Realizing a rapidly switched Unruh-DeWitt detector through electro-optic sampling of the electromagnetic vacuum
Sho Onoe · Thiago L. M. Guedes · Andrey S. Moskalenko · Alfred Leitenstorfer · Guido Burkard · Timothy C. Ralph
Abstract and summary · read the original at the source
In one page
Electro-optic sampling is the ultrafast-optics trick of firing a light pulse shorter than one oscillation of the field you want to look at, then reading that field off the pulse’s polarisation. Sho Onoe and Timothy Ralph in Queensland, with Thiago Guedes, Alfred Leitenstorfer and Guido Burkard at Konstanz and Andrey Moskalenko at KAIST, show that this laboratory technique is the same thing as an Unruh-DeWitt detector — the idealised particle detector theorists use to ask what an accelerating observer sees in empty space. The strong probe pulse plays the part of switching the detector on and off inside a fraction of one cycle, and switched that fast the detector registers real photons out of the vacuum: the virtual particles the uncertainty principle allows for very short intervals are handed over as countable excitations of the probe. The authors locate the crossover — which detected frequencies give genuine detector behaviour and which give ordinary squeezing — and specify the crystal, the pulse and the optics that would put it on a bench.
Why it matters hereChapter 2 argues the vacuum is a structured medium you can measure rather than a bookkeeping convention, and this paper turns the most sensitive vacuum probe in ultrafast optics into a detector whose readings are stated in the language of quantum field theory on a spacetime metric. It belongs with chapter 5 too, because it is a laboratory system standing in for curved-spacetime physics: the fast switch acts as a horizon, and how sharp that horizon is decides whether vacuum entanglement can be harvested.
What it claims
01The nonlinear interaction behind electro-optic sampling is rewritten as an Unruh-DeWitt detector. Driving a zincblende-type crystal with a strong coherent pulse shorter than one cycle of the sampled field gives an interaction unitary of beam-splitter form between the probe mode and a Gaussian subcycle mode of the vacuum, the same form the idealised detector takes when its coupling is switched on and off inside a fraction of a cycle. The strong optical pump plays exactly the role the switching function plays in the detector model.Section IV B and Section VI A; equations 30, 31 and 36 to 40
Published and peer-reviewed02A field mode shorter than its own cycle is not empty even in the Minkowski vacuum. Because such a mode is broad enough in frequency to contain both positive- and negative-frequency components, the expectation value of its photon number is nonzero and grows as the pulse gets shorter; the authors attribute those photons to virtual particles, which the uncertainty principle permits for correspondingly short intervals. In their numerical plot the two quadrature variances begin to depart from shot noise once the Gaussian envelope reaches roughly 0.45 of the inverse carrier frequency.Section III B, equations 21 to 24 and Figure 2 with its caption
Published and peer-reviewed03Two regimes are separated, and the boundary between them is calculated. Using what the authors call the first-order unitary evolution, the electro-optic interaction is either a beam-splitter exchange or a two-mode squeezing. Above the probe carrier frequency it is the beam-splitter case, and that is the genuine detector regime in which vacuum virtual particles are promoted to real near-infrared excitations; below it the process is ordinary squeezing, and the counted photons are one half of a created pair. Deviations of the quadrature variances from unity in the detector regime are the signal of sampled virtual particles.Section VI B and VI D, Figure 5; Conclusion, second paragraph
Published and peer-reviewed04The measurement is specified with laboratory numbers. The probe is centred at 255 terahertz with a 5.8 femtosecond intensity full width at half maximum, focused to a 3 micrometre waist radius and carrying about 5 times ten to the ninth photons; the crystal is 7 micrometres of zinc telluride with an electro-optic coefficient of 4 picometres per volt; the detection band is 1 terahertz wide. The readout is ellipsometry: a band-pass filter, a phase-shifting waveplate, a half-wave plate at one eighth of a right angle, a Wollaston prism splitting the two polarisations, and a photon counter on each output.Section VI C and VI D; Figure 4 with its caption
