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arXiv:quant-ph/9907038

Event-ready entanglement preparation

Mladen Pavičić

Published Jul 9, 1999• 1 citation
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Abstract

We present an experiment which prepare entanglement between photons that nowhere interacted and whose paths nowhere crossed. The experiment puts together two photons from two (non-maximal) singlet-like photon pairs and make them interfere at an asymmetrical beam splitter. As a result one finds polarization correlations between the other companion photons from the pairs whose paths nowhere crossed each other even when no polarization measurements have been carried out on the former photons. The latter set of photons that nowhere interacted are therefore event-ready prepared by their pair-companion photons. The result reveals nonlocality as a property of selection which can even be a preselection. It also reveals that one can predict spin-correlated behaviour of photons in space and at beam splitters by controlling a no-spin observable. [From the Book of Abstracts as appeared at the Workshop.]

DOI
10.48550/arxiv.quant-ph/9907038

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We present an experiment which prepare entanglement between photons that nowhere interacted and whose paths nowhere crossed. The experiment puts together two photons from two (non-maximal) singlet-like photon pairs and make them interfere at an asymmetrical beam splitter. As a result one finds polarization correlations between the other companion photons from the pairs whose paths nowhere crossed each other even when no polarization measurements have been carried out on the former photons. The latter set of photons that nowhere interacted are therefore event-ready prepared by their pair-companion photons. The result reveals nonlocality as a property of selection which can even be a preselection. It also reveals that one can predict spin-correlated behaviour of photons in space and at beam splitters by controlling a no-spin observable. [From the Book of Abstracts as appeared at the Workshop.]
A nonclassical feature of the fourth-order interference at a beam splitter, that genuine photon spin singlets are emitted in predetermined directions even when incident photons are unpolarized, has been used in a proposal for an experiment that imposes a quantum spin correlation on truly independent photons. In the experiment two photons from two such singlets interfere at a beam splitter, and as a result the other two photons—which nowhere interacted and whose paths nowhere crossed—exhibit a 100% correlation in polarization, even when no polarization has been measured in the first two photons. The proposed experiment permits closure of the remaining loopholes in the Bell theorem proof, reveals the quantum nonlocality as a property of selection, and pioneers an experimental procedure for the exact preparation of unequal superposition.
In this contribution the preparation of event-ready entanglement in polarised photons by means of entanglement swapping is studied. This protocol makes use of Gaussian photon-sources. Since the states resulting from this protocol are equivalent to those obtained by mixing two photons in a 50:50 beam splitter, the nature of the emerging entanglement is controversial. Several methods to enhance the fidelity of the outgoing state are studied. It is found that at least quantum non-demolition measurements or a quantum computer of some kind is needed to create event-ready entanglement with entanglement swapping.
We propose a new experiment employing two independent sources of spin correlated photon pairs. Two photons from different unpolarized sources each pass through a polarizer to a detector. Although their trajectories never mix or cross, they exhibit fourth-order-interference-like correlations when the other two photons interfere on a beam splitter even when the latter two do not pass any polarizers at all. A wave packet calculation shows that the experiment permits a very discriminatory test of hidden variable theories.
After reviewing basic concepts and tools in order to quantify entanglement, a theoretical set up is proposed, which is based on the double slit experiment and the Mach-Zehnder interferometer. A particle, which is in a superposition of two paths, can interfere with a qubit on each path before it is scattered by a beam splitter and measured by two separate detectors. After a detailed investigation, it can be shown that it is possible to induce entanglement to the qubits via post-selection. This means that the measurement does not induce entanglement, but can be used to select the entangled qubits after a number of iterations. A remarkable result is, that this works independently from the probability of interaction between the particle and the qubits, which could turn out be very beneficial in a possible realization of the set up.
The simplest possible photon-number-squeezed states containing only two photons and exhibiting sub-poissonian statistics with the Fano factor approaching 0.5 have been used for a proposal of a loophole-free Bell experiment requiring only 67 percent of detection efficiency. The states are obtained by the fourth order interference first of two downconverted photons at an asymmetrical beam splitter and thereupon of two photons from two independent singlets at an asymmetrical beam splitter. In the latter set-up, the other two photons which nowhere interacted and whose paths never crossed appear entangled in a singlet-like correlated state.