Synopsis: A Lens to Focus Spins

A quantum bit stored in the spin excitation of an atomic cloud could be “focused” onto the quantum state of a single atom.

A future quantum computer might use photonic qubits to move information around, but the qubits need to be transferred to a stationary medium such as atoms to perform computations. Given the weakness of the light-matter interaction, the transfer often requires a large ensemble of atoms, which can collectively take on a single photonic qubit. However, standard quantum logic operations are impractical with large ensembles and work best with one atom per qubit. Now a team of theorists has proposed techniques to focus a quantum state spread over many atoms down to nearly a single atom. These techniques could allow multiple photonic qubits to be transferred into a quantum memory made of single-qubit atoms.

Alexander Glaetzle of the University of Oxford in the UK and his colleagues analyze an array of atoms collectively imprinted with the spin state of a photon. The array could be 1D, 2D, or 3D. They show theoretically that if lasers force the interaction between nearest neighbors to have a certain form, then the width of the wave function will shrink over time to a region nearly as small as the spacing between atoms. The team goes on to describe the specific case of Rydberg atoms—highly excited, neutral atoms often used as qubits.

The team presents procedures for a few different kinds of focusing situations. For example, a collective spin excitation of a 2D array of atoms could be focused to an entangled pair of atoms. Or a superposition of several spin states of an atomic ensemble could be focused in a way that separates out the individual states to distinct atoms, allowing them to be manipulated independently with lasers.

This research is published in Physical Review X.

--David Ehrenstein

David Ehrenstein is the Focus Editor for Physics.


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