Pure Dephasing Process Research Articles

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

Maxwell's two-demon engine under pure dephasing noise

The interplay between thermal machines and quantum correlations is of great interest in both quantum thermodynamics and quantum information science. Recently, a quantum Szil\'ard engine has been proposed, showing that the quantum steerability between a Maxwell's demon and a work medium can be beneficial to a work extraction task. Nevertheless, this type of quantum-fueled machine is usually fragile in the presence of decoherence effects. We provide an example of the pure dephasing process, showing that the engine's quantumness can be degraded. Therefore, in this work, we tackle this question by introducing a second demon who can access a control system and make the work medium pass through two dephasing channels in a manner of quantum superposition. Furthermore, we provide a quantum circuit to simulate our proposed concept and test it on IBMQ and IonQ quantum computers.

Experimental detection of qubit-environment entanglement without accessing the environment

Decoherence is an unavoidable effect of the quantum system coupled to an environment, which leads to the system evolving from a pure state to a mixed one. A critical issue here is that the reduction of purity is quantum or classical, which corresponds to whether or not entanglement is generated between the system and environment. We report a proof-of-principle experimental investigation of such entanglement in a simple but paradigmatic case, in which a qubit system interacts with a pure dephasing channel. The pure dephasing process is realized in a linear photonic system, and the experiment is a validation in a very controlled setting. By adopting previous methods to detect qubit-environment entanglement, we prove that such entanglement can be detected by performing local measurements on only one subsystem, even when the other subsystem in the pair cannot be detected. Our experiment paves the way for investigating features of open quantum system dynamics.

Study of single photon source efficiency based on cavity quantum electrodynamics

Considering the pure dephasing process and external-to-internal cavity loss ratio, we calculated the efficiency of a single photon source of quantum dots coupled to an arbitrary detuned single-mode cavity based on cavity quantum electrodynamics. The results show that there is an optimal external-to-internal cavity loss ratio to maximize the efficiency of the single photon source, and in the proper external-to-internal cavity loss ratio range, the pure dephasing process can improve the efficiency of the device in a detuning system. The relationship between efficiency and temperature was further studied and it was found that the efficiency at higher temperature could be higher than that at lower temperature by proper design. This is of great significance for the preparation of high-efficiency single photon sources.

Elucidating the Influence of Sulfur Vacancies on Nonradiative Recombination Dynamics in Cu2ZnSnS4 Solar Absorbers.

We report a time-domain ab initio simulation of charge carrier trapping and relaxation dynamics in pristine and defect-containing kesterite Cu2ZnSnS4 (CZTS) structures. Our simulations show that introduction of a neutral sulfur vacancy in the CZTS system leads to a decrease of the charge recombination rate by a factor of ∼4, and the doubly positively charged sulfur vacancy results in a minor decrease of carrier lifetime, as compared to the pristine CZTS system. The neutral sulfur vacancy weakens the nonadiabatic (NA) electron-phonon coupling by moderately localizing charge density and accelerates the pure dephasing process, extending charge carrier lifetime. Therefore, the neutral sulfur vacancy is electrically benign. The doubly positively charged sulfur vacancy introduces a subgap state which is hardly populated, and recombination of the electron and hole bypassing the trap state dominates. As a result, the recombination rate decreases in the doubly charged sulfur vacancy structure. The reported results identified the key role of the sulfur-related vacancy on charge carrier trapping and relaxation of CZTS materials, carrying important implications for further optimization of CZTS and other thin-film solar cell materials.

Optomechanical Collective Effects in Surface-Enhanced Raman Scattering from Many Molecules

The interaction between molecules is commonly ignored in surface-enhanced Raman scattering (SERS). Under this assumption, the total SERS signal is described as the sum of the individual contributions of each molecule treated independently. We adopt here an optomechanical description of SERS within a cavity quantum electrodynamics framework to study how collective effects emerge from the quantum correlations of distinct molecules. We derive analytical expressions for identical molecules and implement numerical simulations to analyze two types of collective phenomena: (i) a decrease of the laser intensity threshold to observe strong nonlinearities as the number of molecules increases, within very intense illumination, and (ii) identification of superradiance in the SERS signal, namely a quadratic scaling with the number of molecules. The laser intensity required to observe the latter in the anti-Stokes scattering is relatively moderate, which makes it particularly accessible to experiments. We treat the system on the basis of the individual molecules and demonstrate that for ideal systems with identical molecules this approach is equivalent to a description based on collective modes. The basis of individual molecules also allows for describing in a straightforward manner more general systems where the molecules might have different vibrational properties or suffer from pure-dephasing processes. Our results show that the collective phenomena can survive in the presence of the homogeneous and inhomogeneous broadening that might influence experimental results.

