Lamb-Dicke Approximation Research Articles

Cooling dynamics of a free ion in a Bose-Einstein condensate

We investigate the dynamics of an ion moving through a homogeneous Bose-Einstein condensate (BEC) after an initial momentum is imparted. For this, we derive a master equation in the weak-coupling limit and Lamb-Dicke approximation for the reduced density matrix of the ion. We study the time evolution of the ion's kinetic energy and observe that its expectation value, identified as the ion temperature Tion, is reduced by several orders of magnitude in a time on the order of microseconds for a condensate density in the experimentally relevant range between 1013cm−3 and 1014cm−3. We characterize this behavior by defining the duration at half maximum as the time required by Tion to reach half of its initial value, and study its dependence on the system parameters. Similarly, we find that the expectation value of the ion's momentum operator is reduced by nine orders of magnitude on the same timescale, making the ion's position converge to a final value. Based on these results, we conclude that the interaction with the bosonic bath allows for cooling and pinning of the ion by decreasing the expectation value of its kinetic energy and velocity, which constitutes a result of direct relevance for current atom-ion experiments. Published by the American Physical Society 2024

Numerical Simulation of the Performance of Single Qubit Gates for Trapped Ions

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.

Fast multi-qubit global-entangling gates without individual addressing of trapped ions

We propose and study ways speeding up of the entangling operations in the trapped ions system with high fidelity. First, we find a scheme to increase the speed of a two-qubit gate without the limitation of trap frequency, which was considered as the fundamental limit. Second, we study the fast gate scheme for entangling more than two qubits simultaneously. We apply the method of applying multiple frequency components on laser beams for the gate operations. In particular, in order to avoid infinite terms from the coupling to carrier transition, we focus on the phase-insensitive gate scheme here. We carefully study the effect of large excitation of motional mode beyond the limit of Lamb–Dicke approximation by including up to second order terms of the Lamb–Dicke parameter. We study the speed limit of multi-qubit global entangling gates without individual addressing requirements. Furthermore, our gates can be made insensitive to the fluctuation of initial motional phases which are difficult to stabilise in the phase-insensitive gate scheme.

Trapped Ion Quantum Computing Using Optical Tweezers and Electric Fields.

We propose a new scalable architecture for trapped ion quantum computing that combines optical tweezers delivering qubit state-dependent local potentials with oscillating electric fields. Since the electric field allows for long-range qubit-qubit interactions mediated by the center-of-mass motion of the ion crystal alone, it is inherently scalable to large ion crystals. Furthermore, our proposed scheme does not rely on either ground-state cooling or the Lamb-Dicke approximation. We study the effects of imperfect cooling of the ion crystal, as well as the role of unwanted qubit-motion entanglement, and discuss the prospects of implementing the state-dependent tweezers in the laboratory.

Two mode mechanical non-Gaussian squeezed number state in a two-membrane optomechanical system

We consider an optomechanical system with two membranes when a bichromatic laser field with red-sideband and blue-sideband frequencies is applied in the single photon strong coupling regime. It is shown that using the mode selecting method and under the Lamb–Dicke approximation, motion of membranes can evolve to single or two mode squeezed number states. By considering the environmental effect, a Wigner function is plotted for understanding the conditions that lead to the generation of non-Gaussian states. The results show that, in this system, initial states of membranes are important to generation of non-Gaussian mechanical squeezed number states.

Quantum simulation of ‘zitterbewegung’ in a single trapped ion under conditions of parity-keeping and parity-breaking

The motional trembling (‘zitterbewegung’) of a relativistic electron governed by Dirac equation was originally predicted by Schrodinger in the early days of quantum mechanics and simulated in a recent experiment with a single trapped ultracold ion. We investigate stable and instable confinements of a single trapped ion in a Paul trap under different conditions relevant to parity. Since our treatment involves neither restriction of Lamb-Dicke limit nor rotating-wave approximation, we may demonstrate different quantum dynamics of the single trapped ion in a wide range of the trapping parameters. We discuss the origin of the zitterbewegung which is relevant to the stability of the ion trapping.

