Theoretical Quantum Physics Research Group

Our group pursues innovative cutting-edge theoretical research in quantum physics and quantum information processing. We are interested in how it can be applied to the real-world development of quantum-enabled technologies. The main areas of our research are:

Foundations of Quantum Mechanics

The foundation of quantum theory has been a great source of stimulation to the development of quantum information science. Conversely, the insight obtained in quantum information science can help deepen our understanding of the foundational aspects of quantum theory itself.

We try to illuminate the weird nature of quantum theory, especially nonlocal feature induced by entanglement. Our recent work includes:
  • Masato Koashi, Koji Azuma, Shinya Nakamura, and Nobuyuki Imoto, Does "quantum nonlocality without entanglement" have quantum origin?, arXiv:1303.1269 (2013).

Quantum Key Distribution

Quantum mechanics allows us to accomplish tasks that cannot be done with classical means. An important example of such tasks is quantum key distribution (QKD) which distributes a secret key, which is a random bit string, in an information-theoretic-secure manner to ensure the security against any eavesdropping. The secret key is to be used to encode and decode plain texts in one-time-pad method in standard communications.

To achieve information-theoretic security in QKD, the users' devices must not deviate from the mathematical models of those devices that the theory of QKD requires. In practice, there is a gap between the properties of the actual devices and the theoretical models, which is called a security-loophole or a side-channel. Therefore, in order to guarantee the practical security of QKD, we have to fill this gap.

On the other hand, as the key generation rate and achievable distances of current QKD protocols are limited, it is also important to propose more efficient QKD protocols with higher key generation rate covering longer distances.

Considering both theoretical and experimental counter-measures against security loopholes, and proposing more efficient QKD protocols constitute our main research direction, and our recent work includes:
  • Koji Azuma, Kiyoshi Tamaki and William J. Munro, All-photonic intercity quantum key distribution, Nature Commun. 6,10171 (2015).

  • Kiyoshi Tamaki, Marcos Curty, Go Kato, Hoi-Kwong Lo, and Koji Azuma, Loss-tolerant quantum cryptography with imperfect sources, Phys. Rev. A 90, 052314 (2014).

  • Kiyoshi Tamaki, Hoi-Kwong Lo, Chi-Hang Fred Fung, and Bing Qi, Phase encoding schemes for measurement device independent quantum key distribution and basis-dependent flaw Phys. Rev. A 85, 042307 (2012).

Quantum Communication, Networks and Computation

The development of technologies based on the principles of quantum mechanics are likely to be one of the core foundations upon which the twenty-first century will rely. In fact these resources allow tasks, such as unconditionally secure communication, the prime factorization of large integers or simulating complex systems, to be undertaken which are extremely hard to achieve with classical computers. This broad field of quantum information processing can be divided into a number of areas including: quantum metrology, imaging and sensing, quantum communication and quantum computation.

We are investigating how to create quantum information processing devices such as quantum repeaters, networks and computers. Several examples of our recent work include:
  • Koji Azuma, Kiyoshi Tamaki, and Hoi-Kwong Lo, All photonic quantum repeaters, Nature Commun. 6, 6787 (2015).

  • Koji Azuma, Hitoshi Takeda, Masato Koashi, and Nobuyuki Imoto, Quantum repeaters and computation by a single module: Remote nondestructive parity measurement, Phys. Rev. A 85, 062309 (2012).

  • W. J. Munro, A. M. Stephens, S. J. Devitt, K. A. Harrison and Kae Nemoto, Quantum communication without the necessity of quantum memories, Nature Photonics 6, 777 - 781 (2012).

Hybrid Quantum Systems

It is now considered that no single type of physical system possesses all the required attributes for quantum information processing tasks such as computation, communication and sensing. Systems that benefit from an excellent ability to perform multi-qubit gates tend to have short coherence times, while those that have long coherence times are limited in their ability to perform multi-qubit gates. Hybridization of quantum systems from each class, however, allows one to exploit the best properties of the individual systems. Nitrogen-vacancy centers in optical cavities possess an extremely long lived nuclear spin, as well as an electron spin which couples to optical fields, allowing natural remote two-qubit gates to be achieved. Such a hybrid approach naturally enables distributed information processing, where remote nodes are networked together. Superconducting circuits and electron spin ensembles are another example. Here the superconducting qubits provide easy manipulation and processing of information, while a spin ensemble (having long storage times) can act as a quantum memory.

Our group theoretically explores various aspects of these hybrid systems including how they can be realised and used as well as modelling them. Several recent papers include:
  • Kae Nemoto, M. Trupke, S. J. Devitt, A. M. Stephens, B. Scharfenberger, K. Buczak, T. Nobauer, M. S. Everitt, J., Schmiedmayer, and W. J. Munro, Photonic Architecture for Scalable Quantum Information Processing in Diamond, Phys. Rev. X 4, 031022 (2014).

  • Ashley M. Stephens, Jingjing Huang, Kae Nemoto, and William J. Munro, Hybrid-system approach to fault-tolerant quantum communication, Phys. Rev. A 87, 052333 (2013).

  • X. Zhu, Y. Matsuzaki, R. Amsuss, K Kakuyanagi, T. Shimo-Oka, N. Mizuochi, Kae Nemoto, K. Semba, W. J. Munro and S. Saito, Observation of dark states in a superconductor diamond quantum hybrid system, Nature Commun 5, 3424 (2014).