Experimental observations are usually described using theoretical models that make assumptions about the dimensionality of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from the results of measurements performed on it, without any extra assumption? The concept of a dimension witness(1-6) answers this question, as it allows bounding the dimension of an unknown system only from measurement statistics. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario(6). We use photon pairs entangled in polarization and orbital angular momentum(7-9) 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 work opens newavenues in quantum information science, where dimension represents a powerful resource(10-12), especially for device-independent estimation of quantum systems(13-16) and quantum communications(17,18).
- ORBITAL ANGULAR-MOMENTUM
- BELL INEQUALITIES