Landau-Zener Research Articles

Real-time measurement of non-Hermitian Landau–Zener tunneling near band crossings

Real-time measurement of non-Hermitian Landau–Zener tunneling near band crossings

Calculation of ion-ion mutual neutralization rate constants using Landau-Zener theory coupled with trajectory simulations for Ar+-Cl-, Br-, I.

In this computational study, we self-consistently calculate the rate constants of mutual neutralization reactions by incorporating the electron transfer probability, using Landau-Zener state transition theory with inputs derived from abinitio quantum chemistry calculations, into classical trajectory simulations. Electronic structure calculations are done using correlation consistent basis sets with multi-reference configuration interaction to map all the molecular electronic states below the ion-dissociation limit as a function of the distance between the reacting species. Our electronic structure calculations have been significantly improved from our previous work [Liu et al., J. Chem. Phys. 159, 114111 (2023)] through improved selection of molecular electronic configurations maintaining a fine grid of 1a0 over a wide range of bond lengths and accurate treatment of spin-orbit couplings. Non-adiabatic coupling matrix elements are calculated with the three-point central difference method near each avoided crossing to estimate the exact crossing point Rx and coupling parameter Hif, which are inputs to the multi-channel Landau-Zener theory to calculate the electron transition probability. Our approach is applied to estimate the mutual neutralization rate constants for the following ion pairs: Ar+-Cl-,Ar+-Br-,Ar+-I- at ∼133Pa. Our predictions are compared against the experimental data reported by Shuman et al. [J. Chem. Phys. 140, 044304 (2014)]. It is seen that the improvement in the electronic structure calculation results in excellent agreement between the simulation results and the available experimental data to within a factor of ∼2 or ∼±50%.

Multitunneling effect of nonreciprocal Landau-Zener tunneling: Insights from DC field responses

Multitunneling effect of nonreciprocal Landau-Zener tunneling: Insights from DC field responses

Nonresonant Electric Quantum Control of Individual On-Surface Spins.

Quantum control techniques play an important role in manipulating and harnessing the properties of different quantum systems, including isolated atoms. Here, we propose to achieve quantum control over a single on-surface atomic spin using Landau-Zener-Stückelberg-Majorana (LZSM) interferometry implemented with scanning tunneling microscopy (STM). Specifically, we model how the application of time-dependent, nonresonant ac electric fields across the STM tip-surface gap makes it possible to achieve precise quantum state manipulation in an isolated Fe^{2+} ion on a MgO/Ag(100) surface. We propose a protocol to combine Landau-Zener tunneling with LZSM interferometry that permits one to measure the quantum spin tunneling of an individual Fe^{2+} ion. The proposed experiments can be implemented with ESR-STM instrumentation, opening a new venue in the research of on-surface single spin control.

Electroluminescence in n-type GaAs unipolar nanoLEDs.

In this Letter, we report the observation of electroluminescence (EL) at ∼866 nm from n-i-n unipolar (electron-transporting) III-V GaAs nanoLEDs. The devices consist of nanopillars with a top diameter of 166 nm, arranged in a 10 × 10 pillar array. Hole generation through impact ionization and Zener tunneling is achieved by incorporating an AlAs/GaAs/AlAs double-barrier quantum well within the epilayer structure of the n-i-n diode. Time-resolved EL measurements reveal decay lifetimes >300 ps, allowing us to estimate an internal quantum efficiency (IQE) higher than 2% at sub-mA current injection. These results demonstrate the potential for a new, to the best of our knowledge, class of n-type nanoscale light-emitting devices.

Revisiting the Discrepancy between Experimental and Theoretical Predictions of the Adiabaticity of Ti+ + CH3OH.

