Decoherence Dynamics Research Articles

Quantum decoherence dynamics in stochastically fluctuating environments.

We theoretically study the decoherence of a two-level quantum system coupled to noisy environments exhibiting linear and quadratic fluctuations within the framework of a stochastic Liouville equation. It is shown that the intrinsic energy levels of the quantum system renormalize under either the linear or quadratic influence of the environmental noise. In the case of quadratic dependence, the renormalization of the energy levels of the system emerges even if the environmental noise exhibits stationary statistical properties. This is in contrast to the case under linear influence, where the intrinsic energy levels of the system renormalize only if the environmental noise displays nonstationary statistics. We derive the analytical expressions of the decoherence function in the cases where the fluctuation of the frequency difference depends linearly and quadratically on the nonstationary Ornstein-Uhlenbeck noise (OUN) and random telegraph noise (RTN) processes, respectively. In the case of the linear dependence of the OUN, the environmental nonstationary statistical property can enhance the dynamical decoherence. However, the nonstationary statistics of the environmental noise can suppress the quantum decoherence in this case under the quadratic influence of the OUN. In the presence of the RTN, the quadratic influence of the environmental noise does not give rise to decoherence but only causes a determinate frequency renormalization in dynamical evolution. The environmental nonstationary statistical property can suppress the quantum decoherence of the case under the linear influence of the RTN.

Exciton–photocarrier interference in mixed lead-halide-perovskite nanocrystals

The use of semiconductor nanocrystals in scalable quantum technologies requires characterization of the exciton coherence dynamics in an ensemble of electronically isolated crystals in which system–bath interactions are nevertheless strong. In this communication, we identify signatures of Fano-like interference between excitons and photocarriers in the coherent two-dimensional photoluminescence excitation spectral lineshapes of mixed lead-halide perovskite nanocrystals in dilute solution. Specifically, by tuning the femtosecond-pulse spectrum, we show such interference in an intermediate coupling regime, which is evident in the coherent lineshape when simultaneously exciting the exciton and the free-carrier band at higher energy. We conclude that this interference is an intrinsic effect that will be consequential in the quantum dynamics of the system and will thus dictate decoherence dynamics, with consequences in their application in quantum technologies.

Nonzero current due to coherent dynamic electron transport through a dimer with no external bias

We have proposed a mechanism of the interplay of injected coherence and decoherence dynamics of the system in electron transport through an excitonic dimer when it is simultaneously connected to thermocoherent electron source and a thermal electron sink. Due to such an interplay, the variation of current with the energy gap between two excitonic states which is the internal bias of the system, is largely affected and it markedly bears the signature of the decoherence dynamics. We have also shown an extension in the modification of Landauer conductance formula for electron transport through excitonic dimer. The current has the property of an asymmetry in spectral width which can be understood in terms of sum of two Lorentzian profiles. More remarkably we have shown that current due to electron transport through the dimer can also arise at zero external bias which is a purely quantum coherent effect of the source fermionic reservoir having zero thermal occupation number.

Practical Method for Achieving Single-Photon Femtosecond Time-Resolved Spectroscopy: Transient Stimulated Emission.

Recent advances in single-photon femtosecond spectroscopy have highlighted the power of entangled photons in probing the properties of materials, previously inaccessible through semiclassical spectroscopic approaches. In this study, we theoretically propose a new single-photon-based femtosecond time-resolved spectroscopy technique termed single-photon transient stimulated emission (SP-TSE). SP-TSE not only enables the selective investigation of singly excited superposition states but also harnesses the quantum mechanical nature of photons for the efficient data acquisition of transient responses, thereby supporting the feasibility of experimental realization of SP-TSE. The key aspect of SP-TSE is the selective detection of two-photon states produced through stimulated emission using coincidence counting techniques. Our theoretical framework, supported by numerical simulations, demonstrates the efficacy in capturing the pure decoherence dynamics of a model molecular cavity, highlighting its potential to reveal quantum mechanical properties that are difficult to observe with semiclassical femtosecond time-resolved experiments.

Femtosecond Collisional Dissipation of Vibrating D_{2}^{+} in Helium Nanodroplets.

We explored the collision-induced vibrational decoherence of singly ionized D_{2} molecules inside a helium nanodroplet. By using the pump-probe reaction microscopy with few-cycle laser pulses, we captured in real time the collision-induced ultrafast dissipation of vibrational nuclear wave packet dynamics of D_{2}^{+} ion embedded in the droplet. Because of the strong coupling of excited molecular cations with the surrounding solvent, the vibrational coherence of D_{2}^{+} in the droplet interior only lasts for a few vibrational periods and completely collapses within 140fs. The observed ultrafast coherence loss is distinct from that of isolated D_{2}^{+} in the gas phase, where the vibrational coherence persists for a long time with periodic quantum revivals. Our findings underscore the crucial role of ultrafast collisional dissipation in shaping the molecular decoherence and solvation dynamics during solution chemical reactions, particularly when the solute molecules are predominantly in ionic states.