When can quantum decoherence be mimicked by classical noise?

Quantum decoherence arises due to uncontrollable entanglement between a system and its environment. However, the effects of decoherence are often thought of and modeled through a simpler picture in which the role of the environment is to introduce classical noise in the system's degrees of freedom. Here, we establish necessary conditions that the classical noise models need to satisfy to quantitatively model the decoherence. Specifically, for pure-dephasing processes, we identify well-defined statistical properties for the noise that are determined by the quantum many-point time correlation function of the environmental operators that enter into the system-bath interaction. In particular, for the exemplifying spin-boson problem with a Lorentz-Drude spectral density, we show that the high-temperature quantum decoherence is quantitatively mimicked by colored Gaussian noise. In turn, for dissipative environments, we show that classical noise models cannot describe decoherence effects due to relaxation through spontaneous emission of photons/phonons. These developments provide a rigorous platform to assess the validity of classical noise models of decoherence.

Non-Markovian dynamics of a three-level atom in a stochastic field with finite correlation time due to initial correlations

Dynamics of a three-level atom interacting with a stochastically fluctuating environment characterized by finite correlation time is investigated in the cases of a product initial state and a correlated initial state. The correlated initial state is obtained by the state reduction caused by the von Neumann measurement on the quantum system evolved from the product initial state and surrounded by the initially stationary stochastic environment. It is shown that in case of a pure dephasing process under the influence of a Kubo-Anderson type stochastic environment, the projective measurement causes the emergence of correlations between previously independent stochastic processes, their non-stationarity, and also causes the transition from a Markovian-type evolution to a non-Markovian one.

Boson–boson pure-dephasing model with non-Markovian properties

In this paper, we discuss the mechanism of pure-dephasing process with a newly proposed boson–boson model, namely, a bosonic field coupled to another bosonic bath in thermal equilibrium. Our model is fully solvable and can reproduce the pure-dephasing process which is usually described by the well-known spin–boson model, therefore offering a new perspective to understanding decoherence processes in open quantum systems of high dimension. We also show that this model admits a generically non-Markovian dynamics with respect to various different non-Markovian characterizations, i.e., the criteria based on divisibility, quantum regression formula and Wigner function, respectively. The criterion based on Wigner function is firstly proposed in this paper. For the case that the particle number of the pure-dephasing system is constrained to be 0 or 1, we analytically prove its equivalence to the criteria based on trace distance and divisibility.

Double-time correlation functions of two quantum operations in open systems

A double-time correlation function of arbitrary two quantum operations is studied for a nonstationary open quantum system which is in contact with a thermal reservoir. It includes a usual correlation function, a linear response function, and a weak value of an observable. Time evolution of the correlation function can be derived by means of the time-convolution and time-convolutionless projection operator techniques. For this purpose, a quasidensity operator accompanied by a fictitious field is introduced, which makes it possible to derive explicit formulas for calculating a double-time correlation function in the second-order approximation with respect to a system-reservoir interaction. The derived formula explicitly shows that the quantum regression theorem for calculating the double-time correlation function cannot be used if a thermal reservoir has a finite correlation time. Furthermore, the formula is applied for a pure dephasing process and a linear dissipative process. The quantum regression theorem and the the Leggett-Garg inequality are investigated for an open two-level system. The results are compared with those obtained by exact calculation to examine whether the formula is a good approximation.

Phonon-mediated generation of quantum correlations between quantum dot qubits.

We study the generation of quantum correlations between two excitonic quantum dot qubits due to their interaction with the same phonon environment. Such generation results from the fact that during the pure dephasing process at finite temperatures, each exciton becomes entangled with the phonon environment. We find that for a wide range of temperatures quantum correlations are created due to the interaction. The temperature-dependence of the level of correlations created displays a trade-off type behaviour; for small temperatures the phonon-induced distrubance of the qubit states is too small to lead to a distinct change of the two-qubit state, hence, the level of created correlations is small, while for large temperatures the pure dephasing is not accompanied by the creation of entanglement between the qubits and the environment, so the environment cannot mediate qubit-qubit quantum correlations.

