Physicists Create Artificial Molecule On A Chip

NEW HAVEN, Conn., Sept. 24 -- Using integrated circuit fabrication techniques, a team of researchers from Yale University has bound a single photon to a superconducting device engineered to behave like a single atom, forming an artificial molecule. It's the first experimental result in a field Yale professors Robert Schoelkopf and Steven Girvin have dubbed circuit quantum electrodynamics.

The researchers have constructed a miniaturized superconducting cavity whose volume is more than one million times smaller than the cavities used in corresponding current atomic physics experiments. The microwave photon is therefore "trapped," allowing it to be repeatedly absorbed and reemitted by the 'atom' many times before it escapes the cavity. The 'atom' is a superconducting circuit element containing approximately one billion aluminum atoms acting in concert.

In circuit QED experiments, a photon trapped between the transmission lines (tan) couples to the artificial atom, or qubit (purple). The base of the qubit is about 9 microns long. (Photo: D. Schuster and L. Frunzio, Schoelkopf Group, Yale University)CAPTION

The superconducting devices can be operated as qubits (quantum bit or "artificial atom"), the basic element of information storage in the field of quantum computing. In the Sept. 9 issue of the journal Nature, Andreas Wallraff and his colleagues present telltale evidence that their qubit was coupling to a microwave photon, sharing energy in much the same way electrons are shared when two atoms combine to form a molecule. They offered two suggestions for naming the new, combined state: phobit or quton.

Qutons have been made before, the first about 12 years ago. But by using artificial atoms for their qubits instead of real ones and microwave transmission lines instead of optical cavities, the Yale physicists were able to shrink a roomful of experimental apparatus onto a chip less than one square centimeter (or less than ? square inch) in size. They have also improved the coupling between resonator and "atom" by a factor of about 1000, which will help them explore fundamental interactions of light and matter. Soon they will try to control several qubits on one chip, using photons to connect them together in a prototype architecture for quantum computing and quantum cryptography.

This represents a new paradigm in which quantum optics experiments can be performed in a micro-chip electrical circuit using microwaves instead of visible photons and lasers. Because of the tiny cavity volume and large 'atom' size, the photon and 'atom' are very strongly coupled together and energy can be rapidly exchanged between them. Under the peculiar rules of quantum mechanics, the state of the system becomes a coherent superposition of two simultaneous possibilities: the energy is either an excitation of the atom, or it is a photon. It is this superposition that was observed in the Yale experiment.

In addition to allowing fundamental tests of quantum mechanics and quantum optics in a completely new format, this new system has many desirable features for a quantum computer. In a quantum computer the bits of information are replaced by qubits (e.g., an atom), which, paradoxically, can harness quantum uncertainty to vastly speed up certain types of calculations. The ability to couple qubits to photons, demonstrated by the Yale group, could allow qubits on a chip to be wired together via a "quantum information bus" carrying single photons.

For more information, visit: www.eng.yale.edu/rslab/cQED