Securing information has been a concern for more than 4,000 years, but in the times in which we are connecting every single aspect of our businesses and lives, developing secure products and infrastructures has become a global priority. Remarkably, quantum technologies bring unique possibilities for the cryptographic world. In this talk, we will describe recent efforts on the development of a highly integrated quantum entropy source, a key component to generate unpredictable cryptographic keys in any connected device. In particular, we will present the integration of two quantum entropy sources, one in Silicon Photonics and the other in Indium Phosphide. The devices are based on the accelerated phase diffusion process observed in pulsed semiconductor lasers, a macroscopic quantum effect resulting from microscopic spontaneous emission events. Both chip implementations enable Gb/s generation rates in form factors below 2mm x 5mm in indium Phosphide and 0.5mm x 1mm in Silicon Photonics. Our results show progress towards the industrialization of quantum devices using standard semiconductor production lines and processes.
John Bell’s theorem of 1964 states that local elements of physical reality, existing independent of measurement, are inconsistent with the predictions of quantum mechanics (Bell, J. S. (1964), Physics (College. Park. Md). Specifically, correlations between measurement results from distant entangled systems would be smaller than predicted by quantum physics. This is expressed in Bell’s inequalities. Employing modifications of Bell’s inequalities, many experiments have been performed that convincingly support the quantum predictions. Yet, all experiments rely on assumptions, which provide loopholes for a local realist explanation of the measurement. Here we report an experiment with polarization-entangled photons that simultaneously closes the most significant of these loopholes. We use a highly efficient source of entangled photons, distributed these over a distance of 58.5 meters, and implemented rapid random setting generation and high-efficiency detection to observe a violation of a Bell inequality with high statistical significance. The merely statistical probability of our results to occur under local realism is less than 3.74×10-31, corresponding to an 11.5 standard deviation effect.
It is well known that classical states of light exhibit shot noise, characteristic of independent or uncorrelated particles. For phase estimation problems, this leads to a shot-noise limited uncertainty of 1/sqrt[N], where N is the number of particles detected. It is also well known that the shot-noise limit is not fundamental: squeezed states and entangled states can be used for sub-shot-noise phase measurements. The uncertainty principle sets a fundamental limit of 1/N, known as the "Heisenberg" limit. We have recently demonstrated a method, using parametric downconversion and post-selection, to generate entangled "NOON" states suitable for sub-shot-noise phase measurements [M.W. Mitchell et al, Nature 429, 161 (2004)]. We generated a three-photon NOON state and demonstrated three-fold improvement in phase resolution with this state. The relationship between phase resolution and phase uncertainty depends on prior information about the phase being estimated. As in the case of phase measurements with squeezed states, extra precision in one dimension is gained at the cost of reduced precision in other dimensions. Only when prior information is incorporated can entangled-state metrology be applied to beat the shot-noise limit. We illustrate this relationship and discuss adaptive strategies for phase estimation and the possibility of reaching the Heisenberg limit.
Quantum process tomography is often cited as providing all the information that can be known about a given quantum process. In this paper we have shown that even if two processes appear identical under process tomography, it is possible to distinguish them using an interferometric setup. Using this setup, it is possible to gain more information about a process than just tomography provides.
We describe experiments with photon pairs to evaluate, correct for, and
avoid sources of error in optical quantum information processing.
It is well known that a simple beamsplitter can
non-deterministicially prepare or select entangled polarization
states. We use quantum process tomography (QPT) to fully
characterize this effect, including loss and decoherence. The QPT
results identify errors and indicate how well they can
be corrected. To evade decoherence in a
noisy quantum channel, we identify decoherence-free subspaces
using experimental channel characterization, without need for a
priori knowledge of the decoherence mechanism or simplifying
assumptions. Working with pairs of polarization-encoded photonic
qubits, we use tomographic and adaptive techniques to identify 2-
and 3-state decoherence-free subspaces for encoding
decoherence-free qubits and qutrits within the noisy channel.