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Optical Fiber Communication Conference

Quantum information with atomic ensembles

P. Zoller

Published Jan 1, 2001• 0 citations
Publisher

Abstract

We summarize our recent work on quantum information with atomic ensembles. In particular, we describe techniques for storing and manipulating quantum information in collective states of mesoscopic ensembles. Quantum information processing is is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting “dipole blockade” can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. Furthermore, we present schemes for entangling a very large number of atoms of a spinor 1 Bose Einstein condensate based on collisional interaction between atoms. As a final example, we discuss the possiblilty of creating entangled atomic states of a few bosonic atoms in a setup which mimics a “fractional quantum Hall effect” and allows the manipulation of anyonic excitations.

DOI
10.1364/icqi.2001.fqipb1

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We summarize our recent work on quantum information with atomic ensembles. In particular, we describe techniques for storing and manipulating quantum information in collective states of mesoscopic ensembles. Quantum information processing is is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting “dipole blockade” can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. Furthermore, we present schemes for entangling a very large number of atoms of a spinor 1 Bose Einstein condensate based on collisional interaction between atoms. As a final example, we discuss the possiblilty of creating entangled atomic states of a few bosonic atoms in a setup which mimics a “fractional quantum Hall effect” and allows the manipulation of anyonic excitations.
We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting "dipole blockade" can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as nonclassical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.
Quantum optics provides tools for the accurate control of light using atoms and also for manipulating atomic states using light. These techniques are being used for many applications, including quantum information processing and the generation of exotic quantum states. This thesis contains proposals for implementing a photonic quantum memory and a photon-photon gate, which are essential elements of photonic quantum information processing. Furthermore it proposes a scheme for the creation of many-body entangled states. First a proposal for a new quantum memory protocol, called the atomic frequency sweep quantum memory, is presented. A two-level polariton model is derived that explains the coherent storage and retrieval of light through the manipulation of the atomic resonance frequency. This is followed by a scheme for a deterministic photonic controlled-PHASE gate based on the strong interaction between two stationary collective Rydberg excitations in an atomic ensemble. Distortion effects caused by nonuniform interaction are quantified and compensation techniques for these effects are proposed. Finally a proposal is presented that uses Rydberg dressing for the generation of energy cat states in an atomic medium, i.e. superposition states of all the atoms being in the ground or excited state, where these two states are connected by an optical transition and thus have a significant difference in energy. Considering the fragility of the state, the effects of many different imperfections and decoherence sources are quantified. The resulting cat state would allow testing of energy decoherence models with greatly improved sensitivity.
Modern optical techniques allow one to accurately control light using atoms and to manipulate atoms using light. In this Colloquium the author reviews several ideas indicating how such techniques can be used for accurate manipulation of quantum states of atomic ensembles and photons. First a technique is discussed that allows one to transfer quantum states between light fields and metastable states of matter. The technique is based on trapping quantum states of photons in coherently driven atomic media, in which the group velocity is adiabatically reduced to zero. Next, possible mechanisms are outlined for manipulating quantum states of atomic ensembles. Specifically, a ``dipole blockade'' technique is considered in which optical excitation of mesoscopic samples into Rydberg states can be used to control the state of ensembles at the level of individual quanta. It is also noted that even simple processes involving atom-photon correlations can be used to effectively manipulate the ensemble states. Potentially these techniques can be used for implementation of important concepts from quantum information science.
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.
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.