A solvable model for graph state decoherence dynamics

We present an exactly solvable toy model for the continuous dissipative dynamics of permutation-invariant graph states of NN qubits. Such states are locally equivalent to an NN-qubit Greenberger-Horne-Zeilinger (GHZ) state, a fundamental resource in many quantum information processing setups. We focus on the time evolution of the state governed by a Lindblad master equation with the three standard single-qubit jump operators, the Hamiltonian part being set to zero. Deriving analytic expressions for the expectation values of observables expanded in the Pauli basis at all times, we analyze the nontrivial intermediate-time dynamics. Using a numerical solver based on matrix product operators, we simulate the time evolution for systems with up to 64 qubits and verify a numerically exact agreement with the analytical results. We find that the evolution of the operator space entanglement entropy of a bipartition of the system manifests a plateau whose duration increases logarithmically with the number of qubits, whereas all Pauli-operator products have expectation values decaying at most in constant time.

Quantum speed limits of quantum system in colored environments

In this paper, we investigate quantum speed limits (QSLs) of a two-level system in the colored environments which are described by stochastic processes. We present QSL can be affected by the correlation time, the partitions of the constituent elements of the two-level system in different colored environments: the Ornstein–Uhlenbeck environment, simple fluctuating bottleneck environment, and the Dichotomic (two-state) environment. The investigations provide us a potential way to manipulate the quantum speed limit time (QSLT) of the quantum system in the colored environments. Our results could provide different insights on quantum control technologies of QSL and decoherent dynamics for the quantum computation.

Mapping electronic decoherence pathways in molecules.

Establishing the fundamental chemical principles that govern molecular electronic quantum decoherence has remained an outstanding challenge. Fundamental questions such as how solvent and intramolecular vibrations or chemical functionalization contribute to the decoherence remain unanswered and are beyond the reach of state-of-the-art theoretical and experimental approaches. Here we address this challenge by developing a strategy to isolate electronic decoherence pathways for molecular chromophores immersed in condensed phase environments that enables elucidating how electronic quantum coherence is lost. For this, we first identify resonance Raman spectroscopy as a general experimental method to reconstruct molecular spectral densities with full chemical complexity at room temperature, in solvent, and for fluorescent and non-fluorescent molecules. We then show how to quantitatively capture the decoherence dynamics from the spectral density and identify decoherence pathways by decomposing the overall coherence loss into contributions due to individual molecular vibrations and solvent modes. We illustrate the utility of the strategy by analyzing the electronic decoherence pathways of the DNA base thymine in water. Its electronic coherences decay in [Formula: see text]30 fs. The early-time decoherence is determined by intramolecular vibrations while the overall decay by solvent. Chemical substitution of thymine modulates the decoherence with hydrogen-bond interactions of the thymine ring with water leading to the fastest decoherence. Increasing temperature leads to faster decoherence as it enhances the importance of solvent contributions but leaves the early-time decoherence dynamics intact. The developed strategy opens key opportunities to establish the connection between molecular structure and quantum decoherence as needed to develop chemical strategies to rationally modulate it.

Electron Dynamics in Open Quantum Systems: The Driven Liouville-von Neumann Methodology within Time-Dependent Density Functional Theory.

A first-principles approach to describe electron dynamics in open quantum systems driven far from equilibrium via external time-dependent stimuli is introduced. Within this approach, the driven Liouville-von Neumann methodology is used to impose open boundary conditions on finite model systems whose dynamics is described using time-dependent density functional theory. As a proof of concept, the developed methodology is applied to simple spin-compensated model systems, including a hydrogen chain and a graphitic molecular junction. Good agreement between steady-state total currents obtained via direct propagation and those obtained from the self-consistent solution of the corresponding Sylvester equation indicates the validity of the implementation. The capability of the new computational approach to analyze, from first principles, non-equilibrium dynamics of open quantum systems in terms of temporally and spatially resolved current densities is demonstrated. Future extensions of the approach toward the description of dynamical magnetization and decoherence effects are briefly discussed.

