Quantum Measurement and Control

The indistinguishability of non-orthogonal quantum states lies at the heart of quantum mechanics - it underpins the fundamental challenge of quantum state discrimination and has been harnessed as a resource in quantum technologies. Perfect identification of an unknown quantum process that acts on the state of a quantum system (including unitary operations, measurements, and decohering processes) can be achieved via quantum process tomography, but requires infinite uses of the unknown process. Here we experimentally demonstrate that discriminating between non-orthogonal processes can always be achieved with finite uses of the unknown process, in stark contrast to the situation for quantum states. We use either entanglement or an additional known process to deterministically and unambiguously discriminate between non-orthogonal measurement processes, and qubit and quitrit unitary processes. Finally we experimentally demonstrate that non-local multipartite unitary processes can be locally distinguished - i.e. without entanglement. Our processes act on photons and are discriminated with a confidence of >97% in all cases.

Figure shows Quantum process discrimination. (a) The Bloch Sphere showing nonorthogonal measurement directions X and Z and nonorthogonal Hermitian unitaries σz and Ĥ. σz and T̂ can always be discriminated with a finite number of qubits with θarbitrarily acute. (b) Entanglement-assisted QPD. (c) QPD without entanglement. (d) Multipartite unitary discrimination without entanglement.
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