Phonon induced pure dephasing process of excitonic state in colloidal semiconductor quantum dots

We present a theoretical study on the pure dephasing process of colloidal semiconductor quantum dots induced by lattice vibrations using continuum model calculations. By solving the time dependent Liouville-von Neumann equation, we present the ultrafast Rabi oscillations between excitonic state and virtual state via exciton-phonon interaction and obtain the pure dephasing time from the fast decayed envelope of the Rabi oscillations. The interaction between exciton and longitudinal optical phonon vibration is found to dominate the pure dephasing process and the dephasing time increases nonlinearly with the reduction of exciton-phonon coupling strength. We further find that the pure dephasing time of large quantum dots is more sensitive to temperature than small quantum dots.

Maximizing the purity of a qubit evolving in an anisotropic environment

We provide a general method to calculate and maximize the purity of a qubit interacting with an anisotropic non-Markovian environment. Counter to intuition, we find that the purity is often maximized by preparing and storing the qubit in a superposition of non-interacting eigenstates. For a model relevant to decoherence of a heavy-hole spin qubit in a quantum dot or for a singlet-triplet qubit for two electrons in a double quantum dot, we show that preparation of the qubit in its non-interacting ground state can actually be the worst choice to maximize purity. We further give analytical results for spin-echo envelope modulations of arbitrary spin components of a hole spin in a quantum dot, going beyond a standard secular approximation. We account for general dynamics in the presence of a pure-dephasing process and identify a crossover timescale at which it is again advantageous to initialize the qubit in the non-interacting ground state. Finally, we consider a general two-axis dynamical decoupling sequence and determine initial conditions that maximize purity, minimizing leakage to the environment.

Intersublevel dephasing in InAs/GaAs quantum dots below the Reststrahlen band

Using transient four-wave mixing in the terahertz range, we have measured the s-p inter-sublevel dephasing time in self-assembled InAs/GaAs quantum dots for transition energies below the Reststrahlen band. Dephasing times of up to 600 ps at a photon energy of 18 meV have been determined. By comparing pump-probe and four-wave mixing measurements, we show that there is no significant influence of any pure dephasing process at low temperature. The linear temperature dependence is consistent with acoustic phonon scattering.

Infrared spectroscopic properties of goethite: anharmonic broadening, long-range electrostatic effects and Al substitution

The infrared spectrum and its temperature dependence (20–320 K) were collected on a synthetic goethite sample (α-FeOOH). In addition, the infrared powder absorption spectrum of goethite and aluminum-substituted goethite was computed using ab initio quantum mechanical calculations based on density functional theory. This combined experimental and theoretical approach allows (1) the unequivocal assignment of absorption bands to the corresponding vibrational modes, (2) separate identification of the effects of the particle shape and of the aluminum substitution on the infrared spectrum, and (3) a discussion of the anharmonic properties and the origin of the line broadening in goethite. In particular, the two well-resolved OH bending absorption bands show different temperature evolution. Their detailed analysis suggests that the broadening of the band at ~800 cm−1 cannot be described solely by a usual three-phonon process. The strong anharmonic behavior of this mode implies the addition of a four-phonon process, such as a pure dephasing process. In our calculations, the effect of the Hubbard U correction is also investigated and found to be most visible on the OH stretching and bending modes, in relation to the associated structural relaxation. The OH stretching frequencies decrease, leading to a better agreement with experimental frequencies, while the OH bending frequencies increase.

Phonon-induced pure-dephasing of luminescence, multiple exciton generation, and fission in silicon clusters

The size and temperature dependence of the pure-dephasing processes involved in luminescence, multiple exciton generation (MEG), and multiple exciton fission (MEF) are investigated for Sin clusters (n = 5-10, 15) using ab initio molecular dynamics and optical response function theory. The cluster bandgaps correlate with two types of binding energy, indicating that bandgaps can be used to characterize cluster stability. Ranging from 5 to 100 fs, the dephasing times are found to be longest for MEF and shortest for MEG, with luminescence falling in the middle range. Generally, the dephasing is fast, if the orbitals supporting the pair of states involved in the superpositions differ in energy, atomic localization, and number of nodes. The dephasing accelerates with temperature, because more phonon modes are activated, and lower frequency acoustic modes are able to explore the anhamonic part of the potential energy surface. The temperature dependence is stronger for larger clusters, since they possess a wider range of low-frequency anharmonic modes. Our research indicates that rapid dephasing in Si clusters favors generation of independent charge carriers from single and multiple excitons, making the clusters a promising material for photon energy conversion. The simulations of the dephasing processes reported in this work assist in understanding of the exciton evolution pathways in inorganic semiconductor clusters and other nanoscale materials.

