Mikolaj Lasota

Various imperfections in realistic setup components limit the performance of quantum key distribution (QKD) protocols in terms of both the maximum secure transmission distance and the achievable key generation rate. Temporal filtering is an effective way to improve the performance of QKD protocols. The receiver optimizes detection windows to reduce dark counts and channel noise. However, windows cannot be arbitrarily short due to detector timing limitations and the finite temporal bandwidth of signals. Fiber dispersion further broadens pulses, so detection windows and source parameters must be jointly optimized to maximize the secure key rate. In this work, we study the entanglement-based BB84 protocol, where information is encoded in the polarization of photon pairs generated via spontaneous parametric down-conversion (SPDC) with a pulsed pumping laser. We perform the joint optimization of source parameters and detection system used by Alice and Bob in order to improve the performance of the protocol in a realistic set-up. We take into account several key parameters of the source, including the crystal’s phase-matching function width, pump pulse duration, and number of the photon pairs emitted per one use of the source. For the detection system, we optimize the detection windows taking into account several characteristics of the setup, such as, timing jitter, dead time and dark count rate. Finally, we include transmission losses in the quantum channel. Our work is an important step towards refining practical parameter values for realistic QKD systems across wide range of experimental conditions

Numerous imperfections of realistic setup elements required for the implementation of quantum key distribution (QKD) protocols impose strong limitation on their performance, particularly in terms of the obtainable key generation rate. A possible way to increase this quantity in the case of utilizing spontaneous parametric down-conversion (SPDC) source, producing strongly correlated pairs of photons, is to use wavelength-division-multiplexing (WDM) modules in order to direct photons of different wavelengths into separate detection systems utilized by the trusted parties. In this way it can be possible to generate multiple cryptographic keys in paralell, significantly increasing the overall key rate. In this work we theoretically investigate the potential improvement offered by such possibility in the case of entanglement-based version of the BB84 protocol realized with pulsed-pump SPDC source. We optimize the properties of the produced photon pairs and the intensity of the utilized pumping laser in order to maximize the potential improvement over the traditional, non-WDM, setup configuration. We consider both noiseless case, with ideal single-photon detectors utilized by the trusted parties and more practical situation with imperfect binary detectors, in which case we develop formulas for optimizing the detection window when the temporal filtering method is used to reduce the quantum bit error rate.

In this work we perform an analysis of a general setup for subtracting photons from a given state of light and use the obtained results to derive criteria characterizing usefulness of realistic single photon sources in two recently proposed quantum repeater schemes. We also describe possible tests of Bell’s inequalities using these two schemes and find requirements on the statistics of photon sources which would enable us to show their violation in realistic situation.