Realization of modularized quantum light source toward fault-tolerant large-scale universal optical quantum computers
Nippon Telegraph and Telephone Company (NTT), the University of Tokyo, and the Japan Institute of Physics and Chemistry (RIEKN) have jointly developed a fiber-coupled quantum light source (compressed light source). The team said that this is a key technology to achieve fault-tolerant large-scale universal optical quantum computers.
The research results were published on December 22, 2021 in the American scientific journal Applied Physics Letters [1], and was selected as Editor's Pick.
Different ways to implement large-scale general-purpose quantum computers
The whole world is actively carrying out research and development to realize universal quantum computers. Recently, it was reported that the use of superconducting circuits to achieve quantum calculations with about 100 physical qubits. However, to realize a fault-tolerant general-purpose quantum computer, approximately one million physical qubits are needed. Therefore, increasing the number of qubits has become a major challenge for quantum computing. In order to realize one million qubits through superconducting circuits or ion traps, people have taken the method of increasing the number of qubits by integrating its components and parallelizing equipment. On the other hand, optical quantum computers are expected to use time-domain multiplexing technology and measurement-induced quantum operations to achieve absolute large-scale general-purpose quantum computing, which is a completely different method from traditional methods.
Quantum computers that use photons have many advantages. For example, it does not require low temperature and vacuum equipment required by other methods, making it compact. By creating multiple quantum entangled states in the time domain, the number of qubits can be easily increased without the need for micro-integration of circuits or parallelization of devices. Due to the broadband nature of light, high-speed computational processing is possible. In theory, quantum error correction is possible by using continuous light variables that utilize the parity of photons, rather than discrete variables that utilize the presence or absence of photons. This method has a high degree of compatibility with optical communication technologies such as low-loss fibers and high-function optical devices, and has made great progress in the construction of general-purpose large-scale fault-tolerant optical quantum computers.
Compressed light source is the key to realizing light quantum computer
To realize a light quantum computer, one of the most important components is a quantum light source that generates compressed light. This is the source of quantum properties in a light quantum computer, especially a quantum light source that requires fiber coupling. Squeezed light is a non-classical light with an even number of photons and squeezed quantum noise, which is used to generate quantum entanglement. In addition, squeezed light plays an extremely important role in quantum error correction, because quantum error correction is achieved by using the parity of the photon number.
In order to realize a large-scale general-purpose fault-tolerant optical quantum computer, we need a fiber-coupled compressed light source that has highly compressed quantum noise and can maintain photon number parity even in high photon number components. For example, a compression level of more than 65% is required to generate time-domain multiple quantum entanglement (two-dimensional cluster state) that can be used for large-scale quantum computing. However, because it is difficult to produce high-quality compressed light, such equipment has not been developed.
In this research, the research team developed a new fiber-coupled quantum light source that can work at the wavelength of optical communication. By combining it with optical fiber components, continuous wave compressed light was successfully generated in a closed fiber optic system for the first time, with compressed quantum noise exceeding 75%, and sideband frequencies exceeding 6 THz.
This means that the key components in the optical quantum computer have been implemented in a form compatible with optical fibers, while maintaining the broadband characteristics of light. This will help develop optical quantum computers in stable and maintenance-free systems that use optical fibers and optical communication devices. This will greatly promote the development of rack-mounted large-scale optical quantum computers.
Modular quantum light source
In time-domain multiplexing technology, researchers divide continuously flying light into multiple time periods and place information on separate light pulses. In this way, the number of qubits on the time axis can be easily increased without increasing the size of the device (see Figure 1). In addition, by using the parity of the photon number and the continuous variation of light, quantum error correction has been proved theoretically possible.
