Photons on Demand

Will our near future be characterized by supercomputers and artificial atoms? Dr. Robert Schoelkopf, Professor of Applied Physics and Physics, and Dr. Steven Girvin, Deputy Provost for Science and Technology, believe so. Their latest project involves using artificial atoms to generate single microwave photons, called “photons ondemand,” in a circuit.

Their artificial atom is not truly an atom, but a collection of about 1 billion aluminum atoms working in concert. This artificial atom is about three-tenths of a millimeter long and sits inside a superconducting cavity from which single microwave photons are generated.

Since a microwave photon contains 10,000 times less energy than a visible light photon, and because microwaves are also used in cell phone technology, experiments involving the artificial atoms are performed at 0.02 degrees Kelvin in a shielded environment in the Schoelkopf lab.

The artificial atom is engineered to have quantized energy levels, similar to the energy levels that electrons occupy in atoms. When a certain amount of energy is applied in this cavity, it jumps to the next energy level. As that atom loses energy, it returns to the original energy level, and this loss of energy causes the fluorescence of a single microwave photon.

Regular computers operate using units of quantum information called qubits, which have values of either 0 or 1. When the atom in the cavity is excited, it enters a superposition of both 0 and 1, making the outcome unpredictable.

Information can be stored in this superposition and transmitted through a single microwave photon quickly and safely to a second artificial atom, which then acquires the same superposition state.

The present experiment carries out the first step of transferring the quantum information from the artificial atom to the photon field. The second step of transferring the information to a second qubit has been carried out in recent experiments with two artificial atoms in one cavity.

The next step, according to Girvin, would be to excite the atom in the cavity to another energy level, beyond the normal 0 and 1, to investigate the outcome or to try to transmit the quantum information to an atom in a separate cavity.