Multistage Entanglement Swapping

Abstract

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 (BSMs) 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.PACS numbers: 03.67. Bg, 03.67.Mn, 42.50.Dv, 42.50.Xa Entanglement swapping is arguably one of the most important ingredients for quantum repeaters and quantum relays, which lays at the heart of quantum communication [1,2,3,4]. For photonic quantum communication, the distance is largely limited due to decoherence from coupling to the environment and an increasing loss of photons in a quantum channel. This leads to an exponential decay in the fidelity of quantum information. This drawback can eventually be overcome by subdividing larger distances into smaller sections over which entanglement or quantum states can be distributed. The sections are then bridged by entanglement swapping processes [2,3]. The swapping procedure therefore constitutes one of the key elements for a quantum relay [3], and a full quantum repeater [2] if combined with quantum purification [5,6] and quantum memory [7]. As a result, quantum communication becomes feasible despite of realistic noise and imperfections. At the same time, the overhead for the used resources and communication time only increase polynomially with the distance [2,3,4].Experimentally, photonic entanglement swapping has so far been successfully achieved for the case of discrete variables [8,9], and for continuous variable [10], both via a single stage process. However, only after successful multiple swapping, will we be able to have a fully functional quantum repeater. There are additional advantages utilizing a multiple swapping process. For a quantum relay with many segments, it is equivalent to significantly lower the dark-count rate, which is a substantial factor limiting the transmission distance of successful quantum communication [3]. For quantum information carriers possessing mass, multiple swapping processes can speed up the distribution of entanglement by a factor that is proportional to the number of segments used [11]. Moreover, multistage entanglement swapping can improve the protection of quantum states against noise from amplitude errors [11].We report in this letter an experimental demonstration of a multiple entanglement swapping over two stages. This is achieved by utilizing three synchronous spatially independent pairs of polarization entangled photons, and performing...

Inside Quantum Repeaters

“…This can be achieved with an operation known as entanglement swapping [27], [28], [38]-an operation that is effectively a Bell-state measurement on a node that is linked to neighbouring nodes with entangled links (see Fig. 5).…”

A note on entanglement swapping of atomic states through the photonic Faraday rotation

“…The state in (12) is constructed via the interaction of the polarized photon (in the state |ψ 1 = (|L 1 + |R 1 )/ √ 2) with a three-level atom (previously prepared in Figure 2 shows the entire procedure of the case 1. …”

Deterministic and probabilistic entanglement swapping of nonmaximally entangled states assisted by optimal quantum state discrimination

“…Recently, an experiment on delayed ES [37] was implemented [38]. Other experimentally demonstrated applications include multistage ES [39] and generation of GHZ states [40] by means of ES. * miguelangel.solis@cefop.udec.cl Typically, ES is formulated considering maximally entangled states.…”