A path towards quantum computing at room temperature

A new approach to the practical implementation of quantum gates that is room-temperature compatible and only relies on components that have been individually demonstrated.

Army scientists predicted that quantum computer circuits that will no longer need extremely cold temperatures to function could soon be available.

Scientists have demonstrated the practicability of a quantum logic gate comprised of photonic circuits and optical crystals.

One of the significant drawbacks of quantum systems is the fragility of the strange states of the qubits. Most imminent hardware for quantum technology must be kept at very cold temperatures—near zero kelvins—to forestall the extraordinary states being pulverized by associating with the computer’s environment.

Dr. Kurt Jacobs, of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, said, “Any interaction that a qubit has with anything else in its environment will start to distort its quantum state. For example, if the environment is a gas of particles, then keeping it very cold keeps the gas molecules moving slowly, so they don’t crash into the quantum circuits as much.”

Scientists directed various efforts to resolve this issue, but a definite solution is yet to be found. At the moment, photonic circuits that incorporate nonlinear optical crystals have presently emerged as the sole feasible route to quantum computing with solid-state systems at room temperatures.

Unlike quantum systems that use ions or toms to store data, quantum systems that use photons can bypass the cold temperature limitation. However, the photons should even now interact with different photons to perform logic operations. This is the place the nonlinear optical crystals become an integral factor.

Scientists can engineer cavities in the crystals that temporarily trap photons inside. Doing so enables the quantum system to 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.

Meanwhile, scientists 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.

Although, the idea to make quantum logic gates using nonlinear optical crystals is still hypothesizing. There are still doubts about whether this could even lead to practical logic gates.

Now, Army scientists, in collaboration with MIT, have presented a new way to realize a quantum logic gate with this approach using established photonic circuit components.

Jacobs said, “The problem was that if one has a photon traveling in a channel, the photon has a ‘wave-packet’ with a certain shape. 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.”

Scientists noted, “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. The actual construction of a quantum logic gate with this method will first require significant improvements in the quality of certain photonic components.”

Dr. Mikkel Heuck of the Massachusetts Institute of Technology said, “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. However, the process of loading and emitting a wave-packet without distortion is something that we should able to realize with current experimental technology. So that is an experiment that we will be working on next.”

Journal Reference:
  1. Mikkel Heuck, Controlled-Phase Gate Using Dynamically Coupled Cavities and Optical Nonlinearities. DOI: 10.1103/PhysRevLett.124.160501

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