Electric field correlation measurements on the electromagnetic vacuum state.
basic_science · Level V
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- Record sourced from PubMed, PMID 30971847.
- Also identified by DOI 10.1038/s41586-019-1083-9.
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Abstract
Quantum mechanics ascribes to the ground state of the electromagnetic radiation<sup>1</sup> zero-point electric field fluctuations that permeate empty space at all frequencies. No energy can be extracted from the ground state of a system, and therefore these fluctuations cannot be measured directly with an intensity detector. The experimental proof of their existence therefore came from more indirect evidence, such as the Lamb shift<sup>2,3,4</sup>, the Casimir force between close conductors<sup>5,6,7</sup> or spontaneous emission<sup>1,8</sup>. A direct method of determining the spectral characteristics of vacuum field fluctuations has so far been missing. Here we perform a direct measurement of the field correlation on these fluctuations in the terahertz frequency range by using electro-optic detection<sup>9</sup> in a nonlinear crystal placed in a cryogenic environment. We investigate their temporal and spatial coherence, which, at zero time delay and spatial distance, has a peak value of 6.2 × 10<sup>-2</sup> volts squared per square metre, corresponding to a fluctuating vacuum field<sup>10,11</sup> of 0.25 volts per metre. With this measurement, we determine the spectral components of the ground state of electromagnetic radiation within the bandwidth of our electro-optic detection.