Four-way entanglement converted to light

Researchers at the California Institute of Technology have entangled four separate atomic memories, then read out the state by entangling four laser beams.

“The research represents an important achievement in quantum information science by extending the coherent control of entanglement from two to multiple – four – spatially separated physical systems of matter and light,” said Caltech.

The group previously entangled a pair of atomic quantum memories and coherently transferred entangled photons into and out of the quantum memories.

With two quantum memories, they are entangle or not.

“For multi-component entanglement with more than two subsystems there are many possible ways to entangle the subsystems,” said Caltech. “For example, with four subsystems, all of the possible pair combinations could be bipartite entangled but not be entangled over all four components; alternatively, they could share a global quadripartite entanglement.”

Separated by 1mm, four groups of around a million caesium atoms each were trapped in a magnetic field and laser-cooled to within a few hundred millionths of a degree above absolute zero.

Each group has atoms with internal up or down spins that are collectively described by a ‘spin wave’.

It is these spin waves that the Caltech researchers entangled among the four atomic ensembles using techniques demonstrated for two ensembles in 2005.

In the current experiment, entanglement was stored in the atomic ensembles for a variable time, and then read out by transferring it to four laser beams.

“The emitted light from each atom in an ensemble constructively interferes with the light from other atoms in the forward direction, allowing us to transfer the spin wave excitations of the ensembles to single photons,” said Caltech graduate student Akihisa Goban.

The researchers coherently moved the quantum information from the individual sets of multipartite entangled atoms to four entangled beams of light, said the university, forming the bridge between matter and light that is necessary for quantum networks.

“Our work introduces new sets of experimental capabilities to generate, store, and transfer multipartite entanglement from matter to light in quantum networks,” said Caltech graduate student Kyung Soo Choi. “It signifies the ever-increasing degree of exquisite quantum control to study and manipulate entangled states of matter and light.”

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