Quantum physicists turn to the dark state

1 Mar 2016

A team of quantum physicists at Aalto University have managed to tame a so-called “dark state”. The study, published in Nature Communications on 23 February 2016, is part of the Superconducting Qubits and Circuit QED project funded under the Academy of Finland’s Academy Programme Programmable Materials (OMA). The research team is part of the Centre of Excellence in Low Temperature Quantum Phenomena and Devices.

In their experiment, the researchers operated the circuit in a regime where it no longer absorbs or emits electromagnetic waves of a certain frequency, as if it would be hiding under an invisibility cloak – hence the term “dark state”. Then, by using a sequence of carefully-crafted microwave pulses, the team employed the dark state to realise a transfer of population from the ground energy level to the second energy level, without populating the first energy level. The amount of energy transferred in this process corresponds to a single microwave photon with about the same frequency as those in mobile phones or microwave ovens. This was verified by quantum tomography – a technique of reconstructing the wave function (in general the density matrix) by applying rotations in an abstract qubit space followed by measurements.

Besides the relevance for quantum computing, the result also has deep conceptual implications. Much of our understanding of reality is based on the so-called continuity principle: the idea that influences propagate from here to there by going through all places in-between. Real objects do not just appear somewhere from nothing. But the experiment seems to defy this. Like in a great show of magic, quantum physics allows things to materialise here and there, apparently out of nowhere.

Nature Communications article: Stimulated Raman adiabatic passage in a three-level superconducting circuit, K. S. Kumar, A. Vepsäläinen, S. Danilin, and G. S. Paraoanu. DOI: 10.1038/NCOMMS10628

Source: Aalto University press release

Last modified 1 Mar 2016
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