Coherence Decay Research Articles

Non-Markovian models of environmentally-driven disentanglement in molecular charge qubits

Models of quantum disentanglement are developed for nanometer-scale molecular charge qubits (MCQs). Two MCQs, A and B, are prepared in a Bell state and separated for negligible A–B interactions. Interactions between the local environment and each MCQ unravels A–B entanglement during coherent system+environment evolution. Three models are used for dynamics: (1) a previously developed numerical model, in which both AB and environment E are modeled explicitly; (2) an exact, semi-analytic model, in which only the dynamics of AB are calculated, and (3) an approximate model developed from the semi-analytic model and assumptions about randomness in E. In the approximate model, the non-zero coherences of the density operator for AB decay with a Gaussian time dependence. This provides a time scale for system dynamics in the exact models as well. This time scale is related directly to the strength of AB–E interaction. This time scale describes cases where environmental interaction with one target MCQ is dominant, generalizing a previous time scale applicable only when both MCQs have roughly the same strength of interaction with the local environment. Entanglement is measured using two-qubit correlation functions, the dynamics of which are used to demonstrate the effectiveness of the time scale. The early-time decay of coherences and the loss of entanglement are well-characterized as Gaussian, a behavior that Markovian models for memoryless environments cannot capture. The approximate Gaussian model may be used to describe the dynamics of MCQ disentanglement under the influence of environments modeled here, as well as other environments where randomness is present.

Topological Protection and Control of Quantum Markovianity

Under the Born–Markov approximation, a qubit system, such as a two-level atom, is known to undergo a memoryless decay of quantum coherence or excitation when weakly coupled to a featureless environment. Recently, it has been shown that unavoidable disorder in the environment is responsible for non-Markovian effects and information backflow from the environment into the system owing to Anderson localization. This turns disorder into a resource for enhancing non-Markovianity in the system–environment dynamics, which could be of relevance in cavity quantum electrodynamics. Here we consider the decoherence dynamics of a qubit weakly coupled to a two-dimensional bath with a nontrivial topological phase, such as a two-level atom embedded in a two-dimensional coupled-cavity array with a synthetic gauge field realizing a quantum-Hall bath, and show that Markovianity is protected against moderate disorder owing to the robustness of chiral edge modes in the quantum-Hall bath. Interestingly, switching off the gauge field, i.e., flipping the bath into a topological trivial phase, allows one to re-introduce non-Markovian effects. Such a result indicates that changing the topological phase of a bath by a tunable synthetic gauge field can be harnessed to control non-Markovian effects and quantum information backflow in a qubit-environment system.

Rotational-permutational dual-pairing and long-lived spin order.

Quantum systems in contact with a thermal environment experience coherent and incoherent dynamics. These drive the system back toward thermal equilibrium after an initial perturbation. The relaxation process involves the reorganization of spin state populations and the decay of spin state coherences. In general, individual populations and coherences may exhibit different relaxation time constants. Particular spin configurations may exhibit exceptionally long relaxation time constants. Such spin configurations are known as long-lived spin order. The existence of long-lived spin order is a direct consequence of the symmetries of the system. For nuclear spin systems, rotational and permutational symmetries are of fundamental importance. Based on the Schur-Weyl duality theorem, we describe a theoretical framework for the study of rotational and permutational dual-symmetries in the context of long-lived spin order. Making use of the proposed formalism, we derive refined bounds on the number on long-lived spin populations and coherences for systems exhibiting rotational-permutational dual-symmetries.

Decoherence in Conformal Field Theory

Noise sources are ubiquitous in Nature and give rise to a description of quantum systems in terms of stochastic Hamiltonians. Decoherence dominates the noise-averaged dynamics and leads to dephasing and the decay of coherences in the eigenbasis of the fluctuating operator. For energy-diffusion processes stemming from fluctuations of the system Hamiltonian the characteristic decoherence time is shown to be proportional to the heat capacity. We analyze the decoherence dynamics of entangled CFTs and characterize the dynamics of the purity, and logarithmic negativity, that are shown to decay monotonically as a function of time. The converse is true for the quantum Renyi entropies. From the short-time asymptotics of the purity, the decoherence rate is identified and shown to be proportional to the central charge. The fixed point characterizing long times of evolution depends on the presence degeneracies in the energy spectrum. We show how information loss associated with decoherence can be attributed to its leakage to an auxiliary environment and discuss how gravity duals of decoherence dynamics in holographic CFTs looks like in AdS/CFT. We find that the inner horizon region of eternal AdS black hole is highly squeezed due to decoherence.

