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    Quantum Communication: How to Outwit noise

    March 31, 2017, Categories  Physics, Technology/Electronics

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    Sketch of a thermal quantum network, where two nodes (for example, two superconducting qubits located inside separated dilution refrigerators) are connected via a unidirectional quantum communication channel at finite temperature T ch . (b) For the implementation of a noise-resilient transfer protocol, the qubit state is first mapped onto an intermediary oscillator. The oscillator is then coupled to the incoming and outgoing fields of the channel, f in , i ( t ) and f out , i ( t ) , via a tunable decay rate γ i ( t ) , which can be realized, for example, by a flux-tunable quantum interference device [45, 46, 47]. Reuse & Permissions Figure 2 Figure 2 (a) Occupation

    Intracity Quantum Communication via Thermal Microwave Networks: Sketch of a thermal quantum network, where two nodes (for example, two superconducting qubits located inside separated dilution refrigerators) are connected via a unidirectional quantum communication channel at finite temperature T ch . (b) For the implementation of a noise-resilient transfer protocol, the qubit state is first mapped onto an intermediary oscillator. The oscillator is then coupled to the incoming and outgoing fields of the channel, f in , i ( t ) and f out , i ( t ) , via a tunable decay rate γ i ( t ) , which can be realized, for example, by a flux-tunable quantum interference device [45, 46, 47]. Reuse & Permissions Figure 2 Figure 2 (a) Occupation

    Quantum information transfer requires reliable information transfer from one quantum system to the other, which is extremely difficult to achieve. Independently, two research teams – one at the University of Innsbruck and the other at TU Wien (Vienna) – have now developed a new quantum communication protocol. This protocol enables reliable quantum communication even under the presence of contaminating noise. Both groups work with the same basic concept: To make the protocol immune to the noise, they add an additional element, a quantum oscillator, at both ends of the quantum channel.

    “Our goal was to find a way to reliably transfer a quantum state from one place to the other without having to do it several times to make it work,” explains Peter Rabl from the Atominstitut, TU Wien.Superconducting qubits, in particular, are promising elements for quantum technologies. They are tiny circuits that can assume 2 different states at the same time: quantum superposition.

    To transfer this quantum state from one superconducting qubit to another requires microwave photons, which are already used for classic signal transfer. Reliably transferring quantum information via a microwave regime has been considered impossible as the constant thermal noise completely superposes the weaker quantum signal.

    The two groups have now shown that these obstacles are not impossible to overcome as previously assumed. In collaboration with teams from Harvard and Yale (USA) they have been able to develop a transfer protocol that is immune to the inevitable noise. “Our approach is to add another quantum system – a microwave oscillator – as a mediator at both ends of the protocol to couple the qubits instead of coupling them directly to the microwave channel or waveguide,” explains Rabl.

    “We cannot prevent the thermal noise that develops in the quantum channel,” says Benoit Vermersch. “What is important is that this noise affects both oscillators on both ends in the same way. Therefore, we are able to exactly separate the detrimental effect of the noise from the weaker quantum signal through precise coupling to the waveguide.”

    “According to our calculations, we may connect qubits over several hundred meters with this protocol,” says Peter Rabl. “We would still have to cool the channels but in the long term it will be technologically feasible to link buildings or even cities in a quantum physical manner via microwave channels.” http://www.tuwien.ac.at/en/news/news_detail/article/124858/

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