Tuesday, March 15, 2011

Using quantum methods to read classical memories offers surprising advantages

March 14, 2011 by Lisa Zyga

Quantum physicist Stefano Pirandola from the University of York, UK, has published the study on the quantum readout of classical memories in a recent issue of Physical Review Letters.

"This is the first demonstration showing that the use of nonclassical light is beneficial for the readout of digital memories, reminiscent of current optical storage devices," Pirandola told PhysOrg.com.

As Pirandola explains in his study, there is an important difference between classical light – the light that is used in practically all of today’s technology applications – and quantum light. In classical light, the states of the electromagnetic field can be decomposed as probabilistic sums of coherent states. In contrast, when this decomposition is not possible, the states of an electromagnetic field are considered to be nonclassical (quantum). Important examples of nonclassical states are those that are entangled, in particular those with Einstein-Podolsky-Rosen (EPR) correlations. When two modes of light are described by these kinds of entangled states, their position and momentum "quadratures" are extremely correlated with each other.

In the proposed method, a classical digital memory consists of many reflective cells, each of which has two possible reflectivities that represent the states 0 and 1 (the two values of a bit). To read the memory, light is irradiated on the cells, and a detector measures the reflected light to determine each cell’s state. Currently, classical light is used for these kinds of memories. However, when its energy is decreased, classical light can only retrieve a limited amount of information from each cell.

Quantum light, on the other hand, doesn’t face the theoretical limits that classical light does. Pirandola’s calculations showed that EPR transmitters (those that use quantum light) can retrieve much more information than classical transmitters in the regime of few photons. He calculates that the enhancement provided by quantum light can be quite large – even up to 1 bit per cell, which corresponds to the extreme situation where only quantum light can retrieve information, and classical light cannot retrieve any information at all. 

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