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Fortschritte der Physik

Entanglement Swapping as Event-Ready Entanglement Preparation

Pieter Kok, Samuel L. Braunstein

Published May 1, 2000• 4 citations
Publisher

Abstract

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.

DOI
10.1002/(sici)1521-3978(200005)48:5/7<553::aid-prop553>3.0.co;2-k

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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 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.]
We show how to make event-ready multi-partite entanglement between qubits which may be encoded on photons or matter systems. Entangled states of matter systems, which can also act as single photon sources, can be generated using the entangling operation presented in Barrett and Kok (2004 Preprint quant-ph/0408040). We show how to entangle such sources with photon qubits, which may be encoded in the dual-rail, polarization or time-bin degrees of freedom. We subsequently demonstrate how projective measurements of the matter qubits can be used to create entangled states of the photons alone. The state of the matter qubits is inherited by the generated photons. Since the entangling operation can be used to generate cluster states of matter qubits for quantum computing, our procedure enables us to create any (entangled) photonic quantum state that can be written as the outcome of a quantum computer.
We show how to make event-ready multi-partite entanglement between qubits which may be encoded on photons or matter systems. Entangled states of matter systems, which can also act as single photon sources, can be generated using the entangling operation presented in quant-ph/0408040. We show how to entangle such sources with photon qubits, which may be encoded in the dual rail, polarization or time-bin degrees of freedom. We subsequently demonstrate how projective measurements of the matter qubits can be used to create entangled states of the photons alone. The state of the matter qubits is inherited by the generated photons. Since the entangling operation can be used to generate cluster states of matter qubits for quantum computing, our procedure enables us to create any (entangled) photonic quantum state that can be written as the outcome of a quantum computer.
We report an experimental demonstration of entanglement swapping over two quantum stages. By successful realizations of two cascaded photonic entanglement swapping processes, entanglement is generated and distributed between two photons, that originate from independent sources and do not share any common past. In the experiment we use three pairs of polarization entangled photons and conduct two Bell-state measurements: one between the first and second pair, and one between the second and third pair. This results in projecting the remaining two outgoing photons from pair 1 and 3 into an entangled state, as characterized by an entanglement witness. The experiment represents an important step towards a full quantum repeater where multiple entanglement swapping is a key ingredient.
Entanglement swapping is a protocol that details how to create entanglement between previously uncorrelated particles. Its all-photonic version - mediated by the interference of photon pairs generated by separate quantum systems-finds disparate applications in quantum networks. So far, all-photonic entanglement swapping between remote systems has been implemented only using sources that operate probabilistically. However, the scaling up of quantum networks requires deterministic quantum emitters that do not suffer from a trade-off between degree of entanglement and photonpair generation rate. Here, we demonstrate all-photonic entanglement swapping using photon-pairs generated by two separate GaAs quantum dots. The emitters are deterministically embedded in hybrid semiconductor-piezoelectric devices that make the entangled-photons from two dissimilar quantum dots nearly identical. Entanglement swapping is demonstrated with a fidelity as high as 0.71(2), more than 10 standard deviations above the classical limit. The experimental data are quantitatively explained by a theoretical model that also suggests how to boost the protocol performances. Our work opens the path to the exploitation of quantum dot entangled-photon sources in quantum repeater networks.
Preparation of entangled states of photons are useful for quantum computing and communication. In this paper, we present a simplistic protocol of entanglement generation using beam splitters with suitable reflectivity. The photons in an initial state with fully classical probability distribution pass through an optical network, made up of sequential beam splitters and are prepared in maximally entangled states. We also present the detailed theoretical analysis of entangled state generation, for an arbitrary number of photons, fed through the input ports of the beam splitters with equal probability.