Unlike regular time evolution governed by the Schrödinger equation, standard quantum measurement appears to violate time-reversal symmetry. Measurement creates random disturbances (e.g., collapse) that prevents back-tracing the quantum state of the system. The effect of these disturbances is explicit in the results of subsequent measurements. In this way, the joint result of sequences of measurements depends on the order in time in which those measurements are performed. One might expect that if the disturbance could be eliminated this time-ordering dependence would vanish. Following a recent theoretical proposal [A. Bednorz et al 2013 New J.Phys. 15 15 15 023043], we experimentally investigate this dependence for a kind of measurement that creates an arbitrarily small disturbance, weak measurement. We perform various sequences of a set of polarization weak measurements on photons. We experimentally demonstrate that, although the weak measurements are minimally disturbing, their time-ordering affects the outcome of the measurement sequence for quantum systems.A fundamental open question in physics is the role of time in quantum mechanics [1][2][3].While observables such as position and momentum are represented by operators, time in the Schrödinger equation appears only as ordinary number-parameter, just as in classical mechanics [2]. In view of relativistic theories of physics which famously treat time and space on equal footing, this distinction is problematic. In fact, while Heisenberg's uncertainty principle between energy and time appears to suggest that a time operator conjugate to the total energy operator exists, attempts to create such an operator lead to contradictions [4].Similar issues confound attempts to create an observable for the time it takes for a particle to tunnel through a potential barrier, or even the time of arrival of a particle at a detector [5][6][7].Another issue is that it is widely believed that information is conserved in quantum physics. This follows from the unitary time-reversible evolution in the Schrödinger equation. Yet,

In this work, the second harmonic generation from excitonic transitions in semiconductor quantum dots is computationally studied. By integrating a density matrix treatment with a partial configuration interaction approach, we obtain the second order susceptibility as a function of externally applied electric and magnetic fields for highly confined neutral and charged excitons. Our results show an enhancement in the nonlinear response with respect to analogous optical processes based on intraband transitions, and predict their efficient tunability by taking advantage of the interplay between Coulomb effects and field-driven wave function manipulation.

We present the experimental control of Non-Markovian dynamics of open quantum systems simulated with photonic entities. The polarization of light is used as the system, whereas the surrounding environment is represented by light's spatial structure. The control of the dynamics is achieved by the engineering of the environment via spatial interference. Using the behavior of the trace distance, we are able to identify each dynamics and characterize the position of maximum revival of information.

We present designs for variably polarizing beam splitters. These are beam splitters allowing the complete and independent control of the horizontal and vertical polarization splitting ratios. They have quantum optics and quantum information applications, such as quantum logic gates for quantum computing and non-local measurements for quantum state estimation. At the heart of each design is an interferometer. We experimentally demonstrate one particular implementation, a displaced Sagnac interferometer configuration, that provides an inherent instability to air currents and vibrations. Furthermore, this design does not require any custom-made optics but only common components which can be easily found in an optics laboratory.