We revisit the naked transition metal cation (Ti+) and methanol reaction and go beyond the standard Landau-Zener (LZ) picture when modeling the intersystem crossing (ISC) rate between the lowest doublet and quartet states. We use both (i) unconstrained Born-Oppenheimer molecular dynamics (BOMD) calculations with an approximate two-state method to estimate population transfer between spin diabats and (ii) constrained dynamics to explore energetically accessible portions of the NDOF - 1 crossing seam, where NDOF is the total number of internal degrees of freedom. Whereas previous LZ calculations (that necessarily relied on the Condon approximation to be valid) fell short and predicted much slower crossing probabilities than shown in experiment, we show that ISC can occur rapidly because the spin-orbit coupling (SOC) between the doublet and quartet surfaces can vary by 2 orders of magnitude (depending on where in the seam the crossing occurs during dynamics) and the crossing region is revisited multiple times during a dynamics run of a few hundred femtoseconds. We further isolate the two important nuclear coordinates that tune the SOC and modulate the transition, highlighting exactly how and why organometallic ISC can occur rapidly for small systems with floppy internal nuclear vibrational modes.

Dissipative Landau-Zener tunneling in the crossover regime from weak to strong environment coupling

Landau-Zener tunneling, which describes the transition in a two-level system during a sweep through an anti-crossing, is a model applicable to a wide range of physical phenomena. Realistic quantum systems are affected by dissipation due to coupling to their environments. An important aspect of understanding such open quantum systems is the relative energy scales of the system itself and the system-environment coupling, which distinguishes the weak- and strong-coupling regimes. Using a tunable superconducting flux qubit, we observe the crossover from weak to strong coupling to the environment in Landau-Zener tunneling. Our results confirm previous theoretical studies of dissipative Landau-Zener tunneling in the weak and strong coupling limits. We devise a spin bath model that effectively captures the crossover regime. This work is relevant for understanding the role of dissipation in quantum annealing, where the system is expected to go through a cascade of Landau-Zener transitions before reaching the target state.

Measurement of the time-domain Landau-Zener-Stückelberg-Majorana interference sidebands in an <sup>87</sup>Sr optical lattice clock

<sec>Landau-Zener-Stückelberg-Majorana (LZSM) interference has significant application value in quantum state manipulation, extending quantum state lifetime, and suppressing decoherence. Optical lattice clock, with a long coherence time, increases the likelihood of experimentally observing time-domain LZSM interference. Although time-dominant Landau-Zener (LZ) Rabi oscillations have already been observed in optical lattice clock, the time-dominant LZSM interference sidebands in optical lattice clock remain unexplored. This study is based on an <sup>87</sup>Sr optical lattice clock. By periodically modulating the frequency of the 698-nm clock laser and optimizing the parameters of the optical clock system, LZ transitions are achieved under the fast-passage limit (FPL). During the clock detection, two acoustic optical modulators (AOMs) are employed: AOM1 that compensates for the frequency drift of the clock laser and operates continuously throughout the experiment, and AOM2 that performs traditional clock transition detection and generates a cosine modulation signal by using an external trigger from the RF signal generator in Burst mode. Ultimately, the periodically modulated 698-nm clock laser with a frequency of <inline-formula><tex-math id="M1">\begin{document}$\omega (t) = \cos \left[ {\displaystyle\int {\left( {{\omega _{\text{p}}} - A{\omega _{\text{s}}}\cos {\omega _{\text{s}}}t} \right)dt} } \right]$\end{document}</tex-math></inline-formula> is used to probe atoms, and the Hamiltonian is <inline-formula><tex-math id="M2">\begin{document}$ {\hat H_n}(t) = \dfrac{h}{2}[\delta + A{\omega _{\text{s}}}\cos ({\omega _{\text{s}}}t)]{\hat \sigma _z} + \dfrac{{h{g_n}}}{2}{\hat \sigma _x} $\end{document}</tex-math></inline-formula>.</sec><sec>As the modulated laser interacts with the atoms, the interference phenomenon is exhibited in the time domain; adjusting the clock laser detuning allows for probing the time-domain LZSM interference sideband spectra at different detection times. The results show that the time-domain LZSM interference sideband consists of multiple sidebands. Specifically, ±<i>kth</i> order sidebands can be observed at <i>δ</i>/<i>ω</i><sub>s</sub> = <i>k</i>, where <i>k</i> is an integer, representing constructive interference. Additionally, due to the different LZ Rabi oscillation periods for each sideband, the excitation fractions of different sidebands are also different, resulting in different excitation fractions for sidebands at the same clock detection time. When scanning the frequency of the clock laser, small interference peaks will appear next to the +1st, +4th, +5th, +6th<i>,</i> –3th and –4th order sidebands when detection time is an integer period. These peaks all appear on the right side of the sidebands, thus breaking the symmetry of LZSM interference sidebands. In contrast, when the detection time is a half-integer period, the interference sidebands exhibit symmetric distribution. This phenomenon mainly arises from the effective dynamical phase accumulated during the LZSM interference evolution. Moreover, the excitation fraction is higher than that at half-integer period, which holds potential application value in state preparation research. The experimental results are in excellent agreement with theoretical simulations, confirming the feasibility of conducting time-domain LZSM interference studies on the optical lattice clock. In the future, by further suppressing clock laser noise, the optical lattice clock will provide an ideal experimental platform for studying the effects of noise on LZ transition.</sec>

