Physicists Take Next Step Toward Quantum Computing

Using artificial atoms on a chip, physicists from the Yale Institute for Nanoscience and Quantum Engineering (YINQE) have taken the next step toward quantum computing by demonstrating that the particle nature of microwave photons can now be detected, according to a report spotlighted in the Feb. 1 issue of the journal Nature.

Steven Girvin

Quantum theories generally apply best to processes that happen on the smallest scale of atoms and molecules. By making artificial atoms larger—to a size that is nearly visible—and using microwaves as the source of energy, collaborators from theory group of Steven Girvin, the Eugene Higgins Professor of Physics and Applied Physics, and the laboratory of physics professor Robert Schoelkopf created an electronic circuit that stores and measures individual microwave photons. This research, they say, brings quantum mechanics to a larger scale, and they hope to employ it to build new kinds of quantum machines.

The radiation from a microwave oven or cell phone does not seem to have much in common with light, says Schoelkopf, but like light, microwaves are made of individual photons.

“A single microwave photon is quite large, extending over one centimeter in length, and yet has 100,000 times less energy than even a visible photon,” he says. “Unlike a camera, which absorbs the light it detects, our measurement preserves the photons for later use.”

According to Fleury, “Such manipulation of single microwave photons is an important step towards realizing a quantum computer, which could exponentially speed up computations of difficult problems in cryptography, quantum physics and chemistry.”

David Schuster, a graduate student who recently completed this work as part of his thesis, says: “Much like the children’s game ‘telephone,’ current solid state quantum computing schemes can only make nearest-neighbor interactions. This forces distant quantum bits (qubits) to communicate by passing through many intermediates, causing errors. Single microwave photons can be used as mobile carriers of quantum information, allowing distant qubits to communicate directly and avoiding these problems.”

The measurements the scientists made represent the next step in circuit quantum electrodynamics, a field introduced by the same groups at Yale in 2004 to study quantum optics with microwaves using integrated circuits. According to Girvin, the detector they designed works “as if we made an antenna on an atom.” Their results demonstrate that microwaves are particles because the system gives a response representing a discrete number of interactions of the microwave with the atom.

In addition to circuits, microwaves interact with a variety of physical systems, including atomic spins, molecules, and even nuclei. Single microwave photons can act as a bridge between these naturally occurring quantum systems and fabricated electrical circuits, resulting in a hybrid processor of quantum information. The next phase of the work, according to the authors, is to connect multiple “atoms,” using the photons to transfer the information between them.

An interview with Schoelkopf is featured in a Nature Podcast online (see Feb.1, 2007 episode, beginning at minute 14:35). Supplementary information is available online at www.eng.yale.edu/rslab/projects/cQED.html.

Co-authors on the paper are Andrew A. Houck, Joseph A. Schreier, Andreas Wallraff, Jay M. Gambetta, A. Blais, Luigi Frunzio, J. Majer, B. Johnson and Michel H. Devoret, who are all affiliated with YINQE.

The research was supported by funds from the National Security Agency under the Army Research Office, the National Science Foundation, the W.M. Keck Foundation, a Yale Quantum Information and Mesoscopic Physics Fellowship, and Yale.