Currently such circuits require extremely cold temperatures - close to zero Kelvins - to prevent their special states from being destroyed by interacting with the computer's environment. If future devices use quantum technologies that require such cooling to very cold temperatures, say the researchers, then this will make them expensive, bulky, and power hungry.
One of the most likely alternative paths to quantum computing with solid-state systems at room temperatures is seen as the application of transparent crystals with optical nonlinearities. But, say the researchers, the plausibility of such a system had remained in question until now, when they recently became the first to demonstrate the feasibility of a quantum logic gate comprised of photonic circuits and optical crystals.
"Photonic circuits are a bit like electrical circuits, except they manipulate light instead of electrical signals," says Prof. Dirk Englund of the Massachusetts Institute of Technology, who along with colleague Dr. Mikkel Heuck collaborated with the Army scientists in the research. "For example, we can make channels in a transparent material that photons will travel down, a bit like electrical signals traveling along wires."
Unlike quantum systems that use ions or atoms to store information, quantum systems that use photons can bypass the cold temperature limitation. However, the photons must still interact with other photons to perform logic operations. This, say the researchers, is where the nonlinear optical crystals come into play.
Through a method that involves engineering cavities in the crystals that temporarily trap photons inside, the quantum system can establish two different possible states that a qubit can hold: a cavity with a photon (on) and a cavity without a photon (off). These qubits can then form quantum logic gates, which create the framework for the strange states.
In other words, say the researchers, they can use the indeterminate state of whether or not a photon is in a crystal cavity to represent a qubit. The logic gates act