A repeated application of this so-called “entanglement-swapping” can in principle be used to transfer quantum entanglement between distant sites (see Figure 2). Recently, we could demonstrate that by violating a Bell inequality between particles that never interacted with each other. This means that a Bell-state measurement of two of the photons - one each from two pairs of entangled photons - results in the remaining two photons becoming entangled, even though they have never interacted in the past. In fact, the quantum state of a teleportee particle does not have to be well defined and it could thus even be entangled with another photon. This means that an arbitrary unknown quantum state can be teleported. Īn important feature of teleportation is that it provides no information whatsoever about the state being teleported. One thus achieves for these cases instantaneous quantum computation, which can provide additional computational advantage (although with exponentially small probability for certain schemes. If Bob would have used his state beforehand as an input state for a quantum computational operation, the teleportation would define his input only after the actual computation has taken place. Note, that in one of the cases the original state is transferred instantaneously and no active transformation is required at Bob's side. After communicating the classical result to Bob, he can perform one out of four unitary operations to obtain the original state to be teleported. Thus the quantum state to be teleported can be reconstructed at Bob's side. The four possible outcomes of this measurement provide her with two bits of classical information. she projects the two particles into the basis of Bell-states. Alice then performs a Bell-state measurement between the teleportee particle and her shared ancilla, i.e. Suppose, Alice and Bob share an ancilla entangled pair in advance. Quantum physics, however, provides a working strategy. Classically this is an impossible task, since Alice cannot obtain the full information of the state to be teleported without previous knowledge about its preparation. The purpose of quantum teleportation is to transfer an arbitrary quantum state to a distant location, say from Alice to Bob, without transmitting the actual physical object carrying the state. Besides its importance for quantum computation, teleportation is at the heart of the quantum repeater, a concept eventually allowing the distribution of quantum entanglement over arbitrary distances and thus enabling quantum communication over large distances and even networking on a global scale. Teleportation of quantum states is an intriguing concept within quantum physics and a striking application of quantum entanglement. Zeilinger, in Les Houches, 2004 3 Quantum teleportation and entanglement swapping
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