Silicon Photonics for Quantum Fibre Networks

Lead Research Organisation: University of Bristol
Department Name: Faculty of Science


Nowadays secure communication is essential for exchange of sensitive information, while the security based on classical cryptography protocols cannot be absolutely guaranteed. Especially when a full-tolerant quantum computer will be available, the classical encryption and decryption methods will be no longer secure, posing a serious threat to cryptosystems. Quantum cryptography (QCy), a branch of Quantum Communications (QCs), has opened a new era in the security of information transmission. Although big steps in QCs and Quantum Key Distribution (QKD) have been taken over the last 20 years, the future of QCs is still challenged by major barriers. The SQUARE project shatters these barriers by developing the next generation of quantum devices based on silicon photonics, enabling compact, reliable, and efficient components, allowing for the construction of fully connected long-reach, high-rate QKD-secured quantum communication networks. The SQUARE project develops a fully integrated transmitter (Tx) and receiver (Rx) for quantum communications and classical communication based on silicon photonics, which is a powerful means to combine the assets of integrated photonics with CMOS technologies. The driving force behind silicon photonics is that silicon represents a mature integration platform, which brings photonic circuits to a higher integration level allowing for mass-production as it has done for electronic circuits. In this sense, we believe that silicon photonics will play a significant role in future QCs, allowing to build up powerful and cost-effective photonic circuits for quantum communication applications. SQUARE will develop the integrated quantum technologies necessary to breach the limits of current state-of-the-art, and furthermore integrate these technologies directly in quantum subsystems. Hence the components will be developed for specific applications in specific quantum network scenarios. The proposed novel network leverages on the strengths of different technologies to obtain global benefits in terms of reach, sensitivity, complexity, practicality and total power consumption.

Planned Impact

The impact of SQUARE project can be divided in three different areas, Economical and social, Industrial and technological, and Scientific.

As reported in the SOTA and motivation paragraph the security of our daily communications is not guaranteed. In the European Union, the quantity of confidential information is growing every day. From personal data, like medical data over government accounts to bank correspondences, QKD could be implemented with benefit and give unconditional security. Furthermore, key transportation hubs like subway stations, bus stations, highways and airports, as well as other sensitive infrastructural locations such as power plants should be able to be monitored and controlled through secure communication channels.

Economical and social aspect
The work proposed in SQUARE constitutes a first step towards an industrial production of silicon quantum chips, necessary for integrated QKD and usable to other quantum systems. In this sense the opportunity of merging together all the knowledge and know-how related to the consortium background, with the vision of the companies involved directly in the project and in the Advisory Board can provide a fully qualified and trustable output. Although the aims of the proposal are clear and well-defined, a complete business plan would not be relevant at this point, but it could be considered within the total duration time of the project. In fact, we believe that within the next decade, QKD will be widely used by international diplomacy, networks of embassies and defence communications, financial institutions and banks and international corporations in order to increase the level of security.

Industrial and technological aspect
The SQUARE project has the potential to develop new quantum devices based on integrated chips which is necessary for establishing secure communications on a global scale. Our proposal provides an excellent solution offering devices beyond state-of-the-art which will be beneficial for the entire industrial sector. Moreover, the opportunity of exploiting new degrees of freedom, like entanglement distribution, space division multiplexing, can open newopportunities not only for new types of quantum communications, but also for real life applications like multi-users QKD and multi-users secret sharing process. Although the proposed project is very ambitious and not without risk, but it offers the potential for very high-impact research and technology efforts in the future. Knowledge and technology
know-how on a European, even international, scale is crucial for its success.

Scientific Scenario
European scientists were the leaders in the development of the pioneering theory of quantum mechanics in the 1920s as well as in the first implementation of the resulting quantum technology. The SQUARE project, is a very prestigious and high impact project, it will enable European scientists in this domain to maintain the leadership by extending the field into chip devices: Europe will be the first to have an integrated quantum secure communication network capable of connecting different countries and cities, to ensure complete secure communication. While there is certainly risk involved in such an ambitious venture, it will, if successful, open up radically new possibilities for research that ranges from speculative and visionary to experiment-based theory. While the SQUARE consortium is strictly European, the consortium members are in a position to draw on scientific knowledge and technological know-how on an international scale. Collaborations with Japan (NTT) and USA (IBM) are under way, and may well provide opportunities to enhance the project's success.
Description The team at Bristol has made substantial progress in this project so far, with the main three milestones being:
* The generation of photon pairs on-chip across 4 independent photon sources producing 4-dimensional entangled photons or qudits. Qudits allow for a more complex encoding of quantum information, and have been shown to be more resilient to noise in the communication channel than traditional photonic qubits.
* The photons generated have been transferred - using a multi-core optical fibre - to a receiver chip where a good fringe visibility has been observed. To do this, we have used a purposely-designed photonic chip, in combination with a multi-core optical fibre and stabilisation protocols developed in software to compensate for fluctuations of the optical properties in the fibre over time.
* Finally, on the receiver chip, we have performed Bell state measurements result in a reasonably high purity and 4-dimensional states were measured with a nearly-unity purity and absolute fidelity. This demonstrates that the quality of the qudit is preserved in the communication, and that quantum communication protocols using qudits can be tested with this set-up.
In parallel, Bristol has optimised several fabrication steps independently: namely the definition of the structures by eBeam lithography and dry etching of the waveguides and grating couplers. This is essential for the realisation of single-photon detectors integrated on-chip. Said detectors are expected to play a key role in the communication system, and in particular, for the implementation of Measurement-Device-Independent Quantum Key Distribution protocols.
Exploitation Route The results achieved so far pave the way for the realisation of complex quantum communication-secure cost-effective systems and protocols. This has immediate impact on communications and security, where Quantum Key Distribution is already commercialised using different standards.
This results achieved in this project, demonstrate how single photons can be used to encode high-dimensional quantum states (qudits) using standard integrated photonic devices. The project also shows how these qudits can be shared using multi-core optical fibres with other integrated phonic devices where the quantum states can be read out for purposes such as Quantum Key Distribution.
Sectors Digital/Communication/Information Technologies (including Software),Security and Diplomacy

Description Bristol Quantum Information Technologies Workshop 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact BQIT is a three day annual workshop aimed at enabling leading UK and international academics and industrial partners to come together; to explore and discuss future ambitions and challenges in the field of Quantum Information Technologies. Since conception in 2014 BQIT has had 162 speakers and panellists who have presented their work and opinions on a range of topics, from quantum theory to innovation in industry.

My talk was titled "Silicon Photonic Quantum Technologies".
Year(s) Of Engagement Activity 2018
Description Keynote lecture, European Conference on Integrated Optics (ECIO) 2019 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The European Conference on Integrated Optics (ECIO) focused on leading edge research on integrated optics, optoelectronics and nano-photonics and gathers experts from academia and industry to show their latest technical results, and showcase their products and services. I was invited to give the keynote lecture titled "Integrated quantum photonic technologies for applications communications and computation".
Year(s) Of Engagement Activity 2019