<!--intro-->Scientists at the U.S. Department of Energy's Los Alamos National Laboratory have taken another step forward in the quest for a quantum-based computer by demonstrating the existence of a physical state immune to certain types of information-corrupting "noise," which could otherwise disrupt computations based on quantum states. The research appears in a recent issue of the journal Science.<!--/intro--> Quantum computation promises greatly advanced speeds for solving certain mathematical problems, including database searching and number factoring. The essential phenomenon that the Los Alamos team demonstrated is a state in what is called a "decoherence-free subspace." The researchers showed this state's existence using entangled photons, paired particles of light whose conditions are intimately linked. "Our results are a proof-of-principle experiment that will hopefully motivate other quantum researchers to explore this subspace," said Los Alamos' Paul Kwiat, leader of the research effort. "Separate theoretical research has shown that it should be possible to perform quantum computation operations without taking the system out of the decoherence-free subspace, but until now the confirmation of this phenomenon has been lacking." In quantum physics, individual particles have no precise location and can coexist in more than one place at a time. Even distantly separated particles can share nonlocal correlations in a relationship known as "quantum entanglement." The entangled particles in the Los Alamos study are photons the basic particles of light engineered in such a way that they always have correlated polarizations. Polarization is the direction in which the photon's electric field vibrates. A simplified, classical analogy of the concept of entangled photons is two coins, each equally likely to give heads or tails, that somehow always give the identical result, even if they are very far apart and flipped by different people. The experimentation demonstrating the existence of a decoherence-free subspace was facilitated by work at Los Alamos in developing an improved method for creating entangled photons. The entangled photon pairs created by Kwiat's team are produced using two thin, nonlinear optical crystals to split the "parent" photons from a laser into entangled "daughter" photons. In previous research at Los Alamos, these entangled photons have been used for quantum cryptography to create unbreakable cryptographic keys that can be used to lock or unlock encrypted messages. Decoherence is a problem in quantum systems because the fragile quantum superpositions of entangled states are destroyed by unwanted coupling to the environment through which the photons are passing. Decoherence in Kwiat's system is intentionally created by passing the entangled photons through a roughly 10 millimeter piece of quartz. This optical environment produces a collective decoherence in the photons where one particular entangled photon state is, as predicted by quantum theory, essentially decoherence-free. These photons could serve as the basis of information carriers for quantum communications. "If one existed, a quantum computer would be extremely powerful; building one, however, is extremely challenging," said Kwiat. "Our demonstration of the existence of decoherence-free subspace is a small, but not insignificant step toward the goal of large-scale quantum computation." Kwiat and his team are part of a collective effort at Los Alamos to unravel the mysteries of quantum physics with teams working in all major areas of quantum information processing and quantum cryptography. Work leading to the demonstrated existence of decoherence-free subspace was funded in part by the National Security Agency and the Advanced Research and Development Activity Program.