Quantum computing at room temps moves closer to reality

Technology News | May 4, 2020

By Rich Pell

Digital Signal Processing Optoelectronics Materials & processes Memory & Data Storage

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 on two qubits together, and can create “quantum entanglement” between them. This entanglement is automatically generated in a quantum computer, and is required for quantum approaches to applications in sensing.

Such an application of using nonlinear optical crystals to make quantum logic gates, say the researchers, had remained in question until they presented a way to realize a quantum logic gate with this approach using established photonic circuit components.

“The problem was that if one has a photon travelling in a channel, the photon has a ‘wave-packet’ with a certain shape,” says Dr. Kurt Jacobs, of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “For a quantum gate, you need the photon wave-packets to remain the same after the operation of the gate. Since nonlinearities distort wave-packets, the question was whether you could load the wave-packet into cavities, have them interact via a nonlinearity, and then emit the photons again so that they have the same wave-packets as they started with.”

Once they designed the quantum logic gate, the researchers performed numerous computer simulations of the operation of the gate to demonstrate that it could, in theory, function appropriately. Actual construction of a quantum logic gate with this method, say the researchers, will first require significant improvements in the quality of certain photonic components.

“Based on the progress made over the last decade, we expect that it will take about ten years for the necessary improvements to be realized,” says Heuck. “However, the process of loading and emitting a wave-packet without distortion is something that we should able to realize with current experimental technology, and so that is an experiment that we will be working on next.”

For more, see “Controlled-Phase Gate Using Dynamically Coupled Cavities and Optical Nonlinearities.”