“Optical switch” could make quantum computers more stable
CAMBRIDGE, Mass. – An optical switch
that allows one photon to control the quantum state of another could yield a quantum
computer that offers stability and control.
Quantum particles have an odd property
that enables them to be in “superposition”;
i.e., they can be in two states at the same
time. For instance, if a single photon were
fired at a barrier with two slits in it, it
would pass through both of them. Bits in
ordinary computers are represented as either 0 or 1, but a bit made from a quantum
particle – a qubit – could represent both 0
and 1 at the same time. For this reason, a
string of only 16 qubits could represent
64,000 numbers simultaneously.
One difficulty in building quantum
computers, however, is that superpositions
of states can be very fragile. Any environmental interactions could cause a subatomic particle to snap into just one of its
possible states. Although photons are
much more resistant to outside influences
than subatomic particles, they also are
much more difficult to control.
“We have long been interested in the
question if and how it is possible to make
single photons interact with one another,
since this would enable many new quantum devices to be used for secure quantum
communication, photonic quantum circuits
and, ultimately, quantum computers,” said
Vladan Vuletic, the Lester Wolfe Professor
of Physics at MIT.
To achieve this, MIT and Harvard University scientists developed an optical
switch consisting of a small cluster of cesium atoms suspended between two tiny
mirrors in a vacuum cavity. When a photon enters the cavity, it bounces back and
forth between the mirrors, delaying its
emission on the other side. If another photon has already struck the cesium atoms,
then each pass through them delays the
second photon even more. Although a
delay by a single pass through the atom
would be imperceptible, the mirror-lined
cavity enabled the scientists to pass the
photon many times through the atoms – in
their case, 40,000 times.
In the setup for vacuum-induced transparency,
an atomic gas sensor is trapped between two
high-quality mirrors (top and bottom), forming
an optical resonator. The absorption of a probe
beam incident onto the atoms from the side can
be reduced by the vacuum field of the resonator
if it is tuned to an atomic transition. Courtesy
of Haruka Tanji, Harvard/MIT.
