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Quantum rings to rule them all

25/08/17Science & Technology

Dr Manus Hayne, academic lead of the QR-SPLED project, speaks to PEN about quantum communication technologies.

Future quantum and classical optical communication technology requires the mass production of very low cost components which can be operated at, or above, room temperature and at telecoms wavelengths. Now, patents are pending on novel technologies which could lead to the roll out of low-cost, high-speed, unhackable communication for all.

The patents will enable industrial partners to exploit breakthroughs developed by Dr Manus Hayne from Lancaster University’s Department of Physics, a world-authority on self-assembled  GaSb/GaAs quantum rings and their use in devices such as telecoms-wavelength vertical-cavity surface-emitting lasers (VCSELs).

Pan European Networks asked Hayne about his work in this area, as well as his thoughts on the field of quantum technology and related research more generally.

Could you begin by outlining the (potential) role that quantum technologies have in the evolution of telecommunications?

Communications that are guaranteed 100% unhackable by the fundamental laws of quantum mechanics have long been cited as one of the first applications of quantum technologies. Research into this particular field has flourished for a number of years across Europe, and elsewhere in the world, in both academic and industrial laboratories. Presently, when an encrypted message is sent, e.g. over the internet, a ‘key’ is used to encode and decode the message. The key is typically the product of large prime numbers. Both sender and receiver need the key, so it also needs to be sent between them. This system is potentially hackable because the key is just a product of numbers, which could be factorised.

This is certainly a difficult and very long process for a product of large prime numbers, but it becomes easier as computers get more powerful, and would be a trivial task for a quantum computer. Also, a hacker is potentially able to listen in and obtain both the key and the message.

In quantum key distribution (QKD) a randomly generated key is sent between the sender (traditionally called Alice) and receiver (traditionally called Bob) using a source which emits single photons on demand. The randomness in the key is intrinsic to the quantum-mechanical polarisation state of the stream of single photons. If an eavesdropper (traditionally called Eve) tries to intercept the message, it will generate anomalies in the stream of single photons and the interpretation of the key, so Alice and Bob will know that Eve has been trying to listen in. The actual message, which has been encoded using the quantum key, is then sent in a conventional (classical) manner.

Clearly, there are a myriad of applications in which quantum telecommunications could be extremely useful, the obvious ones being in security and financial transactions, but if the costs can be brought down it could become ubiquitous rather than restricted to the very high value communications of large corporations and governments.

What challenges remain in the way of being able to mass produce components at low cost? How does your research via the QR-SPLED project tie into this?

There is no doubt that QKD works. However, enabling the large-scale rollout of QKD (or similar schemes) requires the mass production of cheap quantum components in a way that is compatible with the existing production methods of classical components, such as the LEDs and laser diodes used in everyday applications such as computer mice, laser printers, bar code scanners and, of course, optical communication.

Taking single photon sources as an example, an ideal single photon source has a very demanding list of characteristics. It should be small, cheap, efficient (meaning a high yield of single photons), bright (meaning a high repetition rate), able to operate at or above room temperature, emit photons at the most efficient wavelength range for optical fibres (telecommunications wavelengths), and be directly electrically stimulated i.e a single photon light emitting diode (SPLED). Above all, it should emit no more than one photon at a time – less than one is inconvenient, but manageable.

One of the most promising ways to achieve this is the use of self-assembled quantum dots (QDs). QDs are nanoscale droplets of compound semiconductor material that spontaneously form when a very thin layer of the material is deposited on a mismatched substrate, in a similar way to water droplets forming on glass. The most common example is indium arsenide (InAs) on gallium arsenide (GaAs). Further deposition of the substrate material (e.g. GaAs) can be used to embed the QDs into a component, such as an LED, a laser diode, or a SPLED.

However, while many of the above characteristics have been individually or multiply demonstrated, no system (QD or otherwise) has yet been identified that can achieve all of them at once. The goal of the QR-SPLED project, funded by Innovate UK and the Engineering and Physical Sciences Research Council (EPSRC) in the framework of the UK National Quantum Technologies Programme, is to bring us closer to the ideal SPLED.

