Decoherence and relaxation of a single electron in a one dimensional conductor
A. Marguerite, C. Cabart, C. Wahl, B. Roussel, V. Freulon, D. Ferraro, Ch. Grenier, J. -M. Berroir, B. Plaçais, T. Jonckheere, J. Rech, T. Martin, P. Degiovanni… 展开作者
A. Marguerite, C. Cabart, C. Wahl, B. Roussel, V. Freulon, D. Ferraro, Ch. Grenier, J. -M. Berroir, B. Plaçais, T. Jonckheere, J. Rech, T. Martin, P. Degiovanni, A. Cavanna, Y. Jin, G. Fève
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24 pages, 6 figures To be published in Physical Review B
We study the decoherence and relaxation of a single elementary electronic excitation propagating in a one-dimensional chiral conductor. Using two-particle interferences in the electronic analog of the Hong-Ou-Mandel experiment, we analyze quantitatively the decoherence scenario of a single electron propagating along a quantum Hall edge channel at filling factor 2. The decoherence results from the emergence of collective neutral excitations induced by Coulomb interaction and leading, in one dimension, to the destruction of the elementary quasiparticle. This study establishes the relevance of electron quantum optics setups to provide stringent tests of strong interaction effects in one-dimensional conductors described by Luttinger liquids paradigm.
We analyze the effect of decoherence on the violation of the Clauser-Horne (CH) inequality for the full electron counting statistics in a mesoscopic multiterminal conductor. Our setup consists of an entangler that emits a flux of entangled electrons into two conductors characterized by a scattering matrix and subject to decoherence. Loss of phase memory is modeled phenomenologically by introducing fictitious extra leads. The outgoing electrons are detected using spin-sensitive electron counters. Given a certain average number of incoming entangled electrons, the CH inequality is evaluated as a…
Although interesting per se, decoherence and relaxation of single-electron excitations induced by strong effective screened Coulomb interactions in Quantum Hall edge channels are an important challenge for the applications of electron quantum optics in quantum information and quantum sensing. In this paper, we study intrinsic single-electron decoherence within an ideal single-electron channel with long-range effective Coulomb interactions to determine the influence of the material and sample properties. We find that weak-coupling materials characterized by a high velocity of hot-electron excit…
We present a nonperturbative approach which allows to evaluate the weak localization correction to the conductivity of disordered conductors in the presence of interactions. The effect of the electron-electron interaction on the magnetoconductance is described by the function A(t)\exp (-f(t)). The dephasing time is determined only by f(t), and this time remains finite down to T=0 due to the electron-electron interactions. In order to establish the relation between our nonperturbative analysis and the perturbative results the effect of interaction on the pre-exponent A(t) should be taken into a…
We perform a brief but critical review of the Landauer picture of transport that clarifies how decoherence appears in this approach. On this basis, we present different models that allow the study of the coherent and decoherent effects of the interaction with the environment in the electronic transport. These models are particularly well suited for the analysis of transport in molecular wires. The effects of decoherence are described through the D'Amato-Pastawski model that is explained in detail. We also consider the formation of polarons in some models for the electron-vibrational interactio…
Recently von Delft (cond-mat/0510563v1)(JvD) has successfully re-derived our influence functional for interacting electrons and claimed that within our approach he was able to obtain the electron decoherence rate that vanishes at T=0. In this Comment we demonstrate that this JvD's claim is in error, as it is based on ambiguous and uncontrolled manipulations violating basic principles of quantum theory, such as energy-time uncertainty relation, causality, fluctuation-dissipation theorem, detailed balance and the like. We also briefly address insufficient approximations employed by Marquardt {\i…
Combining scattering matrix theory with non-linear $σ$-model and Keldysh technique we develop a unified theoretical approach enabling one to non-perturbatively study the effect of electron-electron interactions on weak localization and Aharonov-Bohm oscillations in arbitrary arrays of quantum dots. Our model embraces (i) weakly disordered conductors (ii) strongly disordered conductors and (iii) metallic quantum dots. In all these cases at $T \to 0$ the electron decoherence time is found to saturate to a finite value determined by the universal formula which agrees quantitatively with numerous …
The dephasing time in coupled mesoscopic conductors is caused by the fluctuations of the dipolar charge permitted by the long range Coulomb interaction. We relate the phase breaking time to elementary transport coefficients which describe the dynamics of this dipole: the capacitance, an equilibrium charge relaxation resistance and in the presence of transport through one of the conductors a non-equilibrium charge relaxation resistance. The discussion is illustrated for a quantum point contact in a high magnetic field in proximity to a quantum dot.
