Quantum technology for the 21st century

In the laboratory environment, atom interferometry provides a way to measure rotations and accelerations with spectacular accuracy. Very low noise figures are possible, but also extremely stable bias (zero offset) and calibration. This opens the possibility of inertial navigation over long periods without needing recourse to the global navigation satellite system.

I will outline the basic idea of atom interferometry and explain why it is so accurate. I will also discuss the challenges that arise when trying to incorporate this new technology into a practical inertial navigation system.

Extremely long electron and nuclear spin coherence times have recently been demonstrated in isotopically pure Si-28 making silicon one of the most promising semiconductor materials for spin based quantum information. The two level spin state of single electrons bound to shallow phosphorus donors in silicon in particular provide well defined, reproducible qubits and represent a promising system for a scalable quantum computer in silicon. An important challenge in these systems is the realisation of an architecture, where we can position donors within a crystalline environment with approx. 20-50nm separation, individually address each donor, manipulate the electron spins using ESR techniques and read-out their spin states.

We have developed a unique fabrication strategy for a scalable quantum computer in silicon using scanning tunneling microscope hydrogen lithography to precisely position individual P donors in a Si crystal aligned with nanoscale precision to local control gates necessary to initialize, manipulate, and read-out the spin states. During this talk I will focus on demonstrating spin transport and single-shot spin read-out of precisely-positioned P donors in Si. I will also describe our approaches to scale up using rf reflectometry and the investigation of 3D architectures for implementation of the surface code.

In the early 1980's, observation of a magneto-resistance anomaly in metallic thin films was attributed to the phenomenon of weak localisation of electrons and to time-reversal symmetry breaking due to a magnetic field acting upon charged particles.

In the context of our quantum simulation program, we have directly observed weak localisation of ultra-cold atoms in a 2D configuration, placed in a disordered potential created by a laser speckle. In order to manipulate time-reversal symmetry with our neutral atoms, we take advantage of the slow evolution of our system, and we observe the suppression and revival of weak localisation when time reversal symmetry is cancelled and re-established.

Cameras are often marketed in terms of the number of pixels they possess – but do more pixels mean a ‘better’ camera?  Rather than increasing the number of pixels we ask the question 'how can a camera work with a single pixel?'. This talk will link the field of computational ghost imaging to that of single-pixel cameras explaining how spatial structuring of either the illumination or imaging system means that image and video reconstruction can be achieved using just a small number of photodiodes.

Such approaches are particularly useful for imaging at wavelengths where detector arrays are either very expensive or unobtainable. Within QuantIC we are using this technology to make low-cost cameras for imaging the short-wave Infra red, allowing the visualisation of leaking gas.

Quantum imaging strives to develop methods for improved image formation based the use of quantum phenomena. These quantum images can be superior to those created by conventional methods in terms of sharpness, signal-to-noise ratios, ability to work at low light levels, etc. Aims of research in this area include both the development of increased understanding of quantum protocols for superior imaging and the implementation of these protocols. In this presentation we describe several such protocols under recent investigation. One protocol entails ghost imaging, in which coincidence measurements are used to obtain spatial information about an object even though only a non-imaging (“bucket”) detector is used to collect photons that have interacted with the object. This protocol can prove especially useful when the two photons detected in coincidence have very different wavelengths. Another protocol involves the use of Mandel’s induced coherence without induced emission. It has recently been shown that this process can lead to the formation of images carried by photons that are actually never detected. A third example is the development of imaging procedures for use with extremely weak thermal light fields (such as nighttime illumination by star light). The protocol entails subtracting exactly one photon from such a thermal field. This photon-subtracted state displays strong quantum features such as sub-Poissonian noise properties.

Quantum communication offers long-term security especially relevant for government and industry users. On a very fundamental level the security of quantum key distribution relies on the non-orthogonality of the quantum states used. So even coherent states are well suited for this task, the quantum states that describe the light generated by laser systems. Continuous variable quantum key distribution with coherent states uses a technology that is very similar to the one employed in classical coherent communication systems, the backbone of today's internet connections. Here we review recent developments in this field in two regimes: (1) improving QKD equipment by implementing front-end telecom devices and (2) research into satellite QKD for bridging long distances by building upon existing optical satellite links.

In optical fibre systems continuous variable quantum cryptography reaches GHz speed and offers efficient integration with commercially available telecommunication techniques, especially in short links within an inner-city or a data centre. Compact and efficient sending and receiving devices can be made of integrated optical components including quantum random number generators.

Optical free space communication is a reliable means to transmit classical and quantum information. Free space links offer ad-hoc deployment in intra-city communication, air-to-ground or satellite-to-ground scenarios. In quantum communication the experimental effort has so far been devoted mostly to discrete variables such as the polarisation state of single photons. We present experiments investigating free space transmission of quantum continuous variable states using homodyne measurement, rendering the quantum states immune to stray light and enabling daylight operation. Quantum communication with satellites offers a viable solution to bridge long distances. We will discuss our current joint project with Tesat Spacecom / Airbus in the development of quantum key distribution with coherent optical communication in satellite systems.

One of the fundamental challenges in the development of quantum technologies is to control noise in these quantum systems.  A quantum system always suffers from noise due to unwanted interactions with the environment and imperfect controls. The noise effects on each component of our quantum system tend to accumulate, and it is impossible without noise control for the system to maintain its quantum coherence required for quantum information processing.  The control of noise is hence essential for one to realise scalable quantum information systems.  In this presentation, we show how we can overcome noise to create a scalable quantum information system both in terms of its size and running time. To achieve this, we consider an approach in which both of the basic device and system architecture are designed together. Such a device can be realised using a single NV centre embedded within an optical cavity. We show how this device as a fundamental module can be used to construct a scalable quantum information system.

Secure communications based on quantum cryptography is already employed to transmit highly sensitive medical or financial data. Expanding the range of applications to more widespread use will require a dramatic reduction in the cost of deploying the technology.  Central to this challenge is the requirement to avoid expensive dedicated dark fibre for the quantum channel and rather to send the qubits in the ordinary data-carrying network.

Here I review progress on integrating quantum communications into conventional telecom infrastructures. Recent work demonstrates it is possible to send qubits along optical fibres which are simultaneously carrying the high data bandwidths found in modern communication systems. This allows a high bandwidth quantum encryption system to be realised, in which multiple 100 Gb/s data channels are secured by quantum keys transmitted along the same fibre.

Deployment costs can be greatly reduced in conventional fibre optic communications using point to multi-point links to connect many customers to a common point in the network through shared fibre. This concept can also be applied to quantum communications, allowing multiple users to share the fibre and equipment costs. Indeed recent studies show that quantum key distribution can be combined with data transmission in GPON access networks able to connect up to 128 users.

Finally, I will discuss plans to implement these technologies in a large-scale quantum network in the UK.