Quadrature squeezed photons from a two-level system

Measurements of a steady emission of single photons from a quantum dot demonstrate that the fluctuations of the electric field can periodically be 3% below the fundamental quantum limit and confirm the long-standing prediction that the quantum state of single photons can be squeezed. Cheating on principle The minimum uncertainty in a quantum experiment is determined by the Heisenberg limit. The Heisenberg principle cannot be violated, but 'cheating' is possible, most familiarly in the well-known quantum optics technique of 'squeezing'. This involves reducing fluctuations of a single variable of a light state beyond the quantum limit at the expense of enhancing the fluctuations of another. It was predicted more than 30 years ago by Dan Walls and Peter Zoller that this effect should also be achievable for single photons but an experimental verification has been out of reach. Mete Atatüre and colleagues now succeed in a measurement involving steady emission of single or 'antibunched' photons from a quantum dot with high photon detection rate. They demonstrate that the fluctuations of one variable can be 3% below the fundamental quantum limit, confirming the earlier prediction that the quantum state of single photons can be squeezed. 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 1 , photon antibunching 2 and coherent photon emission 3 . One fundamental aspect of resonance fluorescence—squeezing in the form of reduced quantum fluctuations in the single photon stream from an atom in free space—was predicted more than 30 years ago 4 . 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 necessitate stringent experimental conditions for the observation of squeezing with atoms. Attempts to circumvent these issues had to sacrifice antibunching, owing to either stimulated forward scattering from atomic ensembles 5 , 6 or multi-photon transitions inside optical cavities 7 , 8 . 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 9 and reach the necessary conditions for th