Solid State Quantum Networks (SSQN)

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. For this reason, theoretical schemes to extend this distance using "entanglement swapping" and "teleportation" have been established. 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.
In this consortium we propose to work towards such a deterministic quantum network based on semiconductor quantum dot-micropillar cavity systems. We will generate entangled photon sources from the biexciton-exciton cascade of a quantum dot (QD), with a potential fidelity of >90%. Moreover, we will develop a QD-spin micropillar cavity system, which acts as an all-in-one spin-photon-interface and a Bell-state analyser. This component eliminates the need for synchronous arrival of the two photons, and allows a wait-until-success protocol over the timescale of the spin coherence time (microseconds to milliseconds). Further subcomponents will include electro-optically tuneable single photon sources and recently proposed sequentially entangled sources.
With this suite of subcomponents we will be able to realise all the functions required for a scalable quantum network including the final entanglement purification steps. This is in contrast to previous experimental demonstrations of entanglement swapping (and teleportation) which were probabilistic and thus unscalable.
The project involves collaboration between four partners. We will bring together two world-class groups, LPN and Würzburg (UWUERZ), working on micropillar cavities producing highly efficient entangled pair sources (LPN), and strongly-coupled QD-spin-cavity systems (UWUERZ), with the aim of addressing the challenging issues of entangled-pair sources and spin-cavity systems. Theoretical support for novel and practical entanglement schemes will be provided by Imperial College (IMP), and the experimental implementation will be performed by Bristol (BRIS) and LPN, who have world-class expertise in quantum optical communication , QD spins and semiconductor microcavity quantum electro-dynamics.

The project long term vision includes the development of:
- Global secure communication and computation using quantum key distribution via optical fibre network with quantum repeaters.
- A quantum internet: supply of cluster states of entangled photons via the existing optical fibre network to the home and office.
- A spin out technology of ultra-low power quantum switches for classical optical networks.
Despite breakthroughs in miniaturization of quantum optical circuits and atom-cavities, the first quantum computing device is likely to be a very large expensive device, possibly requiring cryogens, sophisticated components and even a technician to operate it. It is still not yet clear exactly which platform will win out, although one might predict that it will almost certainly contain a photonic element. In order for quantum computing to develop into a commercially viable product that is available to business, research labs and eventually the home, a reliable way of transporting a single photons state from one place to another is needed. In fact, one might argue that for successful industry sponsorship of quantum computing, developing a quantum communication network is more important than developing the quantum computer itself. We can then construct a vision of future quantum computing with a quantum network. This could include:
-a high-functioning quantum computer contained within a research/government facility (a cloud quantum-computing facility) performing high-end quantum algorithms,
-a low-cost plug-and-play quantum chip (such as a linear-optical quantum circuit) that is small, inexpensive and requiring minimal expertise to use in the home/office,
- a quantum interconnect, supplying cluster states of entangled photons to the end-user, and possibly relaying back photons output from the low-cost chip for detection.
In our scheme, optical fibres would route single photons from the user to the cloud QC. To overcome losses, we would use QD-pillar spin-photon interfaces at substations placed every 50km or so to perform the function of entanglement swapping, as described in the proposal. By separating the functionality in this way, cooling to <20K could easily be incorporated into large arrays of fibre-coupled entanglement swappers at relatively low cost.
Note that by using this separated functionality, high-level quantum computing facilities may be accessed by the end user in an absolutely secure way. The algorithm may be designed such that the photons sent by the end user to the substation for detection are in a random basis. The end user performs a classical correlation of the single photon detection events with the basis used with the quantum chip. As the end user is the only one with this information, the algorithm used is absolutely secure.
Pathway to Impact:
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 clusterstates 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.