The advent of high-intensity ultrafast laser pulses has opened new opportunities for controlling and designing quantum materials. In particular, terahertz (THz) pulses can resonantly drive optical phonon modes, enabling dynamic manipulation of lattice degrees of freedom. In this work, we investigate the ultrafast quantum thermodynamics of optical phonon mode driven by a THz pulse by treating the phonon as an open quantum system coupled to a thermal environment within a Caldeira-Leggett-type framework. We derive the quantum heat current between the phonon and the bath and analyze its behavior under realistic pulse protocols. Our results demonstrate that ultrafast laser driving can reveal and even induce significant deviations from the commonly adopted Markovian approximation, thereby providing a pathway to probe and control non-Markovian dissipation in driven solid-state systems.
The advent of high-intensity ultrafast laser pulses has opened new opportunities for controlling and designing quantum materials. In particular, terahertz (THz) pulses can resonantly drive optical phonon modes, enabling dynamic manipulation of lattice degrees of freedom. In this work, we investigate the ultrafast quantum thermodynamics of optical phonon mode driven by a THz pulse by treating the phonon as an open quantum system coupled to a thermal environment within a Caldeira-Leggett-type framework. We derive the quantum heat current between the phonon and the bath and analyze its behavior under realistic pulse protocols. Our results demonstrate that ultrafast laser driving can reveal and even induce significant deviations from the commonly adopted Markovian approximation, thereby providing a pathway to probe and control non-Markovian dissipation in driven solid-state systems.
Understanding heat transfer between a quantum system and its environment is of grave importance if reliable quantum devices are to be constructed. Here, the heat transfer between the system and bath in non-Markovian open quantum systems in the process of adiabatic speedup is investigated. Using the quantum state diffusion equation method, the heat current, energy current and the power are calculated during the free evolution and under external control of the system. While the heat current increases with increasing system-bath coupling strength and bath temperature, the non-Markovian nature of the bath can restrict the heat current. Without pulse control, the heat current is nearly equal to energy current. However, with pulse control, the energy current is nearly equal to the power. We show that more non-Markovian baths can be used to better approximate an adiabatic evolution and have a smaller heat current. Our results reveal that the non-Markovian nature of the bath significantly contributes to both the heat transfer rate as well as the effective adiabatic speedup.
Detecting coherent phonons pose different challenges compared to coherent photons due to the much stronger interaction between phonons and matter.This is especially true for high frequency heat carrying phonons, which are intrinsic lattice vibrations experiencing many decoherence events with the environment, and are thus generally assumed to be incoherent.Two photon interference techniques, especially coherent population trapping (CPT) and electromagnetically induced transparency (EIT), have led to extremely sensitive detection, spectroscopy and metrology.Here, we propose the use of two photon interference in a three-level system to sense coherent phonons.Unlike prior works which have treated phonon coupling as damping, we account for coherent phonon coupling using a full quantum-mechanical treatment.We observe strong asymmetry in absorption spectrum in CPT and negative dispersion in EIT susceptibility in the presence of coherent phonon coupling which cannot be accounted for if only pure phonon damping is considered.Our proposal has application in sensing heat carrying coherent phonons effects and understanding coherent bosonic multi-pathway interference effects in three coupled oscillator systems.Phonons are packets of vibrational energy that shares many similarities with its bosonic cousin photons.Advances in nanofabrication has enabled many parallels between the development of photon and phonon control.Parallel developments in passive control techniques include photonic [1] versus phonoic crystals [2], optical [3] versus acoustic metamaterials [4] etc. Development in active manipulation of electromagnetic waves through light-matter interaction have led to creation of nanoscale optical emitters [5] and gates [6] and similar progress have been made in controlling phonons using their interaction with matter especially in the realms of optomechanics [7] and phononic devices [8,9].Phonons span a vast frequency range and while techniques to control and sense lower frequency coherent phonons have been well-developed [10-19], heat carrying coherent terahertz acoustic phonons have been harder to measure directly due to the small wavelength and numerous scattering mechanism at these small wavelengths [20].In the past, THz crystal phonons have been generated and detected in low temperature experiments with defect doped crystals [21-24], with experimental evidence of coherent phonon generation [25][26][27].At the same time, interpretation of non-equilibrium phonon transport, with the advancement of nanoscale electrical heating and ultrafast optical pump-probe techniques, have allowed us to infer phonon coherence from broadband thermal conductivity measurements [28][29][30][31][32].There have been also interest of using defect-based techniques as a thermal probe using perturbation to energy levels due to changes in temperature [33].Furthermore, surface deflection techniques with ultrafast optics have also been used to generate phonons close to THz frequencies in materials [34][35][36][37].Defect-based techniques are attractive compared to both thermal conductivity measurement and deflection techniques due to its ability to directly access atomic length scales where THz phonon wavelength resides.Also, the energy levels in the excited state [21,38,39] and ground state [25,40] electron manifold of these defects can match the phonon energy precisely, resulting in a narrow band phonon detector.In light of the success of defect-based optical absorption techniques in coupling directly to high frequency phonons, we propose the use of two photon interference to measure the coherence properties of these phonons.
