Ultra-Fast real-time optical entanglement generation
A new era pioneered by fast generation of quantum correlations 1000 times higher than the conventional rate

News Highlights

TOKYO - January 29, 2025 - A research team from the Department of Applied Physics, School of Engineering, The University of Tokyo, led by PhD Student Akito Kawasaki, Assistant Professor Warit Asavanant, and Professor Akira Furusawa, and NTT Corporation (Headquarters: Chiyoda Ward, Tokyo; Representative Member of the Board and President: Akira Shimada; hereinafter "NTT"), has succeeded in generating and observing the world's fastest quantum entanglement¹.
　Quantum entanglement is a phenomenon unique to quantum mechanics that involves a peculiar correlation between two or more qubits (or qumodes). This entanglement is the root resource for quantum technologies such as quantum computation, quantum communications, and quantum error correction. For practical evaluation of quantum entanglement, the generation rate (bandwidth) of entanglement is an important parameter in addition to its purity. Conventional optical quantum entanglement generation rates are on the order of kilohertz (kHz) to megahertz (MHz), and time conversion is on the order of tens of microseconds (10^-6 seconds) to tens of nanoseconds (10^-9 seconds). This generation rate limits the clock frequency² of quantum computers in practical use, therefore conventional generation rates could only realize quantum computing systems that are slower than GHz, the clock frequency of current classical computers.
　In this research, using an optical parametric amplifier (OPA)³ (Figure 1) jointly developed by the University of Tokyo and NTT, we achieved the world's fastest generation and real-time measurement of optical quantum entanglement at 60 GHz (picosecond order). Real-time measurement⁴ is essential for quantum technologies that involve real-time information processing, such as quantum computation and quantum communications. This time, real-time quantum measurement of entangled states of light was more than 1000 times faster than conventional measurement. This rate of generation exceeds that of quantum systems using other physical systems and classical computers. This research enables the use of quantum entanglement, which is the root resource of all quantum technologies, in a form that is fast and fully applicable to quantum information processing. This research is expected to have various applications as a basic technology for next-generation ultrafast optical technology.

Figure 1 Optical Parametric Amplifier Used in This Experiment (Left) And Conceptual Diagram of The Module (Right)

Background

Multiple qubits may have complex correlations that cannot be explained by classical physics, and this peculiar correlation is called quantum entanglement. Quantum entanglement is a fundamental resource for many applications of quantum technology. For example, quantum computation and quantum teleportation use quantum entanglement and quantum measurements to achieve various manipulations of quantum states. In quantum error correction, quantum correlation based on entanglement can be successfully used to detect and correct errors without destroying the target quantum information.
　In addition to the purity of the entanglement, its generation rate is also an important parameter when evaluating this quantum entanglement. In quantum information processing applications such as quantum computation and quantum communication, it is necessary to generate and measure a quantum entangled state at high speed in real time. This time scale is determined by the carrier frequency of the physical system used in the quantum system. In particular, the optical system with a carrier frequency of several hundred terahertz (THz) is the physical system that is expected to generate quantum entanglement at the highest speed. However, due to technical limitations, most of the entanglement is limited to the order of MHz at most, and high speed for practical use has not been realized. In quantum computing applications, the generation rate limits the clock frequency of a quantum computer, therefore conventional generation rates can only realize quantum computing systems that are slower than GHz, the clock frequency of current classical computers.

Details of the results

The high-speed entanglement generation and measurement system is shown in Figure 2. In this experiment, the University of Tokyo and NTT used a THz-order bandwidth OPA, which NTT had been developing for many years as an ultra-high-speed optical communication device. By combining this light source with ultrafast real-time measurement using OPAs, ultrafast quantum entanglement generation and measurement were realized. The OPA used in the measurement is the exact same type as the OPA used in the light source, but in the measurement the amplitude information of a certain phase of the light is amplified without degradation of its quantum information to be measured. This technique, which measures the amplified signal with a high-speed receiver for optical communications, was established experimentally through joint research between the University of Tokyo and NTT [1]. In this research, we developed a new phase synchronization method for multiple high-speed measurement systems and applied it to high-speed real-time measurement of entangled states between two parties for the first time in the world. NTT's technology for stable fabrication of OPAs with uniform characteristics plays an important role in the synchronization of these measurement systems.

Figure 2 Experimental System
Note. On the left is the generation system of optical quantum entanglement, and on the right is the high-speed measurement system. Entangled state generation and phase-sensitive amplification are achieved through interaction between pump light and waveguide OPA. Probe light is used for phase control of the experimental system, and the entangled state is measured using a 5G homodyne detector.

Figure 3 shows the measurement results. First, the upper part of Figure 3 shows the real-time measurement results, and the measurement results of the two modes have picosecond scale correlation. The typical time scale of the conventional quantum correlation measurement is at most nanosecond. We succeeded in achieving a literal order of magnitude speedup. The results of frequency domain analysis to confirm that the correlation between the two modes is a quantum correlation are shown in the lower part of Figure 3. This figure suggests the existence of correlations below the classical shot-noise level, and we conclude that quantum entanglement, which cannot be explained by classical physics, has been observed in the entire bandwidth up to 60 GHz.

Figure 3 Experimental Results
Note. Top row: Real-time entanglement measurement. HD1 and HD2 represent the outputs of the two homodyne measurement shown in Figure 2, respectively. Bottom: The frequency domain of entanglement. The left and right sides show the results of the homodyne measurement⁵ at the base of the x and p directions, respectively. In the upper part, the picosecond order quantum correlation is observed in the x and p basis measurements, providing evidence of quantum correlations. The lower 0dB represents the classical shot noise level, below which the blue line is evidence of quantum entanglement.

