Abstract
A loophole-free violation of Bell inequalities is of fundamental importance for demonstrating quantum nonlocality and long-distance device-independent secure communication. However, transmission losses represent a fundamental limitation for photonic loophole-free Bell tests. A local precertification of the presence of the photons immediately before the local measurements may solve this problem. We show that local precertification is feasible by integrating three current technologies: (i) enhanced single-photon down-conversion to locally create a flag photon, (ii) nanowire-based superconducting single-photon detectors for a fast flag detection, and (iii) superconducting transition-edge sensors to close the detection loophole. We carry out a precise space-time analysis of the proposed scheme, showing its viability and feasibility.
- Received 20 November 2011
DOI:https://doi.org/10.1103/PhysRevX.2.021010
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Published by the American Physical Society
Popular Summary
Many predictions of quantum mechanics are undeniably counterintuitive, and none is perhaps more so than “entanglement.” It says that, if two quantum particles are entangled, the information on the state of one of the particles leads to information on the state of the other, no matter how far apart the two particles are separated spatially. This picture of nonlocality is so at odds with the intuitive concept of “local realism” fundamental to classical physics that Einstein, Podolsky, and Rosen suggested in 1935 that quantum mechanics was an incomplete manifestation of a more fundamental theory that fulfilled local realism. Bell’s inequalities, a set of mathematical equations introduced by John Stewart Bell in 1964 that must be satisfied by any quantum theory of local realism, cast this conflict in terms of experimentally testable predictions. Water-tight, or loophole-free, experimental demonstrations of the violation of Bell’s inequality have, therefore, been a long-standing important goal of quantum mechanics. One of the directions in which such demonstrations are pursued uses entangled photons. Transmission losses of photons over large distances, however, have so far represented a critical loophole in detection. In this theoretical paper, we propose a solution to this problem and show that the solution is entirely feasible based on three current photonic measurement technologies.
In a two-photon Bell test, two entangled photons are generated at a source and then propagated to two spatially separated sites, A and B, to be measured. Transmission losses can take place during the propagations of the photons from the source to the two measurement sites. The central idea in our proposed solution is called “local precertification” and consists of a few steps: First, each original photon reaching either of the two sites is split into two “descendent” photons in such a way that one of them, say, photon 1, carries the full information encoded in the original photon. Then, photon 2 is detected by a fast detector and is used as a flag to precertify the presence of photon 1. Finally, photon 1 is detected with a highly sensitive detector. Only events in which photon 2 is detected are counted in the Bell test. This way, the transmission losses do not affect the ultimate detection efficiency.
Are these steps experimentally feasible? The answer is yes. The photon splitting can be done by use of the so-called single-photon down-conversion, a well-established technique in quantum photonics. The current nanowire-based supercondcuting single-photon detectors, with their high detection speed, are ideal for the fast detection of photon 2. The final high-sensitivity detection of photon 1 can be achieved with the use of superconducting transition-edge sensors. A quantitative feasibility analysis of the proposal sets it on a more concrete ground.
We think that our proposed scheme will remove the main obstacle for photon-based loophole-free Bell tests. The developments needed to achieve the local precertification should also have applications in long-distance photonic communication beyond Bell tests.
