Quantum information processing may no longer face entanglement problems

Entanglement, the bizarre quantum-mechanical connection that can exist between particles, is an essential component in many quantum-information-processing applications, such as quantum computation, teleportation, and cryptography. But the connection between the particles can become noisy or dirty, degrading the quality of the entanglement and rendering it useless for quantum information processing.

As reported in the Feb. 22 issue of the journal Nature, a team of researchers led by Paul Kwiat, now a John Bardeen Professor of Electrical and Computer Engineering and Physics at the University of Illinois (Champaign, IL), has demonstrated a way to purify and restore maximally entangled states.

�Entangled states tend not to remain pure, due to interactions with the environment,� says Kwiat. �Just as the connection between two cell phones can become clouded with static and must be filtered, we have implemented a technique that cleans up the static in entangled systems.�

The concept of entanglement is perhaps the most bizarre feature of quantum mechanics, a generally accepted theory that replaces classical mechanics for microscopic phenomena. In quantum mechanics, the properties of entangled photons, for example, are inextricably linked to each other--even if the photons are located on opposite sides of the galaxy. In this sense, quantum mechanics is said to be nonlocal.

In work performed at the Los Alamos National Laboratory (Los Alamos, NM), Kwiat and colleagues Nicolas Gisin and Andre Stefanov (both from the University of Geneva) and researcher Salvador Barraza-Lopez investigated entanglement distillation and hidden nonlocality. First, the researchers created pairs of polarization-entangled photons by passing a laser pulse through two adjacent nonlinear crystals. By varying the linear polarization of the laser pulse, they could change the degree of entanglement. It was then possible to implement a simple distillation procedure to filter out a smaller, but cleaner, subset of entangled photons.

�We basically used the procedure to throw away the unwanted part of the contribution, and what remained was in a perfectly entangled state,� Kwiat said. �In this way, we demonstrated distillation of maximally entangled states from non-maximally entangled inputs.�

When applied to partially mixed states, the distillation procedure generated states that subsequently demonstrated nonlocal correlations, even though the initial states did not. Such hidden nonlocality had been postulated, but never before seen.

�What�s remarkable is that our filtering procedure is a local process, performed individually on only one photon of a pair, and yet the nonlocal nature of quantum entanglement is preserved,� adds Kwiat. �Because there are fewer photons, the final signal is not as strong, but the noise is clearly reduced.