Outlook

In this study, we succeeded in 60 GHz bandwidth real-time measurement of quantum entanglement between two modes. This is more than 1000 times faster than the conventional entanglement measurement, and the method can be extended to entanglement on a larger scale than that between two modes. For example, by combining this technology with the optical quantum computing processor of this research group [2], optical quantum computing at 60 GHz can be realized. This is expected to lead to a future where the clock frequency of optical quantum systems will surpass that of current classical computers. In addition, quantum entanglement is an important resource not only for quantum computation but also for various quantum technologies, and the developed technology is expected to be applied to quantum communications, security, and basic technologies for next-generation ultrafast quantum networks.

Related Information:

[1] "High-speed generation of optical quantum state: Acceleration of optical quantum computers via optical communication technology" (2024/11/1)
https://www.t.u-tokyo.ac.jp/en/press/pr2024-11-01-002

[2] "Generation of time-domain-multiplexed 2-dimensional cluster state" (2019/10/18)
https://www.t.u-tokyo.ac.jp/en/press/foee/press/setnws_201910181425216020578419.html

Information on presenters and researchers:

School of Engineering, The University of Tokyo

NTT Device Technology Laboratories

RIKEN Center for Quantum Computing

Role of each institution:

Paper information:

Journal: Nature Photonics
Title: Real-time observation of picosecond-timescale optical quantum entanglement toward ultrafast quantum information processing
Authors: Akito Kawasaki, Hector Brunel, Ryuhoh Ide, Takumi Suzuki, Takahiro Kashiwazaki, Asuka Inoue, Takeshi Umeki, Taichi Yamashima, Atsushi Sakaguchi, Kan Takase, Mamoru Endo, Warit Asavanant, Akira Furusawa
DOI: 10.1038/s41566-024-01589-7
URL: https://www.nature.com/articles/s41566-024-01589-7

Research grant:

This research was supported by the Japan Science and Technology Agency (JST) Moonshot Research and Development Program Moonshot Goal 6 "Realization of a fault-tolerant universal quantum computer that will revolutionize economy, industry, and security by 2050" (Program Director: Masahiro Kitagawa, Director, Center for Quantum Information and Quantum Biology, Osaka University) Research and Development Project "Development of Large-scale Fault-tolerant Universal Optical Quantum Computers (JPMJMS2064)" (Project Manager: Akira Furusawa, Professor, School of Engineering, The University of Tokyo / Deputy Director, RIKEN Center for Quantum Computing, RIKEN).

Glossary:

¹Quantum entanglement
Quantum entanglement is a quantum mechanical phenomenon, which has a peculiar correlation between physical quantities. Under this correlation, it is known that two parties with quantum entanglement influence each other even if they are located far apart. The phenomenon is unexplainable within classical physics, and Einstein called it "spooky action at distance," pointing out its strangeness. However, many previous experiments have demonstrated the existence of entanglement, and in 2022, the Nobel Prize was awarded to three researchers who contributed to the demonstration of quantum entanglement. This entanglement is expected to be used as a resource to realize various applications of quantum technologies such as quantum computation, quantum communication, and quantum measurement.

²Clock frequency
Clock frequency is a measure of how fast a computer or electronic device is running. It shows how many calculations can be done per second, expressed in hertz (Hz). In general, the higher the clock frequency, the more computation and information processing can be performed in a shorter time, which is advantageous in practice. Classical computers generally used today have a clock frequency of approx. GHz (one billion times per second).

³Optical parametric amplifier (OPA)
In optical information processing, quantum information is written into the amplitude and phase information of light. Optical parametric amplification is a quantum optical phenomenon that amplifies the amplitude in a specific phase direction and attenuates the amplitude in the other phase (squeezing operation). This squeeze operator plays an important role in optical quantum information processing, such as quantum light source generation and high-speed measurement. Optical parametric amplification is experimentally realized by nonlinear optical effects using nonlinear optical crystals. In this experiment, the squeeze operator was achieved using an optical parametric amplifier with a built-in waveguide-shaped nonlinear optical crystal called PPLN crystal.

⁴Real-time measurement
Real-time quantum measurements are essential for applications of quantum information processing such as practical quantum computation and quantum communication. The concept here contrasts with real-time measurement is post-process information processing. This saves all the retrieved information once, and then processes the saved information later. Post-process information processing is not practical for quantum information processing, even though it is suitable for proof-of-principle experiments because of the enormous amount of data stored and the delay of information processing. In optical systems, quantum measurement called homodyne measurement is commonly used as a real-time measurement. While homodyne measurement can simultaneously read out the phase and amplitude information of light in real time, precise phase control is required, making it a highly difficult measurement for experiments. In this research, a new method of phase control is developed to achieve two high-speed homodyne measurements and synchronization between them.

⁵Homodyne measurement
In quantum information processing using light, quantum information is encoded for the amplitude and phase of light. Homodyne measurement is a quantum measurement technique that can measure the amplitude of light in a specific phase direction. Light power measurement by a photodetector is often used as a general measurement of light, but power measurement is not suitable for quantum information processing because it cannot read the phase information of light. Measurement-induced quantum computation is a type of quantum computation method that can perform quantum computation by repeatedly performing homodyne measurement and feedforward operation according to the measurement result.

About NTT

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