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Quantum Divorce: When Entanglement Doesn't Work Out

A phenomenon called entanglement sudden death can break critical quantum links



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It sounds like a tiebreaker round in a mixed martial arts bout: entanglement sudden death, or ESD. In actuality it is a mysterious phenomenon by which entangled quantum objects—two electrons, for example, whose properties are linked by some instantaneous connection across space—can suddenly break off their relationship.

In classical physical systems, decay is usually asymptotic—that is, the correlation between two objects approaches, but never reaches, zero. But ESD does not follow this pattern—when it strikes, the degree of entanglement drops to zero in a finite time, severing the quantum link clean.

In a review paper in this week's Science, physicists Ting Yu of the Stevens Institute of Technology in Hoboken, N.J., and Joseph Eberly of the University of Rochester address the phenom and what it means for the future of quantum computing. (They wrote a similar paper in the same journal in 2007 as a commentary on a concurrent study that was among the first to experimentally verify the existence of ESD.) Essentially, Yu and Eberly wrote in this week's paper, ESD remains a mystery, and one that might cause problems in developing quantum systems that rely on entanglement, such as the transmission and storage of information.

In particular, ESD can arise when two sources of environmental "noise" act to disrupt an entangled state. Each source would individually induce a more gradual asymptotic decay, but in tandem they can trigger ESD. This might have practical implications in quantum computing, as ours is a world of constant fluctuation—even a vacuum is alive with vibrations of energy. "In reality," Yu says, "multiple noises are almost universal."

Quantum memory—the long-term storage of information in quantum bits, or qubits—will be most vulnerable to ESD, Yu says, since more transitory processes could theoretically be sped up enough to be completed before ESD kicked in.

Christopher Monroe, a physicist who heads a leading quantum information group at the University of Maryland, College Park, believes that the speed of practical quantum systems will obviate most ESD concerns. "I see this concept as an interesting new angle in describing entanglement," he says, "although it probably does not impact or add roadblocks to the quest to build large entangled states for applications in quantum information science and fundamental quantum physics."

If viable quantum computers or information-processing systems are to be built, Monroe notes, "we need nearly perfect entangled states," adding that the strength of such entanglements would likely preclude ESD. "The [upper] thresholds for sudden-death entanglement are way below the more stringent thresholds we are currently aiming for," he says.

At the same time, "the concept of quantum entanglement remains murky," Monroe says, and concepts such as ESD "hopefully can help us get a better glimpse at what entanglement is and means."

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