STANFORD, Calif., May 19, 2008 -- What was once considered more theoretical than practical, computers based on the powerful properties of quantum mechanics now have the potential to revolutionize information technology and security.
Engineers and physicists from Stanford and the University of California-Santa Barbara have demonstrated a progenitor of an essential component of quantum computers, a logic gate, which enables interaction between just two particles of light.
The Stanford team, led by electrical engineering assistant professor, Jelena Vuckovic, attributes the key advance to a solid-state device that can reliably produce an interaction between photons. The team nestles a tiny ball of indium arsenide molecules (quantum dot) within a cavity on a photonic crystal. A chip of semiconducting gallium arsenide is precisely drilled with holes, giving it the ability to trap photons so they interact with the quantum dot.
Jelena Vuckovic, assistant professor of electrical engineering, with her team in the Nanoscale and Quantum Photonics Lab. Her team includes, from left, doctoral students Ilya Fushman, Dirk Englund and Andrei Faraon.
"We have demonstrated a system composed of a single quantum dot in a cavity that can be used to realize such a gate, and we demonstrated that two photons can be made to interact with each other via this system," says Stanford applied physics doctoral student Ilya Fushman. "So we showed that such a gate is possible and demonstrated the first necessary steps in that direction."
According to Vuckovic, previous demonstrations were done only with systems that required complicated atom-trapping techniques that were not as practical as the semiconductor chip implementation because they would be difficult to extend to the hundreds or thousands of logic nodes required for a quantum computer.The new devise, however, is made with materials and manufacturing processes that are familiar to computer chip makers.
In computing, a logic gate is built to accept a set of inputs and, depending on their properties, provide a specific output. In the binary logic found in today's electrical computers, a certain gate will yield a “1” only if all of its inputs are “1”s. Otherwise it will yield a “0.” Similarly, a quantum photonic gate would work by detecting the properties of input photons from two light beams, called "control" and "signal," and then producing an output based on those by flipping the polarization of one of the input photons.
In their experiment, the researchers shined two beams of photons upon the quantum dot. When a photon from the signal beam struck the dot alone, it was re-emitted without modification. If a photon from the control beam got there first, then the amount of time that the photon from the signal beam spent in the cavity changed. That difference in time, called a phase shift, can be mapped to a difference in photon polarization.
According to Vuckovic, the team has demonstrated that when the two photons are identical, a phase shift of 12.6 degrees is achieved. This is only a fraction of the 180-degree rotation required to make a full logic gate, but by combining several of the devices in a row, her team expects to attain the needed effect. When the signal and control photons are allowed to differ, the phase shifts can be up to 45 degrees.
Other challenges include eliminating manufacturing imperfections and reliably placing the quantum dots right where they need to be within the crystals, but the team is optimistic.
"We are hopeful that these engineering challenges can be overcome to open the path to chip-based, high-fidelity quantum logic with photons," says Vuckovic.
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