Researchers generate high-quality quantum light with modular waveguide device

For the first time, researchers have successfully generated strong non-classical light using a modular waveguide light source. The achievement is a critical step toward creating faster and more practical optical quantum computers.

“Our goal is to dramatically improve information processing by developing faster quantum computers that can perform any type of computation without errors,” said research team member Kan Takase of the University of Tokyo. “While there are several ways to create a quantum computer, light-based approaches hold promise because the information processor can operate at room temperature and the computational scale can be easily expanded.”

In the magazine Optica Publishing Group Optics Express, a multi-institutional team of researchers from Japan describes the waveguide optical parametric amplifier (OPA) module they created for quantum experiments. By combining this device with a specially designed photon detector, they were able to generate a light state known as Schrödinger’s cat, a superposition of coherent states.

“Our method of generating quantum light can be used to increase the computing power of quantum computers and make the information processor more compact,” Takase says. “Our approach outperforms conventional methods, and the modular waveguide OPA is easy to operate and integrate into quantum computers.”

Generating strong non-classical light

Continuous undulating light is used to generate the different quantum states needed to run quantum computers. For best computing performance, the compressed light source should have very low light loss and be broadband, meaning it covers a wide range of frequencies.

“We want to increase the clock frequency of optical quantum computers, which can basically reach terahertz frequencies,” Takase says. “Higher clock frequencies allow faster execution of computational tasks and make it possible to shorten the delay lines in the optical circuits. This makes optical quantum computers more compact and also makes it easier to develop and stabilize the overall system.”

OPAs use nonlinear optical crystals to generate pressed light, but conventional OPAs do not generate the quantum light with the properties needed for faster quantum computers. To meet this challenge, researchers from the University of Tokyo and NTT Corporation developed an OPA based on a waveguide-type device that achieves high efficiency by confining light to a narrow crystal.

By carefully designing the waveguide and manufacturing it with precision processing, they were able to create an OPA device with much smaller propagation loss than conventional devices. It can also be modularized for use in various experiments with quantum technologies.

Designing the right detector

The OPA device is designed to create compressed light at telecommunications wavelengths, a wavelength range that tends to exhibit low losses. To complete the system, researchers needed a powerful photon detector operating at telecom wavelengths. However, standard semiconductor photon detectors do not meet the performance requirements for this application.

For example, researchers from the University of Tokyo and the National Institute of Information and Communications Technology (NICT) have developed a detector specially designed for quantum optics. The new superconducting nanostrip photon detector (SNPD) uses superconductivity technology to detect photons.

“We combined our new waveguide OPA with this photon detector to generate a very non-classical or quantum state of light called the Schrödinger cat,” Takase says. “Generating this state, which is difficult with conventional, low-efficiency waveguide OPAs, confirms the high performance of our waveguide OPA and opens up the possibility of using this device for a wide variety of quantum experiments.”

The researchers are now looking at how to combine high-speed measurement techniques with the new waveguide OPA to get closer to their goal of ultrafast optical quantum computers.