Scientists in the group of Christine Silberhorn in Paderborn and the group of Werner Vogel in Rostock have demonstrated a new method to identify nonclassical correlations between two parties sharing a quantum state. The method allows to characterize quantum states and resources in a simple and straight forward way. The results are published in <link http: journals.aps.org prl abstract physrevlett.115.023601>Uncovering Quantum Correlations with Time-Multiplexed Click Detection, Phys. Rev. Lett. 115, 023601 (2015).
Nonclassical effects are profoundly interesting because they allow us to look into the quantum world and find novel phenomena. Generally speaking, nonclassicality lies at the heart of quantum enhancement over classical protocols for any application. Thus, it is utterly important to discriminate between classical and nonclassical effects.
In the Integrated Quntum Optics research group at Paderborn, nonclassical states can be routinely produced in nonlinear crystals. However, the verification of nonclassicality is nontrivial as most single photon detectors are non-ideal and could falsely show nonclassicality even for classical states. One such detector is the time multiplexing detector with avalanche photo diodes. It is often used to resolve the number of photons, even though the diodes are mere 'click' detectors. Deducing the true photon number probabilities from the measured click-statistics is difficult as their exact relation depends on the design of the time multiplexing detector including the number of beam splitters, the detector response functions and the losses in the system.
In previous work, Jan Sperling and coworkers in the group of Werner Vogel have developed a rigorous theory for such time multiplexing detectors that allows verification of nonclassicality directly from the measured click statistics without intermediate steps like loss inversion.
In this collaboration, Georg Harder and colleagues took experimental data from a relatively bright quantum state (with 16 photons on average per pulse), which was shared between two spatially separated parties. Even though the utilized detectors could only resolve up to 8 photons, it was possible to certify nonclassical correlations between the two parties by applying the click-counting theory. Remarkably, the nonclassical correlations could be identified even though each party alone did not have nonclassical states; the nonclassicality was hidden in the correlations between the two systems.
This demonstrates the power of the applied method in real world quantum systems. Being a relatively simple method - only the directly measured click statistics are sufficient - it has the potential to become a useful tool for quantum state characterization.
Publication on Certifying Nonclassical Quantum Correlations
