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Quadrature squeezed photons from a two-level system
Carsten H. H. Schulte, Jack Hansom, Alex E. Jones, Clemens Matthiesen, Claire Le Gall, Mete Atature
TL;DR
Resonance-fluorescence squeezing from a two-level system is difficult to observe under weak excitation and low collection conditions. The authors use phase-dependent homodyne intensity-correlation detection with a quantum dot, observing below-vacuum quadrature variance alongside antibunching. The coexistence reflects the nonclassical wave- and particle-like properties of the photons.
Problem
Squeezing from a two-level system was predicted, but its observation requires weak excitation, while modest atomic oscillator strength and low collection efficiency impose stringent conditions.
Source: p. 1Method
The experiment combines a quantum dot’s resonance fluorescence with a phase-tunable local oscillator and measures phase-dependent intensity correlations.
Source: p. 2, METHODS · p. 5Results
At s = 0.1, the in-phase quadrature variance is below vacuum while resonance fluorescence remains antibunched.
Source: p. 3Takeaways & Limitations
In the weak-excitation regime, coherence between the vacuum and single-photon states accompanies both quadrature squeezing and photon antibunching.
Source: p. 4Squeezing vanishes for s ≥ 1, and negative variance is measurable only within a small phase range at s < 1.
Source: p. 4
Abstract
from arXiv · showhide
Resonance fluorescence arises from the interaction of an optical field with a two-level system and has played a fundamental role in the development of quantum optics and its applications. Despite its conceptual simplicity it entails a wide range of intriguing phenomena, such as the Mollow-triplet emission spectrum and coherent photon emission. One fundamental aspect of resonance fluorescence, reduced quantum fluctuations in the single photon stream from an atom in free space, was predicted more than 30 years ago. However, the requirement to operate in the weak excitation regime, together with the combination of modest oscillator strength of atoms and low collection efficiencies, has continued to cast stringent experimental conditions for the observation of squeezing with atoms. Attempts to circumvent these issues had to sacrifice antibunching due to either stimulated forward scattering from atomic ensembles or multiphoton transitions inside optical cavities. Here, we use an artificial atom with a large optical dipole enabling 100-fold improvement of the photon detection rate over the natural atom counterpart and reach the necessary conditions for the observation of quadrature squeezing in single resonance-fluorescence photons. Implementing phase-dependent homodyne intensity-correlation detection, we demonstrate that the electric field quadrature variance of resonance fluorescence is 3\% below the fundamental limit set by vacuum fluctuations, while the photon statistics remain antibunched. The presence of squeezing and antibunching simultaneously is a fully nonclassical outcome of the wave-particle duality of photons.
METHODS
A quantum-dot interferometer combines resonance fluorescence with an attenuated laser local oscillator, while monitoring and postselecting data to control background and spectral drift. Phase-dependent measurements use detector intensities to bin correlation histograms; phase noise and finite timing resolution constrain the measured squeezing.
METHODS
- A polarising beam splitter separates the resonantly driven quantum dot’s fluorescence from the laser, which is attenuated for use as the local oscillator; a second beam splitter recombines the fields.The setup has 1% collection efficiency, and the quantum-dot resonance is tuned through the Stark effect to vary the phase without moving interferometer parts.Source: METHODS · p. 5
- Laser leakage and spectral wandering are monitored, and correlation histograms are postselected using thresholds on leakage and phonon-sideband count rate.The phonon sideband is detected on a third single-photon detector; quantum-dot emission is monitored continuously.Source: METHODS · p. 5
- Because phase is not actively controlled during the measurements, detector intensities serve as a phase proxy, and reference fringes map intensity bins to phase bins.The binning cannot distinguish positive from negative phase, but the correlation functions are symmetric around zero.Source: METHODS · p. 5
- Low count rates increase shot noise and can worsen phase binning, while phase noise and finite timing resolution reduce the measured squeezing.Phase noise includes fluctuations from spectral wandering and interferometric instability; its extent is used as a fitting parameter in the theoretical curves.Source: METHODS · pp. 5–6