Electric field correlation measurements on the electromagnetic vacuum state

TL;DR

This paper reports the first direct measurement of electric field correlations in the electromagnetic vacuum's zero-point fluctuations using terahertz electro-optic detection, revealing their spectral and coherence properties.

Contribution

It introduces a novel direct measurement technique for vacuum field fluctuations in the terahertz range, providing new insights into their spectral composition and coherence.

Findings

Measured vacuum electric field fluctuations with a peak value of 0.25 V/m.

Determined the spectral composition of the electromagnetic ground state.

Analyzed the temporal and spatial coherence of vacuum fluctuations.

Abstract

Quantum mechanics ascribes to the ground state of the electromagnetic radiation 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 came thus from more indirect evidence, such as the Lamb shift, the Casimir force between close conductors or spontaneous emission. A direct method to determine the spectral characteristics of vacuum field fluctuations has been missing so far. In this work, we perform a direct measurement of the field correlation on these fluctuations in the terahertz frequency range using electro-optic detection in a non-linear 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 $6.2\cdot 10^{-2}~V^2/m^2$, corresponding to a fluctuating vacuum field of 0.25 V/m. With this measurement, we determine the spectral composition of the ground state of electromagnetic radiation which lies within the bandwidth of electro-optic detection.