NTT and Tohoku University Pioneer Quantum Leap with Single-Photon Coherent Ising Machines
NTT Research and Tohoku University have jointly achieved a significant scientific milestone, pushing the boundaries of quantum computing by developing a highly energy-efficient and powerful form of Coherent Ising Machines (CIMs). This collaboration promises to advance the frontier of quantum-inspired computing, offering a scalable solution to complex optimization problems across multiple domains, including AI, logistics, and unsupervised machine learning.
News Highlights
- NTT Research PHI Lab and Tohoku University publish a paper detailing breakthroughs in single-photon Coherent Ising Machines.
- The research introduces a quantum-enhanced CIM capable of converting quantum entanglement into classical correlation, improving performance over conventional CIMs.
- The partnership is advancing large-scale “cyber CIMs”—software-simulated CIMs run on high-performance computing (HPC) platforms, scalable up to 100 million spins.
The paper, titled “Single photon coherent Ising machines for constrained optimization problems”, appears in the Quantum Science and Technology journal and outlines the foundational theory, methodology, and surprising results from the joint effort.
A Powerful Alliance in Quantum Informatics
This breakthrough stems from a collaboration between the Physics and Informatics (PHI) Lab at NTT Research, Inc., a subsidiary of Japan’s NTT Group (TYO:9432), and the Graduate School of Information Sciences (GSIS) at Tohoku University. The partnership began in 2023 under a joint research agreement (JRA), bringing together leading researchers from both institutions to explore the computational potential of CIMs through advanced simulation and quantum modeling.
The project is led by Professor Hiroaki Kobayashi of Tohoku University and Dr. Yoshihisa Yamamoto, Director of the PHI Lab at NTT Research. Together, they aim to scale CIM technology using classical HPC systems to simulate what they call “cyber CIMs”—a digital environment where quantum-inspired models can be tested and refined at scale.
From Biological Inspiration to Quantum Design
At the core of this research is the Coherent Ising Machine, a type of unconventional computing architecture based on degenerate optical parametric oscillators (DOPOs). These machines map complex problems into an Ising model, a well-known mathematical framework used to describe magnetic spin systems. This allows CIMs to simulate and solve combinatorial optimization problems efficiently.
The PHI Lab is rethinking conventional computing paradigms by merging principles from quantum optics, classical electronics, and even neuroscience. Inspired by how biological brains process information, the lab is committed to developing hybrid analog-digital computers that are both efficient and scalable.
A New Era: The Single-Photon CIM
Unlike previous experimental CIMs, which used pulses containing about 100 million photons, this new model works with only a single photon per pulse—a difference of eight orders of magnitude. The initial expectation was that such weak light signals would lead to noise and instability, significantly degrading performance.
Surprisingly, the opposite was observed. Numerical simulations demonstrated that single-photon CIMs outperformed conventional high-power models, solving optimization problems with higher accuracy and consistency.
This unexpected result signals a quantum breakthrough. Because photon counts are so low, quantum effects dominate the system, allowing researchers to investigate its behavior using quantum theory rather than classical models.
Quantum Enhancement Through Entanglement and Feedback
What gives the single-photon CIM its edge is a subtle yet powerful quantum mechanical mechanism. Within the CIM, a beam splitter at the measurement port generates entangled photon pairs—one internal and one extracted for measurement. These photons retain a high degree of quantum correlation despite the inherently noisy environment.
The critical insight is that the CIM doesn’t just produce entanglement—it also converts it immediately into classical correlations through its measurement and feedback loop. This hybrid process ensures that fragile quantum states do not decay uselessly but are transformed into robust signals that reinforce the machine’s optimization process.
This “quantum-classical bridge” is absent in conventional CIMs, which rely on a large number of photons and exhibit behavior that is better described using classical physics. The quantum entanglement in the single-photon CIM facilitates a more accurate and stable encoding of information across the oscillator network.
Building the Future: Cyber CIM and High-Performance Computing
To make CIM technology broadly accessible, NTT Research and Tohoku University are collaborating on large-scale cyber CIM simulators that leverage HPC platforms. These simulators will allow scientists and developers to run CIM experiments in software, bypassing the need for specialized optical hardware in early development phases.
Tohoku University is currently optimizing the performance of these simulators using techniques such as:
- Vectorization and parallelization of computation kernels
- Efficient cache memory data management
- Scaling CIM simulations to 100 million spins with sparse connections
The ultimate goal is to create a CIM simulation platform that can solve complex, NP-hard problems quickly and efficiently, offering a practical tool for sectors ranging from artificial intelligence to supply chain optimization.
Dr. Yamamoto emphasized the broader vision, stating:
“By combining quantum optical formalism and digital electronic platforms, our work with Tohoku University brings us one step closer to building a large-scale CIM simulator. This will enable cyber CIMs that offer users an accessible and efficient way to solve stochastic differential equations describing DOPO networks with quantum feedback.”
Real-World Applications and Industrial Relevance
While CIMs are still in the research and simulation phase, the technology holds potential for real-world applications across multiple industries, including:
- Machine learning, particularly in unsupervised clustering and classification
- Operations research, for tasks such as scheduling and logistics
- Drug discovery, through accelerated molecular modeling
- Energy optimization, such as in smart grids or renewable distribution
This partnership between NTT Research and Tohoku University could help bridge the gap between quantum theory and practical engineering, offering enterprises new tools to solve computationally expensive problems in near real-time.
Conclusion: From Theory to Technology
The collaboration between NTT Research and Tohoku University represents more than just academic progress—it lays the groundwork for a new generation of computing that merges the best of quantum mechanics with digital systems. By demonstrating the feasibility and superiority of single-photon CIMs, and outlining a clear path to simulation and eventual physical realization, this joint initiative stands at the cutting edge of quantum-inspired technology.
As the research continues, the focus will shift to developing physical implementations of single-photon CIMs and improving the performance of large-scale cyber CIM environments. These developments promise to usher in energy-efficient, high-speed solutions to some of the most intractable problems facing technology, science, and industry today.
About NTT Research
NTT Research opened its offices in July 2019 in Silicon Valley to conduct basic research and advance technologies as a foundational model for developing high-impact innovation across NTT Group’s global business. Currently, four groups are housed at NTT Research facilities in Sunnyvale: the Physics and Informatics (PHI) Lab, the Cryptography and Information Security (CIS) Lab, the Medical and Health Informatics (MEI) Lab, and the Physics of Artificial Intelligence (PAI) Group.
The organization aims to advance science in four areas: 1) quantum information, neuroscience and photonics; 2) cryptographic and information security; 3) medical and health informatics; and 4) artificial intelligence. NTT Research is part of NTT, a global technology and business solutions provider with an annual R&D investment of thirty percent of its profits.
