An overwhelming majority of experiments in classical and quantum physics make a priori assumptions about the dimension of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from t
he results of measurements performed on it, without any extra assumption? The concept of a dimension witness answers this question, as it allows one to bound the dimension of an unknown classical or quantum system in a device-independent manner, that is, only from the statistics of measurements performed on it. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario. We use pairs of photons entangled in both polarization and orbital angular momentum to generate ensembles of classical and quantum states of dimensions up to 4. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Our results open new avenues for the device-independent estimation of unknown quantum systems and for applications in quantum information science.
We present an experimental demonstration of the full control of the frequency correlations of entangled photon pairs. The joint spectrum of photon pairs is continuously varied from photons that exhibit anticorrelation in frequency to photons that exh
ibit correlation in frequency, passing through the case of uncorrelated photons. Highly entangled frequency-anticorrelated photon pairs were obtained even when an ultrafast laser was used as a pump. The different kinds of correlations are obtained without changing neither the wavelength, nor the nonlinear crystal.
We demonstrate a technique that allows to fully control the bandwidth of entangled photons independently of the frequency band of interest and of the nonlinear crystal. We show that this technique allows to generate nearly transform-limited biphotons
with almost one octave of bandwidth (hundreds of THz) which corresponds to correlation times of just a few femtoseconds. The presented method becomes an enabling tool for attosecond entangled-photons quantum optics. The technique can also be used to generate paired photons with a very high degree of entanglement.
We demonstrate experimentally a new technique to control the bandwidth and the type of frequency correlations (correlation, anticorrelation, and even uncorrelation) of entangled photons generated by spontaneous parametric downconversion. The method i
s based on the control of the group velocities of the interacting waves. This technique can be applied in any nonlinear medium and frequency band of interest. It is also demonstrated that this technique helps enhance the quality of polarization entanglement even when femtosecond pulses are used as a pump.
