Distributed entangled photonic states and applications

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Since the birth of Quantum Mechanics, it has been debated exactly why quantum systems behave the way they do. Particles that can instantaneously affect each other's state without interacting, quantities that can never be measured with certainty and the notion that the wavefunction is about information, rather than concrete physicality, are puzzles that every physicist ponders. Whatever its mystery, however, new science often gives rise to new technologies, and in the last 25 years physicists have realized that these strange properties of quantum mechanics may indeed be used for revolutionary new information processing machines. Among these are: quantum cryptography, ultra-fast computation, measurements with unprecedented precision and a form of teleportation. Several of these rely on quantum entanglement, which is now recognized as the crucial property enabling these new technologies. A major problem for exploiting entanglement is that it is quickly destroyed when the particles possessing these special correlations interact with the environment, though light, vibrations or collisions. The rapid loss of entanglement decreases their performance at the aforementioned tasks disappointingly.The first part of our research addresses this problem with a technique called distillation. It involves purifying a collection of particles to obtain a few highly correlated and isolated ones. The particles we study are photons resulting from the interaction between lasers and non-linear materials. Measuring some of the photons in a specifically designed combination of mirrors, detectors and half mirrors transforms degraded entangled states into cleaner ones. Preliminary experiments have succeeded in the last years in producing such states, studying their deterioration or trying to enhance their performance without separating them. Our aim is to separate the groups of photons, act on them locally (as would be required in a real communications system) and enhance their useful entanglement. To achieve this goal several tools need to be developed. First, photons have a very large number of possible characteristics, such as wavelength or direction. We need to ensure that the desired entanglement exists in the quantum state of the light in the right way that we can exploit it. Second, the large number of dimensions required to specify the state might normally require a similarly large number of measurements to characterize it. We need to develop mathematical methods where rigorous bounds on the amount of entanglement can be extracted with partial measurements. Third, these partial measurements are somewhat elaborate and need novel detectors. Such detectors have been built, showing promising results. The complexity of these photon number and photo-correlation detectors needs to be studied in more detail. To do so we will use techniques based on recently demonstrated quantum detector tomography protocols. This process, which has analogies to medical imaging, allows us to build up a full picture of the operation of our new detectors, and to thereby show that they can indeed quantify the entanglement we have distilled. Another aspect of our project is the science of measurements, metrology. The challenge lies in measuring a certain quantity with a limited amount of resources, in this case, photons. It is known that using entangled photons gives a precision beyond what can be obtained using classical light. Therefore we need to craft special quantum states of light (similar to the ones used in the first part of the project) with few photons that are designed to attain this super-classical measurement precision even when there are environmental disturbances, so that they will be useful in real-world applications. We believe that both these research projects will improve and develop the tools for quantum communication and high-precision metrology.