Hamiltonian non-Hermicity: Accurate dynamics with the multiple Davydov D2Ansätze.

We examine the applicability of the numerically accurate method of time dependent variation with multiple Davydov Ansätze (mDA) to non-Hermitian systems. As illustrative examples, three systems of interest have been studied, a non-Hermitian system of dissipative Landau-Zener transitions, a non-Hermitian multimode Jaynes-Cummings model, and a dissipative Holstein-Tavis-Cummings model, all of which are shown to be effectively described by the mDA method. Our findings highlight the versatility of the mDA as a powerful numerical tool for investigating complex many-body non-Hermitian systems, which can be extended to explore diverse phenomena such as skin effects, excited-state dynamics, and spectral topology in the non-Hermitian field.

Bloch-Zener oscillation with engineered Floquet energy bands in synthetic temporal lattices.

Here we experimentally demonstrate the dynamics of Bloch-Zener oscillations (BZOs) in a synthetic temporal lattice formed by the optical pulses in coupled fiber loops. By periodically modulating the phases imposed to the optical pulses in linear driven lattices, a two-band Floquet system with tunable bandgaps is realized, and the related BZOs that occurred in this system are displayed. On this basis, by manipulating the phase difference and coupling angle of the synthetic lattice, the widths of 0-gap and π-gap are tuned feasibly so that a wide variety of the interplays between Bloch oscillations and Landau-Zener tunneling (LZT) are exhibited. As an application, the temporal Mach-Zehnder interferometer utilizing BZOs is realized, where the output patterns could be modulated by the coupling rate of the synthetic lattice. This work lays the foundation for exploring BZO physics with synthetic dimensions, which may find applications in temporal pulse controlling and optical signal processing.

Simulating the Landau-Zener sweep in deeply sub-Ohmic environments.

With the goal to study dissipative Landau-Zener (LZ) sweeps in realistic solid-state qubits, we utilize novel methods from non-Markovian open quantum system dynamics that enable reliable long-time simulations for sub-Ohmic environments. In particular, we combine a novel representation of the dynamical propagator, the uniform time evolving matrix product operator method, with a stochastic realization of finite temperature fluctuations. The latter greatly reduces the computational cost for the matrix product operator approach, enabling convergence in the experimentally relevant deeply sub-Ohmic regime. Our method allows the exact simulation of dynamical protocols with long operation times, such as the LZ sweep, in challenging parameter regimes that are realized in current experimental platforms.

Modulation of spin–orbit coupled Bose–Einstein condensates: analytical characterization of acceleration-induced transitions between energy bands

Abstract The effects of modulating spin-orbit coupled Bose–Einstein condensates are analytically studied. A sinusoidal driving of the coupling amplitude is shown to induce significant changes in the energy bands and in the associated spin-momentum locking. Moreover, in agreement with recent experimental results, gravitational acceleration of the modulated system is found to generate transitions between the modified energy bands. The applicability of the Landau–Zener (LZ) model to the understanding of the experimental findings is rigorously traced. Through a sequence of unitary transformations and the reduction to the spin space, the modulated Hamiltonian, with the gravitational potential incorporated, is shown to correspond to an extended version of the LZ scenario. The generalization of the basic LZ model takes place along two lines. First, the dimensionality is enlarged to combine the description of the external dynamics with the internal-state characterization. Second, the model is extended to incorporate two avoided crossings emerging from the changes induced in the energy bands by the modulation. Our approach allows a first-principle derivation of the effective model-system parameters. The obtained analytical results provide elements to control the transitions.

