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Quantum Communications and Quantum Imaging

Towards quantum networking using collective Rydberg qubits (Conference Presentation)

Yefeng Mei, Yin Qiang Li, Huy N. Nguyen, Brian W. Yang, P. R. Berman, A. Kuzmich

Published Oct 3, 2022• 0 citations
Publisher

Abstract

Collective qubits between atomic ground and Rydberg states can be converted, on-demand, into single photons, making them well-suited for scalable quantum network-type protocols. We demonstrate long-lived many-body Rabi oscillations and multi-particle entanglement, as well as study the dynamics of interaction-induced dephasing for collective Rydberg qubits held in a state-insensitive optical lattice trap. Both excitation blockade and spin-wave dephasing can contribute to suppression of multiple excitations, allowing for deterministic preparation of collective atomic qubits, single photons, and atom-photon entanglement for quantum information processing.

DOI
10.1117/12.2626509

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Collective qubits between atomic ground and Rydberg states can be converted, on-demand, into single photons, making them well-suited for scalable quantum network-type protocols. We demonstrate long-lived many-body Rabi oscillations and multi-particle entanglement, as well as study the dynamics of interaction-induced dephasing for collective Rydberg qubits held in a state-insensitive optical lattice trap. Both excitation blockade and spin-wave dephasing can contribute to suppression of multiple excitations, allowing for deterministic preparation of collective atomic qubits, single photons, and atom-photon entanglement for quantum information processing.
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.
The amplification of quantum information carried by a single quantum excitation is a recurring challenge across diverse quantum platforms. The coupling between a single qubit and a mesoscopic ensemble of spins, for example, can be leveraged to realize non-destructive detection of the qubit state. However, realizing robust couplings between such systems is experimentally challenging and typically requires programmable quantum gates or native long-range interactions. Here, we introduce a platform that couples a single qubit, encoded in the ground-to-Rydberg transition of a control atom, to a Rydberg-dressed target ensemble of ground-state atoms trapped in an optical lattice. We show that the state of the control qubit can be coherently mapped onto the ensemble via a qubit-controlled collective phase shift. By Rydberg-dressing the ensemble, the controlled phase shift per target atom becomes independent of the number of target atoms, making the protocol intrinsically insensitive to atom-number fluctuations and atom loss, which are the dominant experimental imperfections in our system. Exploiting the collective response of up to eight target spins, we demonstrate the efficacy of the scheme by realizing non-destructive detection of a single Rydberg excitation with a state-assignment fidelity of F=99.81−1.47+0.17 %\mathcal{F} = 99.81^{+0.17}_{-1.47}\,\%F=99.81−1.47+0.17​%. Our approach demonstrates the key ingredients for high-fidelity transfer of quantum information from a single qubit to a mesoscopic ensemble, opening a route to non-destructive mid-circuit readout of Rydberg states and to efficient interfaces between single qubits and photonic modes.
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.
Recent experimental advances in preparing regular defect-free 2D arrays of atoms with optical traps have offered new opportunities to engineer strong collective couplings between light and atoms as quantum optical dipoles. Applications of such collective light-matter interactions include the realization of atom-thick mirrors, retrieval of quantum memories with high efficiency, or the implementation of topological quantum optics. I will present novel applications in the context of quantum networking. First, I will discuss the design of a chiral photonic quantum link, where distant single atoms interact by exchanging photons propagating in a unique direction in free space. This is achieved by coupling both single atoms, representing qubits implemented using a pair of Rydberg states, to bi-layer atomic arrays which act as quantum phased-array antennas. Exploiting the combination of a “Rydberg-dressing” laser and a control laser driving the atomic arrays, an effective Rydberg dipole-dipole interaction can be engineered between atoms and atomic arrays as quantum emitters, allowing to match the spatial modes of spontaneously emitted and absorbed photons to a Gaussian mode of interest. In this way, we realize a chiral quantum interface, i.e., where atoms couple to photonic modes with a unique direction of propagation. This setup provides a basic building block of a novel platform for quantum networks in free space, i.e., without requiring coupling atoms to modes of cavities or nanostructures, which I will illustrate with the deterministic coherent transfer of quantum states between single atoms with high fidelity. Second, I will discuss the collective radiation properties of two distant single-layer atomic arrays acting as quantum memories. This system can support a long-lived non-local superposition state of atomic excitations exhibiting strong subradiance. This “dark” mode consists of an atomic excitation which is delocalized between the two distant atomic arrays, in such a way that the overall radiation is strongly suppressed by quantum interference. I will describe the preparation of these states and their application as resource of non-local entanglement. In particular, by weakly driving the arrays, the state of the quantum memories, localized in each array, can be transferred to this “non-local” subradiant state, allowing again for a deterministic coherent quantum state transfer with high fidelity between the memories. Finally, I will discuss experimental realizations using cold atoms in optical trap arrays, and the effect of experimental imperfections in the arrays, such as finite defect probability, temperature, and depth of the optical traps.
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.