The Solid State Quantum Network

Quantum communication, the transfer of quantum superposition states over long
distances, is presently limited to about 200km (both in optical fibre and free space) due to
unavoidable photon absorption losses. This research aims to extend this
distance using "entanglement swapping" and "teleportation". By
concatenating short entanglement swapping sub-sections it is in principle possible to
generate entangled (correlated) bits over very long distances with bit rate only limited by the
losses in one short section. If realised this would extend quantum communication
applications such as quantum cryptography and quantum teleportation out to distances of
thousands of kilometres.

By the end of the project we hope to have demonstrated the components for a quantum
network that will allow the teleportation of single photon states over 1000km. This opens up
possibilities in two directions for real-life applications: fibre-based ground systems, and
ground-satellite communications. Fibre-based ground systems will allow secure
communications between two parties, and a fibre-based source of entangled cluster states of
photons for home quantum computing or quantum "cloud" computing (transmission of cluster states to a cloud quantum-computing base). Ground-satellite communications will allow, for
the first time, unprecedented proof-of-principle tests of quantum and relativistic physics.

Our proof of principle research will be done using well-understood dots and cavities
operating at 900-980nm using off the shelf detectors. This proof of principle will be
transferred to less well developed 1.3 micron dots and eventually to 1.55 micron wavelengths to be
compatible with fibre-based quantum communication system. Work on longer wavelength
growth is ongoing at UWUERZ and any progress could be rapidly incorporated. We also
believe that any first demonstration of a spin-photon entanglement swapping module at
950nm will stimulate worldwide interest from industry and academia in transferring the
technology to longer fibre communication wavelengths. At 1.3 micron the cavity fabrication
technology can be the same GaAs/AlGaAs growth while deeper dots will enhance electron
isolation and maybe allow higher temperature operation. We also are witnessing rapid
improvement in single photon detector technology (such as superconducting detectors)
which will provide efficient long wavelength single photon detection by the end of this project.

All partners involved in this project have strong links with industrial partners with interests in
quantum technologies. For example, Bristol has a strong connection with Toshiba Research,
HP Labs, Hitachi, GCHQ, Oclaro, Nokia, IDQuantique and Picoquant, whilst UWUERZ have
links to QuTools, Thales, and Becker & Hickl.

While the goals of this project are, in principle, towards a practical quantum network, the
physics of the electron spin system may be studied in more detail in these cavity systems. In
addition, the new element of strong-coupling to a spin system may give rise to novel physics,
while the electron-nuclear interaction may allow ultra-long-term (hours) memories to be
contained within the nuclear spin. Electron spin quantum memories are a suggested solution
for the need for quantum memories connected by optical circuits over short scales, for
example in quantum chips. These require that the QD be embedded into a photonic device.
The scientific output of this project will feed into these wider goals.

All the partners will continue to engage with their contacts in industry, keeping them informed
via research papers, conference presentations and informal visits. We note that any
quantum-optical devices in the future are likely to be based on solid-state technology. One
possible short term output from this project will be the demonstration of ultra-low power
(attojoule) all optical switching through strongly coupled cavity schemes. This in itself could
revolutionise classical optical communications. Another significant long-term impact of this
project will be the training of PhD students and postdoctoral researchers who will later move
into telecommunications and microelectronics industries. At present, quantum information
still has an esoteric reputation: while many physicists are now conversant with the idea of
quantum information, quantum physics is hardly touched undergraduate engineering
courses. Here we will train both physicists and engineers, many of whom may later go into
industry bringing their expertise with them.