We show that in low dimensional disordered conductors, the quasiparticle decay and the relaxation of the phase are not exponential processes. In the quasi-one dimensional case, both behave at small time as $e^{- (t/τ_{in})^{3/2}}$ where the inelastic time $τ_{in}$, identical for both processes, is a power $T^{2/3}$ of the temperature. This result implies the existence of an unusual distribution of relaxation times that we obtain.
We develop a new unified theoretical approach enabling us to non-perturbatively study the effect of electron-electron interactions on weak localization in arbitrary arrays of quantum dots. Our model embraces (i) weakly disordered conductors (ii) strongly disordered conductors and (iii) metallic quantum dots. In all these cases at $T \to 0$ the electron decoherence time is determined by the universal formula $τ_{φ0}\sim gτ_D/\ln (E_C/δ)$, where $g$, $τ_D$, $E_C$ and $δ$ are respectively dimensionless conductance, dwell time, charging energy and level spacing of a single dot. In the case (i) thi…
We consider the relaxation of a uniform current in a planar 2D conductor with account taken of electromagnetic retardation effects. If the 2D conductivity is larger than the speed of light, the straightforward solution for an infinite plane gives a negative relaxation rate. However if one starts from a conducting cylinder of finite radius and then increases it to infinity, the relaxation rate just tends to zero while remaining positive. We suggest that recent unusual plasmon-dispersion curves obtained by V. A. Volkov and A. A. Zabolotnykh [arXiv:1605.00430] result from the incorrect finite-to-…
We establish a connection between the influence functional approach of Golubev and Zaikin (GZ) and Keldysh diagrammatic perturbation theory for calculating the decoherence time of interacting electrons in disordered metals; we show how the standard diagrams for the Cooperon self energy can be recovered from GZ's influence functional $e^{- (i S_R + S_I)}$. This allows us to shed light on GZ's claim that $S_R$ is irrelevant for decoherence: $S_R$ generates as many important self energy diagrams as $S_I$; GZ's neglect of $S_R$ is permissible only at high temperatures ($T > \hbar / τ_{el}$).
This lecture note reviews a variety of transport and thermodynamic measurements of electron decoherence time in low-dimensional conductors at low temperature. The mechanism of dephasing by electron interaction mediated by an arbitrarily small number of magnetic impurities is neither applicable to our linear-response measurements of weak localization, nor is it observed in the recent high-field measurements of the decoherence time in nominally pure quasi-one dimensional gold wires. The source of decoherence, after extensive experiments, still appears to be intrinsic.
We revisit the concepts of spin relaxation and spin decoherence of two level (spin-1/2) systems. From two toy-models, we clarify two issues related to the spin relaxation and decoherence: 1) For an ensemble of two-level particles each subjected to a different environmental field, there exists an ensemble relaxation time $T_1^*$ which is fundamentally different from $T_1$. When the off-diagonal coupling of each particle is in a single mode with the same frequency but a random coupling strength, we show that $T_1^*$ is finite while the spin relaxation time of a single spin $T_1$ and the usual en…
A unified approach to decoherence and relaxation of energy resolved single electron excitations in Integer Quantum Hall edge channels is presented. Within the bosonization framework, relaxation and decoherence induced by interactions and capacitive coupling to an external linear circuit are computed. An explicit connexion with high frequency transport properties of a two terminal device formed by the edge channel on one side and the linear circuit on the other side is established.
We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is two times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) …
We investigate the heat flow between different terminals in an interacting coherent conductor when inelastic scattering is present. We illustrate our theory with a two-terminal quantum dot setup. Two types of heat asymmetries are investigated: electric asymmetry $Δ_E$, which describes deviations of the heat current in a given contact when voltages are exchanged, and contact asymmetry $Δ_C$, which quantifies the difference between the power measured in two distinct electrodes. In the linear regime, both asymmetries agree and are proportional to the Seebeck coefficient, the latter following at l…
The quantum nature of a microscopic system can only be revealed when it is sufficiently decoupled from surroundings. Interactions with the environment induce relaxation and decoherence that turn the quantum state into a classical mixture. Here, we study the timescales of these processes for a qubit encoded in the collective state of a set of magnetic atoms deposited on a metallic surface. For that, we provide a generalization of the commonly used definitions of $T_{1}$ and $T_{2}$ characterizing relaxation and decoherence rates. We calculate these quantities for several atomic structures, incl…
A brief overview is presented of recent work which investigates the time-dependent relaxation of charge and its spontaneous fluctuations on mesoscopic conductors in the proximity of gates. The leading terms of the low frequency conductance are determined by a capacitive or inductive emittance and a dissipative charge relaxation resistance. The charge relaxation resistance is determined by the ratio of the mean square dwell time of the carriers in the conductor and the square of the mean dwell time. The contribution of each scattering channel is proportional to half a resistance quantum. We dis…
We show that, in low dimensional conductors, the quasiparticle decay and the relaxation of the phase are not exponential processes. In quasi-one dimension, they scale as $e^{- (t/τ_N)^{3/2}}$ where the characteristic time $τ_{in}$, identical for both processes, is a power $T^{2/3}$ of the temperature. This result implies a distribution of relaxation times.
