AQP Seminar: Photonic Experiments on Quantum Measurement: Reversing, Recycling, and Delocalizing Measurements

Photonic Experiments on Quantum Measurement: Reversing, Recycling, and Delocalizing Measurements

Dr. Lee Rozema (Vienna Center for Quantum Science and Technology (VCQ))

Abstract: In textbook quantum mechanics, the projection postulate states that measuring a quantum system collapses it into an eigenstate of the measured observable. In typical photonic experiments, however, measurement means absorbing a photon at a detector, thereby destroying the system and leaving no post-measurement state. By implementing von Neumann measurement interactions that couple a system to an auxiliary probe, we can separate the coherent measurement interaction from the final destructive readout, allowing us to experimentally explore quantum measurement across new regimes. In this talk, I will present three recent photonic experiments that manipulate this measurement interaction in distinct ways. First, I will examine what happens when a measurement is coherently reversed in a Wigner’s Friend scenario, showing that an external agent can turn a Friend’s laboratory into a universal quantum computer even when the Friend is restricted to non-universal classical operations. Second, moving to multi-photon interactions, we use a photonic CNOT gate to couple one photon of an entangled photon pair to an independent ancilla photon. This allows us torecycle the entanglement and demonstrate the first sequential double Bell inequality violation with two physically distinct observers measuring separate photons. Finally, I will show how embedding this measurement apparatus inside a photonic quantum switch enables time-delocalized measurements in an indefinite causal order. This represents a step towards closing a critical loophole for fundamental tests of causal non-separability and opens applications such as indefinite causal key distribution.

This event is supported by InstituteQ, Finland's National Quantum Institute.