Quantization rule for transition suppression in dynamic two-level system

The transmission probability in a generic two-quasi-state system is addressed. An approximate expression for the transmission probability between the two quasi-states is derived for the case where the energy gap between the two states varies. Unlike the Landau-Zener formula, the temporal change can have a generic form and can vanish several times. In particular, when the energy gap vanishes twice, an analytical quantization rule for the suppression of transmission between two quasi-states is derived.

Coherent Acoustic Control of Defect Orbital States in the Strong-Driving Limit

We use a bulk acoustic wave resonator to demonstrate coherent control of the excited orbital states in a diamond nitrogen-vacancy (NV) center at cryogenic temperature. Coherent quantum control is an essential tool for understanding and mitigating decoherence. Moreover, characterizing and controlling orbital states is a central challenge for quantum networking, where optical coherence is tied to orbital coherence. We study resonant multiphonon orbital Rabi oscillations in both the frequency and time domain, extracting the strength of the orbital-phonon interactions and the coherence of the acoustically driven orbital states. We reach the strong-driving limit, where the physics is dominated by the coupling induced by the acoustic waves. We find agreement between our measurements, quantum master-equation simulations, and a Landau-Zener transition model in the strong-driving limit. Using perturbation theory, we derive an expression for the orbital Rabi frequency versus the acoustic drive strength that is nonperturbative in the drive strength and agrees well with our measurements for all acoustic powers. Motivated by continuous-wave spin-resonance-based decoherence protection schemes, we model the orbital decoherence and find good agreement between our model and our measured few-to-several-nanoseconds orbital decoherence times. We discuss the outlook for orbital decoherence protection. Published by the American Physical Society 2024

Self‐Selective Crossbar Synapse Array with n‐ZnO/p‐NiOx/n‐ZnO Structure for Neuromorphic Computing

AbstractArtificial synapse devices are essential elements for highly energy‐efficient neuromorphic computing. They are implemented as crossbar array architecture, where highly selective synaptic weight updates for training and sneak leakage‐free inference operations are required. In this study, self‐selective bipolar artificial synapse device is proposed with n‐ZnO/p‐NiOx/n‐ZnO heterojunction, and its analog synapse operation with high selectivity is demonstrated in 32 × 32 crossbar array architecture without the aid of selector devices. The built‐in potential barrier at p‐NiOx/n‐ZnO junction and the Zener tunneling effect provided nonlinear current–voltage characteristics at both voltage polarities for self‐selecting function for synaptic potentiation and depression operations. Voltage‐driven redistribution of oxygen ions inside n–p–n oxide structure, evidenced by x‐ray photoelectron spectroscopy, modulated the distribution of oxygen vacancies in the layers and consequent conductance in an analog manner for the synaptic weight update operation. It demonstrates that the proposed n–p–n oxide device is a promising artificial synapse device implementing self‐selectivity and analog synaptic weight update in a crossbar array architecture for neuromorphic computing.

Floquet analysis perspective of driven light–matter interaction models

In this paper, we analyze the harmonically driven Jaynes–Cummings and Lipkin–Meshkov–Glick models using both numerical integration of time-dependent Hamiltonians and Floquet theory. For a separation of time scales between the drive and intrinsic Rabi oscillations in the former model, the driving results in an effective periodic reversal of time. The corresponding Floquet Hamiltonian is a Wannier–Stark model, which can be analytically solved. Despite the chaotic nature of the driven Lipkin–Meshkov–Glick model, moderate system sizes can display qualitatively different behaviors under varying system parameters. Ergodicity arises in systems that are neither adiabatic nor diabatic, owing to repeated multi-level Landau–Zener transitions. Chaotic behavior, observed in slow driving, manifests as random jumps in the magnetization, suggesting potential utility as a random number generator. Furthermore, we discuss both models in terms of a Floquet Fock state lattice.