Experimental simulation and limitations of quantum walks with trapped ions

We examine the prospects of discrete quantum walks (QWs) with trapped ions. In particular, we analyze in detail the limitations of the protocol of Travaglione and Milburn (2002 Phys. Rev. A 65 032310) that has been implemented by several experimental groups in recent years. Based on the first realization in our group (Schmitz et al 2009 Phys. Rev. Lett.103 090504), we investigate the consequences of leaving the scope of the approximations originally made, such as the Lamb–Dicke approximation. We explain the consequential deviations from the idealized QW for different experimental realizations and an increasing number of steps by taking into account higher-order terms of the quantum evolution. It turns out that these already become significant after a few steps, which is confirmed by experimental results and is currently limiting the scalability of this approach. Finally, we propose a new scheme using short laser pulses, derived from a protocol from the field of quantum computation. We show that this scheme is not subject to the above-mentioned restrictions and analytically and numerically evaluate its limitations, based on a realistic implementation with our specific setup. Implementing the protocol with state-of-the-art techniques should allow for substantially increasing the number of steps to 100 and beyond and should be extendable to higher-dimensional QWs.

Comparing cavity and ordinary laser cooling within the Lamb–Dicke regime

Cavity-mediated cooling has the potential to become one of the most efficient techniques to cool molecular species down to very low temperatures. In this paper we analyse cavity cooling with single-laser driving for relatively large cavity decay rates κ and relatively large phonon frequencies ν. It is shown that cavity cooling and ordinary laser cooling are essentially the same within the validity range of the Lamb–Dicke approximation. This is done by deriving a closed set of rate equations and calculating the corresponding stationary state phonon number and cooling rate. For example, when ν is either much larger or much smaller than κ, the minimum stationary state phonon number scales as κ2/16ν2 (strong confinement regime) and as κ/4ν (weak confinement regime), respectively.

Validity of Lamb–Dicke Approximations in Ion-Trap Systems

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 logic gates with two-level trapped ions beyond Lamb–Dicke limit

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.

Generation of Schrödinger cat state of a single trapped cold ion

The fidelity of the generated Schrödinger 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.

Spontaneous recoil effects of optical pumping on trapped atoms

The recoil effects of spontaneous photon emissions during optical pumping of a trapped three-level atom are exactly calculated. Without resort to the Lamb-Dicke approximation, and considering arbitrary detuning and saturation of the pump laser, the density of recoil shifts in phase space is derived. It is shown that this density is not of Gaussian shape, and that it becomes isotropic in phase space only for a branching ratio corresponding to fluorescence scattering but unfavorable for optical pumping. The dependence of its anisotropy on the laser saturation is discussed in the resonant case, and the mapping of moments of the atom's center-of-mass motion due to the pumping is presented. Moreover, it is shown how optimum parameters for protecting the center-of-mass quantum state from pump-induced disturbance depend on the specific property to be protected.

Rabi oscillations in a quantum dot-cavity system coupled to a nonzero temperature phonon bath

We study a quantum dot strongly coupled to a single high-finesse optical microcavity mode. We use a rotating wave approximation (RWA) method, commonly used in ion–laser interactions, together with the Lamb–Dicke approximation to obtain an analytic solution of this problem. The decay of Rabi oscillations because of the electron–phonon coupling is studied at arbitrary temperature and analytical expressions for the collapse and revival times are presented. Analyses without the RWA are presented as means of investigating the energy spectrum.

Generation of Entangled Displacement Fock State of Trapped Ion Manipulated by a Laser Beam

The interaction of a single two-level two-mode trapped ion with a laser beam has been studied theoretically. With application of a unitary transformation, an analytical solution to this quantum system has been obtained without performing the Lamb–Dicke approximation. In this system the entangled displacement Fock state is produced.

