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Quantum optics

Quantum optics

Trapped ions smash entanglement record

07 Apr 2011
Entangled ions in a row

Physicists in Austria have smashed the record for entangled quantum bits – or qubits – that could one day form the basis of a quantum computer. Breaking their previous record of eight entangled calcium-ion qubits, the researchers entangled 14 calcium-ion qubits.

Quantum computing exploits the peculiar laws of quantum physics to process certain calculations, such as searching or factorizing, much faster than any of today’s computers. Whereas conventional bits of information can take only the values 0 or 1, a quantum computer’s qubits exist in a mixed-up superposition of both. This uncertainty allows any number of qubits, N, to be lumped together – or “entangled”, in quantum speak – to represent a huge 2N working dimensions, and then processed in parallel.

In 2005, a group of researchers led by Rainer Blatt at the University of Innsbruck in Austria set a new record by entangling eight qubits formed of calcium ions in an electromagnetic trap. That alone represents 28 or 256 dimensions and could potentially allow a calculation that would take a week on a classical computer to be performed in a few seconds.

16,384 dimensions

Now, Blatt’s group has again broken the entanglement record, this time with 14 qubits, or an equivalent of 16,384 dimensions. “If one wants to calculate the dynamics of such a system, it’s like simulating the bouncing of a ball in 16,384 dimensions,” says Thomas Monz, a member of Blatt’s group. “Such calculations on a classical computer are still possible but, depending on the quantum system under investigation, can take quite a long time to calculate… For current supercomputers, simulations [are limited] to approximately 43 qubits.”

The Innsbruck team performed the entanglement by manipulating the 14 calcium ions with laser light inside an electromagnetic trap. When they shone the laser on the particles, their spins became correlated in clockwise and anticlockwise directions, forming a coherent unit.

“This is a very beautiful experiment, which shows the mastery of the Innsbruck group in the manipulation of complex quantum systems,” says Serge Haroche, a quantum physicist at the Collège de France, in Paris.

Unwelcome side-effect

Yet, Blatt’s group did find an unwelcome side-effect of the entanglement. Usually, the entanglement decay in a linear fashion, which means that outside noise will destroy the entanglement in a process of “decoherence” at a speed proportional to the number of qubits. Double the number of qubits, and decoherence will act twice as fast. However, the researchers found that in their system the speed of decoherence is proportional to the square of the number of qubits – in other words, it’s much faster.

This so-called ‘superdecoherence’ has been observed before, but not “in a system specifically aimed at realizing quantum computation,” Hennrich says. It could pose a challenge for researchers hoping to use many ions for quantum computation.

Still, the experiment does show that quantum-physics rules do apply, even for 14 particles. This is significant because many physicists, notably Erwin Schrödinger, worried whether there should be some sort of new physics that prompts the transition from quantum, the world of the small, to classical, the world of the big.

The work “shows that at the level of 14 entangled particles, there is no evidence of unknown physics producing the quantum-to-classical transition that Schrödinger and others have agonized about,” says Dietrich Leibfried, a quantum physicist at the National Institute of Standards and Technology in Colorado, US. “The decoherence of up to 14 entangled spins can be completely explained by the mundane sources of noise present in the current implementation.”

The research is published in Phys. Rev. Lett. 106 130506.

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