A tiny, tunable superconducting circuit can place a single microwave photon in two frequencies simultaneously, potentially leading to the long-sought “optics table on a chip.” Researchers at the National Institute of Standards and Technology have developed the chip-scale device, a microwave version of a common optics experiment in which a beamsplitter sends a photon into either of two possible paths across a table of lasers, lenses and mirrors. The experiment also created the first microwave-based bit for linear optical quantum computing. It was described in the July 3 online edition of Nature Physics (doi: 10.1038/nphys2035). This type of quantum computer is envisioned as storing information in either the polarization of single photons or the path of a light beam. In contrast, a microwave version would store information in a photon’s frequency. NIST’s prototype “optics table on a chip” places a microwave photon in two frequencies at once. Courtesy of D. Schmidt, NIST. The newly developed circuit combines components that are 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). The NIST scientists tuned the SQUID properties to couple two resonant frequencies of the cavity. Next, they manipulated a photon to make it oscillate between different superpositions of the two frequencies. The photons switched back and forth from equal proportions of both frequencies to an uneven 75/25 split. The experimental setup trapped photons in the cavity instead of sending them across an optical table. Although it is already technically feasible to produce quantum states in chip-scale devices such as superconducting resonators, the scientists said they can now manipulate them just as they do in traditional optics setups. They can control how the new circuit couples various quantum states of the resonator over time. As such, they can create sequences of interactions to make simple optical circuits and reproduce traditional optics experiments. For example, they can make interferometers based on the frequency of a single photon, or produce special quantum states of light, such as “squeezed” light.