General measurements of a photonic qubit

Dug Young Kim, Cho, Ra '

Geoff J. Pryde, Jeremy L. O’Brien, AG White +2 more

Geoff J. Pryde, Jeremy L. O’Brien, Andrew G. White +3 more

Two-qubit entangling gates allow the realization of new types of generalized quantum measurement. We discuss, and use photonic systems to demonstrate, two instances of this: two-qubit entangling measurements realizing superior discrimination of locally prepared two-qubit quantum states relative to what is achievable with local measurements and classical communication; and nondestructive weak measurements with postselection, leading to quantum weak values.

Geoff J. Pryde, Jeremy L. O’Brien, Andrew G. White +2 more

Measuring the polarization of a single photon typically results in its destruction. We propose, demonstrate, and completely characterize a quantum nondemolition (QND) scheme for realizing such a measurement nondestructively. This scheme uses only linear optics and photodetection of ancillary modes to induce a strong nonlinearity at the single-photon level, nondeterministically. We vary this QND measurement continuously into the weak regime and use it to perform a nondestructive test of complementarity in quantum mechanics. Our scheme realizes the most advanced general measurement of a qubit to date: it is nondestructive, can be made in any basis, and with arbitrary strength.

M. A. Solís-Prosser, Leonardo Neves

We propose a measurement strategy which can, probabilistically, reproduce the statistics of any observable for spatially encoded photonic qubits. It comprises the implementation of a two-outcome positive operator-valued measure followed by a detection in a fixed transverse position, making the displacement of the detection system unnecessary, unlike previous methods. This strategy generalizes a scheme recently demonstrated by one of us and co-workers, restricted to measurement of observables with equatorial eigenvectors only. The method presented here can be implemented with the current technology of programmable multipixel liquid-crystal displays. In addition, it can be straightforwardly extended to high-dimensional qudits and may be a valuable tool in optical implementations of quantum information protocols with spatial qubits and qudits.

R. B. Dalton, Geoff Gillett, Geoff J. Pryde +4 more