Researchers see the way to quantum computing at room temperature

Army researchers predict that quantum computer circuits that will no longer need extremely cold temperatures to function could become reality after about a decade.

For years, solid-state quantum technology operating at room temperature seemed remote. Although the application of transparent crystals with optical nonlinearities had emerged as the most likely route to this milestone, the plausibility of such a system was always in doubt.

Now, Army scientists have officially confirmed the validity of this approach. Dr. Kurt Jacobs of the Army Research Laboratory of the US Army Combat Capability Development Command. In collaboration with Dr. Mikkel Heuck and Prof. Dirk Englund of the Massachusetts Institute of Technology, he became the first to demonstrate the feasibility of a quantum logic gate composed of photonic circuits and optical crystals.

“If future devices using quantum technologies require cooling to very cold temperatures, then this will make them expensive, bulky, and energy-hungry,” Heuck said. “Our research aims to develop future photonic circuits that can manipulate the entanglement required for quantum devices at room temperature.”

Quantum technology offers a range of future advances in computing, communications, and remote sensing.

To perform any type of task, traditional classical computers work with totally determined information. Information is stored in many bits, each of which can be enabled or disabled. A classic computer, when it receives an input specified by a number of bits, can process this input to produce a response, which is also given as a number of bits. A classic computer processes one input at a time.

By contrast, quantum computers store information in qubits that can be in a strange state where they are both on and off at the same time. This allows a quantum computer to explore the responses to many inputs at the same time. While you can’t generate all the answers at once, you can generate relationships between these answers, allowing you to solve some problems much faster than a classic computer.

Unfortunately, one of the main drawbacks of quantum systems is the fragility of the strange states of the qubits. Most prospective hardware for quantum technology must be kept in extremely cold temperatures, close to zero degrees Kelvin, to prevent special states from being destroyed by interacting with the computer environment.

“Any interaction that has a qubit with anything else in its environment will begin to distort its quantum state,” Jacobs said. “For example, if the environment is a particulate gas, keeping it very cold causes the gas molecules to move slowly, so they don’t collide as much in quantum circuits.”

Researchers have led various efforts to solve this problem, but no definitive solution has yet been found. At the moment, photonic circuits incorporating nonlinear optical crystals have now emerged as the only viable route for quantum computing with solid-state systems at room temperature.

“Photonic circuits are a bit like electrical circuits, except that they manipulate light rather than electrical signals,” said Englund. “For example, we can make channels in a transparent material that the photons will travel down, a bit like the electrical signals that travel along the wires.”

Unlike quantum systems that use ions or atoms to store information, quantum systems that use photons can avoid cold temperature limitation. However, the photons must still interact with other photons to perform logical operations. This is where nonlinear optical crystals come into play.

Researchers can design cavities in crystals that temporarily trap photons inside them. Through this method, the quantum system can establish two different possible states that a qubit can contain: a cavity with a photon (on) and a cavity without a photon (off). These qubits can form quantum logic gates, which create the framework for strange states.

In other words, researchers can use the undetermined state of whether or not a photon is in a crystal cavity to represent a qubit. Logic gates act on two qubits together, and can create a “quantum entanglement” between them. This tangle is automatically generated on a quantum computer and is required for quantum approaches to detection applications.

However, the scientists based the idea of ​​making quantum logic gates using nonlinear optical crystals entirely on speculation, up to this point. While it showed immense promise, doubts remained as to whether this method could even lead to practical logic gates.

The application of nonlinear optical crystals had remained in doubt until researchers from the Army laboratory and MIT came up with a way to realize a quantum logic gate with this approach using established photonic circuit components.

“The problem was that if you have a photon traveling on a channel, the photon has a ‘wave pack’ in a certain shape,” Jacobs said. “For a quantum gate, you need the photon wave packets to stay the same after the gate operation. Since nonlinearities distort the wave packets, the question was whether you could load the wave packet into the cavities, do they interact through nonlinearity, and then they emit the photons again so they have the same wave packets that they started with. “

Once they designed the quantum logic gate, the researchers performed numerous computer simulations of the gate’s operation to demonstrate that it could, in theory, function properly. Building a real quantum logic gate with this method will first require significant improvements in the quality of certain photonic components, the researchers said.

“Based on the progress made in the past decade, we expect it to take around ten years for the necessary improvements to be made,” Heuck said. “However, the process of loading and emitting a wave packet without distortion is something that we should be able to do with current experimental technology, so it is an experiment that we will work on next.”

Physical Review Letters He published the team’s findings in a peer-reviewed article on April 20.