Quantum repeater

A quantum repeater (often also: "quantum repeater") is an important component for quantum communication over long distances. As with (classic) repeaters in digital communication technology , repeaters are used between the transmitter and receiver. The concept of the quantum repeater was introduced in 1998 by Hans-Jürgen Briegel , Wolfgang Dür , Juan Ignacio Cirac and Peter Zoller . Since, in contrast to classical signals, the amplification or copying of the signal is not possible in a quantum mechanical state ( no-cloning principle ), the quantum repeater is based on quantum entanglement , the principle of entanglement swapping ( e.g. entanglement exchange, see sketch) and entanglement distillation (or purification, see sketch).

Principle of entanglement swapping

Principle of entanglement cleaning

Between the stations of the transmitter Alice and the receiver Bob there are a number of repeater stations , each of which can receive, store and transmit classical and quantum mechanical signals. The storage of quantum mechanical states takes place in a quantum memory . For the quantum repeater protocol, states are initially entangled in the quantum memories of neighboring repeater stations. Now each of the inner repeater stations shares at least two entangled states, one with each neighbor. Such a pair of entangled states enables entanglement swapping to generate entanglement between states of the neighboring stations. Entanglement swapping at each station creates entanglement between the first and the last repeater station, which then enables the loss-free transmission of quantum states via quantum teleportation from Alice to Bob. Possible errors of entanglement and the entanglement swapping generating redundant by the generation of entangled pairs and can entanglement distillation are collected. ${\displaystyle R_{1},R_{2},\dots ,R_{n}}$ ${\displaystyle R_{2},\dots ,R_{n-1}}$

Under suitable conditions, the quantum repeater enables communication with only polynomially increasing resources (i.e. duration, number of stations, number of required qubits and measurements) despite losses which increase exponentially with distance . The quantum repeater represents a certain form of quantum error correction . Since the proposal of the first repeater protocol, there have been a large number of variants, improvements and implementation proposals, in particular of which the requirements regarding the number of qubits to be stored simultaneously in the repeater stations and the To reduce the quality of the quantum memory required for this . Although all individual steps that are required for a quantum repeater could already be demonstrated experimentally, it has not yet been possible to realize a complete quantum repeater.

Web links

• https://qt.eu/understand/underlying-principles/quantum-repeaters/

credentials

1. ↑ H.-J. Briegel et al. : Quantum Repeaters: The Role of Imperfect Local Operations in Quantum Communication . In: Phys. Rev. Lett. . 81 , 1998, pp. 5932-5935. arxiv : quant-ph / 9803056 .
2. ↑ Dagmar Bruß: Quantum Information. Fischer Taschenbuch Verlag, Frankfurt am Main 2003, ISBN 3-596-15563-0 , pp. 105ff
3. ↑ Yu-Ao Chen, Bo Zhao, Shuai Chen, Jörg Schmiedmayer, Jian-Wei Pan : Experimental demonstration of a BDCZ quantum repeater node . In: Nature . tape 454 , no. 7208 , August 2008, ISSN  1476-4687 , p. 1098–1101 , doi : 10.1038 / nature07241 ( nature.com [accessed September 5, 2019]).
4. ^ R. Hanson, M. Markham, DJ Twitchen, SC Benjamin, NH Nickerson: Entanglement distillation between solid-state quantum network nodes . In: Science . tape 356 , no. 6341 , June 2, 2017, ISSN  0036-8075 , p. 928-932 , doi : 10.1126 / science.aan0070 , PMID 28572386 ( sciencemag.org [accessed September 5, 2019]).