Cloning Entangled Qubits to Scales One Can See

TL;DR

This paper explores amplifying one photon of an entangled pair to macroscopic scales, analyzing how different cloning methods affect entanglement detection and the robustness of the amplified states against losses.

Contribution

It introduces a theoretical framework for amplifying entangled photons with various cloning machines and examines the detection of entanglement using coarse-grained measurements.

Findings

Correlations with lossy threshold detectors can confirm initial entanglement.

Amplified states may not retain entanglement detectable by coarse measurements.

The study highlights the detection loophole in Bell tests with macroscopic quantum states.

Abstract

By amplifying photonic qubits it is possible to produce states that contain enough photons to be seen with a human eye, potentially bringing quantum effects to macroscopic scales [1]. In this paper we theoretically study quantum states obtained by amplifying one side of an entangled photon pair with different types of optical cloning machines for photonic qubits. We propose a detection scheme that involves lossy threshold detectors (such as human eye) on the amplified side and conventional photon detectors on the other side. We show that correlations obtained with such coarse-grained measurements prove the entanglement of the initial photon pair and do not prove the entanglement of the amplified state. We emphasize the importance of the detection loophole in Bell violation experiments by giving a simple preparation technique for separable states that violate a Bell inequality without closing this loophole. Finally we analyze the genuine entanglement of the amplified states and its robustness to losses before, during and after amplification.