The effect of time-varying electromagnetic fields on electron coherence is investigated. A sinusoidal electromagnetic field produces a time varying Aharonov-Bohm phase. In a measurement of the interference pattern which averages over this phase, the effect is a loss of contrast. This is effectively a form of decoherence. We calculate the magnitude of this effect for various electromagnetic field configurations. The result seems to be sufficiently large to be observable.
The quantum coherence of the two-site XYZ model with Dzyaloshinsky-Moriya (DM) interactions in an external inhomogenous magnetic field is studied. The DM interaction, the magnetic field and the measurement basis can be along different directions, and we examine the quantum coherence at finite temperature. With respect to the spin-spin interaction parameter, we find that the quantum coherence decreases when the direction of measurement basis is the same as that of the spin-spin interaction. When the spin-lattice interaction is varied, the coherence always increases irrespective of the relation …
The dynamics of coherence in a two-qubit Heisenberg XXX model with magnetic field in the presence of the intrinsic decoherence is investigated. We discuss the influence of dipole parameter, spin distance, magnetic field and initial state parameter on coherence of the long time limit situation. Then, we discuss how decoherence changes with time with the initial state parameter and the decoherence parameter.
We investigate the evolution of a Zeeman coherence grating induced in a cold atomic cesium sample in the presence of an external magnetic field. The gratings are created in a three-beam light storage configuration using two quasi-collinear writing laser pulses and reading with a counterpropagating pulse after a variable time delay. The phase conjugated pulse arising from the atomic sample is monitored. Collapses and revivals of the retrieved pulse are observed for different polarizations of the laser beams and for different directions of the applied magnetic field. While magnetic field inhomog…
We study the influence of an external driving field on the coherence properties of a qubit under the influence of bit-flip noise. In the presence of driving, two paradigmatic cases are considered: (i) a field that results for a suitable choice of the parameters in so-called coherent destruction of tunneling and (ii) one that commutes with the static qubit Hamiltonian. In each case, we give for high-frequency driving a lower bound for the coherence time. This reveals the conditions under which the external fields can be used for coherence stabilization.
The dynamics of single electron and nuclear spins in a diamond lattice with different 13C nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free induction decays of nuclear spin Bell states and single nuclear coherences among 13C nuclear spins are compared and analyzed. Reduction of a free induction decay time T2* and a coherence time T2 upon increase of nuclear spin concentration has been found. For diamond material with depleted concentration of nuclear spin, T2* as long as 30 micros…
The study of quantum coherence in condensed matter systems is a broad avenue to be explored toward the enhancement of its quantum properties by means of material engineering. In this regard, the present work reports a study of the influence of temperature, pressure and magnetic fields on the quantum coherence of a Cu(II) metal-silicate framework. We calculate the $l_1$ trace norm quantum coherence as a function of the magnetic susceptibility of the compound, which allows us to evaluate the effects of these external parameters on the degree of coherence of the material. Our results show that th…
The elimination of decoherence of two-state quantum systems interacting with a thermal reservoir through an external controllable driving field is discussed in the present paper. The restriction equation with which the external controllable driving field should agree will be derived. Based on this, we obtain the time-development equation of the off-diagonal elements of density operator in the supersymmetric multiphoton two-state quantum systems, which is helpful for studying the polarization evolution in this two-state quantum model.
We study the effect of Landau-Zener (LZ) tunneling caused by the varying sweeping rate of external field, formulating and approximately solving the problem with many levels of the LZ tunneling rate. Comparing with the steadily vary about sweeping field, the LZ tunneling rate will be essentially changed because of the unsteady variation of the sweeping field in time. Thus could help us to make the particles with in lower states transit periodically to upper states within the finite time.
