Search

We report the enhancement of quantum correlations for biphotons generated via spontaneous four-wave mixing in an ensemble of cold two-level atoms. This enhancement is based on the filtering of the Rayleigh linear component of the spectrum of the two emitted photons, favoring the quantum-correlated sidebands reaching the detectors. We provide direct measurements of the unfiltered spectrum presenting its usual triplet structure, with Rayleigh central components accompanied by two peaks symmetrically located at the detuning of the excitation laser with respect to the atomic resonance. The filtering of the central component results in a violation of the Cauchy-Schwarz inequality to $4.8 \pm 1.0 \leq 1$ for a detuning of 60 times the atomic linewidth, representing an enhancement by a factor of four compared with the unfiltered quantum correlations observed at the same conditions., Comment: 5 pages, 4 figures

Quantum correlations in four-wave-mixing from ensembles of cold two-level atoms may prevail without filtering over background light with well-known classical interpretations, such as Rayleigh scattering, as recently experimentally demonstrated in [Phys. Rev. Lett. {\bf 128}, 083601 (2022)]. Here we provide an extended investigation of this effect, in which we detail the experimental procedure and the variation of the quantum correlation with various parameters of the system. Particularly, we show that the decay rate of the quantum correlations changes with the number of atoms in the sample, providing another indication of its superradiant nature. The non-classical aspects of the signal occur for short timescales, but the long timescales carry as well a lot of information on the classical correlations of the system. This slow classical regime presents also two clearly distinct time scales, which we explain by two different pathways for the creation of biphotons. From the global analysis of the data in all its timescales, we are able to derive an empirical expression to fit the data, resulting in information on, among other parameters, the sample's temperature and superradiant acceleration. In general, the reported quantum correlations present a dependence on critical parameters of the system, such as optical depth and excitation power, that is quite different from other systems used for biphoton generation, and are more robust to changes in these parameters. This opens the possibility of exploring this process for efficient generation of narrowband biphotons or of other quantum-correlated photonic states of higher order., Comment: 13 pages, 15 figures

Quantum metrology offers significant improvements in several quantum technologies. In this work, we propose a Gaussian quantum metrology protocol assisted by initial position-momentum correlations (PM). We employ a correlated Gaussian wave packet as a probe to examine the dynamics of Quantum Fisher Information (QFI) and purity based on PM correlations to demonstrate how to estimate the PM correlations and, more importantly, to unlock its potential applications such as a resource to enhance quantum thermometry. In the low-temperature regime, we find an improvement in the thermometry of the surrounding environment when the original system exhibits a non-null initial correlation (correlated Gaussian state). In addition, we explore the connection between the loss of purity and the gain in QFI during the process of estimating the effective environment coupling and its effective temperature.

We explore the effects of Markovian bath coupling and initial position-momentum correlations on the coherence and purity of Gaussian quantum states. Our analysis focuses on the roles these factors play in the dynamics of quantum coherence, coherence lengths, and state purity. Our results reveal that initial position-momentum correlations have a remarkable impact on the quantum properties of the mixed state. These correlations lead to opposing behaviors in coherence and purity: as quantum coherence increases in response to stronger correlations, purity diminishes, and vice versa. This inverse relationship illustrates the phenomenon where, governed by these initial correlations, a state with greater mixing can display enhanced quantum coherence compared to a less mixed state. We also observe an unanticipated coherence freezing phenomenon, quantified by the relative entropy of coherence. Notably, this freezing is driven by initial position-momentum correlations, although the final frozen value is independent of these correlations.

The Gouy phase is essential for accurately describing various wave phenomena, ranging from classical electromagnetic waves to matter waves and quantum optics. In this work, we employ phase-space methods based on the cross-Wigner transformation to analyze spatial and temporal interference in the evolution of matter waves characterized initially by a correlated Gaussian wave packet. First, we consider the cross-Wigner of the initial function with its free evolution, and second for the evolution through a double-slit arrangement. Different from the wave function which acquires a global Gouy phase, we find that the cross-Wigner acquires a Gouy phase difference due to different evolution times. The results suggest that temporal like-Gouy phases are important for an accurate description of temporal interference. Furthermore, we propose a technique based on the Wigner function to reconstruct the cross-Wigner from the spatial intensity interference term in a double-slit experiment with matter waves.

We report the experimental verification of nonclassical correlations for a four-wave-mixing process in an ensemble of cold two-level atoms, confirming theoretical predictions by Du et al. in 2007 for the violation of a Cauchy-Schwarz inequality in the system, and obtaining $R = (1.98\pm0.03) \nleq 1$. Quantum correlations are observed in a nano-seconds timescale, in the interference between the central exciting frequency and sidebands dislocated by the detuning to the atomic resonance. They prevail without filters over the noise background coming from linear scattering from the same optical transition. These correlations are fragile with respect to processes that disturb the phase of the atomic excitation, but are robust to variations in number of atoms and to increasing light intensities., Comment: 8 pages, 6 figures