Designed, not yet built05The first-order unitary treatment predicts quadrature variances below the shot-noise level at some detected frequencies, where ordinary first-order perturbation theory does not. Sub-shot-noise variance is one of the trademarks of quantumness, so the authors put it forward as the working regime in which the existence of virtual photons carrying quantum correlations in the electromagnetic ground state can be verified experimentally. That measurement is the one to watch.Section VI D, final paragraph; Abstract, final sentence
What to watch06Fast switching acts as a horizon, and the shape of the switch decides what can be harvested. In Unruh-Davies and Hawking radiation a horizon separates the two members of a virtual pair and leaves them delocalised and entangled; switching the detector on and off quickly does the same job. With a Gaussian pump the horizon is soft, so the probe picks up both partners in the same mode and the authors see no strong decoupling — the squeezing visible in their two figures is the signature of that. Searching for pump profiles sharp enough to observe vacuum entanglement between regions of spacetime is the research direction they name.Conclusion, final paragraph
What to watch
Read it · abstract
Abstract
A new theoretical framework to describe the experimental advances in electro-optic detection of broadband quantum states, specifically the quantum vacuum, is devised. Electro-optic sampling is a technique in ultrafast photonics which, when transferred into the quantum domain, can be utilized to resolve properties of a sampled quantum state via its interaction with a strong coherent probe pulse at ultrafast timescales. By making use of fundamental concepts from quantum field theory on spacetime metrics, the nonlinear interaction behind the electro-optic effect is shown to be equivalent to a stationary Unruh-DeWitt detector coupled to a conjugate field during a very short time interval. When the coupling lasts for a time interval comparable to the oscillation periods of the detected field mode (i.e., the subcycle regime), virtual particles inhabiting the field vacuum are transferred to the detector in the form of real excitation. We demonstrate that this behavior can be rigorously translated to the scenario of electro-optic sampling of the quantum vacuum, in which the (spectrally filtered) probe works as an Unruh-DeWitt detector, with its interaction-generated photons arising from virtual particles inhabiting the electromagnetic vacuum. Our analysis accurately encapsulates the quantum nature of the vacuum, and we propose the specific working regime in which we can experimentally verify the existence of virtual photons with quantum correlations in the electromagnetic ground state.
Sho Onoe, Thiago L. M. Guedes, Andrey S. Moskalenko, Alfred Leitenstorfer, Guido Burkard and Timothy C. Ralph, Physical Review D 105, 056023, published 24 March 2022; preprint arXiv:2103.14360, 26 March 2021.
(Abstract only — see the rights note above. On this site, the companion electro-optic sampling sheets are Guedes and colleagues on the back action the probe itself causes at /library/stm-b32988c49d, Yang and colleagues on subcycle tomography of quantum light at /library/stm-87e1461aa4, and Herter and colleagues on experimentally separating vacuum fluctuations from source radiation at /library/stm-d38d7e2394.)
The way in
https://doi.org/10.1103/PhysRevD.105.056023Published as Physical Review D 105, 056023 (2022) under the APS default licence, which is not a Creative Commons licence; the publisher PDF refuses automated requests. The preprint is on arXiv as 2103.14360, version 1 posted 26 March 2021 under the arXiv.org perpetual non-exclusive distribution licence, checked on the arXiv record on 2026-09-08, where no Creative Commons statement appears. So this sheet carries the summary, the claims and the authors’ own abstract and sends the reader to the source. The abstract below is the published one, as carried on the DOI record; the claims are read against the preprint, which was then titled ’Realizing an Unruh-DeWitt detector through electro-optic sampling of the electromagnetic vacuum’ and whose section numbering is used in the locators. Sho Onoe and Timothy C. Ralph write from the Centre for Quantum Computation and Communication Technology at the University of Queensland; Thiago L. M. Guedes, Alfred Leitenstorfer and Guido Burkard from the Department of Physics at the University of Konstanz; Andrey S. Moskalenko from the Department of Physics at KAIST in Daejeon.
How to cite it
Sho Onoe, Thiago L. M. Guedes, Andrey S. Moskalenko, Alfred Leitenstorfer, Guido Burkard, Timothy C. Ralph (2022) Realizing a rapidly switched Unruh-DeWitt detector through electro-optic sampling of the electromagnetic vacuum. doi:10.1103/PhysRevD.105.056023
Where it sits in the curriculum