Studying quantum properties in solid-state systems is a significant avenue for research. In this scenario, double quantum dots (DQDs) appear as a versatile platform for technological breakthroughs in quantum computation and nanotechnology. This work inspects the thermal entanglement and quantum coherence in two-coupled DODs, where the system is exposed to an external stimulus that induces an electronic transition within each subsystem. The results show that the introduction of external stimulus induces a quantum level crossing that relies upon the Coulomb potential changing the degree of quant…
We investigate experimentally and theoretically the temporal evolution of the spin of the conduction band electron and that of the valence band heavy hole, both confined in the same semiconductor quantum dot. In particular, the coherence of the spin purity in the limit of a weak externally applied magnetic field, comparable in strength to the Overhauser field due to fluctuations in the surrounding nuclei spins. We use an all-optical pulse technique to measure the spin evolution as a function of time after its initialization. We show for the first time that the spin purity performs complex temp…
In this letter we present a method for increasing the coherence time of praseodymium hyperfine ground state transitions in Pr^3+:Y_2SiO_5 by the application of a specific external magnetic field. The magnitude and angle of the external field is applied such that the Zeeman splitting of a hyperfine transition is at a critical point in three dimensions, making the first order Zeeman shift vanishingly small for the transition. This reduces the influence of the magnetic interactions between the praseodymium ions and the spins in the host lattice on the transition frequency. Using this method a pha…
We present a general analysis for the interaction of a probe laser radiation with a coherently prepared molecular Raman medium. We describe a general formalism which includes dispersive effects, such as group velocity and group velocity dispersion (GVD). When dispersion is negligible, the analysis is especially simple and insightful. We show that molecular oscillations result in a modulated instantaneous susceptibility of the medium. The effect of the time-varying susceptibility on a probe laser pulse is two-fold: the output frequency becomes modulated because of the time-varying phase velocit…
We demonstrate fast and accurate control of the evolution of collective atomic coherences in an Erbium doped solid using external electric fields. This is achieved by controlling the inhomogeneous broadening of Erbium ions emitting at 1536 nm using an electric field gradient and the linear Stark effect. The manipulation of atomic coherence is characterized with the collective spontaneous emission (optical free induction decay) emitted by the sample after an optical excitation, which does not require any previous preparation of the atoms. We show that controlled dephasing and rephasing of the a…
We propose to use quantum coherence as ultimate proof of quantum nature in the radiation that appears by means of the Dynamical Casimir Effect in experiments with superconducting microwave waveguides. We show that, unlike previously considered measurements of quantum correlations such as entanglement and discord, quantum coherence does not require a threshold value of the external pump amplitude and is highly robust to thermal noise.
The following work is motivated by the conceptual problems associated with the wave-particle duality and the notion of the photon. Two simple classical models for radiation from individual emitters are compared, one based on sines with random phasejumps, another based on pulse trains. The sum signal is calculated for a varying number of emitters. The focus lies on the final signal's statistical features quantified by means of the temporal coherence function and the temporal coherence length. We show how these features might be used to experimentally differentiate between the models. We also po…
The effect of the inevitable coupling to external degrees of freedom of a quantum computer are examined. It is found that for quantum calculations (in which the maintenance of coherence over a large number of states is important), not only must the coupling be small but the time taken in the quantum calculation must be less than the thermal time scale, $\hbar/k_B T$. For longer times the condition on the strength of the coupling to the external world becomes much more stringent.
We present a way to trap a single Rydberg atom, make it long-lived and preserve an internal coherence over time scales reaching into the minute range. We propose to trap using carefully designed electric fields, to inhibit the spontaneous emission in a non resonant conducting structure and to maintain the internal coherence through a tailoring of the atomic energies using an external microwave field. We thoroughly identify and account for many causes of imperfection in order to verify at each step the realism of our proposal.
