Historical research guide · 0524 · 2026
Experimentally separating vacuum fluctuations from source radiation
Alexa Herter · Frieder Lindel · Laura Gabriel · Stefan Yoshi Buhmann · Jérôme Faist
In one minute
Fermi asked how two atoms sitting in empty space start to talk to each other. Does the link come from the vacuum’s own jitter, which both atoms feel at the same instant, or from a photon one atom emits that reaches the other a light-travel time later? For ninety years that split was a bookkeeping choice rather than a measurement. Herter, Lindel, Gabriel, Buhmann and Faist have now measured the two halves separately. They replace the atoms with two femtosecond laser pulses, 110 femtoseconds long and 50 micrometres apart, crossing a zinc-telluride crystal cooled to 4 kelvin, and read the polarisation twist each pulse picks up. Turning one wave plate chooses which half you see: two quarter-wave plates record the vacuum’s correlations, which peak at zero delay, while swapping one for a half-wave plate records source radiation, which appears only after the travel time. The two halves track the quantum fluctuation-dissipation theorem exactly.
Connection
The vacuum is this site’s first subject, and this is a laboratory reading of its fluctuations with the emitter’s own radiation cleanly separated out — the measurement that shows the field is really there rather than an artefact of the source. The same split, vacuum fluctuation against radiation reaction, is the machinery chapter 3 uses to build inertia from the zero-point field, and the authors name analogue curved spacetimes and event horizons as the next place to point the technique.
From the source
Vacuum-field fluctuations and source radiation are detected individually, separated by their causal properties, in an all-optical analogue of Fermi’s two-atom problem — an experiment long considered unfeasible.Abstract; Sect. I Introduction, closing paragraph; Sect. III Discussion, first paragraph
The vacuum-induced correlation oscillates symmetrically about zero time delay with its maximum at zero delay, while the source-radiation correlation is asymmetric and occurs mainly for positive time delays, confirming its causal nature.Sect. II Results, Fig. 2 (b)
The measured signals verify the time-domain fluctuation-dissipation theorem at the quantum level: the two correlations are related by a Hilbert transform, showing as a phase shift of pi over two in time and, in frequency, as the source-radiation imaginary part equalling the vacuum-induced correlation multiplied by minus one half.Sect. II Results, final two paragraphs; Fig. 2 (c)
With 110 femtosecond pulses in a zinc-telluride crystal at 4 kelvin and a beam separation of 50 micrometres, the peak correlation exceeds 5 volts squared per metre squared and the signal bandwidth extends to 4.5 terahertz.Sect. II Results, paragraphs 4–6; Methods, Experimental setup
Dalibard, Dupont-Roc and Cohen-Tannoudji’s 1982 prescription — demanding that the operators describing the two contributions each be Hermitian and therefore physical observables — is confirmed by these results, giving a general approach applicable to any effect involving these two fundamental quantum contributions.Sect. III Discussion, first paragraph
The demonstrated half-wave plate and quarter-wave plate configuration should be able to detect the correlations caused by a single terahertz photon within a thermal photon pool at room temperature, and opens the way to entanglement harvesting from the vacuum state and to probing quantum correlations of relativistic fields in analogue curved spacetimes, including event horizons.Sect. III Discussion, paragraphs 3–4
The way in
https://doi.org/10.1038/s41467-026-69142-4How to cite it
Alexa Herter, Frieder Lindel, Laura Gabriel, Stefan Yoshi Buhmann, Jérôme Faist (2026) Experimentally separating vacuum fluctuations from source radiation doi:10.1038/s41467-026-69142-4