Modular Waveguide Represents Step Toward Faster Quantum Computers

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TOKYO, April 14, 2022 — Researchers at the University of Tokyo generated strongly nonclassical light using a modular waveguide-based light source. The demonstration, which the researchers said is the first of its kind, is poised to benefit future work aimed at creating faster and more practical optical quantum computers.

Continuous-wave squeezed light is used to generate the various quantum states necessary to perform quantum computing. For optimal computing performance, the squeezed light source must exhibit very low levels of light loss and be broadband.

In the work, the researchers generated a state of light known as Schrödinger or Schrödinger’s cat, which is a superposition of coherent states. The team created a waveguide optical parametric amplifier (OPA) module for its quantum experiments and combined the device with a specially designed photon detector to generate the quantum light.

OPAs use nonlinear optical crystals to generate squeezed light, though conventional OPAs do not generate light that has the properties necessary to increase the speed of quantum computing. With NTT Corp., the University of Tokyo researchers built an OPA based on a waveguide-type device that achieved high efficiency by confining light to a narrow crystal. They designed the OPA device to create squeezed light at 1545 nm, which is within the telecommunications wavelengths — a region that tends to exhibit low losses.

Standard photon detectors based on semiconductors do not meet the performance requirements for the intended application; to complete the system the researchers needed a photon detector that worked at these wavelengths. They developed a detector designed specifically for quantum optics that uses superconductivity technology to detect photons. The researchers called the detector the superconducting nanostrip photon detector.

Researchers developed a new waveguide optical parametric amplifier (OPA) module, which they combined with a specially designed photon detector (pictured) to generate strongly nonclassical light that can be used for quantum experiments. Courtesy of Kan Takase, University of Tokyo
Researchers developed a new waveguide OPA module, which they combined with a specially designed photon detector (pictured) to generate strongly nonclassical light that can be used for quantum experiments. Courtesy of Kan Takase/University of Tokyo.
The OPA device in the system demonstrated much lower propagation loss than conventional devices. The researchers said it could also be modularized for use in various experiments with quantum technologies.

“Our method for generating quantum light can be used to increase the computing power of quantum computers and to make the information processer more compact,” said research team member Kan Takase from the University of Tokyo. “Our approach outperforms conventional methods, and the modular waveguide OPA is easy to operate and integrate into quantum computers.”

Takase said that the team’s goal is to improve information processing by developing faster quantum computers that can perform any type of computation with errors.

“Although there are several ways to create a quantum computer, light-based approaches are promising because the information processor can operate at room temperature and the computing scale can be easily expanded,” Takase said.

More specifically, Takase said, the team aims to increase the clock frequency of optical quantum computers. These clocks can in principle achieve terahertz frequencies, he said. “Higher clock frequencies enable faster execution of computational tasks and allow the delay lines in the optical circuits to be shortened. This makes optical quantum computers more compact while also making it easier to develop and stabilize the overall system.”

The research was published in Optics Express (

Published: April 2022
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
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continuous wave
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