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Chinese Physics Letters

Validity of Lamb–Dicke Approximations in Ion-Trap Systems

Lan Hai-jiang, Miao Zhang, Lian-Fu Wei

Published Jan 1, 2010• 0 citations
Publisher

Abstract

The Lamb–Dicke (LD) approximation (LDA) is usually utilized to simplify the treatments for the dynamics of ion-trap systems, where the so-called LD parameters should be sufficiently small. In this Letter, based on the quantum dynamics of a single trapped ion beyond the LDA, we discuss the fidelities of the control-NOT (CNOT) gate generated by performing the usual LDA. It is shown that the fidelity of the generated CNOT gate under the LDA is sufficiently high for the current LD experiments, e.g., it reaches 99.9% for η = 0.20. The validity of the LDA is also discussed by calculating these fidelities for slightly larger LD parameters, e.g., η = 0.4, etc.

DOI
10.1088/0256-307x/27/1/010304

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The Lamb–Dicke (LD) approximation (LDA) is usually utilized to simplify the treatments for the dynamics of ion-trap systems, where the so-called LD parameters should be sufficiently small. In this Letter, based on the quantum dynamics of a single trapped ion beyond the LDA, we discuss the fidelities of the control-NOT (CNOT) gate generated by performing the usual LDA. It is shown that the fidelity of the generated CNOT gate under the LDA is sufficiently high for the current LD experiments, e.g., it reaches 99.9% for η = 0.20. The validity of the LDA is also discussed by calculating these fidelities for slightly larger LD parameters, e.g., η = 0.4, etc.
Quantum controlled-NOT (CNOT) gates with single trapped cold ions have been experimentally demonstrated [C. Monroe et al., Phys. Rev. Lett. 75, 4714 (1995)] by using auxiliary atomic levels (besides the qubit ones) and under the usual Lamb-Dicke (LD) limit (wherein the so-called LD parameter, which represents the interaction strength between the internal and external degrees of freedom of the driven ion, is required to be sufficiently small). It has been shown [C. Monroe et al., Phys. Rev. A 55, R2489 (1997); L. F. Wei et al., 65, 062316 (2002)] that, beyond the LD limit, these gates could also be implemented for certain specific LD parameters. Here, we further show that, without any auxiliary atomic level, the desirable quantum logic gates could still be realized with sufficiently high fidelities for arbitrary LD parameters, as long as the durations of the applied laser pulses are accurately set within the decoherence times of the ions.
The effects of higher-order Lamb–Dicke approximations (LDAs), on engineering motional quantum states of a single trapped cold ion, are discussed. By measuring the internally electronic states of a single trapped cold ion, which is driven by two classical laser beams tuning respectively to the first upper and lower sidebands, we show that the externally macroscopic vibrational states superposed by either even or odd number states are generated. Under the LDA (i.e., only the first order effect of LD parameter η is considered), these states just are the usual even and odd coherent states. For relative large values of η, we showed that the effects of higher-order LDAs, i.e., terms relating to ηk, k ≥ 2, are not negligible. They might significantly enhance certain nonclassical properties, e.g., squeezing and antibunching effects of the generated macroscopic quantum states.
The fidelity of the generated Schrodinger Cat state (SCS) of a single trapped ion in the Lamb–Dicke approximation is discussed. The results show that the fidelity significantly decreases with the values of Lamb–Dicke parameter η and coherent state amplitude α increasing. For η = 0.20 and α = 3, the typical values of experimental parameters, the fidelity is rather low (30%). A scheme for generating the SCS is proposed without making the Lamb–Dike approximation in laser–ion interaction, and the fidelity of the generated SCS is about 99% for the typical values of experimental Lamb–Dicke parameters.
In the system with two two-level ions confined in a linear trap, this paper presents a simple scheme to realize the quantum phase gate (QPG) and the swap gate beyond the Lamb–Dicke (LD) limit. These two-qubit quantum logic gates only involve the internal states of two trapped ions. The scheme does not use the vibrational mode as the data bus and only requires a single resonant interaction of the ions with the lasers. Neither the LD approximation nor the auxiliary atomic level is needed in the proposed scheme. Thus the scheme is simple and the interaction time is very short, which is important in view of decoherence. The experimental feasibility for achieving this scheme is also discussed.
Trapping ions outside the Lamb–Dicke limit have been proven to be useful for the laser-cooling and quantum computing. Under the supposition of the Rabi frequency much smaller than the Lamb–Dicke parameter, we can use a simple method to analytically solve the system with a single cold ion trapped and radiated beyond the Lamb– Dicke limit, in the absence of the rotating-wave approximation (RWA). Discussion has been made for the limitation of our approach and the comparison of our results with the solutions under the RWA.
Finite gate errors limit performance of modern quantum computers. In this work, we study single qubit gate fidelities for trapped ions. For this we have numerically solved Schrödinger equation using full Hamiltonian of the system for one, two, three and four ions. This approach allows us to analyze gate errors beyond the LambDicke approximation and to take into account not only a finite occupation of the phonon modes, but also the effects related to the ion–phonon entanglement. As a result, we show how infidelity of the global single qubit gates depend on the initial phonon mode occupations, the Lamb–Dicke parameter, Rabi frequency and the number of ions.