= PROSPECTS FOR APPLICATIONS AND COMMERCIAL EXPLOITATION =
The main thrust of the ongoing investigations still belongs to basic research. However, a few areas can be already identified which are closer to potential applications and even for commercial exploitation. Quantum communication
In the 1990's Europe took clearly the lead in quantum cryptography with groups like BT and Oxford/DERA team in the UK, Geneva University in Switzerland and the Universities of Innsbruck and of Vienna in Austria. However, today the competition is hard. About simultaneously as the European company id Quantique (www.idQuantique.com), a company in the US announces a commercial quantum cryptography product (http://www.MagiQtech.com). Another European company which developed a commercial quantum key distribution scheme is Elsag plc. A serious European competitor in entanglement based quantum cryptography in Singapore, where Christian Kurtsiefer and his team, in collaboration with researchers from NIST, are building and testing QKD at NUS. In the last two years Japan appeared very strongly on the global scene with major industrial players devoting entire development teams to quantum key distribution systems: NEC, Mitsubishi, Toshiba and NTT among others (the 2 first ones did already present prototypes). Moreover the Japanese government widely supports university research in quantum communication. It is noteworthy that Japan opted almost exclusively for weak-laser-pulse quantum cryptography in optical fibers at a wavelength of 1550nm using time-bin encoding. Considering the various technologies, both the US and Japan compete directly with Europe in Quantum Cryptography based on weak pulses. In contrast, the European leadership in entanglement-based quantum cryptography and quantum communication is uncontested at present.
Quantum computing
Quantum information processing in the sense of fault tolerant quantum computing for large-scale numerical algorithms (for example Shor's algorithm) is the ultimate goal. Within the next few years one expects few-qubit quantum computer with applications to quantum repeaters, for example. On the intermediate time scale one goal is to beat classical computations on whatever (non-trivial) problem. This could be achieved, for example, with specialized quantum computing, as with quantum simulators (see section 4.3.4) where a system with more than 30 qubits is already beyond the reach of any foreseeable classical machine.
Quantum metrology
Entangled states provide instances of the most fragile objects ever known, because they are extremely sensitive to interaction with the environment. 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 synchronization via the use of frequency-entangled pulses: for instance, in the latter case, picosecond resolution at 3 km distance has been attained. Entangled photon pairs can be used also for absolute calibration of detectors independently of black-body radiation (i.e. of temperature), without the need to refer to a standard source.
Large scale laser interferometers with kilometer 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 limit. Scientists in Europe and Australia have recently demonstrated the required quantum noise squeezing of laser light at kilohertz frequencies.
State-of-the-art atom clocks developed in Europe 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 60 nm) 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. On the other hand, the quantum regime is being entered 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.
Quantum technologies
A simple example is the use of quantum randomness to generate random numbers. Such random numbers are truly random, in contrast to the pseudo-random numbers that classical computers generate. Using tools from quantum communication, one can develop such a quantum random generator that is much faster than the other physical generators, e.g. those based on the rather slow thermal fluctuations. A first commercial product is available, see http://www.idquantique.com.
It is also possible to generate quantum entanglement between the spatial degrees of freedom of light, which enables us 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.
The success in quantum science and engineering has created several extremely valuable optical tools that operate exclusively under the rules of quantum mechanics and offer practical optical measurement and characterization techniques (quantum optical metrology) that have clear advantages over existing technologies. The main step 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 polarization 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 polarization and frequency entanglement in CSPDC gave rise to a polarization mode dispersion measurement technique that provides an order of magnitude enhancement in the resolution.