Information and Telecommunications
Scalable absolutely secure optical fibre networks using quantum cryptography
Project Leader:
Researchers :
N. Godbout (École Polytechnique de Montréal)
G. Brassard (Montréal); B. King (McMaster);
R. Laflamme, A.H. Majedi (Waterloo); H.K. Lo, A.M. Steinberg, R.L.
Williams (Toronto)
For ages, mathematicians have dreamt of a system that would allow people to exchange
messages in absolute secrecy. Around the middle of last century, Shannon proved that this is
possible if and only if the legitimate participants share a random secret key which is as long as
the message they wish to transmit. But Shannon’s theorem did not take account of the quantum
world in which we live. When information is appropriately encoded as quantum states, such as
single polarized photons, any eavesdropping attempt to access it yields partial knowledge at best
and entails a probability of spoiling the information irreversibly. This unavoidable disturbance can
be detected by the legitimate users. In principle, this phenomenon can be exploited to design a
quantum cryptographic system that is unconditionally secure even against an eavesdropper
whose computing power and technology are restricted only by the laws of nature, with no need
for a long shared secret key. The question of security for practical implementations is more
subtle, though, and the research team will work on such proofs.
The project aims at leveraging the Canadian expertise in theoretical and experimental quantum
information processing to make significant advances in the research and development of practical
and scalable quantum key distribution systems over optical fibre. In particular, the team will
implement a quantum cryptographic network that uses its novel idea of frequency multiplexing to
provide secure communication between several users, with no need for a dedicated fibre link
between each pair of users. The next step will be to use quantum entanglement to make the
quantum cryptographic network secure even against insider threats. A longer-term goal is the
development of quantum repeaters that will allow for virtually unlimited distance in quantum
cryptographic links. This is challenging because a quantum repeater cannot be a simple amplifier,
as in classical networks. In order to preserve secrecy, a quantum repeater must use quantum
error correction techniques to regenerate quantum information without actually measuring it. This
requires the ability to implement quantum logic gates between two different qubits – a non-trivial
task, since photons do not per se interact with one another. That is why the team includes experts
in quantum information processing, performing basic and applied research on the quantum circuit
serving as a repeater.
The advances made through this collaborative work will accelerate the development of a
Canadian “quantum optical toolbox”: a set of concepts, proofs, techniques, devices, simulation
tools and highly qualified personnel, which can be put to use in the long term for the development
of other applications of quantum optics. Such applications include interaction-free measurements,
sub-diffraction limit imaging, control and measurement of chemical reactions, etc. Of particular
interest in this toolbox will be the development of a highly efficient single-photon source at
standard telecom wavelengths that is guaranteed to emit one and only one photon on demand.
Affiliates involved in this project:
Communications Security Establishment, Canadian
Photonics Fabrication Centre, National Research
Council Institute for Microstructural Sciences, Mike
Lazaridis, MetroPhotonics, ITF Optical Technology
Inc., Department of National Defence