7 Jul 2011 Gaithersburg - Researchers at the National Institute of Standards and Technology (NIST) have created a tunable superconducting circuit on a chip that can place a single microwave photon (particle of light) in two frequencies, or colours, at the same time.
This curious "superposition", a hallmark of the quantum world, is a chip-scale, microwave version of a common optics experiment in which a device called a beam-splitter sends a photon into either of two possible paths across a table of lasers, lenses and mirrors. The new NIST circuit can be used to create and manipulate different quantum states, and is thus a prototype of the scientific community's long-sought "optics table on a chip".
Described inNature Physics, the NIST experiments also created the first microwave-based bit for linear optical quantum computing. This type of quantum computer is typically envisioned as storing information in either the path of a light beam or the polarization (orientation) of single photons. In contrast, a microwave version would store information in a photon's frequency. Quantum computers, if they can be built, could solve certain problems that are intractable today.
The new NIST circuit combines components used in superconducting quantum computing experiments - a single photon source, a cavity that naturally resonates or vibrates at particular frequencies, and a coupling device called a SQUID (superconducting quantum interference device). Scientists tuned the SQUID properties to couple together two resonant frequencies of the cavity and then manipulated a photon to make it oscillate between different superpositions of the two frequencies. For instance, the photon could switch back and forth from equal 50/50 proportions of both frequencies to an uneven 75/25 split. This experimental setup traps photons in a "box" (the cavity) instead of sending them flying across an optical table.
"This is a new way to manipulate microwave quantum states trapped in a box", stated NIST physicist José Aumentado, a co-author of the new paper. "The reason this is exciting is it's already technically feasible to produce interesting quantum states in chip-scale devices such as superconducting resonators, and now we can manipulate these states just as in traditional optics set-ups."
NIST researchers can control how the new circuit couples different quantum states of the resonator over time. As a result, they can create sequences of interactions to make simple optical circuits and reproduce traditional optics experiments. For example, they can make a measurement tool called an interferometer based on the frequency/colour of a single photon, or produce special quantum states of light such as "squeezed" light.
E. Zakka-Bajjani, F. Nguyen, M. Lee, L.R. Vale, R.W. Simmonds and J. Aumentado are the authors of "Quantum superposition of a single microwave photon in two different 'colour' states" published inNature Physicsand posted on-line on July 3, 2011.