'Optics table on a chip' could help solve computing problems

The tunable superconducting circuit can place a single microwave photon (particle of light) in two frequencies, or colours, simultaneously.

‘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 setups,’ said José Aumentado, a physicist at the US National Institute of Standards and Technology (NIST).

The device is essentially 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.

However, 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).

The 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.

They 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.

‘This is a new way to manipulate microwave quantum states trapped in a box,’ Aumentado said.