SoftBank's Initiatives Toward the Realization of the Quantum Internet ~Successful Transmission of Photons Carrying Entanglement in Commercial Optical Fibers in the Tokyo Metropolitan Area~

1. Integration of the Current Internet and Next-Generation Technologies

The Internet that we use in our daily lives has become an indispensable part of modern society, enriching our lives through activities such as information searches, online payments, and video streaming. This is because the Internet enables the processing of vast amounts of information by connecting computers and devices worldwide. However, the rapid development and widespread adoption of technologies such as artificial intelligence (AI) and the Internet of Things (IoT) have led to an exponential increase in the volume of information exchanged over the Internet. As a result, the burden on data processing has grown significantly, raising concerns that conventional computing systems may struggle to keep up with this rising demand. In particular, the limitations of classical computing technologies are becoming increasingly evident in areas such as large-scale combinatorial optimization and complex data analysis.　Therefore, there is a growing demand for information processing systems that surpass the capabilities of traditional Internet infrastructure based on classical computing.

In response to these societal demands, the concept of the "quantum internet" has emerged, which holds the potential to integrate with next-generation computing technologies. Currently, global efforts are underway to develop "quantum computers", which leverage principles of quantum mechanics and are expected to surpass the computational capabilities of classical computers in the near future. However, scaling up quantum computing systems remains a significant challenge in the pursuit of universal quantum computers capable of solving a wide range of problems. To address this, researchers are focusing on distributed quantum computing, a technology that interconnects multiple quantum computers located in different places via a network to collaboratively perform computations.

Nevertheless, if conventional Internet infrastructure is used as the network for distributed quantum computing, it will not significantly enhance the computational power of quantum computers. In contrast, a quantum internet would enable an exponential increase in computational power relative to the scale of interconnected quantum computers. This would allow for large-scale problem-solving through distributed processing, making it possible to tackle challenges that a single quantum computer alone would struggle to handle.

2. What is the Quantum Internet?

Atoms and molecules exhibit not only particle-like properties but also wave-like properties such as interference and superposition. These characteristics arise from the quantum nature of atoms and molecules. Quantum systems behave in ways that defy our everyday intuition. One phenomenon unique to quantum systems is "entanglement."

Quantum entanglement refers to strong correlations between the states of multiple quantum particles generated in a special way and cannot be explained by classical methods. For example, when the state of one particle in an entangled pair is measured, the state of the other is instantly determined, regardless of the distance between them. There are several methods to generate entanglement. In the case of using light (photons) to induce entanglement, one photon of an entangled pair can be sent through an optical fiber, leading to entanglement between distant photon pairs.

The quantum internet is a network that can transmit entangled photons to various locations. By building such a network, entangled states can be shared globally. This enables functions unavailable in the current internet, such as information-theoretically secure cryptographic protocols that even quantum computers cannot break, quantum teleportation of quantum states over long distances, and high-precision time synchronization between two points, in addition to the distributed quantum computing.

The realization of a quantum internet requires technologies for generating entangled photons and reliably transmitting them through optical fibers. However, photons are highly susceptible to losses and disturbances in optical fibers, making long-distance quantum communication technically challenging. Therefore, research and development are underway to use "quantum repeaters" that relay entanglement along the communication path, as illustrated in Figure 1.

Figure 1. Quantum repeater’s architecture (Source: https://lquom.com/en/products/）

3. SoftBank's Initiatives Toward the Realization of the Quantum Internet

SoftBank Corp. ("SoftBank") is working towards the practical implementation of a hybrid network that integrates conventional Internet technologies with quantum internet technologies, aiming to establish the infrastructure for a future quantum society (Figure 2).

Figure 2. Schematic of a hybrid network that integrates the current Internet and a quantum internet

In 2023, SoftBank in collaboration with LQUOM Inc. ("LQUOM"), which is developing quantum communication systems and related technologies for the realization of a quantum internet, launched an experiment to transmit photons carrying entanglement using optical fiber deployed in the Tokyo metropolitan area.

September 2023: SoftBank Corp. and LQUOM Launch Quantum Communication Field Study of Entangled Photons in Tokyo Metropolitan Area

The optical fibers deployed by telecommunication operators run along utility poles and underground, exposing them to various environmental factors such as vibrations from subways and passing vehicles, wind and rain, and temperature fluctuations due to seasonal and day-night changes. Additionally, optical fibers have slight inherent losses, causing a certain probability of photon loss during long-distance transmission.

Therefore, for the practical realization of a quantum internet, it is crucial to verify whether entangled photons can be reliably transmitted through commercially deployed optical fibers.

This experiment focused on:

・Evaluating the transmission environment of commercial optical fibers in the Tokyo metropolitan area
・Long-distance transmission of photons carrying entanglement

Experiment 1: Evaluation of the transmission environment using commercial optical fiber in the Tokyo metropolitan area

There are multiple methods for quantum relaying, and in all methods, photons generated at each site are transmitted to an intermediate relay node where photon interference is used to establish entanglement between the sites. If the phase of the photons that pass through the optical fiber fluctuates significantly, photons fail to interfere properly at the relay node, and the fidelity of the entanglement formed between the sites decreases. Therefore, to build a future long-distance quantum internet using quantum repeaters, it is crucial to control the phase of the photons passing through the optical fibers with high precision.