We review recent studies on spin decoherence of electrons and holes in quasi-two-dimensional quantum dots, as well as electron-spin relaxation in nanowire quantum dots. The spins of confined electrons and holes are considered major candidates for the realization of quantum information storage and processing devices, provided that sufficently long coherence and relaxation times can be achieved. The results presented here indicate that this prerequisite might be realized in both electron and hole quantum dots, taking one large step towards quantum computation with spin qubits.
We have rederived the controversial influence functional approach of Golubev and Zaikin (GZ) for interacting electrons in disordered metals in a way that allows us to show its equivalence, before disorder averaging, to diagrammatic Keldysh perturbation theory. By representing a certain Pauli factor (1-2 rho) occuring in GZ's effective action in the frequency domain (instead of the time domain, as GZ do), we also achieve a more accurate treatment of recoil effects. With this change, GZ's approach reproduces, in a remarkably simple way, the standard, generally accepted result for the decoherence…
We investigate the spin relaxation and decoherence in a single-electron graphene quantum dot with Rashba and intrinsic spin-orbit interactions. We derive an effective spin-phonon Hamiltonian via the Schrieffer-Wolff transformation in order to calculate the spin relaxation time T_1 and decoherence time T_2 within the framework of the Bloch-Redfield theory. In this model, the emergence of a non-monotonic dependence of T_1 on the external magnetic field is attributed to the Rashba spin-orbit coupling-induced anticrossing of opposite spin states. A rapid decrease of T_1 occurs when the spin and or…
It is shown that the dephasing rate in Coulomb coupled mesoscopic structures is determined by charge relaxation resistances. The charge relaxation resistance together with the capacitance determines the RC-time of the mesoscopic structure and at small frequencies determines the voltage fluctuation spectrum. Self-consistent expressions are presented which give the charge relaxation resistance and consequently the dephasing rate in terms of the diagonal and off-diagonal elements of a generalized Wigner-Smith delay-time matrix. Dephasing rates are discussed both for the equilibrium state and in t…
We explore an instantaneous decoherence correction (IDC) approach for the decoherence and energy relaxation in the quantum-classical dynamics of charge transport in organic semiconducting crystals. These effects, originating from environmental fluctuations, are essential ingredients of the carrier dynamics. The IDC is carried out by measurement-like operations in the adiabatic representation. While decoherence is inherent in the IDC, energy relaxation is taken into account by considering the detailed balance through the introduction of energy-dependent reweighing factors, which could be either…
The conduction electrons' dephasing rate, $τ_ϕ^{-1}$, is expected to vanish with the temperature. A very intriguing apparent saturation of this dephasing rate in several systems was recently reported at very low temperatures. The suggestion that this represents dephasing by zero-point fluctuations has generated both theoretical and experimental controversies. We start by proving that the dephasing rate must vanish at the $T\to 0$ limit, unless a large ground state degeneracy exists. This thermodynamic proof includes most systems of relevance and it is valid for any determination of $τ_ϕ$ from …
Theory of the ac spin injection from a ferromagnetic electrode into a normal conductor through a tunnel or Schottky contact is developed. Diffusion and relaxation of non-equilibrium spins results in a frequency dependent complex impedance controlled by the spin relaxation rates and the resistances involved. Explicit expression for the impedance is presented. Experimental investigation of the frequency dependence of the impedance should allow measuring spin relaxation times in both conductors, their effective resistances, and also the parameters of the contact controlling the spin injection.
