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Journal of Physics B Atomic Molecular and Optical Physics

Generation of multi-atom entangled states and implementation of controlled-phase gating using photonic modules

Wan Li Yang, Hua Wei, Fei Zhou, Mang Feng

Published Feb 16, 2009• 6 citations
Publisher

Abstract

Following a recent proposal (Devitt et al 2007 Phys. Rev. A 76 052312), we study the application of photonic modules on quantum information processing. We show how to generate various entangled states of atoms in a scalable way. In addition, we design two-qubit controlled-phase gating (CPhase) using photonic modules with the qubits encoded in either the photonic polarization or the atomic levels.

DOI
10.1088/0953-4075/42/5/055503

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Following a recent proposal (Devitt et al 2007 Phys. Rev. A 76 052312), we study the application of photonic modules on quantum information processing. We show how to generate various entangled states of atoms in a scalable way. In addition, we design two-qubit controlled-phase gating (CPhase) using photonic modules with the qubits encoded in either the photonic polarization or the atomic levels.
In the emerging field of quantum information technology the two basic subfields are quantum communication and quantum computation. Photonic qubits are considered as most promising information carriers for this new technology due to the immense advantage of suffering negligible decoherence. Additionally, the very small photon-photon interactions can be replaced by inducing effective nonlinearities via measurements which allow for the implementation of crucial two-qubit gate operations. Although the spontaneous parametric down-conversion gives access to the generation of highly entangled few-photon states, such as four-qubit cluster states which allow to demonstrate the new concept of the one-way quantum computer, its applicability is highly limited due to the poor scaling of the simultaneous emission of more than one-entangled photon pair. Therefore of particular interest is the reversible mapping of qubits from photon states to atomic states. This might allow the implementation of photonic quantum repeaters for long-distance quantum communication or the generation of arbitrary multi-photon states as required for linear-optics quantum computing. Thus for the realization of such a quantum network several approaches for achieving the required quantum control between matter and photons have been studied during the past few years. Recent experiments demonstrating the generation of narrow-bandwidth single photons using a room-temperature ensemble of 87Rb atoms and electromagnetically induced transparency should emphasize the progress towards such a quantum network.
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.
Photons are preferable carriers of quantum information having little interaction with environment. Most crucial elements of photonic quantum information processing are two qubit gates. Although usage of controlled-not gates for photonic two qubit operations is common, controlled-phase gates provide more versatile utilization by enabling desired phase difference to be generated between qubits. Ability to determine the phase after fabrication yields extra benefit. We demonstrate a design of integrated photonic controlled-phase gate with programmable phase difference. We used a unique and practical polarization rotator which is ring resonator with a single notch.
In recent years, quantum state engineering and quantum information processing using microwave fields and photons have received increasing attention. In addition, multiqubit gates play an important role in quantum information processing. In this work, we propose to encode a photonic qubit via two arbitrary orthogonal eigenstates (with eigenvalues $\ifmmode\pm\else\textpm\fi{}1$, respectively) of the photon-number parity operator. With such encoding, we then present a single-step method to realize a multi-target-qubit controlled-phase gate with one photonic qubit simultaneously controlling $n\ensuremath{-}1$ target photonic qubits, by employing nnn microwave cavities coupled to one superconducting flux qutrit. This proposal can be applied not only to implement nonhybrid multi-target-qubit controlled-phase gates using photonic qubits with various encodings, but also to realize hybrid multi-target-qubit controlled-phase gates using photonic qubits with different encodings. The gate realization requires only a single-step operation. The gate operation time does not increase with the number of target qubits. Because the qutrit remains in the ground state during the entire operation, decoherence from the qutrit is greatly suppressed. As an application, we show how to apply this gate to generate a multicavity Greenberger-Horne-Zeilinger (GHZ) entangled state with general expression. Depending on the specific encodings, we further discuss the preparation of several nonhybrid and hybrid GHZ entangled states of multiple cavities. We numerically investigate the circuit-QED experimental feasibility of creating a three-cavity spin-coherent hybrid GHZ state. This proposal can be extended to accomplish the same tasks in a wide range of physical systems, such as multiple microwave or optical cavities coupled to a three-level natural or artificial atom.
In this talk I shall present a brief overview on some recent exciting experimental progress towards scalable quantum information processing (QIP), e.g., quantum communication, quantum computation and quantum simulation, via manipulation of photonic qubits. Building on our long experience in research on multi-photon entanglement, we are working on a number of experiments in the field of QIP with particular emphasis on linear optical quantum computation and free space quantum communication. It is foreseen that with ongoing development, quantum communication over 1000 km scale and coherent quantum manipulation of up to a few tens photonic qubits shall be feasible in the next five years.
Entanglement is one of the most vital properties of quantum mechanical systems, and it forms the backbone of quantum information technologies. Taking advantage of nano/microfabrication and particularly complementary metal-oxide-semiconductor manufacturing technologies, photonic integrated circuits (PICs) have emerged as a versatile platform for the generation, manipulation, and measurement of entangled photonic states. We summarize the recent progress of quantum entanglement on PICs, starting from the generation of nonentangled and entangled biphoton states, to the generation of entangled states of multiple photons, multiple dimensions, and multiple degrees of freedom, as well as their applications for quantum information processing.