We present a model for an autonomous quantum thermal machine comprised of two qubits capable of manipulating and even amplifying the local coherence in a non-degenerate external system. The machine uses only thermal resources, namely, contact with two heat baths at different temperatures, and the external system has a non-zero initial amount of coherence. The method we propose allows for an interconversion between energy, both work and heat, and coherence in an autonomous configuration working in out-of-equilibrium conditions. This model raises interesting questions about the role of fundament…
In contrast to the traditional Spontaneous Generated Coherence (SGC), Field Generated Coherence (FGC)-based atomic scheme is presented for spontaneous emission cancellation. It is easy to achieve externally controllable experimental trapping condition in this 4-field-driven 5-level atomic system. Consequently, due to the FGC the decay from the central dressed \emph{bare-energy-state} of the set of upper three closely spaced hyperfine decaying states of Sodium D2 line is completely cancelled under the trapping condition, exhibiting a novel phenomenon of a \textit{dark bare-energy-state}% . Exte…
We carry out a theoretical study of the collective spontaneous emission (superradiance) from an ultrathin film comprised of three-level atoms with $V$-configuration of the operating transitions. As the thickness of the system is small compared to the emission wavelength inside the film, the local-field correction to the averaged Maxwell field is relevant. We show that the interplay between the low-frequency quantum coherence within the subspace of the upper doublet states and the local-field correction may drastically affect the branching ratio of the operating transitions. This effect may be …
Measuring small separations between two optical sources, either in space or in time, constitute an important metrological challenge as standard intensity-only measurements fail for vanishing separations. Contrarily, it has been established that appropriate coherent mode projections can appraise arbitrarily small separations with quantum-limited precision. However, the question of whether the optical coherence brings any metrological advantage to mode projections is still a point of debate. Here, we elucidate this problem by experimentally investigating the effect of varying coherence on estima…
Within the framework of a general three-level problem, the dynamics of the nitrogen-vacancy (NV) spin is studied for the case of a special type of external driving consisting of a set of continuous fields with decreasing intensities. Such a set has been proposed for minimizing coherence losses. Each new driving field with smaller intensity is designed to protect against the fluctuations induced by the driving field at the preceding step with larger intensity. We show that indeed this particular type of external driving minimizes the loss of coherence, using purity and entropy as quantifiers fo…
Recently it was demonstrated that long-lived quantum coherence exists during excitation energy transport in photosynthesis. It is a valid question up to which length, time and mass scales quantum coherence may extend, how to one may detect this coherence and what if any role it plays for the dynamics of the system. Here we suggest that the selectivity filter of ion channels may exhibit quantum coherence which might be relevant for the process of ion selectivity and conduction. We show that quantum resonances could provide an alternative approch to ultrafast 2D spectroscopy to probe these quant…
Aspects of coherence and decoherence are analyzed within the optical Bloch equations. By rewriting the analytic solution in an alternate form, we are able to emphasize a number of unusual features: (a) despite the Markovian nature of the bath, coherence at long times can be retained; (b) the long-time asymptotic degree of coherence in the system is intertwined with the asymptotic difference in level populations; (c) the traditional population-relaxation and decoherence times, $T_1$ and $T_2$, lose their meaning when the system is in the presence of an external field, and are replaced by more g…
A binary mixture of interacting Bose-Einstein condensates (BEC) in the presence of fragmentation-driving external lattice potentials forms two interdependent mean-field lattices made of each component. These effective mean-field lattices, like ordinary optical lattices, can induce additional fragmentation and phase coherence loss of BECs between lattice sites. In this study, we consider the nonequilibrium dynamics of two hyperfine states of one-dimensional Bose-Einstein condensates, subjected to state-dependent optical lattices. Our numerical calculations using the truncated Wigner approximati…
Vacuum induced coherence in a strongly coupled cavity consisting of a three-level system is studied theoretically. The effects of the strong coupling to electromagnetic field vacuum are examined by solution of an open-system quantum master equation. The numerical results show that the system exhibits population trapping, and the numerical results are interpreted with analytical expressions derived from a new basis in the weak excitation regime. We further show that the generated effects can be probed with weak external fields. Moreover, it is shown that the induced coherence can be controlled …
We consider N identical two-level atoms coupled to an optical cavity, which is coherently driven by an external field. In the limit of small atomic excitation, the reflection and transmission coefficients for both fields and intensities are calculated analytically. In addition, the frequency content of the cavity field and hence also the emission spectrum of the cavity is determined. It is discussed in particular how individual collisional dephasing and common atomic energy-level fluctuations prevent the cavity field from being in a coherent state, which in turn affects the outgoing fields.