Finding the Dynamics of an Integrable Quantum Many‐Body System via Machine Learning

AbstractThe dynamics of the Gaudin magnet (“central‐spin model”) is studied using machine‐learning methods. This model is of practical importance, for example, for studying non‐Markovian decoherence dynamics of a central spin interacting with a large bath of environmental spins and for studies of nonequilibrium superconductivity. The Gaudin magnet is also integrable, admitting many conserved quantities. Machine‐learning methods may be well suited to exploiting the high degree of symmetry in integrable problems, even when an explicit analytic solution is not obvious. Motivated in part by this intuition, a neural‐network representation (restricted Boltzmann machine) is used for each variational eigenstate of the model Hamiltonian. Accurate representations of the ground state and of the low‐lying excited states of the Gaudin‐magnet Hamiltonian are then obtained through a variational Monte Carlo calculation. From the low‐lying eigenstates, the non‐perturbative dynamic transverse spin susceptibility is found, describing the linear response of a central spin to a time‐varying transverse magnetic field in the presence of a spin bath. Having an efficient description of this susceptibility opens the door to improved characterization and quantum control procedures for qubits interacting with an environment of quantum two‐level systems.

Dynamic phases induced by two-level system defects on driven qubits

Recent experimental evidences point to two-level defects, located in the oxides and on the interfaces of the Josephson junctions, as the major constituents of decoherence in superconducting qubits. How these defects affect the qubit evolution with the presence of external driving is less well understood since the semiclassical qubit-field coupling renders the Jaynes–Cummings model for qubit-defect coupling undiagonalizable. We analyze the decoherence dynamics in the continuous coherent state space induced by the driving and solve the master equation endowed with an extra decay-cladded driving term via a Fokker–Planck equation. The solutions for diffusion propagators as Gaussian distributions show four distinct dynamic phases: four types of convergence paths to limit cycles of varying radius by the distribution mean, which are determined by the competing external driving and the defect decays. The qubit trajectory resulted from these solutions is a super-Poissonian over displaced Fock states, which reduces to a Gibbs state of effective temperature decided by the defect at zero driving limit. Furthermore, the Poincare map shows the dependence of the rate of convergence on the initial state. In other words, the qubit evolution can serve as an indicator of the defect coupling strength through the variation of the driving strength as a parameter.

Electron Spin Decoherence Dynamics in Magnetic Manganese Hybrid Organic-Inorganic Crystals: The Effect of Lattice Dimensionality.

Organic-inorganic metal hybrids with their tailorable lattice dimensionality and intrinsic spin-splitting properties are interesting material platforms for spintronic applications. While the spin decoherence process is extensively studied in lead- and tin-based hybrids, these systems generally show short spin decoherence lifetimes, and their correlation with the lattice framework is still not well-understood. Herein, we synthesized magnetic manganese hybrid single crystals of (4-fluorobenzylamine)2MnCl4, ((R)-3-fluoropyrrolidinium)MnCl3, and (pyrrolidinium)2MnCl4, which represent a change in lattice dimensionality from 2D and 1D to 0D, and studied their spin decoherence processes using continuous-wave electron spin resonance spectroscopy. All manganese hybrids exhibit nanosecond-scale spin decoherence time τ2 dominated by the symmetry-directed spin exchange interaction strengths of Mn2+-Mn2+ pairs, which is much longer than lead- and tin-based metal hybrids. In contrast to the similar temperature variation laws of τ2 in 2D and 0D structures, which first increase and gradually drop afterward, the 1D structure presents a monotonous rise of τ2 with the temperatures, indicating the strong correlation of spin decoherence with the lattice rigidity of the inorganic framework. This is also rationalized on the basis that the spin decoherence is governed by the competitive contributions from motional narrowing (prolonging the τ2) and electron-phonon coupling interaction (shortening the τ2), both of which are thermally activated, with the difference that the former is more pronounced in rigid crystalline lattices.

Spin-vibronic coherence drives singlet-triplet conversion.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

Self-Oscillations in a Nanogap Spin Hall Nano-Oscillator with a Perpendicularly Magnetized External Film

We study spin-current-induced magnetodynamics in a planar nanogap spin Hall nano-oscillator (SHNO) based on $\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_{4}$ with a large perpendicular magnetic anisotropy (PMA) at in-plane field geometry. Two distinct dynamic modes are observed in the spin-torque ferromagnetic resonance (FMR) and the generated microwave spectra. The primary mode has a frequency above the FMR frequency and a significant blueshift of the frequency with the increasing current $I$, consistent with the propagating spin-wave mode. The secondary higher-frequency mode with a much weaker power intensity, discrete frequency jump, and a slight redshift of frequency is only observed at low fields below the saturation field of 2 kOe, suggesting that it is likely related to the emerged domain region with in-plane magnetization due to grain-boundary defects, which is further supported by the frequency enhancement with reducing temperature. The primary propagating mode decreases its frequency and broadens its minimum linewidth by reducing the temperature, indicating that the frequency-drift instability dominates its dynamical decoherence due to the boundary-pinning effect. Furthermore, the micromagnetic simulation reproduces the experimentally observed propagating spin-wave and in-plane domain modes, and provides their spatial characteristics. Our demonstrated propagating spin waves in a SHNO with an out-of-plane magnetization extended bilayer can facilitate long-distance mutual synchronization in the SHNO network arrays for enhancing coherence and power as spin-wave sources in magnon-based devices or spin-based neuromorphic computing.

