Historical research guide · 0512 · 2013
Dynamical Casimir effect in a Josephson metamaterial
Pasi Lähteenmäki · G. S. Paraoanu · Juha Hassel · Pertti J. Hakonen
In one minute
Pasi Lähteenmäki, Sorin Paraoanu, Juha Hassel and Pertti Hakonen at Aalto University built a mirror out of electronics and used it to shake light out of the vacuum. Their cavity is a seven-millimetre superconducting waveguide holding 250 SQUIDs, the tiny superconducting loops whose inductance a magnetic field can tune. Changing that field changes how fast microwaves travel down the line, by a few per cent, which changes the cavity’s effective length exactly as a moving wall would — and no real wall can be shaken anywhere near fast enough. Driven at 10.8 gigahertz, twice the cavity’s 5.4-gigahertz resonance, the device emits real microwave photons in pairs whose two frequencies always add up to the drive frequency. The team measured the full covariance matrix of the outgoing field, and when they detuned the cavity the emitted spectrum split into the two-peaked shape theory predicts. Amplified thermal noise would have given a lopsided spectrum; theirs is symmetric. The photons came out of the ground state.
Connection
Chapter 2 rests on the vacuum being a real medium with modes you can push on, and this is the second independent demonstration that pushing on them yields countable photons — read it with the Chalmers measurement at /library/stm-b8989fce8d, a different group, a different circuit, the same result. For chapter 6 the interest is in the knob: the speed of light inside the metamaterial is set by a magnetic flux, so the boundary is moved by a current rather than by a motor, and the energy that leaves the cavity was drawn from the ground state of the field.
From the source
Photon production by a mechanically moving mirror is non-negligible only when the mirror velocity approaches the speed of light — a mirror on a 1 gigahertz nanomechanical oscillator moving at an amplitude of 1 nanometre would create only about ten to the minus ninth Casimir photons per second — which is why the boundary has to be made electrical rather than mechanical.Introduction, paragraph beginning ’The experimental verification’
A metamaterial of 250 SQUIDs embedded in a 7 millimetre superconducting coplanar waveguide, flux-modulated at about 10.8 gigahertz, varies the speed of light in the line by a few per cent around half the vacuum value and so changes the effective electrical length of the 5.4 gigahertz cavity by up to 10 millimetres; the resonance quality factor multiplies the length modulation, and the field inside sees this as a moving boundary.Paragraph beginning ’In our work’; Figure 1a to 1c and the supplementary derivation of the effective length modulation
Casimir photons are generated as correlated pairs at frequencies symmetric about half the drive frequency, with energy conservation satisfied so that the two frequencies sum to the drive; the authors extract the full covariance matrix of the two sidebands from simultaneously triggered vector signal analysers and find non-zero off-diagonal elements, the two-mode squeezing correlations, growing linearly with the pump amplitude.Abstract; Figures 2b, 2c and 2d; equation 1
When the cavity is detuned from half the drive frequency the emitted noise spectrum splits into two nearly equal peaks, matching the theoretical prediction at 50 millikelvin external and 60 millikelvin internal dissipation temperature, and this bimodal structure is taken as direct evidence of the dynamical Casimir effect.Figures 3a, 3b and 3d; abstract, sentence on the bimodal sparrow-tail structure
The observed photon flux cannot be assigned to amplification of thermal fluctuations: the sample sits at 50 millikelvin, a thermal occupation of 0.0056 quanta at 5.4 gigahertz, the measurements were made where the parametric gain of the driven cavity is nearly unity, and a single quantum of classical noise in the internal modes would make the spectrum strongly asymmetric where the measured pattern is symmetric.Paragraph beginning ’The samples were fabricated’; Figure 3c and the closing paragraph
The correlated microwave photon pairs open the way to Einstein-Podolsky-Rosen type experiments and to quantum information processing with continuous variables.Final sentence of the paper
The way in
https://doi.org/10.1073/pnas.1212705110How to cite it
Pasi Lähteenmäki, G. S. Paraoanu, Juha Hassel, Pertti J. Hakonen (2013) Dynamical Casimir effect in a Josephson metamaterial doi:10.1073/pnas.1212705110