The novelty of our approach in the project is twofold. Firstly, we will be using self-assembled gallium antimonide (GaSb) quantum rings (QRs) rather than InAs QDs. These are a highly-specialised type of self-assembled QD, even in the semiconductor research community, with some unique characteristics. The most striking one is in their geometry: they really are nanoscale rings, rather than dots. Also, being made of GaSb, they have some fundamental differences to conventional InAs QDs in how they confine the charge that generates the photons. The result is that they readily emit at telecoms wavelengths, and their properties may also prove advantageous for operating at room temperature. The second aspect of our novel approach is that we plan to go right ahead in this one-year project and embed them into an industrially compatible semiconductor material growth and device fabrication process.

How important is industrial collaboration here? Was this easy to achieve?

Working with companies is a very important aspect of my research, built into my formal responsibilities within the physics department, part of my wider role in the university, and even a major part of my undergraduate teaching. I am also co-founder and director of the fledgling spin-out company Lancaster Material Analysis, which provides cross-sectional materials and device analysis services to industrial and academic clients.

In the particular case of the QR-SPLED project, industrial collaboration is absolutely crucial to the whole ethos and practical implementation of the research. In terms of ethos, the genuine objective of the research was always to make SPLEDs in a way that was entirely compatible with current conventional semiconductor photonic device component volume manufacturing. In terms of practical implementation, the industrial partners will make specialist contributions that are the foundation of their businesses, executing their tasks with a level of expertise and professionalism that will match, if not exceed, anything available in academia. Specifically, the semiconductor company IQE will provide production-quality wafers with pre-prepared epitaxial growth, and Compound Semiconductor Technologies (CST) will process the material into devices on their production line.

In my experience, collaborating with companies is not very different from collaborating with academics, in the sense that any successful collaboration should always bring together actors with complementary skills to achieve some common goals. Also common to both types of collaborations is that they involve people. Granted, academics like to publish papers and businesses like to develop profitable new products, so there has to be some common ground, but with continuous advances in technology and the need to innovate, this is often achievable.

For the QR-SPLED project, the involvement of the industrial partners was heavily de-risked by a previous project in which we successfully made classical devices, i.e. QR LEDs and vertical-cavity surface-emitting lasers (VSCELS). The QR VCSELs are particularly exciting; operating with record low threshold current densities (<1 Acm-2), and emitting at temperatures up to 110°C and at telecoms wavelengths. Indeed, a follow-up Innovate UK project to establish the commercial viability of QR VCSELs is due to start in the autumn. This extraordinary success makes us dare to hope that room-temperature, telecoms-wavelength QR-SPLEDs may not be so crazy after all.

What are your thoughts on EU initiatives to help the evolution of quantum technologies (the new Horizon 2020 flagship programme, for instance)?

I lived and worked in Belgium for ten years and have been involved in the preparation, co-ordination and evaluation of EU projects for many years, so I am delighted that the EU is following the UK’s example by investing in quantum technologies. It will provide a fantastic opportunity for UK researchers to collaborate with leading European laboratories and companies, especially in cases where there is specific expertise in mainland Europe that is not available in the UK.

Having said that, my perhaps controversial view is that a downside of the sheer scale of UK investment in quantum technologies to date is that it may deter UK researchers and institutions from participating in what is likely to be a highly competitive EU programme with a significant administrative burden for participating organisations. Take, for example, the recent QuantERA call, to which the EPSRC committed a relatively large €2.3m, compared with other countries. At the same time there was a joint EPSRC/Innovate UK quantum technology call to which EPSRC committed £5m (~€5.6m) and Innovate UK £10m.

Looking to the QR-SPLED project and beyond, what are your hopes for the long(er) term? Where will your future research priorities lie?

The QR-SPLED is a feasibility study, and we are genuinely treating it as such. Although telecoms wavelength emission from this system is almost inevitable, it remains to be seen whether QRs can reliably emit single photons, and at what temperatures. If the results of the project are promising, we intend to apply for follow-on funding with our existing industrial partners to further develop the technology, perhaps with some systems and end-user companies on-board too.

In the meantime, we plan to press on with the QR VCSEL work, which has proven to be a very promising spin-off on the road towards QR-SPLEDs. QR VCSELs have a number of potential markets in which demand is clearly established. These include fibre to the home or business, and replacing incumbent shorter-wavelength VCSELs in datacoms. Besides that, we have an insanely ambitious research programme on the development of fast, low-voltage, non-volatile memories, but that is another story.

 

Dr Manus Hayne

Academic lead – QR-SPLED project

Lancaster University

http://www.lancaster.ac.uk/physics/about-us/people/manus-hayne

 

This article will appear in Pan European Networks: Science & Technology issue 24, which will be published in September, 2017.

Pan European Networks Ltd