Spontaneous symmetry breaking in nonsteady modes of open quantum many-body systems

In a quantum many-body system coupled to the environment, its steady state can exhibit spontaneous symmetry breaking when a control parameter exceeds a critical value. In this study, we consider spontaneous symmetry breaking in non-steady modes of an open quantum many-body system. Assuming that the time evolution of the density matrix of the system is described by a Markovian master equation, the dynamics of the system is fully characterized by the eigenmodes and spectrum of the corresponding time evolution superoperator. Among the non-steady eigenmodes with finite lifetimes, we focus on the eigenmodes with the highest frequency, which we call the most coherent mode. For a dissipative spin model, it is shown that the most coherent mode exhibits a transition from a disordered phase to a symmetry-broken ordered phase, even if the steady state does not show singular behavior. We further argue that the phase transition of the most coherent mode induces a qualitative change in the decoherence dynamics of highly entangled states, i.e., the Schr\"odinger's cat states.

Schrödinger cat states of a 16-microgram mechanical oscillator

According to quantum mechanics, a physical system can be in any linear superposition of its possible states. Although the validity of this principle is routinely validated for microscopic systems, it is still unclear why we do not observe macroscopic objects to be in superpositions of states that can be distinguished by some classical property. Here we demonstrate the preparation of a mechanical resonator in Schrödinger cat states of motion, where the ∼1017 constituent atoms are in a superposition of two opposite-phase oscillations. We control the size and phase of the superpositions and investigate their decoherence dynamics. Our results offer the possibility of exploring the boundary between the quantum and classical worlds and may find applications in continuous-variable quantum information processing and metrology with mechanical resonators.

Probing many-body dynamics in a two-dimensional dipolar spin ensemble

The most direct approach for characterizing the quantum dynamics of a strongly interacting system is to measure the time evolution of its full many-body state. Despite the conceptual simplicity of this approach, it quickly becomes intractable as the system size grows. An alternate approach is to think of the many-body dynamics as generating noise, which can be measured by the decoherence of a probe qubit. Here we investigate what the decoherence dynamics of such a probe tells us about the many-body system. In particular, we utilize optically addressable probe spins to experimentally characterize both static and dynamical properties of strongly interacting magnetic dipoles. Our experimental platform consists of two types of spin defects in nitrogen delta-doped diamond: nitrogen-vacancy colour centres, which we use as probe spins, and a many-body ensemble of substitutional nitrogen impurities. We demonstrate that the many-body system’s dimensionality, dynamics and disorder are naturally encoded in the probe spins’ decoherence profile. Furthermore, we obtain direct control over the spectral properties of the many-body system, with potential applications in quantum sensing and simulation.

Intrinsic decoherence dynamics in the three-coupled harmonic oscillators interaction

A system of three-coupled quantized fields is studied under the intrinsic decoherence scheme given by the Milburn equation. Taking the closed analytical form of this relation, without second-order approximation, we arrive at an explicit analytical solution for the wavefunction dynamics. We show the decaying behavior in the number of modes of each quantized field for suitable initial conditions. One of the modes goes coherently driven, and the others are vacuum states. We discuss the redistribution of the excitations.

Qubit-environment entanglement outside of pure decoherence: Hyperfine interaction

In spin-based architectures of quantum devices, the hyperfine interaction between the electron spin qubit and the nuclear spin environment remains one of the main sources of decoherence. This paper provides a short review of the current advances in the theoretical description of the qubit decoherence dynamics. Next, we study the qubit-environment entanglement using negativity as its measure. For an initial maximally mixed state of the environment, we study negativity dynamics as a function of environment size, changing the numbers of environmental nuclei and the total spin of the nuclei. Furthermore, we study the effect of the magnetic field on qubit-environment disentangling time scales.

Scanning qubit probe of edge states in a topological insulator

In this work, we propose a novel qubit-based sensor with the ability to characterize topological edge states in low-dimensional systems. A composite system is studied, consisting of a qubit coupled to a topologically nontrivial Su-Schrieffer-Heeger chain between semi-infinite lead channels. This qubit probe utilizes decoherence dynamics which, under a weak-coupling framework, are related to the environment's local density of states. Qubit decoherence rate measurements along a sample therefore provide the means to extract edge state profiles. The environment's influence on the qubit's subspace is captured by an effective projective treatment, leading to an analytical decoherence rate expression. We demonstrate that the scanning qubit probe identifies and yields a complete spatial characterization of the topological edge states within the composite system.