We calculate current (shot) noise in a metallic diffusive conductor generated by spin imbalance in the absence of a net electric current. This situation is modeled in an idealized three-terminal setup with two biased ferromagnetic leads (F-leads) and one normal lead (N-lead). Parallel magnetization of the F-leads gives rise in spin-imbalance and finite shot noise at the N-lead. Finite spin relaxation results in an increase of the shot noise, which depends on the ratio of the length of the conductor ($L$) and the spin relaxation length ($l_s$). For $L\gg l_s$ the shot noise increases by a facto…
The control of discrete quantum states in solids and their use for quantum information processing is complicated by the lack of a detailed understanding of the mechanisms responsible for qubit decoherences. For spin qubits in semiconductor quantum dots, phenomenological models of decoherence currently recognize two Basic stages; fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields and a much slower process of irreversible relaxation of spin qubit polarization due to dynamics of the nuclear spin bath induced by comp…
We present an experiment where we tune the decoherence in a quantum interferometer using one of the simplest object available in the physic of quantum conductors : an ohmic contact. For that purpose, we designed an electronic Mach-Zehnder interferometer which has one of its two arms connected to an ohmic contact through a quantum point contact. At low temperature, we observe quantum interference patterns with a visibility up to 57%. Increasing the connection between one arm of the interferometer to the floating ohmic contact, the voltage probe, reduces quantum interferences as it probes the el…
In this lectures, we have described some essential features of loss of coherence by a qubit coupled to the environment. We have first presented well known semiclassical arguments that relate both decoherence and relaxation to the environmental noise. Then we have shown that models with pure decoherence (but no relaxation in qubit states) can be exactly solvable. As an example, we have treated in detail the model of fluctuating background charges which is believed to describe one of the most important channels for decoherence for the charge Josephson junction qubit. We have shown that the decoh…
We have extended Monte Carlo simulations of hopping transport in completely disordered 2D conductors to the process of external charge relaxation. In this situation, a conductor of area $L \times W$ shunts an external capacitor $C$ with initial charge $Q_i$. At low temperatures, the charge relaxation process stops at some "residual" charge value corresponding to the effective threshold of the Coulomb blockade of hopping. We have calculated the r.m.s$.$ value $Q_R$ of the residual charge for a statistical ensemble of capacitor-shunting conductors with random distribution of localized sites in s…
We present a theoretical analysis of several aspects of nonequilibirum cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamical equations which describe the nonequilibrium cotunneling in a convenient and compact way. These equations describe the time evolution of the spin variables of the QD explicitly in terms of the response and correlation functions of the free reservoir variables. This scheme not on…
We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of ma…
Long-range quantum communication for spin qubits is an important open problem. Here we study decoherence of an electron spin qubit that is being transported in a moving quantum dot. We focus on spin decoherence due to spin-orbit interaction and a random electric potential. We find that at the lowest order, the motion induces longitudinal spin relaxation, with a rate linear in the dot velocity. Our calculated spin relaxation time ranges from sub $μ$s in GaAs to above ms in Si, making this relaxation a significant decoherence channel. Our results also give clear indications on how to reduce the …
We study the localization and decay properties as well as the thermal conductance of one-dimensional plasmons. Our model contains a Luttinger-liquid part with spatially random plasmon velocity and interaction parameter as well as a nonlinearity that is cubic in density. The scaling of the decay rate of plasmons is obtained in several regimes. At sufficiently high frequencies, it describes the inelastic life time of localized plasmon excitations that crosses over to the clean result with lowering frequencies. For higher frequencies, we analyze implications of many-body-localization effects that…
We study theoretically the phonon-induced relaxation and decoherence processes in the hybrid qubit in silicon. Hybrid qubit behaves as a charge qubit when the detuning is close to zero and as spin qubit for large detuning values. It is realized starting from an electrostatically defined double quantum dot where three electrons are confined and manipulated through only electrical tuning. By employing a three-level effective model for the qubit and describing the environment bath as a series of harmonic oscillators in the thermal equilibrium states, we extract the relaxation and decoherence time…
We develop a "chain-boson model" master equation, within the Born-Markov approximation, for a few superconducting quantum interference devices (SQUIDs) coupled into a chain and exchanging their angular momenta with a low temperature phonon bath. Our master equation has four generators; we concentrate on the damping and diffusion and use them to study the relaxation and decoherence of a Heisenberg SQUID chain whose spectrum exhibits critical point energy-level crossings, entangled states, and pairs of resonant transitions. We note that at an energy-level crossing the relevant bath wavelengths a…
