High-speed optical quantum state generation 1,000 times faster than conventional methods achieved by the University of Tokyo

A research team comprising Graduate Student Akito Kawasaki, Assistant Professor Warit Asavanant, and Professor Akira Furusawa of the Graduate School of Engineering at the University of Tokyo, together with Assistant Professor Rajveer Nehra of the University of Massachusetts and NTT, the National Institute of Information and Communications Technology (NICT), and RIKEN announced on November 1 that they succeeded in increasing the generation rate of optical quantum states that exhibit strong quantum (nonclassical) properties called Schrödinger cat states by about 1,000 times compared to conventional methods.

There are various challenges to overcome in the practical applications of quantum computers. A large computational scale (scalability) and fault tolerance are major challenges that need to be achieved for any physical systems. The challenge with respect to realizing large-scale systems is whether it is possible to expand from small-scale systems of few hundred qubits, which are currently in the research and development stage, to large-scale systems of several million qubits, which are required for practical use. Especially in complex quantum systems that are vulnerable to disturbances, there is no clear method for expanding from small to large systems. This has been a bottleneck in realizing practical quantum computers in many physical systems.

To address this challenge, in 2019, a research group from the Furusawa Laboratory at the University of Tokyo realized a scalable optical quantum computing platform, demonstrating its strengths that set it apart from other physical systems. Furthermore, as noise and errors are always present in real systems, a strategy is needed to correctly execute quantum information processing, even in environments where errors may occur, to achieve error tolerance. One way to achieve this is to create "logical qubits" that encode quantum information in a way that allows the detection and correction of errors.

In this regard, this research group was the first in the world to successfully generate the most promising logical qubit, known as the Gottesman-Kitaev-Preskill (GKP) qubit, and the discovery was announced in 2024. However, this method of generating GKP qubits requires the use of multiple quantum states with strong nonclassical properties called Schrödinger cat states. When this method is applied to conventional optical systems, the generation rate of the Schrödinger cat states remains at the order of kHz, which poses the problem that the generation rate of GKP qubits that use multiple cat states is even slower. Currently, this generation rate limits the speed of quantum computation. Without solving the aforementioned problem, it will be difficult to realize fault-tolerant optical quantum computers with GKP qubits.

This generation rate limitation stems from two sources. The first is the frequency band of a light source with controlled quantum fluctuations called squeezed light, which serves as a quantum light source. In addition to the light source, quantum measurements impose constraints. The bandwidth of the squeezed light source determines not only the generation rate but also the shape of the wave packet of generated quantum states.

To accurately observe the quantum state defined by this wave packet, a homodyne detector that can observe a sufficiently wider frequency band is required. Typical values for each element in conventional quantum optics experiments are limited by constraints that the bandwidth of squeezed light sources is limited to the order of MHz at most and the bandwidth of homodyne detectors is limited to about 100 MHz to ensure high quantum efficiency.

In this study, the frequency bandwidth of the light source and measurement was greatly improved using an optical parametric amplifier (OPA), which was developed primarily by NTT, as a squeezed light source, and a superconducting photon detector, which was jointly developed by the University of Tokyo and NICT. As a result, the bandwidth of a conventional squeezed light source of few megahertz was expanded to 6 THz (terahertz, expansion by about 1 million times), the detector was accelerated from 100 MHz to 70 GHz (700 times), and high-speed optical quantum states were successfully generated.

The generation rate reached about 1 MHz, which is about three orders of magnitude (1000 times) higher than conventional Schrödinger cat state generation. The researchers also succeeded in measuring the wave packet shape on a sub-nanosecond scale (previously measured on the order of tens to hundreds of nanoseconds), demonstrating the usefulness of high-speed measurements achieved in this study.

In this research the performance of the photon detector limited the generation rate. If this limitation can be overcome in the future, the generation rate is expected to be 1000 times higher than the current result, and the research group expects to realize ultrafast optical quantum computers.

Journal Information
Publication: Nature Communications
Title: Broadband generation and tomography of non-Gaussian states for ultra-fast optical quantum processors
DOI: 10.1038/s41467-024-53408-w