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
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
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, email@example.com).