In the presence of shot noise the charge on a mesoscopic conductor fluctuates. We are interested in the charge fluctuations which arise if the conductor is in the proximity of a gate to which it is coupled by long range Coulomb forces only. Specifically we consider a gate coupled to the edge of a Hall bar subject to a quantizing magnetic field which contains a quantum point contact. The gate is located away from the quantum point contact. We evaluate the charge relaxation resistance for this geometry. The charge relaxation resistance determines the current fluctuations and potential fluctuatio…
The Landauer scattering approach to 4-probe resistance is revisited for the case of a d-dimensional disordered resistor in the presence of decoherence. Our treatment is based on an invariant-embedding equation for the evolution of the coherent reflection amplitude coefficient in the length of a 1-dimensional disordered conductor, where decoherence is introduced at par with the disorder through an outcoupling, or stochastic absorption, of the wave amplitude into side (transverse) channels, and its subsequent incoherent re-injection into the conductor. This is essentially in the spirit of B{ü}tt…
Recent experiments on nanoscale conductors coupled to microwave cavities put in prospect transport investigations of electron-photon interplay in the deep quantum regime. Here we propose a pump-probe scheme to investigate the transient dynamics of individual electron-photon excitations in a double quantum dot-cavity system. Excitations pumped into the system decay via charge tunneling at the double dot, probed in real time. We investigate theoretically the short-time charge transfer statistics at the dot, for periodic pumping, and show that this gives access to vacuum Rabi oscillations as well…
We theoretically study decoherence of two localized spins interacting via the RKKY interaction in one-, two-, and three-dimensional electron gas. We derive the kinetic equation for the reduced density matrix of the localized spins and show that energy relaxation caused by singlet-triplet transition is suppressed when the RKKY interaction is ferromagnetic. We also estimate the decoherence time of the system consisting of two quantum dots embedded in a two dimensional electron gas.
Conduction electrons in disordered metals and heavily doped semiconductors at low temperatures preserve their phase coherence for a long time: phase relaxation time $τ_ϕ$ can be orders of magnitude longer than the momentum relaxation time. The large difference in these time scales gives rise to well known effects of weak localization, such as anomalous magnetoresistance. Among other interesting characteristics, study of these effects provide quantitative information on the dephasing rate $1/τ_ϕ$. This parameter is of fundamental interest: the relation between $\hbar/τ_ϕ$ and the temperature $T…
Using a nonequilibrium renormalization group method we study the real-time evolution of spin and current in the anisotropic Kondo model (both antiferromagnetic and ferromagnetic) at finite magnetic field $h_0$ and bias voltage $V$. We derive analytic expressions for all times in the weak-coupling regime $\max\{V,h_0,1/t\}\gg T_c$ ($T_c=$ strong coupling scale). We find that all observables decay both with the spin relaxation and decoherence rates $Γ_{1/2}$. Various $V$-dependent logarithmic, oscillatory, and power-law contributions are predicted. The low-energy cutoff of logarithmic terms is g…
Strain-induced gradients of local electric fields in semiconductor quantum dots can couple to the quadrupole moments of nuclear spins. We develop a theory describing the influence of this quadrupolar coupling (QC) on the spin correlators of electron and hole "central" spins localized in such dots. We show that when the QC strength is comparable to or larger than the hyperfine coupling strength between nuclei and the central spin, the relaxation rate of the central spin is strongly enhanced and can be exponential. We demonstrate a good agreement with recent experiments on spin relaxation in hol…
Using the 'drift-diffusion-Langevin' equation, we have quantitatively analyzed the effects of electron energy relaxation via their interaction with phonons, generally in presence of electron-electron interaction, on shot noise in diffusive conductors. We have found that the noise power $ S_I(ω)$ (both at low and high observation frequencies $ω$) drops to half of its 'mesoscopic' value only at $β\gtrsim 100,$ where $β$ is the ratio of the sample length $L$ to the energy relaxation length $l_{% {\rm ph}}$ (the latter may be much larger then the dephasing length). It means in particular that at l…
Beijing Institute of Technology（北京理工大学）；International Center for Quantum Materials, Beijing Institute of Technology, Zhuhai（北京理工大学珠海量子材料国际中心）；Beijing Normal University（北京师范大学）
We examine the effect of multilevels on decoherence and dephasing properties of a quantum system consisting of a non-ideal two level subspace, identified as the qubit and a finite set of higher energy levels above this qubit subspace. The whole system is under interaction with an environmental bath through a Caldeira-Leggett type coupling. The model interaction we use can generate nonnegligible couplings between the qubit states and the higher levels upto $N\sim 10$. In contrast to the pure two-level system, in a multilevel system the quantum information may leak out of the qubit subspace thro…
