Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.
Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.
The advances in laser cooling and trapping techniques allow samples of ultra-cold atoms to be created and controlled for the study of quantum physics. In this thesis, we focus on using ensembles of cold atoms to investigate novel quantum effects and their applications in quantum optics. Realization of a scalable intercontinental quantum system, e.g. a global quantum internet, relies on quantum memories with long lifetimes on the order of seconds. We have reduced the differential light shifts of the atomic ground states to a sub-Hz level and achieved a lifetime of 16 seconds. Achieving fast and efficient quantum operations, on the other hand, requires strongly-interacting systems. We explore highly excited Rydberg atoms towards this goal and demonstrate Rydberg excitation blockade, many-body Rabi oscillations, trapping of Rydberg atoms, deterministic single photon source, and atom-photon entanglement. Finally, we integrate the two essential capabilities for quantum information processing, fast quantum state generation and long-term storage, by simultaneously exploiting Rydberg levels for interactions and ground atomic levels for the storage of quantum state. Our work advances the control of coherence properties and interactions in cold atomic ensembles to a new level and opens new opportunities for studies of complex quantum systems.
Interacting Rydberg excitations in cold atomic ensembles can exhibit large quantum nonlinearities that enable engineering of strong interactions between individual photons. Consequently, Rydberg ensembles are a promising platform for quantum information applications and the study of more fundamental physics of few- and many-body phenomena with interacting photons. This thesis presents a series of experiments that study and exploit different regimes of Rydberg-mediated interactions. We report the realization of an efficient on-demand single-photon source. The strong long-range Rydberg interactions allow the excitation of only a single collective Rydberg state within the entire atomic medium. The collective excitation can be subsequently retrieved as a single-photon. We use this scheme to generate highly pure and highly indistinguishable photons, which are suitable for scalable quantum information applications. These photons can be compatible with other atomic systems due to their narrow bandwidth, which makes building practical hybrid quantum systems feasible. Here, we demonstrate high visibility two-photon quantum interference between our Rydberg-produced photons, and photons emitted by a remote single-trapped ion. We also study Rydberg atoms under electromagnetically induced transparency conditions, where coherent superpostions of photons and Rydberg excitations propagate through the atomic medium as lossless dark-state polaritons. In the experiment, we use the external control fields to tune the interactions to a many-body regime where we can observe resonant scattering of dark-state polaritons to lossy channels. We show that the enhanced scattering process arises as a pure three-body effect.
Single photons are the best carriers of quantum information for long-distance transmission. Nevertheless, maximal achievable distance is limited by the exponential decay of photons as a function of link length. The protocol of quantum repeater provides a promising solution by replacing direction transmission with segmented entanglement distribution and entanglement connection via swapping. The quantum repeater necessitates a key element of quantum memory for making efficient interconnections. An atomic ensemble is very suitable for this purpose due to the collective enhanced interaction. Single photons are stored as collective excitations in an atomic ensemble. Thus a comprehensive study of the physics relating to collective excitations is crucially important for improving the quantum memory performance and its reachable applications in quantum repeater and quantum network. In this article, we review our experimental work on cold atomic ensembles in recent years, focusing on the coherent manipulation of collective excitations. We first briefly introduce the general concept of collective excitations and the preparation process through spontaneous Raman scattering, and we review our experimental work on extending the coherence time, such as suppressing motional dephasing by increasing the spin-wave wavelength, by confining atoms with a three-dimensional optical lattice. Afterwards, we discuss about the retrieval process of collective excitations and review our experiments on using a ring-cavity enhanced setup to improve the retrieval efficiency. The coherent qubit operation in a quantum memory is very useful for enabling new functionalities for a quantum network, in a subsequent section, we thus review our work on developing Raman-based coherent operations for single excitations. Afterwards, we mention our experiments on creating a pair of atom-photon entanglement by interfering two modes of a collective excitation. Improving the entanglement preparation efficiency is crucially important, and Rydberg-based interaction provides a promising solution. Our experimental work in this direction is also reviewed. Additionally, as an application in coherent manipulation with collective excitations, we show several experiments on using excitations in remote atomic memories and demonstrating basic functionality of quantum repeater and quantum network. In short, significant progress has been made in the coherent manipulation of single collective excitations in cold atomic ensembles, and further improvement will be accelerated by the Rydberg-enabled interactions; practical applications in quantum repeater and quantum network is foreseeable in the near future.
