Scheme for the protective measurement of a single photon using a tunable quantum Zeno effect

This paper presents a proof-of-principle scheme for the protective measurement of a single photon. In this scheme, the photon is looped arbitrarily many times through an optical stage that implements a weak measurement of a polarization observable followed by a strong measurement protecting the state. The ability of this scheme to realize a large number of such interaction-protection steps means that the uncertainty in the measurement result can be drastically reduced while maintaining a sufficient probability for the photon to survive the measurement.

Figure 1

Proposed scheme for a photonic implementation of a protective measurement based on the quantum Zeno effect. A vertically polarized signal photon resulting from a down-conversion process (DC) is reflected into the optical loop at the polarizing beam splitter (PBS). Upon leaving the PBS, brief activation of a Pockels cell (PC1) by a pulse generator (PG), which is triggered by the arrival of the idler photon at a detector (D), changes the polarization of the signal photon to horizontal. The signal photon then traverses an interaction–protection stage consisting of two half-wave plates (HWPs), a pair of birefringent crystals (BCs), and a linear polarizer (PL). HWP1 prepares the initial quantum state $|\psi \rangle =cos\theta |H\rangle +sin\theta |V\rangle$. BC implements a small polarization-dependent spatial displacement of the photon, weakly coupling polarization and spatial degrees of freedom. PL realizes the state protection by projecting the photon back onto the initial state $|\psi \rangle$. HWP2 rotates the polarization to horizontal, thereby readying the photon for transmission at the PBS for its next round trip through the loop. After $N$ such round trips, a second Pockels cell (PC2) is activated to rotate the polarization to vertical and switch the photon out of the loop to be detected at the spatially resolving photon imager (IMG).