We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly-interacting condensate, experimentally measured via the contrast in an interference experiment, as well as the response to an external driving become maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. This can be understood as a stochastic resonance of the many-particle system. Even stronger effects are observed when dissipation acts in concurrence with strong inter-particle interactions, restorin…
Spins in solids are an ideal candidate to act as a memory and interface with superconducting qubits due to their long coherence times. We spectroscopically investigate erbium-167-doped yttrium orthosilicate as a possible microwave-addressed memory employing its microwave frequency transitions that occur without applying an external magnetic field. We obtain coherence times of 380 $μ$s in a ground state spin transition and 1.48 ms in an excited state spin transition. This is 28 times longer compared to previous zero field measurements, as well as 200 times longer than a previous microwave memor…
Quantum coherence and interference effects in atomic and molecular physics has been extensively studied due to intriguing counterintuitive physics and potential important applications. Here we present one such application of using quantum coherence to generate and enhance gain in extreme ultra-violet(XUV)(@58.4nm in Helium) and infra-red(@794.76nm in Rubidium) regime of electromagnetic radiation. We show that using moderate external coherent drive, a substantial enhancement in the energy of the lasing pulse can be achieved under optimal conditions. We also discuss the role of coherence. The pr…
We study the dynamics of ultracold atoms trapped in optical double-well potentials in presence of noise generated by an external environment. When prepared in a Fock number state, the system shows phase coherence in the averaged density profile obtained using standard absorption image techniques, that disappears in absence of noise. This effect indicates that also in many-body systems an external environment may enhance quantum coherence, instead of destroying it.
We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly-interacting condensate as well as the response to an external driving show a pronounced stochastic resonance effect: Both quantities become maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. Even stronger effects are observed when dissipation acts in concurrence with strong inter-particle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence sign…
We study nonequilibrium properties of small and chaotic quantum systems, i.e., non-integrable systems whose size is small in the sense that the separations of energy levels are non-negligible as compared with other relevant energy scales. The energy change $ΔE$ induced by a slowly-varying external field $λ(t)$ is evaluated when the range of the variation $Δλ$ is large so that the linear response theory breaks down. A new statistical theory is presented, by which we can predict $<ΔE>$, the average of $ΔE$ over a finite energy resolution $δE$, as a function of $λ(t)$ if we are given the density …
We investigated the depth dependence of coherence times of nitrogen-vacancy (NV) centers through precisely depth controlling by a moderately oxidative at 580°C in air. By successive nanoscale etching, NV centers could be brought close to the diamond surface step by step, which enable us to trace the evolution of the number of NV centers remained in the chip and to study the depth dependence of coherence times of NV centers with the diamond etching. Our results showed that the coherence times of NV centers declined rapidly with the depth reduction in their last about 22 nm before they finally d…
The quantumness of Dirac paticles for quantized fields in a dilatonic black hole is estimated by means of quantum channel. We develop a general Bloch vector representation of quantum channel in black hole spacetimes beyond single mode approximation. The nonclassicality of Dirac particles can be measured by the minimization of quantum coherence over all orthonormal basis sets. The quantumness of the channel decreases as the dilaton parameter increases. The interplay between the external reservoir noise and dilaton black hole on the dynamical behavior of quantum coherence and steerability is inv…
We demonstrate theoretically the noise-stimulated enhancement of quantum coherence in a superconducting flux qubit. First, an external classical noise can increase the off-diagonal components of the qubit density matrix. Second, in the presence of noise, the Rabi oscillations survive for times significantly longer than the Rabi decay time in a noiseless system. These Rabi oscillations appear as a modulation of the forced response of the qubit to the ac driving field. These effects can be considered as a manifestation of quantum stochastic resonance and are relevant to experimental techniques, …
The existence of vacuum fluctuations is one of the most important predictions of modern quantum field theory. In the vacuum state, fluctuations occurring at different frequencies are uncorrelated. However, if a parameter in the Lagrangian of the field is modulated by an external pump, vacuum fluctuations stimulate spontaneous downconversion processes, creating squeezing between modes symmetric with respect to half of the frequency of the pump. Here we show that by double parametric pumping of a superconducting microwave cavity, it is possible to generate another fundamental type of correlation…
