Single atom entangles two photons
Sam Wormley
http://physicsweb.org/articles/news/11/6/13/1
21 June 2007
Researchers in Germany and the UK are the first to entangle pairs of
photons using a single atom. Their technique is based on an optical
cavity containing one rubidium atom, and could provide a way to
process information in a quantum computer (Sciencexpress).
When two photons are entangled, the quantum state of one photon is
revealed automatically by measuring the state of the other -- a
property that is crucial to the operation of quantum computers.
Whereas ordinary computers use bits of information that are either 1
or 0, a quantum computer would use quantum bits of information, or
qubits, that can be in a superposition of both 1 and 0 at the same
time. A 1 could represent, say, a horizontally polarized photon,
while 0 represent a vertically polarized photon. By combining N such
qubits, these could entangled to represent 2N values at the same
time, which would, in principle, allow a quantum computer to
outperform a classical computer for certain tasks.
Some physicists believe that quantum computers could involve
entangled photons moving from node to node in an optical system
\u2013 with the nodes performing logical operations on the photons.
In order for such a computer to work, it would need a way of
transferring quantum information from the entangled photons to the
nodes and vice versa. It would also require a way of generating
entangled photon pairs and sending them off in the appropriate
directions. Little of this technology is available today and hence
quantum computation remains a distant dream.
Such nodes could consist of an atom trapped by a standing wave of
light in an optical cavity. However, researchers have so far only
been able to get such an atom to emit a single photon that is
entangled with the atom itself. Now, however, Gerhard Rempe and
colleagues at the Max Planck Institute for Quantum Optics in
Garching, Germany and Axel Kuhn at the UK's Oxford University, have
extended this technique to use a single atom to create an entangled
pair of photons.
The team first fired a laser pulse at a trapped atom, causing it to
emit a single photon. As a result of this process, the atom and the
photon are entangled. A microsecond or so later, a second laser pulse
was fired at the atom, causing it to emit a second photon. Crucially,
the second pulse cause the entanglement to be transferred from the
atom to the second photon, and the two photons become an entangled
pair.
Rempe told Physics Web that the photons could then be sent to
interact with two different atoms. As a result of this interaction,
the two atoms would become entangled with each other. This, he said,
could form the basis of a "quantum repeater", which is an essential
component of a quantum computer.
According to Rempe, an important benefit of the scheme is that an
entangled photon pair can be produced "at the push of a button",
unlike other methods, which create entangled pairs in random manner.
The researchers have managed to operate their scheme with a 1.3%
probability that a photon pair is entangled. While this is on par
with other entanglement schemes for quantum computing, the team is
currently working to better localize the atom inside the cavity,
which Rempe said will improve the efficiency of the system.