In the first experiment, we evaluated the phase fluctuations of light transmitted through commercial optical fibers. Specifically, laser light from a classical light source installed at SoftBank's headquarters was injected into an optical fiber approximately 32 km in length. The light traveled through the fiber, passed through SoftBank's data center in Tokyo, and then returned, at which point its phase was measured (Figure 3).

Furthermore, we continuously monitored the phase fluctuations over several months to assess their stability across different times of the day and seasonal variations.

Figure 3. Experimental system for evaluating the transmission environment of commercial optical fibers

The experimental results are presented in Figures 4. Figure 4 illustrates the daily temperature variations in Tokyo, and shows the corresponding phase changes, expressed in terms of the variation of the optical fiber length at each time. The phase change is the relative value to the initial measurement time.

These results indicate that the phase of the light passing through the commercial optical fiber slightly varies throughout the day. This phase change is considered to be caused by the expansion and contraction of the optical fiber due to changes in ambient temperature. For instance, the temperature rises around 9 a.m., and the optical fiber length also increases due to its expansion. In contrast, the temperature decreases around 3 a.m., 3 p.m., and 9 p.m., and the optical fiber length decreases due to its contraction.

In addition to these significant changes caused by daily temperature fluctuations, minor variations were also observed on shorter timescales. When converted to fiber length, the magnitude of these phase changes ranges from several to tens of micrometers per second. Such phase changes can be corrected in real time by dynamically adjusting the length of the optical fiber using actuators (e.g., piezoelectric elements).

Figure 5 shows the phase variations at different frequencies, derived from Fourier transforms of the phase measurements taken at 3 a.m. and 3 p.m. on the same day. Both actual measurements and the RMS (root mean square) values are plotted. A comparison of the results reveals significant differences in the phase variations below 100 Hz. The difference at low frequencies can be attributed to traffic-induced vibrations (from vehicles and trains) and structural resonances in buildings caused by wind. In addition, the pronounced phase changes at 3 p.m. can be attributed to increased social activity, as well as higher ambient temperatures and wind speeds during the daytime compared to nighttime.

Figure 4. Left: Temperature changes in Tokyo on June 14, 2024, based on data from the Japan Meteorological Agency
Right: Time variation of optical fiber length at each time

Figure 5. Frequency spectrum of the changes in optical fiber length

Experiment 2: Long-distance transmission of photons carrying entanglement

In the second experiment, we generated narrow-linewidth entangled photons using the cavity-enhanced quantum light source LQ-PS-100 developed by LQUOM (Figure 6) and transmitted them through the commercial optical fiber used in the previous experiment. This experiment aimed to verify whether these photons could be transmitted with sufficient quality for practical use. Specifically, we sent one of the two photons produced by the quantum light source to the data center, while measuring the other photon immediately after its generation in the laboratory to evaluate the time difference in photon arrival (Figure 7). Since high sensitivity was required for photon detection, the photon detector must have high detection efficiency. For this experiment, we used a superconducting single-photon detector under development by Hamamatsu Photonics K.K.

Figure 6. Cavity-enhanced quantum light source developed by LQUOM (LQ-PS-100)

Figure 7. Experimental system for the transmission of photons carrying entanglement using commercial optical fibers

The obtained results are shown in Figure 8. The figure also includes the results of a laboratory experiment where both photons carrying entanglement generated by the quantum light source were detected in the laboratory. This corresponds to a scenario with no loss or noise from transmission through the commercial optical fiber. The horizontal axis is the corrected time difference in the arrival of the two photons, considering the differences in the lengths of the optical fibers through which each photon of the pair (photon 1 and photon 2 in Figure 7) travels. The vertical axis is the number of photon pairs detected per second. To compare with the results from the experiment using the commercial optical fiber, the number of photon pairs detected in the laboratory experiment was multiplied by a factor of 30 in the data analysis to account for the transmission loss in the commercial optical fiber.

The figure shows that, when accounting for optical fiber loss, the difference in the number of detected photon pairs between the laboratory experiment and the commercial optical fiber experiment is as expected. In addition, the number of detected photon pairs is highest when the arrival time difference is close to zero, indicating that the signal degradation due to loss and noise in the commercial optical fiber is minimal. Furthermore, the results remained stable over multiple days, both during the day and at night. Therefore, this experiment demonstrates that photons carrying entanglement can be stably transmitted over SoftBank's commercial network.

Figure 8. Relation between the two-photon arrival time difference and the number of photon pairs detected per second

4. Future Prospects of the Quantum Internet Based on Two Experimental Results

In this study, we evaluated the transmission environment of commercial optical fibers deployed in the Tokyo metropolitan area and found that phase compensation for the long-distance transmission of entanglement is feasible. In addition, experiments using quantum light sources and single-photon detectors demonstrated that photons carrying entanglement can be stably transmitted over commercial optical fibers.

SoftBank and LQUOM will verify the foundational technologies necessary for the implementation of the quantum internet and contribute to its early societal implementation, assessing potential use cases for enhancing future network service value.