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 at the University of Illinois – has demonstrated a way to "purify" and restore maximally entangled states. "Entangled states tend not to remain pure, due to interactions with the environment," said Kwiat, a John Bardeen Professor of Electrical and Computer Engineering and Physics at the UI. "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, Kwiat and his colleagues – physics professor Nicolas Gisin and graduate student Andre Stefanov (both from the University of Geneva) and Salvador Barraza-Lopez, an undergraduate student from Mexico – 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. Then, the researchers implemented 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, the researchers reported. 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," Kwiat said. "The final signal is not as strong – because there are fewer photons – but the noise is clearly reduced."
