For the first time, a team of scientists from Yale University has successfully coupled a single photon to a single superconducting qubit (quantum bit or "artificial atom"). In "Yale scientists bring quantum optics to a microchip," you'll discover that it is now possible to perform quantum optics experiments in a micro-chip electrical circuit using microwaves instead of visible photons and lasers. This is another important step towards quantum computers where bits of data are replaced by qubits, or atoms. Because it's now possible to couple qubits to photons, this could allow qubits on a chip to be wired together via a "quantum information bus" carrying single photons. Read more...
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. The work is a collaboration of the laboratory of Professor Robert Schoelkopf and the theory group of Professor Steven Girvin in the Departments of Applied Physics and Physics at Yale University.
The Yale 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.
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.
The research work has been published by the journal Nature under the name "Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics." Here are two links to the abstract and to an early version of the full paper (PDF format, 4 pages, 79 KB). Here is the text of the abstract, written in almost plain English.
The interaction of matter and light is one of the fundamental processes occurring in nature, and its most elementary form is realized when a single atom interacts with a single photon. Reaching this regime has been a major focus of research in atomic physics and quantum optics for several decades and has generated the field of cavity quantum electrodynamics. Here we perform an experiment in which a superconducting two-level system, playing the role of an artificial atom, is coupled to an on-chip cavity consisting of a superconducting transmission line resonator. We show that the strong coupling regime can be attained in a solid-state system, and we experimentally observe the coherent interaction of a superconducting two-level system with a single microwave photon. The concept of circuit quantum electrodynamics opens many new possibilities for studying the strong interaction of light and matter. This system can also be exploited for quantum information processing and quantum communication and may lead to new approaches for single photon generation and detection.
You also might want visit the Schoelkopf Lab of Circuit Quantum Electrodynamics. Finally, if you're very interested by the work of these researchers, you might want to read a related work, "Coherent Coupling of a Single Photon to a Cooper Pair Box." Here is a link to this other paper (PDF format, 8 pages, 1.42 MB), which contains a very interesting illustration shown below.
||This shows an integrated circuit for cavity Quantum Electrodynamics (QED). (Credit: Link above)|
In the above image, you can see the superconducting niobium coplanar waveguide resonator (A) and how the capacitive coupling to the input and output lines is controlled by adjusting the length and separation of the finger capacitors formed in the center conductor (B). Finally, you can see the "false color electron micrograph of a Cooper pair box (blue) fabricated onto the silicon substrate (green) into the gap between the center conductor (top) and the ground plane (bottom) of a resonator (beige) using electron beam lithography and double angle evaporation of aluminum" (C).
Sources: Yale University news release, September 8, 2004; Nature Issue 431, Pages 162-167, September 9, 2004; and other Yale University web pages