Tailoring Photon Statistics with an Atom-Based Two-Photon Interferometer.

Controlling the photon statistics of light is paramount for quantum science and technologies. Recently, we demonstrated that transmitting resonant laser light past an ensemble of two-level emitters can result in a stream of single photons or excess photon pairs. This transformation is due to quantum interference between the transmitted laser light and the incoherently scattered photon pairs [Prasad et al., Nat. Photonics 14, 719 (2020)NPAHBY1749-488510.1038/s41566-020-0692-z]. Here, using the dispersion of the atomic medium, we actively control the relative quantum phase between these two components. We thereby realize a tunable two-photon interferometer and observe interference fringes in the normalized photon coincidence rate. When tuning the relative phase, the coincidence rate varies periodically, giving rise to a continuous modification of the photon statistics from antibunching to bunching. Beyond the fundamental insight that there exists a tunable quantum phase between incoherent and coherent light that dictates the photon statistics, our results lend themselves to the development of novel quantum light sources.

Light-Matter Interaction at the Transition between Cavity and Waveguide QED.

Experiments based on cavity quantum electrodynamics (QED) are widely used to study the interaction of a light field with a discrete frequency spectrum and emitters. More recently, the field of waveguide QED has attracted interest due to the strong interaction between propagating photons and emitters that can be obtained in nanophotonic waveguides, where a continuum of frequency modes is allowed. Both cavity and waveguide QED share the common goal of harnessing and deepening the understanding of light-matter coupling. However, they often rely on very different experimental setups and theoretical descriptions. Here, we experimentally investigate the transition from cavity to waveguide QED with an ensemble of cold atoms that is coupled to a fiber-ring resonator, which contains a nanofiber section. By varying the length of the resonator from a few meters to several tens of meters, we tailor the spectral density of modes of the resonator while remaining in the strong coupling regime. When increasing the resonator length, we observe a continuous transition from the paradigmatic Rabi oscillations of cavity QED to non-Markovian dynamics reminiscent of waveguide QED.

Collective Radiation of a Cascaded Quantum System: From Timed Dicke States to Inverted Ensembles.

The collective absorption and emission of light by an ensemble of atoms is at the heart of many fundamental quantum optical effects and the basis for numerous applications. However, beyond weak excitation, both experiment and theory become increasingly challenging. Here, we explore the regimes from weak excitation to inversion with ensembles of up to 1000 atoms that are trapped and optically interfaced using the evanescent field surrounding an optical nanofiber. We realize full inversion, with about 80% of the atoms being excited, and study their subsequent radiative decay into the guided modes. The data are very well-described by a simple model that assumes a cascaded interaction of the guided light with the atoms. Our results contribute to the fundamental understanding of the collective interaction of light and matter and are relevant for applications ranging from quantum memories to sources of nonclassical light to optical frequency standards.

Observation of Coherent Coupling between Super- and Subradiant States of an Ensemble of Cold Atoms Collectively Coupled to a Single Propagating Optical Mode.

We discuss the evolution of the quantum state of an ensemble of atoms that are coupled via a single propagating optical mode. We theoretically show that the quantum state of N atoms, which are initially prepared in the timed Dicke state, in the single excitation regime evolves through all the N-1 states that are subradiant with respect to the propagating mode. We predict this process to occur for any atom number and any atom-light coupling strength. These findings are supported by measurements performed with cold cesium atoms coupled to the evanescent field of an optical nanofiber. We experimentally observe the evolution of the state of the ensemble passing through the first two subradiant states, leading to sudden, temporary switch-offs of the optical power emitted into the nanofiber. Our results contribute to the fundamental understanding of collective atom-light interaction and apply to all physical systems, whose description involves timed Dicke states.

Collective Radiative Dynamics of an Ensemble of Cold Atoms Coupled to an Optical Waveguide.

We experimentally and theoretically investigate collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our analysis unveils the microscopic dynamics of the system, showing that collective interactions between the atoms and a single guided photon gradually build up along the atomic array in the direction of propagation of light. These results are supported by time-resolved measurements of the light transmitted and reflected by the ensemble after excitation via nanofiber-guided laser pulses, whose rise and fall times are shorter than the atomic lifetime. Superradiant decays more than 1 order of magnitude faster than the single-atom free-space decay rate are observed for emission in the forward-propagating guided mode, while at the same time, no speed-up of the decay rate is measured in the backward direction. In addition, position-resolved measurements of the light that is transmitted past the atoms are performed by inserting the nanofiber-coupled atomic array in a 45-m-long fiber ring resonator, which allow us to experimentally reveal the progressive growth of the collective response of the atomic ensemble. Our results highlight the unique opportunities offered by nanophotonic cold atom systems for the experimental investigation of collective light-matter interaction.