Noise effects and tomography of remote entangled spins in quantum dots

We investigate how decoherence affects the entanglement established between two quantum dots in microcavities, and propose a tomographic scheme able to measure the entangled state. The scheme we consider establishes the entanglement via the exchange and measurement of a photon. Making the realistic assumption of noise dominated by pure-dephasing processes, we find that the photon must be exchanged and measured on time scales shorter than the quantum dots' characteristic dephasing time for appreciable levels of entanglement to be achieved. The tomographic scheme is able to reconstruct the full density matrix of the quantum dots, and requires only single spin rotations and the injection of an additional photon. Remarkably, we find that the additional photon need not be exchanged and measured on a time scale shorter than the dephasing time for accurate tomography, and also allows many to be used in order to increase the measurement signal.

Experimental demonstration of decoherence-induced spontaneous symmetry breaking

We experimentally investigate the variations of exchange-symmetry properties of the four Bell states in an exchange-symmetric pure dephasing process with a two-photon system generated from spontaneous parametric down-conversion (SPDC). Experiment results show that under such an exchange-symmetric local-noise Hamiltonian, the exchange-symmetry property remains unchanged for two of the three symmetric Bell states, i.e., the states $|\ensuremath{\Phi}\ensuremath{\rangle}{}^{\ifmmode\pm\else\textpm\fi{}}=\frac{1}{\sqrt{2}}(|00\ensuremath{\rangle}\ifmmode\pm\else\textpm\fi{}|11\ensuremath{\rangle})$. For the antisymmetric Bell state $|\ensuremath{\Psi}\ensuremath{\rangle}{}^{\ensuremath{-}}=\frac{1}{\sqrt{2}}(|01\ensuremath{\rangle}\ensuremath{-}|10\ensuremath{\rangle})$, the exchange-symmetry property increases and achieves a maximum value of 0.5 at the asymptotic limit. However, for the third exchange-symmetric Bell state $|\ensuremath{\Psi}\ensuremath{\rangle}{}^{+}=\frac{1}{\sqrt{2}}(|01\ensuremath{\rangle}+|10\ensuremath{\rangle})$, the exchange-symmetry property breaks, surviving with a probability of 0.5 at the asymptotic limit, which provides some evidence supporting such decoherence-induced spontaneous-symmetry-breaking phenomena.

Phonon-Induced Pure Dephasing of Two-Electron Spin States in Vertical Quantum Dot Molecules

Phonon-induced dephasing of two-electron spin states in two vertically stacked self-assembled GaAs/InGaAs quantum dots is studied. A pure dephasing process due to elastic phonon scattering is found to dominate at low temperatures. This process is independent of the spin‐orbit coupling and does not require the presence of a magnetic field. It relies on interdot tunneling and the Pauli principle, which make the double quantum dot gate for spin based quantum computing possible, and therefore cannot be avoided.

Vibrational spectroscopy of polyatomic materials: Semiempirical calculations of anharmonic couplings and infrared and Raman linewidths in naphthalene and PETN crystals

We have developed a computational approach that yields anharmonic vibrational couplings in molecular crystals. The approach is based on anharmonic vibrational potential-energy surface reconstruction starting from a normal-mode vibrational basis. The method was implemented for semiempirical Hamiltonians with periodic boundary conditions, with applications to crystalline naphthalene and pentaerythritol tetranitrate. For each material, we predicted infrared and Raman linewidths, and vibrational anharmonic couplings associated with up- and down-conversions, as well as pure-dephasing processes. Comparison is made to experimental data for Raman linewidths and averaged anharmonic couplings; reasonable agreement is obtained, suggesting that implementation of the method within a first-principles electronic structure framework is warranted.