Solid State Quantum Networks (SSQN)

Lead Research Organisation: University of Bristol
Department Name: Electrical and Electronic Engineering


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 entangld 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.

Planned Impact

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.
Description This grant involved a collaboration between four groups:
BRISTOL: Photonics and Quantum Photonics group, University of Bristol
UWUERZ: Phisic Dept, University of Wuerzburg
LPN: Laboratoire de Photonique et de Nanostructures
IMP: Imperial College London, Blackett Labs

We bring together two world-class groups, LPN in
Marcoussis near Paris and University of 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. The
University of Bristol is studying the experimental implementations and future protocols while theoretical
support is provided by Imperial College (IMP).
The fabrication groups UWUERZ and LPN have made various pillar microcavity systems for bright single
and pair photon sources and for spin photon interfaces. These systems have been characterised at LPN,
UWUERZ and BRISTOL and initial fabrication problems have been largely solved. This has led to significant
advances in bright sources with an >80% efficient single photon source being developed using LPN's
technology where pillars are fabricated aligned to single quantum dots. In year 2/3 UWUERZ have also
made significant progress (geared by a national funded project) in making indistinguishable single photon
sources. The success in indistinguishable single photon sources has allowed simple CNOT gate
demonstrations to be made at LPN. Using two sources, LPN could also demonstrate a cavity enhancement
of the two photon interference from remote QD sources.
The development of spin measurement schemes came to fruition in year 3. In LPN a positive (hole)
charged dot in a medium Q microcavity showed a spin dependent reflectivity and was able to be spin
pumped. A modulation doped dot in a low-Q pillars was found by UBRIS to have an electron spin
dependent reflectivity signature. In both cases several degrees of spin dependent Faraday rotation were
seen. The LPN has extended the deterministic fabrication of QD-cavity devices to obtain an electrical
control of the QD charge state. The development of spin photon interfaces can now begin in earnest.
LPN have looked at the non-linear saturation of strongly coupled cavity dot systems and have seen nonlinearity
at an estimated 8-photon level. Results in Bristol on a strongly coupled dot cavity also showed non-linearity at the few photon level which we fitted accurately with a semiclassical model and a fully quantum explanation.
Exploitation Route These findings have contributed to the development of my fellowship EP/M024458/1. They are also relevant yo the areas below
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Healthcare,Other

Description EPSRC QT fellowship
Amount £1,500,000 (GBP)
Funding ID EP/M024458/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 05/2015 
End 04/2020
Description European Research Council
Amount £2,250,000 (GBP)
Funding ID 247462 QUOWSS 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 02/2010 
End 02/2015
Description Future Photonics Hub, Innovation Partnership Fund, Southampton University
Amount £10,000,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2017 
End 03/2018
Description IL6 Russia - Non Newton
Amount £146,078 (GBP)
Funding ID 352345416 
Organisation British Council 
Sector Charity/Non Profit
Country United Kingdom
Start 02/2018 
End 02/2021
Description RS-MOST travel grant 
Organisation National Cheng Kung University
Country Taiwan, Province of China 
Sector Academic/University 
PI Contribution We are characterising the light scattered by Blue phase liquid crystals and investigating the improved stability of crystals in microstructures fabricated in our two photon lithography sysem
Collaborator Contribution Providing Blue phase liguid crystal samples and collaborative advice
Impact We have discovered a new way to stabilise Blue phase liquid crystal and are writing a paper on this.
Start Year 2017
Organisation University of Southampton
Country United Kingdom 
Sector Academic/University 
PI Contribution This is a collaborative grant spanning two institutes. We are optical characterisation and device applications
Collaborator Contribution They are materials development, growth and fabrication
Impact This grant is an outcome of an informal collaboration dating back to 2007
Start Year 2007
Description SSQN Imperial college 
Organisation Imperial College London
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution These were members of our CHISTERA project SSQN. We contributed experimental expertise
Collaborator Contribution They contributed theory
Impact n
Start Year 2009
Description University of Wurzburg 
Organisation University of Wurzburg
Country Germany 
Sector Academic/University 
PI Contribution Design of pillar microcavity samples for characterisation in Bristol
Collaborator Contribution Growth and fabrication of microcavity samples
Impact Successful samples showing a spin dependent response. High efficiency single photon source samples.
Start Year 2010
Description Invited Speaker and Panel Member at Royal Society Frontier of Science Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Ruth Oulton was invited to represent the field of Quantum Cryptography at the prestigious Royal Society UK-Russia Frontiers of Science meeting in Kazan. This meeting acts to promose scientific exchange in a broad range of fields between two countries, and was accompanied by a press conference.
Year(s) Of Engagement Activity 2013