quantum memories are fascinating because they provide a way to study how quantum effects such as entanglement can be transferred between physical systems of widely different nature, between light and matter systems.

What is Quantum Repeaters?

Quantum memories will enable quantum repeaters that can allow longer distance quantum communication networks. There are two important issues in using quantum states for communication. First, the No-cloning theorem says that it is impossible to duplicate an unknown quantum state and the Heisenberg uncertainty principle says that it is impossible to know a quantum state.

Practical quantum repeaters based on trapped ions

Quantum memory lifetimes of longer than 30 minutes can be realized in trapped ions. In addition, low (< 10-3) errors per entanglement swapping operation can be reached, which is crucial for implementing efficient quantum repeaters. The remaining challenge is to couple ions efficiently to telecom photons to realize entanglement distribution between repeater stations separated by approximately 30 km. We plan to efficiently couple memory ions to photons in the telecom range through intermediate ions that are suitable for transduction. Efficient coupling of ions to photons in a miniaturized high quality fiber cavity may eventually enable entanglement over terrestrial (1000s of km) distances that can persist for tens of minutes.

Electromagnetically Induced Transparency Quantum Memory

Both warm and cold quantum memory based on electromagnetically induced transparency (EIT) in cesium atomic ensembles. Using EIT, can store photons in warm ensembles for several µs before being released on demand. Current trend includes implementing cold ensembles via a magneto-optical trap (MOT) to achieve longer storage times. Noise is a big challenge for EIT because a very strong pump beam is aligned (spatially and spectrally) with the single photon beam. New techniques to remove the strong beam while preserving the single photon beam. With EIT quantum memory it is possible to implement a highly accurate spectrometer.