Quantum coherence of multiqubit states in correlated noisy channels

The long-time maintenance of quantum coherence is crucial for its practical applications. We explore decoherence process of a multiqubit system passing through a correlated channel (phase flip, bit flip, bit-phase flip, and depolarizing). The results show that the decay of coherence was evidently delayed when the consecutive actions of the channel on the sequence of qubits has some classical correlations. In particular, the relative entropy of coherence for a system with large number of qubits is more robust than that with small number of qubits. We also provide an explanation for the delayed decoherence by exploring the interplay between the change of the unlocalized quantum coherence and the total correlation gain of the multiqubit system.

On the modelling of wavepacket scattering noise with coherence effects.

An investigation of a wavepacket model for free-jet and jet-surface interaction noise was conducted. The source term for the axisymmetric mode was extracted from a Mach 0.9 jet large eddy simulation and employed to adjust the parameters of a simple source model. Streamwise coherence decay, in particular, was considered. The source model was propagated with both the free-field and tailored Green's function for a semi-infinite flat plate positioned at a distance of r/D = 1 from the jet axis. Significant deviations were observed in the prediction of the low-angle directivity of the isolated jet as well as in the reproduction of the characteristics of the source field. However, the effects of trailing edge noise were well reproduced. The installed jet case, at the region dominated by trailing-edge scattering, showed very little sensitivity to the coherence decay, a crucial feature in the isolated jet case. In this sense, the modelling of the installed-jet case proved to be much simpler.

Super Damping of Mechanical Vibrations

We report the phenomenon of coherent super decay (CSD), where a linear sum of the displacement of several damped oscillators can collectively decay much faster than the individual ones in the first stage, followed by stagnating ones after more than 97% of the energy has been dissipated. The parameters of the damped oscillators for CSD are determined by the process of response function decomposition, which is to use several slow decay response functions to approximate the response function of a fast decay resonator. Evidence established in experiments and in finite element numerical simulations not only strongly supported the numerical investigations, but also uncovered an unexplored region of the tuned mass damper (TMD) parameter space where TMD’s with total mass less than 0.2% of a stainless steel plate can damp its first resonance at 100 Hz up to a damping ratio of 4.6%. Our findings also shed light onto the intriguing underline relationships between complex functions with different singular points.

Probing the dephasing time of crystals via spectral properties of high-order harmonic generation

Coherent time is a characteristic time in the extreme nonlinear optics regime and thus generally introduced as the dephasing time in the simulations of the solid-state high-harmonic generation. This characteristic time linked with the coherent decay of quantum trajectory controls the emergence of the spectral splittings in the high harmonic spectroscopy, which can been attributed to the temporal interference between two adjacent harmonic emission channels. To reproduce the harmonic peaks and spectral splittings, a temporal two-slit interference model in the momentum space is introduced. In addition, mechanisms of spectral splitting provide a state-of-the-art avenue to probe the characteristic elapse between electron and hole. Based on the subfemtosecond resolution of the spectral interference opening and closing zones in the wavelength-dependent high-order harmonic spectra, we also propose an alternative scheme to probe the dephasing time of crystals by the feasible laser pulses in present experimental setups.

Anomalous decay of coherence in a dissipative many-body system

Decoherence is ubiquitous in quantum physics, from the conceptual foundations to quantum information processing or quantum technologies, where it is a threat that must be countered. While decoherence has been extensively studied for simple, well-isolated systems such as single atoms or ions, much less is known for many-body systems where inter-particle correlations and interactions can drastically alter the dissipative dynamics. Here we report an experimental study of how spontaneous emission destroys the spatial coherence of a gas of strongly interacting bosons in an optical lattice. Instead of the standard momentum diffusion expected for independent atoms, we observe an anomalous sub-diffusive expansion, associated with a universal slowing down $\propto 1/t^{1/2}$ of the decoherence dynamics. This algebraic decay reflects the emergence of slowly-relaxing many-body states, akin to sub-radiant states of many excited emitters. These results, supported by theoretical predictions, provide an important benchmark in the understanding of open many-body systems.

Non-Abelian Geometric Dephasing.

We study the adiabatic dynamics of degenerate quantum states induced by loop paths in a control parameter space. The latter correspond to noisy trajectories if the system is weakly coupled to environmental modes. On top of conventional dynamic dephasing, we find a universal non-Abelian geometric dephasing (NAGD) contribution and express it in terms of the non-Abelian Berry connection and curvature. We show that NAGD implies either decay or amplification of coherences as compared to the coherences when only dynamic dephasing is present. The full NAGD matrix structure can be probed through interference experiments. We outline such a detection scheme for modified Majorana braiding setups.