The coherence effects induced by external photons coupled to matter waves inside a Mach-Zehnder three-grating interferometer are analyzed. Alternatively to atom-photon entanglement scenarios, the model considered here only relies on the atomic wave function and the momentum shift induced in it by the photon scattering events. A functional dependence is thus found between the observables, namely the fringe visibility and the phase shift, and the transversal momentum transfer distribution. A good quantitative agreement is found when comparing the results obtained from our model with the experime…
We demonstrate a new way to extend the coherence time of separated Bose-Einstein condensates that involves immersion into a superfluid bath. When both the system and the bath have similar scattering lengths, immersion in a superfluid bath cancels out inhomogeneous potentials either imposed by external fields or inherent in density fluctuations due to atomic shot noise. This effect, which we call superfluid shielding, allows for coherence lifetimes beyond the projection noise limit. We probe the coherence between separated condensates in different sites of an optical lattice by monitoring the c…
In the conventional two-point measurement scheme of quantum thermodynamics, quantum coherence is destroyed by the first measurement. But as we know the coherence really plays an important role in the quantum thermodynamics process, and how to describe the work statistics for a quantum coherent process is still an open question. In this paper, we use the full counting statistics method to investigate the effects of quantum coherence on work statistics. First, we give a general discussion and show that for a quantum coherent process, work statistics is very different from that of the two-point m…
We investigate the transfer of coherence from atoms to a cavity field initially in a statistical mixture in a two-photon micromaser arrangement. The field is progressively modified from a maximum entropy state (thermal state) towards an almost pure state (entropy close to zero) due to its interaction with atoms sent across the cavity. We trace over the atomic variables, i.e., the atomic states are not collapsed by a detector after they leave the cavity. We find that by applying an external classical driving field it is possible to substantially increase the field purity without the need of pre…
We investigate experimentally and theoretically the coherent and incoherent processes in open 3-level ladder systems in room temperature gases and identify in which regime electromagnetically induced transparency (EIT) occurs. The peculiarity of this work lies in the unusual situation where the wavelength of the probe field is shorter than that of the coupling field. The nature of the observed spectral features depends considerably on the total response of different velocity classes, the varying Doppler shifts for bichromatic excitation fields, on optical pumping to additional electronic state…
Ellipticity-Controlled Bright-Dark Coherence Transition in Monolayer WSe2
Qufu Normal University（曲阜师范大学）；Shandong Jianzhu University（山东建筑大学）；Shandong University（山东大学）；Chinese Academy of Sciences（中国科学院）；Beijing Computational Science Research Center（北京计算科学研究中心）
Coherence-enhanced light harvesting has not been directly observed experimentally, despite theoretical evidence that coherence can significantly enhance light-harvesting performance. The main experimental obstacle has been the difficulty in isolating the effect of coherence in the presence of confounding variables. Recent proposals for externally controlling coherence by manipulating the light's degree of polarization showed that coherent efficiency enhancements would be possible, but were restricted to light-harvesting systems weakly coupled to their environment. Here, we show that increases …
We present an experimental investigation on the spectral characteristics of an artificial atom "transmon qubit" constituting a three-level cascade system (Ξ-system) in the presence of a pair of external driving fields. We observe two different types of Autler-Townes (AT) splitting: type I, where the phenomenon of two-photon resonance tends to diminish as the coupling field strength increases, and type II, where this phenomenon mostly stays constant. We find that the types are determined by the cooperative effect of the decay rates and the field strengths. Theoretically analyzing the density-ma…
We predict several effects associated with the optical response of systems prepared in a nonequilibrium state by impulsive optical excitations. The linear response depends on the phase of the electric field even if the initial nonequilirbium state has only populations, no coherences. Initial quantum coherences induce additional phase dependence which also shows new resonances in nonlinear wave mixing. In systems strongly driven by an external optical field, the field frequency generates a phase dependent probe absorption. This gives further control to manipulate the relative contribution to th…
