Historical research guide · 0821 · 2026
Kerr-induced non-Gaussianity of a bright ultrafast quantum state
Andrei Rasputnyi · Ilya Karuseichyk · Gerd Leuchs · Francesco Tani · Denis Seletskiy · Maria Chekhova
In one minute
Bright squeezed vacuum is what you get when you pump an optical amplifier hard with nothing fed into it: the vacuum’s own fluctuations are amplified into a real pulse — here 25 femtoseconds long, carrying roughly a trillion photons — that still behaves quantum mechanically. Nobody had been able to map a state that bright, because ordinary quantum detectors saturate long before you reach it. Rasputnyi, Karuseichyk, Leuchs, Tani, Seletskiy and Chekhova re-purpose an f-2f interferometer, a routine instrument in ultrafast laboratories, to sample the pulse’s phase-space picture one shot at a time — twelve thousand shots in all. They then send the pulse through a 1.5 millimetre slab of fused silica, whose refractive index depends on the pulse’s own intensity. The measured distribution bends from an ellipse into a clean S: the fingerprint of a phase shift that grows with brightness, and the first non-Gaussian shaping of a macroscopic quantum state seen directly. They also show it is not classical light in disguise.
Connection
Chapter 2 argues that the vacuum is a real, structured medium you can act on rather than an empty stage. This is that argument at its most literal: a pulse born entirely out of vacuum fluctuations, amplified to a trillion photons, measured photon-statistic by photon-statistic, and then deliberately reshaped by a piece of glass.
From the source
This is the first direct phase-space tomography of a bright, ultrafast quantum state: a single-shot f-2f interferometer, borrowed from carrier-envelope-phase metrology, samples the Husimi Q-function of a 25 femtosecond bright squeezed vacuum pulse with a mean photon number near one trillion, without attenuating the pulse and without probabilistic post-selection.Abstract; Sect. 1, final paragraphs; Sect. 2
Sending that state through a 1.5 millimetre slab of fused silica applies a Kerr phase proportional to the peak intensity, which in phase space is a rotation by an angle proportional to the squared distance from the origin; the measured distribution shears from an elongated Gaussian into the characteristic S-shaped profile, matching the classical Liouville simulation for the two weaker interaction settings.Sect. 3, figures 2(a)–2(f)
At a mean photon number near one trillion the state can never be pure: losses of order one part in that photon number already halve its purity, and the 18 per cent loss of the real setup leaves a purity around one part in a million — yet the minimum quadrature variance stays below the vacuum level, so the state remains non-classical throughout.Sect. 4, opening paragraphs; Sect. 6, third paragraph
After linear loss the bright squeezed vacuum is exactly decomposable into a statistical mixture of squeezed coherent states, valid for any loss fraction — unlike a classical chaotic field, whose constituents would be plain coherent states, the constituents here keep strong quadrature squeezing, so the observed shape is the envelope of an ensemble of genuinely quantum states.Sect. 4, mixture equations (1) and (2); Sect. 6, penultimate paragraph
Phase squeezing makes a state far more sensitive to the Kerr effect than a coherent state of the same brightness: at 8 decibels of squeezing and the same nonlinear phase measured in the experiment, the simulated Wigner-negativity volume is several orders of magnitude larger and its exponential decay with photon number is more than ten times slower, so substantial negativity should persist to thousands of displacement photons.Sect. 5, figure 4(c)
Global Wigner negativity stays hidden behind loss and technical noise, and at the tightest focus four-wave mixing and spectral broadening start to compete with the pure Kerr effect — the named routes forward are distillation and purification protocols to isolate the pure squeezed coherent components, and schemes built on strong-field light-matter interaction.Sect. 3, closing paragraph; Sect. 6, final paragraph
The way in
https://doi.org/10.1364/OPTICA.599324How to cite it
Andrei Rasputnyi, Ilya Karuseichyk, Gerd Leuchs, Francesco Tani, Denis Seletskiy, Maria Chekhova (2026) Kerr-induced non-Gaussianity of a bright ultrafast quantum state doi:10.1364/OPTICA.599324