Two-point wavepacket modelling of jet noise

This paper is focused on the study of a kinematic wavepacket model for jet noise based on two-point statistics. The model contains physical parameters that define its structure in terms of wavenumber, envelope shape and coherence decay. These parameters, which are necessary to estimate the sound pressure levels radiated by the source, were educed from a large-eddy simulation database of a Mach 0.4, fully turbulent jet. The sound pressure levels predicted by the model were compared with acoustic data and the results show that when the parameters are carefully educed from the data, the sound pressure levels generated are in good agreement with experimentally measured values for low Strouhal numbers and polar angles. Furthermore, here we show that a correct representation of both coherence decay and wavepacket envelope shape are key aspects to an accurate sound prediction. A Spectral Proper Orthogonal Decomposition (SPOD) of the model source was also performed motivated by the search for a low-rank model capable of capturing the acoustic efficiency of the full source. It is shown that only a few SPOD modes are necessary to recover acoustically important wavepacket traits.

Rydberg electromagnetically induced transparency in a large Hilbert space

We study Rydberg electromagnetically induced transparency (EIT) of cesium atoms in a magnetic field. The ladder level scheme consists of ground ($6{S}_{1/2}$), excited ($6{P}_{3/2}$), and Rydberg ($48{D}_{5/2}$) levels. The relevant 96 relevant magnetic sublevels are coupled to each other via coherent coupling and decay. A quantum Monte Carlo wave-function (QMCWF) approach is employed to solve the quantum master equation. The simulated EIT probe-absorption spectra and their magnetic-field dependence are compared with results of a cold-atom experiment, in which we perform an in situ, atom-based measurement of a rapidly decaying eddy-current magnetic field. The EIT spectrum in the magnetic field has two dominant lines with a Zeeman splitting of 5.6 MHz per gauss, which are employed to measure the magnetic field. The QMCWF results show good agreement with the experiment, exhibit additional spectroscopic features, and provide insights into the optical-pumping dynamics, radiation-pressure effects, and the relation of these phenomena with the EIT behavior.

Electromagnetically induced squeezing of atomic spin

We present a special quantum effect named electromagnetically induced squeezing of atomic spin in the conventional Λ-type atomic system driven by a strong pump field and a relatively weak probe field in an atomic ensemble. Nearly perfect atomic spin squeezing in the y component can be achieved with fully on resonance at high pump-field intensity, large optical depth of the atomic ensemble, and a low coherence decay rate between the two lower levels. This method would greatly facilitate the generation of atomic spin squeezing by using only coherent laser fields, and may find potential applications in realistic quantum metrology, quantum computation, and quantum information processing.

Decoherence of up to 8-qubit entangled states in a 16-qubit superconducting quantum processor

We report on the coherence of Greenberger–Horne–Zeilinger (GHZ) states comprised of up to 8 qubits in the IBM ibmqx5 16-qubit quantum processor. In particular, we evaluate the coherence of GHZ states with N = 1, …, 8 qubits, as a function of a delay time between state creation and measurement. We find that the decay in coherence occurs at a rate that is linear in the number of qubits. This is consistent with a model in which the dominant noise affecting the system is uncorrelated across qubits.

Two-photon induced ultrafast coherence decay of highly excited states in single molecules

Coherence is a key aspect of a large variety of processes, ranging from the coherent delocalisation of excitation energy, which is important for energy transfer in supramolecular nanostructures, to coherence between electronic states of a single quantum system, which is essential for quantum optical applications. Coherent control schemes exploit this quantum mechanical property by actively manipulating the outcome of dynamical processes. Moreover, this technique allows measuring dynamical processes under the influence of dephasing. However, going beyond the ensemble averaged situation, i.e. working on the level of single quantum systems, is highly challenging for quantum systems embedded in a solid matrix at elevated temperature. Since interactions between the quantum system and its specific local environment are a priori unknown, this requires a reliable approach to retrieve the relevant parameters governing the ultrafast coherent dynamics. Here, we present measurements of the ultrafast coherence decay of two-photon accessible excited states in single organic molecules embedded in a disordered environment at room temperature. We combine this experimental approach with a quantum dynamics identification procedure, which yields a minimum three-level model to describe the obtained data with very good agreement. In particular, we are able to retrieve the ultrafast (coherent) excited state dynamics in single molecules and demonstrate its sensitivity to the local nanoenvironment from molecule to molecule. This work provides a robust approach to measure and analyse ultrafast quantum dynamics in complex nanosystems.

