Optics Probe Quantum Catalysis

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 + α²)^–1^/²(t|0> + α1>), 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.