Monday, December 01, 2008

Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift

This is an amazing feat.

A group at ETH-Zurich and Sherbrooke, Canada, managed to show how quantum vacuum fluctuation can produce a Lamb shift type effect in a many-body system that is a superconducting electronic circuit[1]!

Abstract: Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states.

Again, an amazing accomplishment. Note also that, as with the Schrodinger Cat-type experiment, this work also use superconducting system to be able to demonstrate the long-range coherence that is required in such an experiment. As Carver Mead has said, a superconductor is the clearest manifestation of quantum effects at the macroscopic scale. That's why it is typically used in these studies.


[1]A. Fragner et al. "Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift", Science v.322, p.1357 (2008).

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