by M. Sellars

06 September 2010
from NatureAsia Website


A quantum leap in efficiency brings quantum information processing a step closer to reality.

Author affiliation
1. Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia
2. Jack Dodd Centre, Physics Department, University of Otago, Dunedin 9016, New Zealand
3. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China


Light fulfils a fundamental role in communicating information, from the objects we see to the radio we hear and the data we pass between computers on optical fibers.


Storing light as a form of data, however, has proved to be a difficult task. Scientists have managed to transfer the quantum information of a light field to an atomic gas, but the recall of information has been far below the threshold required for practical application of the technology.


Researchers at the Australian National University in collaboration with scientists at the University of Otago in New Zealand and Shanxi University in China 1 have now developed a solid-state quantum system that offers four times the recall efficiency of previous systems.

Quantum memory is based on the transfer of quantum information from a light pulse to another system, such as an ultracold atomic gas or a warm atomic vapor. However, the best information retrieval efficiency so far achieved using such media is just 17%.

Looking for a more reliable technique, Morgan Hedges and colleagues used an yttrium-orthosilicate crystal imbedded with ions of the rare earth praseodymium. Auxiliary laser light and an electric-field produced by electrodes on either side of the crystal controlled the light absorption properties of the ions.


This system (Fig. 1) could store the quantum information of a light pulse and release it on demand with an efficiency of 69% after 1.3 μs.

Fig. 1

A solid-state quantum memory system stores quantum information

with efficient retrieval for up to 1.3 μs.


The technique works whether the light field is made up of just a single photon or as many as 500 photons. The researchers observed that only the photons that went into the crystal came out again.

“It would only take a few strays to render the device useless,” explains Hedges.

Efficiency of 69% may not seem very high, but storage of a quantum state need not be perfect. Error-correction techniques can salvage a state so long as losses amount to less than 50%.


This is the first time this all-important 50% threshold for quantum memory has been exceeded.

“In the future, we hope to grow crystals made of the rare-earth ions themselves,” says Hedges. “This will simplify the process of crystal preparation.”



1. Hedges, M.P.,1 Longdell, J.J.,2 Li, Y.3 & Sellars, M.J.1 Efficient quantum memory for light. Nature 465, 1052 (2010).