QUROPE Quantum Information Processing and Communication in Europe

Quantum Random Number Generators (QRNG)

Our information based society consumes lots of random numbers for a wide range of applications like, e.g., cryptography, PIN numbers, lotteries, numerical simulations, etc. The production of random numbers at high rates is technically challenging; at the same time, given the pervasiveness of the deployment of random numbers, poor random number generators can be economically very damaging. Today, there are three kinds of random number generators on the market: computer-based pseudo-random number generators, discretised thermal noise and quantum based. The first kind produces sequences of numbers that look random, but are in fact the result of a deterministic process. The second kind is based on the complexity of thermal noise; however thermal relaxation times make these random number generators relatively slow, in the range of tens of Kbit/second. On the other hand, quantum physics provides the only truly source of randomness in Nature. Moreover, in the basic configuration (a photon impinging on a beam splitter followed by two detectors associated to the bit values 0 and 1) the origin of the randomness is clearly identified. Today's commercial quantum random number generators produce about 4 Mbit/second. Their drawback is a significant cost compared to thermal noise based devices, but one expects that (near) future QRNG will provide higher rates at lower costs.

Quantum Metrology

Quantum entanglement provides instances of quantum states of objects that can be designed to be very robust to unwanted noise, while at the same time being extremely sensitive to a quantity we need to measure. This sensitivity can be exploited to overcome the classical limits of accuracy in various kinds of measurements, for example in ultra-high-precision spectroscopy, or in procedures such as positioning systems, ranging and clock synchronisation via the use of frequency-entangled pulses. For instance, in the latter case, picosecond resolution at 3 km distance has been attained. Large scale laser interferometers with kilometre arm lengths are currently being built or started operating in Europe, the USA and Japan with the hope to achieve the first direct detection ever of gravitational waves and thus to open a new field of astronomy. For these detectors the classical sensitivity limit is a serious restriction. It is likely that for the first detection one will have to implement continuous variable entangled light beams in the two interferometer arms to overcome the classical limits. A collaboration of scientists from Europe, USA and Australia (LIGO Scientific Cluster) has recently reported 3dB quantum noise reduction in the sensitivity of the German GEO 600 gravitational wave detector through injection of squeezed laser light at kilohertz frequencies.

State-of-the-art atom clocks have reached the level of accuracy limited by quantum noise of atoms. Entanglement of atoms in clocks may allow surpassing this limit by generation of spin squeezed states of atoms. Work towards this goal is going on in Europe and in the US.

Single quantum particles can be used as nanoscopic probes of external fields. Along these lines, atomic-scale (up to few nanometers) resolution in the measurement of the spatial structure of an optical field via a single ion, as well as sub-shot-noise atomic magnetometry via spin squeezing and real-time feedback have been already experimentally demonstrated. Solid state implementations of quantum sensors exploit the quantum features of artificial atoms such as defect color centers, most prominently nitrogen vacancies in diamond. They are now being used as ultrasensitive probes for magnetic and electric fields, with enhanced resolution through quantum control techniques. While electron spin resonance in diamond NV centers was known for a long time, it took the understanding of interaction between a spin with a many-body spin bath, i.e. quantum many body physics, to develop such exquisite magnetic field sensors that surpass existing sensing capabilities by many orders of magnitude. It allows performing NMR on a single nuclear spin, and it is expected to yield to single molecule NMR at ambient condition. The quantum properties of these single spins within fluorescent particles are now also being used to study in-situ dynamical probes of biological environments, for example by optically detecting magnetic resonance of individual fluorescent nanodiamonds that are distinguished through their individual Rabi frequency inside living cells. Such single-spin probes in biological systems may open up a host of new possibilities for quantum-based imaging in the life sciences.

The quantum regime is being explored and applied also in the manipulation of nanomechanical devices like rods and cantilevers of nanometer size, currently under investigation as sensors for the detection of extremely small forces and displacements. Several groups in both Europe and the US have now achieved the preparation of nano- and micromechanical systems in their motional quantum ground states and measurement sensitivities beyond the standard quantum limit through squeezed motional states are within reach.

One of the main steps in the development of quantum correlation and quantum entanglement tools was a practical design of ultra-bright sources of correlated photons and development of novel principles of entangled states engineering. This also includes entangled states of higher dimensionality and entangled quantum states demonstrating simultaneous entanglement in several pairs of quantum variables (hyper-entanglement), and calibration of single-photon detectors without any need for using traditional blackbody radiation sources. This unique possibility of self-referencing present in the optical system that is distributed in space-time is the main advantage of quantum correlation and entanglement. The fact that spontaneous parametric down-conversion (SPDC) is initiated by vacuum fluctuations serves as a universal and independent reference for measuring the optical radiation brightness (radiance). It gives the possibility of accurately measuring the infrared radiation brightness without the need of using very noisy and low sensitivity infrared detectors. Development of periodically poled nonlinear structures has opened the road for practical implementation of sources with high intensity of entangled-photon flux and with ultra-high spectral bandwidth for biomedical coherence imaging. Recent demonstrations have shown the possibilities for multi-photon interferometry beyond the classical limit. It has been shown that weak field homodyning could yield enhanced resolution in phase detection. First experimental implementations of quantum ellipsometry indicated the high potential of quantum polarisation measurement. The basic physical principles of optical coherence tomography with dispersion cancellation using frequency entangled photon pairs for sub-micron biomedical imaging have been demonstrated in model environments. The use of quantum correlations led to the design of a new technique for characterizing chromatic dispersion in fibers. The intrinsically quantum interplay between the polarisation and frequency entanglement in CSPDC gave rise to a polarisation mode dispersion measurement technique that provides an order of magnitude enhancement in the resolution.

Quantum Imaging

It is possible to generate quantum entanglement between the spatial degrees of freedom of light, an aspect which enables one to use quantum effects to record, process and store information in the different points of an optical image, and not only on the total intensity of light. One can then take advantage of a characteristic feature of optical imaging, which is its intrinsic parallelism. This opens the way to an ambitious goal, with a probable significant impact in a mid-term and far future: that of massively parallel quantum computing. In a shorter perspective, quantum techniques can be used to improve the sensitivity of measurements performed in images and to increase the optical resolution beyond the wavelength limit, not only at the single photon counting level, but also with macroscopic beams of light. This can be used in many applications where light is used as a tool to convey information in very delicate physical measurements, such as ultra-weak absorption spectroscopy, Atomic Force Microscopy etc. Detecting details in images smaller than the wavelength has obvious applications in the fields of microscopy, pattern recognition and segmentation in images, and optical data storage, where it is now envisioned to store bits on areas much smaller than the square of the wavelength. Furthermore, spatial entanglement leads to completely novel and fascinating effects, such as "ghost imaging", in which the camera is illuminated by light which did not interact with the object to image, or "quantum microlithography", where the quantum entanglement is able to affect matter at a scale smaller than the wavelength.

Key references
[1] L. Childress and R. Hanson, MRS Bulletin 38, 134 (2013).
[2] J. Nunn, Nature Physics pp. 1–2 (2013).
[3] G. Waldherr, J. Beck, P. S. Neumann, R. Said, M. L. Nitsche, M. J. Markham, D. Twitchen, J. Twamley, F. Jelezko, and J. Wrachtrup, Nature Nanotechnology 7, 105 (2012).
[4] L. P. McGuinness, Y. Yan, A. A. Stacey, D. T. Simpson, L. Hall, D. Maclaurin, S. Prawer, P. Mulvaney, J. Wrachtrup, F. E. Caruso, et al., Nature Nanotechnology 6, 358 (2011).