Electrical Breakdown of Excitonic Insulators

We propose a new electrical breakdown mechanism for exciton insulators in the BCS limit, which differs fundamentally from the Zener breakdown mechanism observed in traditional band insulators. Our new mechanism results from the instability of the many-body ground state for exciton condensation, caused by the strong competition between the polarization and condensation energies in the presence of an electric field. We refer to this mechanism as “many-body breakdown.” To investigate this new mechanism, we propose a BCS-type trial wave function under finite electric fields and use it to study the many-body breakdown numerically. Our results reveal two different types of electric breakdown behavior. If the system size is larger than a critical value, the Zener tunneling process is first turned on when an electrical field is applied, but the excitonic gap remains until the field strength reaches the critical value of the many-body breakdown, after which the excitonic gap disappears and the system becomes a highly conductive metallic state. However, if the system size is much smaller than the critical value, the intermediate tunneling phase disappears since the many-body breakdown happens before the onset of Zener tunneling. The sudden disappearance of the local gap leads to an “off-on” feature in the current-voltage (I−V) curve, providing a straightforward way to distinguish excitonic insulators from normal insulators. Published by the American Physical Society 2024

Strong-Field Bloch Electron Interferometry for Band-Structure Retrieval.

When Bloch electrons in a solid are exposed to a strong optical field, they are coherently driven in their respective bands where they acquire a quantum phase as the imprint of the band shape. If an electron approaches an avoided crossing formed by two bands, it may be split by undergoing a Landau-Zener transition. We here employ subsequent Landau-Zener transitions to realize strong-field Bloch electron interferometry, allowing us to reveal band structure information. In particular, we measure the Fermi velocity (band slope) of graphene in the vicinity of the K points as (1.07±0.04) nm fs^{-1}. We expect strong-field Bloch electron interferometry for band structure retrieval to apply to a wide range of material systems and experimental conditions, making it suitable for studying transient changes in band structure with femtosecond temporal resolution at ambient conditions.

Quantum Work Statistics at Strong Reservoir Coupling.

Determining the statistics of work done on a quantum system while strongly coupled to a reservoir is a formidable task, requiring the calculation of the full eigenspectrum of the combined system and reservoir. Here, we show that this issue can be circumvented by using a polaron transformation that maps the system into a new frame where weak-coupling theory can be applied. Crucially, this polaron approach reproduces the Jarzynski fluctuation theorem, thus ensuring consistency with the laws of stochastic thermodynamics. We apply our formalism to a system driven across the Landau-Zener transition, where we identify clear signatures in the work distribution arising from a non-negligible coupling to the environment. Our results provide a new method for studying the stochastic thermodynamics of driven quantum systems beyond Markovian, weak-coupling regimes.

Bloch-Landau-Zener oscillations in a quasi-periodic potential

Bloch oscillations and Landau-Zener tunneling are ubiquitous phenomena which are sustained by a band-gap spectrum of a periodic Hamiltonian and can be observed in dynamics of a quantum particle or a wavepacket in a periodic potential under action of a linear force. Such physical setting remains meaningful for aperiodic potentials too, although band-gap structure does not exist anymore. Here we consider the dynamics of noninteracting atoms and Bose-Einstein condensates in a quasiperiodic one-dimensional optical lattice subjected to a weak linear force. Excited states with energies below the mobility edge, and thus localized in space, are considered. We show that the observed oscillatory behavior is enabled by tunneling between the initial state and a state (or several states) located nearby in the coordinate-energy space. The states involved in such Bloch-Landau-Zener oscillations are determined by the selection rule consisting of the condition of their spatial proximity and condition of quasiresonances occurring at avoided crossings of the energy levels. The latter condition is formulated mathematically using the Gershgorin circle theorem. The effect of the inter-atomic interactions on the dynamics can also be predicted on the bases of the developed theory. The reported results can be observed in any physical system allowing for observation of the Bloch oscillations, upon introducing incommensurablity in the governing Hamiltonian. Published by the American Physical Society 2024