Harnessing quantum processes is an efficient method to generate truly indeterministic random numbers, which are of fundamental importance for cryptographic protocols, security applications or Monte-Carlo simulations. Recently, quantum random number generators based on continuous variables have gathered a lot of attention due to the potentially high bit rates they can deliver. Especially quadrature measurements on shot-noise limited states have been studied in detail as they do not offer any side information to potential adversaries under ideal experimental conditions. However, they may be subject to additional classical noise beyond the quantum limit, which may become a source of side information for eavesdroppers. While such eavesdropping attacks have been investigated in theory in some detail, experimental studies are still rare. We experimentally realize a continuous variable eavesdropping attack, based on heterodyne detection, on a trusted quantum random number generator and discuss the limitations for secure random number generation that arise.
We introduce a technique to determine photon correlations of optical light fields in real time. The method is based on ultrafast phase-randomized homodyne detection and allows us to follow the temporal evolution of the second-order correlation function g (2) (0) of a light field. We demonstrate the capabilities of our approach by applying it to a laser diode operated in the threshold region. In particular, we are able to monitor the emission dynamics of the diode switching back and forth between lasing and spontaneous emission with a g (2) (0)-sampling rate of 100 kHz.
Continuous variable quantum state tomography is one of the most powerful techniques to study the properties of light fields in quantum optics. However, the need for a fixed phase reference has so far prevented widespread usage in other fields such as semiconductor spectroscopy. Here, we introduce non-stationary quantum state tomography, which adapts the technique to the special requirements of ultrafast spectroscopy. In detail, we gain access to the amplitude and phase of light fields with a temporal resolution of about 100 fs without the need for a fixed phase reference. Further, we show how our technique allows us to perform conditional studies of stochastic dynamics that are inaccessible experimentally by conventional means, and demonstrate the capabilities experimentally by monitoring the stochastic dynamics of a thermal light field on the sub-ps scale. Finally, we discuss differences and similarities to more standard Hanbury Brown-Twiss photon correlation experiments, which may be considered as the discrete variable analogues of our technique.
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