Optical quantum memory in atomic ensembles

Short answer: Laboratory papers report optical quantum memories in atomic ensembles with efficiency above 50%, the level needed to beat the no-cloning limit without post-selection. A 2016 cold-atom experiment reports up to 87% efficiency and a 1/e coherence time up to 1 ms, and a 2017 paper reports 65% storage of single photons. These are single-memory experiments, not network deployments.

Efficiency Thresholds and No-Cloning Limits

A critical benchmark for optical quantum memory is a total storage and recall efficiency above 50%. This threshold is necessary to beat the no-cloning limit without post-selection. It is also a requirement for error correction in specific photonic quantum computing protocols. While earlier experimental demonstrations achieved efficiencies greater than 50%, their coherence times at that efficiency level were limited to tens of microseconds. The 2016 cold-atom paper reports maximum efficiencies up to 87% for short storage times. For single photons, a 2017 paper reports a storage efficiency of 65%, which the authors say exceeds the 50% criterion that single-photon storage had not previously met.

Coherence Time and Fiber Loop Comparison

The practical implementation of long-distance quantum communication requires a memory with long coherence time. An ideal optical fiber loop exhibits a 50% loss over a duration of 100 microseconds. The authors of the 2016 paper say no universal quantum memory had beaten this time-efficiency limit until then. Experiments using gradient echo memory in cold atomic ensembles have reported a 1/e coherence time up to 1 millisecond. Experimental data from these cold atom systems demonstrates greater than 50% efficiency for storage times up to 0.6 milliseconds. The authors say this verifies quantum storage ability beyond the ideal fiber limit. The authors verified this using heterodyne tomography of small coherent states.

Cold Atom Implementations and Single Photons

Cold atomic ensembles, specifically laser-cooled rubidium-87 atoms, serve as a platform for high-efficiency storage. One study utilized an ultra-high optical depth ensemble to achieve 87% storage efficiency for times on the order of the pulse width. Another study integrated a highly controllable single-photon source with an optimized quantum storage based on electromagnetically induced transparency. This system demonstrated an optical storage of single photons with a 65% efficiency. The nonclassical characteristics of the storage were verified by the well-maintained nonclassical and single-photon nature of the retrieved photons. These results indicate that cold atomic systems can handle single-photon inputs while maintaining quantum properties.

Warm Vapour Systems and Multi-Pulse Storage

Warm rubidium vapour systems offer an alternative platform for coherent optical memory. Experiments using the Gradient Echo Memory scheme in warm vapour have shown 87% efficient recall of light pulses. The 2011 authors describe this as the highest efficiency measured to date for any coherent optical memory suitable for quantum information applications at that time. The system demonstrated the storage and recall of up to 20 pulses. The Gradient Echo Memory scheme relies on an external field that induces a linear atomic frequency gradient in the direction of propagation. This gradient encodes the spectral components of the input light linearly along the length of the cell. Recall is achieved by reversing the field gradient without the use of pi-pulses.

FAQ

What efficiency is required to beat the no-cloning limit?

A universal quantum storage requires a storage efficiency above 50% to beat the no-cloning limit. This threshold is also necessary for error correction in certain photonic quantum computing protocols. Achieving this level without added noise allows the memory to outperform classical limits.

How long can cold atom memories store information?

Cold atomic ensemble experiments have reported a 1/e coherence time up to 1 millisecond. Greater than 50% efficiency has been demonstrated for storage times up to 0.6 milliseconds. This duration surpasses the 100-microsecond loss limit of an ideal optical fiber loop.

Can warm vapour systems store multiple pulses?

Yes, warm rubidium vapour systems have demonstrated the storage and recall of up to 20 pulses. These systems utilize the Gradient Echo Memory scheme to manipulate stored information. They have achieved 87% efficient recall of light pulses, showing potential as a quantum memory platform.

How is single-photon storage verified?

Single-photon storage is verified by checking the nonclassical characteristics of the retrieved photons. Experiments have shown that the retrieved single photons maintain their nonclassical and single-photon nature. This confirms that the storage process preserves the quantum properties of the input light.

What mechanism does Gradient Echo Memory use?

Gradient Echo Memory uses an external field to induce a linear atomic frequency gradient. This gradient encodes the spectral components of input light linearly along the cell length. Recall is achieved by reversing the field gradient, allowing for spectral compression and dispersion control of stored pulses.