Unraveling Two-Photon Entanglement via the Squeezing Spectrum of Light Traveling through Nanofiber-Coupled Atoms.

We observe that a weak guided light field transmitted through an ensemble of atoms coupled to an optical nanofiber exhibits quadrature squeezing. From the measured squeezing spectrum we gain direct access to the phase and amplitude of the energy-time entangled part of the two-photon wave function which arises from the strongly correlated transport of photons through the ensemble. For small atomic ensembles we observe a spectrum close to the line shape of the atomic transition, while sidebands are observed for sufficiently large ensembles, in agreement with our theoretical predictions. Furthermore, we vary the detuning of the probe light with respect to the atomic resonance and infer the phase of the entangled two-photon wave function. From the amplitude and the phase of the spectrum, we reconstruct the real and imaginary part of the time-domain wave function. Our characterization of the entangled two-photon component constitutes a diagnostic tool for quantum optics devices.

Probing Surface-Bound Atoms with Quantum Nanophotonics.

Quantum control of atoms at ultrashort distances from surfaces would open a new paradigm in quantum optics and offer a novel tool for the investigation of near-surface physics. Here, we investigate the motional states of atoms that are bound weakly to the surface of a hot optical nanofiber. We theoretically demonstrate that with optimized mechanical properties of the nanofiber these states are quantized despite phonon-induced decoherence. We further show that it is possible to influence their properties with additional nanofiber-guided light fields and suggest heterodyne fluorescence spectroscopy to probe the spectrum of the quantized atomic motion. Extending the optical control of atoms to smaller atom-surface separations could create opportunities for quantum communication and instigate the convergence of surface physics, quantum optics, and the physics of cold atoms.

Observation of Collective Superstrong Coupling of Cold Atoms to a 30-m Long Optical Resonator.

We report on the observation of collective superstrong coupling of a small ensemble of atoms interacting with the field of a 30-m long fiber resonator containing a nanofiber section. The collective light-matter coupling strength exceeds the free-spectral range and the atoms couple to consecutive longitudinal resonator modes. The measured transmission spectra of the coupled atom-resonator system provide evidence of this regime, realized with a few hundred atoms with an intrinsic single-atom cooperativity of 0.13. These results are the starting point for studies in a new setting of light-matter interaction, with strong quantum nonlinearities and a new type of dynamics.

Chiral quantum optics.

Advanced photonic nanostructures are currently revolutionizing the optics and photonics that underpin applications ranging from light technology to quantum-information processing. The strong light confinement in these structures can lock the local polarization of the light to its propagation direction, leading to propagation-direction-dependent emission, scattering and absorption of photons by quantum emitters. The possibility of such a propagation-direction-dependent, or chiral, light-matter interaction is not accounted for in standard quantum optics and its recent discovery brought about the research field of chiral quantum optics. The latter offers fundamentally new functionalities and applications: it enables the assembly of non-reciprocal single-photon devices that can be operated in a quantum superposition of two or more of their operational states and the realization of deterministic spin-photon interfaces. Moreover, engineered directional photonic reservoirs could lead to the development of complex quantum networks that, for example, could simulate novel classes of quantum many-body systems.

Fiber ring resonator with a nanofiber section for chiral cavity quantum electrodynamics and multimode strong coupling.

We experimentally realize an optical fiber ring resonator that includes a tapered section with a subwavelength-diameter waist. In this section, the guided light exhibits a significant evanescent field which allows for efficient interfacing with optical emitters. A commercial tunable fiber beam splitter provides simple and robust coupling to the resonator. Key parameters of the resonator such as the out-coupling rate, free spectral range, and birefringence can be adjusted. Thanks to the low taper- and coupling-losses, the resonator exhibits an unloaded finesse of F=75±1, sufficient for reaching the regime of strong coupling for emitters placed in the evanescent field. The system is ideally suited for trapping ensembles of laser-cooled atoms along the nanofiber section. Based on measured parameters, we estimate that the system can serve as a platform for optical multimode strong coupling experiments. Finally, we discuss the possibilities of using the resonator for applications based on chiral quantum optics.