Figure 1 Using time-domain multiplexing technology to generate large-scale quantum entangled states
Using low-loss optical fiber as the propagation medium of flying optical qubits, combined with optical communication devices, can freely and stably generate large-scale quantum entangled states. Specifically, only 4 compressed light sources, 2 fibers (delay lines) of different lengths, and 5 beam splitters (see Figure 2) are needed to generate the large-scale two-dimensional cluster states necessary for general-purpose quantum computing. This method does not require integration or large-scale equipment. It makes it possible to implement universal quantum computing on the scale of practical equipment in the rack, while the method of using superconducting circuits or trapping ions requires component integration or equipment parallelization. In addition, this method can make use of the high frequency of light for high-speed calculations. This means that not only high-speed quantum algorithms can be implemented, but their clock frequencies can also be very high, making optical quantum computers the ultimate high-speed information processing technology.
Figure 2 Realize the basic components of general quantum computing to generate large-scale optical quantum entangled states
So far, researchers have demonstrated various optical quantum operations, realizing this kind of optical quantum computer by using a spatial optical system composed of many high-precision aligned mirrors. This is to minimize the loss of light and enhance the interference between lights as much as possible. However, if the mirror is slightly misaligned, the desired characteristics cannot be obtained, and the optical path must be re-adjusted for each experiment. For these reasons, to realize a practical optical quantum computer, an optical system close to the optical waveguide, such as an optical integrated circuit or optical fiber, must be used, which has good operational stability and maintenance-free. In particular, the most basic element in an optical quantum computer is compressed light. This non-classical light has squeezed quantum noise of wave amplitude or phase, which is a pair of non-exchangeable physical quantities. Since this kind of light is difficult to generate and is easily degraded due to optical loss, the light from a fiber-coupled compressed light source tends to be poor. In particular, more than 65% of the squeezed light necessary to generate a large-scale quantum entangled state (two-dimensional cluster state) for time-domain multiplexing has not been realized in a fiber enclosed structure.
As shown in Figure 3, the researchers developed a low-loss fiber-coupled quantum light source module (optical parametric amplification module). By updating the manufacturing method of the periodically polarized lithium niobate (PPLN) waveguide (the main part of the module), low loss has been achieved. Using the optical communication device assembly technology developed by NTT, a low-loss optical fiber coupling module is realized.
Figure 3 Newly developed quantum light source (optical parametric amplifier)
When connecting fiber optic components, the researchers successfully measured compressed light, where the quantum noise was compressed to more than 75%, and the bandwidth exceeded 6 THz (see Figure 4). This means that the quantum state required by the optical quantum computer can even be generated and measured in a completely closed system in an optical fiber. Therefore, the fiber-coupled quantum light source they developed will make it possible to realize a stable and maintenance-free optical quantum computer on a practical scale, which will greatly promote future development.
Figure 4 Measurement results of quantum noise level. The compressed noise level is more than 75% less noise than the shot noise level
In short, the researchers adopted a new method in which the first module generates compressed light and the second module converts optical quantum information into classical optical information. The optical parametric amplifier is used to achieve optical amplification that maintains the parity of the photon number. Unlike traditional balanced homodyne detection technology, this measurement method can amplify and convert quantum signals into classical optical signals without converting them into electrons. Therefore, it can achieve extremely fast measurements. This technology can be used to realize all-optical quantum computers in the future, and will greatly contribute to the realization of extremely fast all-optical quantum computers running at a THz clock frequency.
In the next step, the researchers stated that they will combine the various optical quantum operations they have developed so far to develop an optical quantum computer composed of optical fiber components. In addition, the team will improve the quantum noise compression capability of the quantum light source to realize a fault-tolerant large-scale general-purpose optical quantum computer.
NTT said: "So far, people think that integrated circuits are essential for realizing large-scale quantum computers. However, this success shows that integrated circuits are not necessary. By using the modules and optical fiber components we have developed, we can achieve large-scale Light quantum computers. With this result, the realization of large-scale quantum computers has become possible. It can be said that a disruptive technology was born."
Link: [1]https://aip.scitation.org/doi/full/10.1063/5.0063118
[2]https://group.ntt/en/newsrelease/2021/12/22/211222a.html
