Inquantum mechanics,entanglement swapping is a protocol to transferquantum entanglement from one pair ofparticles to another, even if the second pair of particles have never interacted. This process may have application inquantum communication networks andquantum computing.
Entanglement swapping has two pairs of entangled particles: (A, B) and (C, D). Pair of particles (A, B) is initially entangled, as is the pair (C, D). The pair (B, C) taken from the original pairs, is projected onto one of the four possibleBell states, a process called aBell state measurement. The unmeasured pair of particles (A, D) can become entangled. This effect happens without any previous direct interaction between particles A and D.[2][3]
Entanglement swapping is a form ofquantum teleportation.In quantum teleportation, the unknown state of a particle can be sent from one location to another using the combination of a quantum and classical channel. The unknown state is projected by Alice onto aBell state and the result is communicated to Bob through the classical channel.[4] In entanglement swapping, the state from one of the two sources is the quantum channel of teleportation and the state from the other source is the unknown being sent to Bob.[5]: 876
The mathematical expression for the swapping process is:[5]: 876
In this expression, refers to an entangled state of pair of particles (X,Y) while BSM indicates Bell state measurement. A Bell state is one of four specific states of representing two particles with maximal entanglement; a Bell state measurement projects a quantum state onto this basis set.[6]: 813
In the field ofquantum cryptography, it helps secure communication channels better. By utilizing swapped entanglements between particles' pairs, it is possible to generate secure encryption keys that should be protected against eavesdropping.[7]
Entanglement swapping also serves as a core technology for designingquantum networks, where many nodes-like quantum computers or communication points-link through these special connections made by entangled links. These networks may support safely transferring quantum information over long routes.[8]
Entanglement swapping may allow the construction of quantum repeaters to stretch out quantum communication networks by allowing entanglement to be shared over long distances. Performing entanglement swapping at certain points acts like relaying information without loss.[9][10]
Bernard Yurke andDavid Stoler showed theoretically in 1992 that entanglement does not require interaction of the final measured particles.[11][5]: 876 [6]: 786 Using a three componentGreenberger–Horne–Zeilinger state, they showed thatMermin's device, athought experiment model designed to explain entanglement, was equivalent to anEPR experiment where the correlated particles had never directly interacted.[11]
The term entanglement swapping was coined by physicistsMarek Żukowski,Anton Zeilinger,Michael A. Horne, andArtur K. Ekert in their 1993 paper. They refined the concept to show one can extend entanglement from one particle pair to another using a method calledBell state measurement.[12]
In 1998Jian-Wei Pan working in Zeilinger's group conducted the first experiment on entanglement swapping. They used entangledphotons to show successful transfer of entanglement between pairs that never interacted.[3] Later experiments took this further, making it work over longer distances and with more complexquantum states.[citation needed]
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