An entangled state of two quantum bits (qubits) can be created and stabilized using interactions that are normally thought to be detrimental, according to two international groups of researchers. While the groups studied two different physical systems, they have both shown that an entangled quantum system can be produced and maintained by coupling it to an environment that dissipates energy.
Entanglement is a purely quantum-mechanical phenomenon and that plays a crucial role in quantum-information systems. It allows two particles, such as photons or electrons, to have a much closer relationship than predicted by classical physics, and it is this relationship that could be exploited in quantum computers and cryptography. Physicists are keen to create and maintain entangled states in a variety of quantum systems, but a common problem that arises while creating these states is decoherence – the destruction of entanglement that occurs when a quantum system interacts with its surroundings. To avoid this, researchers try to keep an entangled system completely isolated from its environment – a difficult task.
But in recent years, new theoretical work has shown that the dissipative interactions with the environment could instead be used to preserve coherence, and the method has already been used to stabilize the state of a single qubit. Now though, the two research groups have gone one step further by actually using dissipation to create and sustain an entangled state in two different systems – one team uses ions, while the other uses superconducting qubits.
Anders Sørensen and Florentin Reiter at the University of Copenhagen, along with Nobel laureate David Wineland, John Gaebler, and colleagues at the National Institute of Standards and Technology (NIST) in the US produced a stable entangled state between two trapped beryllium ions. They used a set of tailored interactions including "sympathetic cooling" by two magnesium ions in the same trap.
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