Measurements of the single-point and joint turbulence statistics of high subsonic jets using hot-wire anemometry

This paper describes a systematic and comprehensive hot-wire investigation into the turbulent statistics of low-, moderate- and high-speed subsonic jets. Experiments were performed to obtain the one-point and two-point statistics of a single-stream, unheated jet turbulence field over a broad region of the jet plume. Results show that hot-wires can be used to measure both the one-point and two-point statistics of the high turbulence intensity, noise-producing regions of unheated, compressible, subsonic jets. For the two-point measurements, probe pairings are performed over the three orthogonal axes. Analysis of the experimental data reveals four main conclusions: (1) both the statistical and joint moments of the turbulence scale well with the local jet shear layer half-width; (2) a simple relationship exists between the statistics of the velocity fluctuations and the square of the velocity fluctuations; (3) a simple relationship exists between the longitudinal and transverse length-scales, and (4) a semi-empirical model has been developed to predict the cross-correlation coefficients, power spectral density, frequency-dependent length-scales and coherence decay of the turbulent velocity field. From the second and third conclusions, it is shown that, in the locations near an eddy’s centre of rotation (i.e. the midpoint of the jet shear layer), the turbulence statistics can be described as quasi-homogeneous and quasi-frozen. The joint statistical moments, therefore, can be inferred simply from single-point tests. These results will help to develop models for predicting jet mixing noise, highlighting the situations in which the simplifying assumptions are inadequate.Graphical abstract

Inducing nontrivial qubit coherence through a controlled dispersive environment

We show how the dispersive regime of the Jaynes-Cummings model may serve as a valuable tool to the study of open quantum systems. We employ it in a bottom-up approach to build an environment that preserves qubit energy and induces varied coherence dynamics. We then present the derivation of a compact expression for the qubit coherence, applied here to the case of a finite number of thermally populated modes in the environment. We also discuss how the model parameters can be adjusted to facilitate the production of short-time monotonic decay (STMD) of the qubit coherence. Our results provide a broadly applicable platform for the investigation of energy-conserving open system dynamics which is fully within the grasp of current quantum technologies.

Decay Rate of the l1 Norm Coherence in Single-Qubit Systems

We examine to what extent the l1 norm of coherence through an open quantum system is affected by noise. To discuss the effect of the noise, we give a definition of the decay rate of the l1 norm of coherence, i.e., the value of the coherence of initial states divided by the coherence of final states. Then we use the measure of the decay of coherence to discuss to what extent several noisy channels affects the coherence. We find that the decay rate is independent of the initial state parameters but only related to parameters of the phase flip channels, the depolarizing channels, and the amplitude channels. However, the decay rate is related to the initial state parameters and parameters of the bit flip channels. Contrary to the view at first glance, we find that the bit flip channels even have cohering power.

Determination of N2-N2 and N2-O2 S-branch Raman linewidths using time-resolved picosecond pure rotational coherent anti-Stokes Raman scattering.

N2-N2 and N2-O2 S-branch Raman linewidths have been determined using picosecond dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS). Time-resolved rotational CARS measurements were performed in gas-phase N2 and air for temperatures up to 1900 K in order to determine the time constants of the coherence decay to subsequently calculate the S-branch Raman linewidths. Coherence decay time traces and the resulting S-branch Raman linewidths are presented for N2-N2 and N2-O2 collisions. Therewith, we reduce the gap of widely missing S-branch linewidth data in the temperature regime of many combustion processes. Further, we demonstrate that the standard monoexponential fitting of the coherence decay, as it is commonly done for nitrogen, is not applicable to oxygen.

Realization of a Rydberg-Dressed Ramsey Interferometer and Electrometer.

We present the experimental realization and characterization of a Ramsey interferometer based on optically trapped ultracold potassium atoms, where one state is continuously coupled by an off-resonant laser field to a highly excited Rydberg state. We show that the observed interference signals can be used to precisely measure the Rydberg atom-light coupling strength as well as the population and coherence decay rates of the Rydberg-dressed states with subkilohertz accuracy and for Rydberg state fractions as small as one part in 10^{6}. We also demonstrate an application for measuring small, static electric fields with high sensitivity. This provides the means to combine the outstanding coherence properties of Ramsey interferometers based on atomic ground states with a controllable coupling to strongly interacting states, thus expanding the number of systems suitable for metrological applications and many-body physics studies.