A scheme of quantum phase gate for trapped ion

We propose a scheme to implement two-qubit controlled quantum phase gate(CQPG) via a single trapped two-level ion located in the standing wave field of a quantum cavity, in which the trap works beyond the Lamb–Dicke limit. When the light field is resonant with the atomic transition |g> ↔ |e> of the ion located at the antinode of the standing wave, we can perform CQPG between the internal and external states of the trapped ion; while the frequency of the light field is chosen to be resonant with the first red sideband of the collective vibrational mode of the ion located at the node of the standing wave, we can perform CQPG between the cavity mode and the collective vibrational mode of the trapped ion. Neither the Lamb–Dicke approximation nor the assistant classical laser is needed. Also we can generate a GHZ state if assisted with a classical laser.

Trapping State in a System of Single Three-level Trapped IonDriven by External Fields

We investigate the interaction of a single three-level trapped ion withtwo laser beams. By applying a unitary transformation and a smallrotating transformation, an exact solution to this quantum system isobtained without performing the Lamb–Dicke approximation, and thetrapping state is observed.

An analytical solution to the nonlinear model for N trapped ions driven by a single laser field

We have studied the nonlinear model for interaction of N ions in a trap with a laser beam. By applying two successive unitary transformations, this nonlinear quantum system can be transformed into the multi-levels Jayness–Cummings model. In the end, we can obtain an analytical solution to the two-ion system without performing the Lamb–Dicke approximation, and observe the super-revival phenomenon.

Quantum logic gates with a two-level trapped ion in a high-finesse cavity beyond the Lamb–Dicke limit

In the system with a two-level ion confined both in a linear trap and in a high-Q single-mode cavity, we present a simple scheme to realize the basic two-qubit logic gates such as the quantum phase gate (QPG), the SWAP gate and the controlled-NOT (CNOT) gate beyond the Lamb–Dicke (LD) limit. We realize the three kinds of two-qubit quantum phase gates, i.e. QPG operation involving the cavity mode as well as the vibrational mode of the trapped ion, QPG operation involving the internal states as well as the vibrational mode of the trapped ion and QPG operation involving the internal states of the trapped ion as well as the cavity mode. The controlled-NOT gate can be implemented from a QPG operation through a rotation of the second qubit before and after the QPG operation. We can also perform the SWAP gate operation involving the ionic internal states of the trapped ion and the two-mode bosonic basis. The logic gates involving the cavity mode as well as the vibrational mode of the trapped ion are insensitive to spontaneous emission, and the logic gates involving the internal states as well as the vibrational mode of the trapped ion are insensitive to the decay of the cavity, which is an important feature for the practical implementation of quantum computing. Neither the LD approximation nor the auxiliary atomic level is needed in our scheme. Experimental feasibility for achieving our scheme is also discussed.

Emission spectrum of a harmonically trapped two-level atom

We investigated theoretically the emission spectrum for a harmonically trapped two-level atom; our study is not limited in Lamb-Dicke approximation. We have shown that the effects of the atomic motion on the emission spectrum are dependent on the initial state of the system, the Lamb-Dicke parameter $\ensuremath{\eta}$, and the detuning parameter.

Engineering quantum pure states of a trapped cold ion beyond the Lamb-Dicke limit

Based on the conditional quantum dynamics of laser-ion interaction, we propose an efficient theoretical scheme to deterministically generate quantum pure states of a single trapped cold ion without performing the Lamb-Dicke approximation. An arbitrary quantum state can be created by using a series of classical laser beams with selected frequencies, initial phases and durations. As special examples, we further show how to create or approximate several typical macroscopic quantum states, such as the phase state and (even/odd) coherent states. Unlike previous schemes operated in the Lamb-Dicke regime, the present one does well for arbitrary strength coupling between the internal and external degrees of freedom of the ion. The experimental realizability of this approach is also discussed.