Entanglement Between a Photon and A Trapped Atom

Entanglement between a photon and a trapped atom has been directly observed for the first time, offering a method for establishing links between quantum memories over appreciable distances. Entanglement--a sort of arranged marriage between two or more particles--has usually been directly measured between species of the same kind, such as all photons or all atoms.

In recent experiments, however, University of Michigan researchers achieve inter-species entanglement by trapping a cadmium ion with electric fields. They put the trapped cadmium's outer electron into an excited (high-energy) state.

The atom immediately decays to one of two ground (low-energy) states--let's call them A and B--while emitting a photon. State A represents the case in which the spin of the atom's outer electron is lined up with the spin of the atom's nucleus; B represents the case in which the electron's spin is opposite to that of the nucleus. The photon's polarization--the direction of its electric field--correlates with the resulting ground state of the atom. In other words, if the atom decays to state A, the photon's electric field rotates clockwise, and if it decays to state B, counterclockwise.

Because each path is equally likely, quantum mechanics forces us to consider both decay routes as occurring at the same time. So once the atom decays, both it and the photon essentially carry out both possibilities--each enters a "superposition" of two states. Meanwhile, their properties remain interdependent--or correlated--with each another. As a result, the atom and photon are in an entangled superposition. While the individual participants are in fuzzy, unresolved states, the terms of their marriage are perfectly defined.

However, measuring the photon--the act of observing it--forces the photon to make a commitment. Upon measurement it must assume one polarization state or another--clockwise or counterclockwise. And this in turn forces the atom to collapse into state A (if the polarization is clockwise) or state B (if polarization is counterclockwise). One could conduct powerful logic operations based on these interdependencies.

This cross-species entanglement technique has shortcomings--researchers cannot actively create an entangled state but must wait for it to occur by detecting the photon, so the entanglement is immediately destroyed and efficiency is not high. However, if two remotely located trapped atoms simultaneously decay in the same way as reported in this experiment, and the two emitted photons are jointly detected after interfering on a beamsplitter, then the two atoms become entangled and available for subsequent use for long-distance quantum computing and quantum communication. (Blinov et al., Nature, 11 March 2004; contact Chris Monroe, crmonroe@umich.edu).