Quantum information is a rapidly developing field that merges the principles of quantum mechanics with information theory to explore new ways of processing and transmitting information. Qubits, which utilize superposition and entanglement, offer the potential for exponentially faster computation and secure communication methods that are fundamentally different from classical systems. This thesis contributes to the future of quantum information by studying two key platforms using neutral atoms: atomic arrays and atomic ensembles. It summarizes essential theoretical and experimental methods in the field and discusses key findings. To achieve entanglement among atomic qubits, Rydberg interactions are utilized, with two main effects discussed in this thesis. The Rydberg blockade effect is introduced where atoms trapped in an ensemble can form a two-level superatom. A state-insensitive optical lattice is used to achieve long coherence time for both the ground and Rydberg states. Long-lived Rabi oscillations and Ramsey interference are reported to demonstrate the collective enhancement of √N in a superatom. Furthermore, Rydberg dephasing is separated from Rydberg blockade and is investigated as a method to generate single photons from multiply excited states. This thesis presents both theoretical models and temporal evolution measurements of Rydberg dephasing from an initially unentangled Rydberg spinwave into an entangled single photon state. Additionally, this thesis examines the collective atomic dipole moment of a superatom, ⟨E⟩. Theoretical and experimental work are conducted to study the dipole moment when the phase-matched emission is combined with a coherent field in a homodyne detection method. This work advances our understanding of Rydberg interactions and collective dipole moments, contributing to the rapidly developing field of quantum information. The goal is to pave the way for the next era of quantum information through collaborative efforts and innovative research.
Interacting ultracold atomic quantum gases provide an ideal test bed to study strongly correlated quantum matter. The interactions between atoms in such an ultracold quantum gas are typically short-ranged and well described by an effective contact potential. Introducing longer-range interactions promises the realization of novel quantum phases which are absent in systems with only short-range interactions. Trapped atoms, resonantly laser-coupled to highly excited, strongly interacting Rydberg states have been proposed as a versatile platform to realize such long-range interacting quantum matter. In a first experiment presented in this thesis, we combined the single-atom-sensitive local preparation and detection enabled by a quantum gas microscope to prepare and microscopically characterize an ensemble of rubidium-87 atoms trapped in an optical lattice in the regime of strong Rydberg blockade. There, we observe collectively enhanced optical coupling in an effective two-level system, the so-called “superatom”, and infer the presence of entanglement. Detuned optical coupling to Rydberg states, termed “Rydberg dressing”, has been proposed as an alternative approach to induce long-range interactions. Rather than directly exciting an atom to a Rydberg state, the properties of the latter are admixed to a ground state, which consequently acquires long-range interactions. In the context of this thesis, we have designed a laser system in the ultraviolet spectral range to admix Rydberg P-state character to the ground state of atoms in an optical lattice. In an emerging Ising quantum magnet, the presence of Rydberg-dressed interactions was demonstrated by an interferometric technique combined with spatially resolved spin correlation measurements. The theoretically predicted tunability of the isotropy and range of the interaction were confirmed experimentally. While these initial measurements exhibited unexpected dissipation, in a subsequent experiment in a one-dimensional spin chain this dissipation was overcome. We substantiated the improved coherence times by tracking the time evolution of the magnetization upon suddenly switching on interactions and observing coherent collapse and revival dynamics, one of the hallmarks of coherent quantum evolution. Our results establish Rydberg-dressed interactions in ultracold gases and pave the way to realize and study novel exotic quantum phases.
Rydberg quantum optics achieves optical non-linearities at the single-photon level by mapping the strong dipolar interactions between Rydberg atoms in cold atomic gases onto light fields using electromagnetically-induced transparency and photon storage. The non-linearities are a direct consequence of the long-range character of the interaction which allows a single photon to modify the optical response in a volume containing many atoms. In this thesis, the long-range character of the resulting effective photon-photon interaction is directly observed as photons propagating in non-overlapping optical modes are stored as collective Rydberg excitations in adjacent and non-overlapping microscopic clouds of 87Rb atoms. While stored, van-der-Waals interactions imprint spatially non-uniform phase shifts in the collective excitations. These distort the photons' retrieval modes resulting in anti-correlated retrieval between the original modes.In this first demonstration of contactless effective interactions between photons, these effects are observed between photons separated by more than 15 times their wavelength, well above the optical diffraction limit.This represents a promising step towards the implementation of scalable, multichannel quantum optical devices such as quantum gates. The experiments are enabled by a new, specialised experimental setup centred around a pair of in-vacuo aspheric lenses. These provide optical resolution of order 1 µm to optically trap and address the ensembles separated by distances well below the range of Rydberg interactions. The ensembles are prepared in approximately 100 ms thanks to efficient loading of a magneto-optical trap (MOT) from an atomic beam produced by a 2D MOT. Combined with the ability to recycle the ensembles > 20000 times, effective cycle times exceeding 100 kHz enable the acquisition of large datasets for the analysis of photon statistics within a matter of minutes.