Optimal photon localization

Koji Azuma, Go Kato

We derive an optimal bound for arbitrary entanglement manipulation based on the transmission of a pulse in coherent states over a lossy channel followed by local operations and unlimited classical communication (LOCC). This stands on a theorem to reduce LOCC via a local unital qubit channel to local filtering. We also present an optimal protocol based on beam splitters and a quantum nondemolition (QND) measurement on photons. Even if we replace the QND measurement with photon detectors, the protocol can achieve near-optimal performance, outperforming known entanglement generation schemes.

Subhasish Chakraborty, Miriam Scherer

Kengo Matsuyama, Ming Ji, Holger Friedrich Hofmann +1 more

We experimentally investigate nonlocal contextual relations between complementary photon polarizations by adapting the entanglement and the local polarizations of a two-photon state to satisfy three deterministic conditions demonstrating both quantum contextuality and nonlocality. The key component of this adaptive input state control is the variable degree of entanglement of the photon-pair source. Local polarization rotations can optimize two of the three correlations, and the variation of the entanglement optimizes the third correlation. Our results demonstrate that quantum contextuality is based on a nontrivial trade-off between local complementarity and quantum correlations.

John F. Clauser

Tamoghna Das, Marcin Karczewski, Antonio Mandarino +2 more

Bell nonclassicality of a single-photon superposition in two modes, often referred to as ``nonlocality of a single photon,'' is one of the most striking nonclassical phenomena discussed in the context of foundations of quantum physics. Here we show how to robustly violate local realism within the weak-field homodyne measurement scheme for any superposition of one photon with vacuum. Our modification of the previously proposed setups involves tunable beam splitters at the measurement stations, and the local oscillator fields significantly varying between the settings, optimally being on or off. As photon-number-resolving measurements are now feasible, we advocate for the use of the Clauser-Horne Bell inequalities for detection events using precisely defined numbers of photons. We find a condition for optimal measurement settings for the maximal violation of the Clauser-Horne inequality with weak-field homodyne detection, which states that the reflectivity of the local beam splitter must be equal to the strength of the local oscillator field. We show that this condition holds not only for the vacuum--one-photon qubit input state, but also for the superposition of a photon pair with vacuum, which suggests its generality as a property of weak-field homodyne detection with photon-number resolution. Our findings suggest a possible path to employ such scenarios in device-independent quantum protocols.

Roberto Beneduci, Franklin E. Schroeck