Daniel S. Burgess
In the everyday world, if you stick a $5 bill
and a $10 bill in your wallet, and later pull out the $5, you can
expect the other still to be $10. In the weirdness of the quantum
world, of course, things are far less intuitive. A team at Universität
Konstanz and at Humboldt Universität in Berlin, both in Germany, has
described an experiment that is analogous to pulling out the $5 bill
and finding that the $10 has become something very different.
A Ti:sapphire laser and a beamsplitter expose the weirdness of the
quantum world. Researchers at Universität Konstanz and at Humboldt
Universität have investigated how the interaction of two classical
states could generate a highly nonclassical coherent superposition of
states. Courtesy of Alexander I. Lvovsky.
The work, which is part of a larger effort to develop basic tools for
quantum applications, focused on the entangling properties of a
beamsplitter. In it, a single-photon Fock state and a coherent state
are incident on a beamsplitter with a reflectivity of 0.925. The
researchers discovered that, although the former emerged unchanged, the
latter transformed into the coherent superposition of the vacuum and
single-photon Fock states.
Alexander I. Lvovsky of Universität Konstanz compared the phenomenon to
a catalytic reaction in chemistry. "Catalysis is a type of chemical
reaction which is facilitated by a certain agent (a catalyst), which
is, however, not consumed in the reaction. We have shown how a similar
principle works in quantum optics. A catalyst -- a single photon --
facilitates conversion of a classical, coherent state of light into a
highly nonclassical 'Schrödinger's kitten,' a coherent superposition of
microscopic states, without being lost."
The researchers prepared the single-photon Fock states |1> by the
down-conversion in BBO of 1.6-ps pulses from a Spectra-Physics
Ti:sapphire laser. The laser also produced the target coherent state
|1>. A PerkinElmer single-photon detector monitored the Fock state
photon's output channel from the beamsplitter and signaled a homodyne
detector to measure the other channel for the final state of the
target, which the researchers found to approximate (t2 + *2)
21/2 (
t|0> +
*|1>), where
t2 is the transmission of the beamsplitter.
Computing applications
Lvovsky, whom the Deutsche Forschungsgemeinschaft awarded an Emmy
Noether grant to fund his group's research, said that, although the
phenomenon is remarkable in its own right, the work has a practical
purpose. "This experiment is an implementation of the first of two
stages that compose an elementary gate in the Knill-Laflamme-Milburn
linear optics quantum computation proposal." To continue the work, the
group hopes to exploit the nonlocal properties of single photons to
implement quantum teleportation.