The coherent dynamics of relaxing spin qubits driven by a classical bichromatic field comprising a strong resonant component and a weaker component with a frequency close to the strong-field Rabi frequency is studied. The double dressing by the bichromatic field modifies dephasing and dissipation processes. We demonstrate that detuning of the weaker-field frequency from the strong-field Rabi frequency prolongs the decay of Rabi oscillations between some doubly dressed states. The sensitivity of Rabi oscillations to the modified detuning-dependent relaxation is illustrated for nitrogen-vacancy …
Phonon-related decoherence effects in a quantum double-well two-level subsystem coupled to a solid are studied theoretically by the example of deformation phonons. Expressions for the reduced density matrix at T=0 are derived beyond the Markovian approximation by means of explicit solution of the non-stationary Schrodinger equation for the interacting electron-phonon system at the initial stage of its evolution. It is shown that as long as the difference between the energies of the electron in the left and the right well greatly exceeds the energy of the electron tunneling between the minima o…
Quantum Hall edge channels at integer filling factor provide a unique test-bench to understand decoherence and relaxation of single electronic excitations in a ballistic quantum conductor. In this Letter, we obtain a full visualization of the decoherence scenario of energy (Landau) and time (Levitov) resolved single electron excitations at filling factor $ν=2$. We show that the Landau excitation exhibits a fast relaxation followed by spin-charge separation whereas the Levitov excitation only experiences spin-charge separation. We finally suggest to use Hong-Ou-Mandel type experiments to probe …
We derive a general expression for the conductivity of a disordered conductor with electron-electron interactions (treated within the standard model) and evaluate the weak localization correction delta sigma_{wl} employing no approximations beyond the accuracy of the definition of delta sigma_{wl}. Our analysis applies to all orders in the interaction and extends our previous calculation by explicitly taking into account quantum fluctuations around the classical paths for interacting electrons (pre-exponent). We specifically address the most interesting low temperature limit and demonstrate th…
In this article we review our work on the dynamics and decoherence of electron and hole spins in single and double quantum dots. The first part, on electron spins, focuses on decoherence induced via the hyperfine interaction while the second part covers decoherence and relaxation of heavy-hole spins due to spin-orbit interaction as well as the manipulation of heavy-hole spin using electric dipole spin resonance.
We consider an electrostatic qubit, interacting with a fluctuating charge of single electron transistor (SET) in the framework of exactly solvable model. The SET plays a role of the fluctuating environment affecting the qubit's parameters in a controllable way. We derive the rate equations describing dynamics of the entire system for both weak and strong qubit-SET coupling. Solving these equation we obtain decoherence and relaxation rates of the qubit, as well as the spectral density of the fluctuating qubit's parameters. We found that in the weak coupling regime the decoherence and relaxation…
We revisit the problem of the decoherence and relaxation of a central spin coupled to a bath of conduction electrons. We consider both metallic and semiconducting baths to study the effect of a gap in the bath density of states (DOS) on the time evolution of the density matrix of the central spin. We use two weak coupling approximation schemes to study the decoherence. At low temperatures, though the temperature dependence of the decoherence rate in the case of a metallic bath is the same irrespective of the details of the bath, the same is not true for the semiconducting bath. We also calcula…
Mechanisms of 'environmental decoherence' such as surface scattering, Elliot-Yafet process and precession mechanisms, as well as their influence on the spin phase relaxation are considered and compared. It is shown that the 'spin ballistic' regime is possible, when the phase relaxation length for the spin part of the wave function (WF)is much greater than the phase relaxation length for the 'orbital part'. In the presence of an additional magnetic field, the spin part of the electron's WF acquires a phase shift due to additional spin precession about that field. If the structure length is chos…
Direct phonon spin-lattice relaxation of an electron qubit bound by a donor impurity or quantum dot in SiGe heterostructures is investigated. The aim is to evaluate the importance of decoherence from this mechanism in several important solid-state quantum computer designs operating at low temperatures. We calculate the relaxation rate $1/T_1$ as a function of [100] uniaxial strain, temperature, magnetic field, and silicon/germanium content for Si:P bound electrons. The quantum dot potential is much smoother, leading to smaller splittings of the valley degeneracies. We have estimated these spli…
We study the dynamics of dissipative spins for general spin-spin coupling. We investigate the population dynamics and relaxation of the purity in the white noise regime, in which exact results are available. Inter alia, we find distinct reduction of decoherence and slowdown of purity decay around degeneracy points. We also determine in analytic form the one-phonon exchange contribution to decoherence and relaxation in the ohmic quantum noise regime valid down to zero temperature.