Many-body localization, which prevents a many-body quantum systems from reaching thermal equilibrium has opened the possibility to study out-of-equilibrium dynamics of isolated quantum systems, leading to new phases of matter such as the discrete time crystal. However, little is known about these effects when the quantum many-body system is coupled to an external environment. An intuitive assumption is that out-of-equilibrium dynamics and quantum coherences would not survive due to thermalization with external degrees of freedom. The present work focuses on open quantum many-body systems and aims at the question how an external reservoir can preserve such out-of-equilibrium effects. In the first part, common generic reservoirs are assumed to focus on many-body effects of the systems of interest. As a first many-body system, an optically driven many-emitter phonon laser is investigated. The full quantum mechanical treatment reveals additional resonances caused by collective effects of the many-emitter setup. Optically addressing these resonances results in an enhancement of phonon intensities within the acoustic cavity. The Heisenberg spin-chain has achieved the role of a standard model to study many-body localization. As a second many-body system, the model is generalized to an open quantum system via boundary reservoirs which are driving the system out-of equilibrium and a spin-current is induced. When driven far from equilibrium, the transport is very much dependent on the external reservoirs. In contrast, it is demonstrated that a setup with long-range coupling between the spins shows transport behavior independent of the external environment. In the second part of this work, the coupling to the reservoir is structured by assuming a boundary condition. Experimentally this is achieved by placing a distant mirror close to the system. A common Born-Markov approximation leading to factorizing system and reservoir states is insufficient to describe these feedback dynamics. In this work, an approach is used to treat the reservoir states as part of the many-body system which is based on matrix product states using the quantum stochastic Schrödinger equation. Focusing on the reservoir statistics of a single two-level system, it is demonstrated that by combining time-dependent optical excitation with time-delayed feedback via structuring the reservoir, individual photon probabilities are controlled by changing external degrees of freedom. Finally, this method is generalized to a many-body system including a coupling to an external reservoir beyond Born-Markov approximation. It is shown that feedback dynamics stabilize a discrete time crystal against dissipation to the environment. Out-of-equilibrium dynamics and many-body localization of the discrete time crystal survive for long times even in the presence of an external environment and become independent of the coupling to the reservoir. The developed method benefits from small entanglement entropy of the many-body localized system as well as from a suppression of entanglement between system and reservoir due to the feedback dynamics. This allows to consider up to forty spins with individual structured reservoirs in a numerically exact manner.
Ultrafast laser probing has revolutionized the study of quantum materials. Quantum magnets realize some of the most complex and entangled many-body states in condensed matter. Here, the authors unify ultrafast physics and quantum magnetism by pump-probe experiments on the frustrated magnet SrCu2(BO3)2, using terahertz light pulses to create coherent driving of the lattice phonons, which in turn creates a special singlet spin excitation. Their phonon-based method introduces a universal mechanism for controlling nonequilibrium quantum many-body physics on ultrafast timescales.
Time-periodic quantum systems are present all over science and technology. They have been successfully studied using Floquet theory, and a broad diversity of interesting phenomena such as coherent destruction of tunnelling, dynamical localization and creation of new phases of matter have been discovered. Furthermore, periodic driving has also been introduced as a tool for engineering novel quantum systems that have no static analogue. Besides these remarkable achievements for isolated systems, in reality, systems always interact with their surroundings. Taking the effect of the environment into account, time-periodic quantum systems have been mainly described using the Floquet-Markov approach, which consists in deriving a weak coupling master equation in the Floquet basis. In this thesis, we focus on how to overcome these standard approximations in the description of driven open quantum systems. We start by providing a general review on the topics of isolated driven systems, open quantum systems and the Floquet-Markov approach, before presenting the main results of this work. Under the effect of high-frequency driving, we provide an analytic solution to a large class of open quantum systems. Our method relies on discrete symmetries of the system-bath Hamiltonian and grants access to the time evolution operator of the full system, including bath degrees of freedom. The solution can be interpreted as the stroboscopic evolution of a family of observables under the influence of an effective static Hamiltonian. This provides a flexible simulation procedure of nontrivial Hamiltonians. The results are exemplified with the driven spin-boson model, where a coherent control of dissipation is observed. By a novel combination of Floquet theory, full counting statistics and a Markovian embedding strategy (reaction coordinate mapping), more intricate scenarios with additional driven system-environment interactions are addressed. Hereby, we can access the dynamics and thermodynamics of a periodically driven thermal machine. For the particular case where the working medium is a two-level system, we identify four different operating regimes, including a heat engine and a refrigerator. As the coupling strength is increased, their efficiency and coefficient of performance decrease and the refrigerator regime disappears. Notably, our model reproduces the setup of laser cooling of trapped ions in a specific parameter limit. Following the same combination of methods, we investigate electron pumping through a single quantum dot with periodically modulated energy and tunnelling amplitudes. In the high-frequency regime, the direction of the current can be controlled with the driving phase and a rectification effect that is exclusive to the non-Markovian character of the model is observed. In the low-frequency regime, we apply an additional transformation to view the model as three independent transport channels and observe that the coupling strength also controls the current direction. Studying charge fluctuations we validate that our model behaves as a single-electron source.
In this paper, we investigate the strong-coupling quantum thermodynamics of a hybrid quantum system far from equilibrium, based on the renormalization theory of quantum thermodynamics we developed recently [Phys. Rev. Research 4, 023141 (2022)]. The strong-coupling hybrid system consists of a superconducting microwave cavity and a spin ensemble of the NV centers in diamond under external driving. The non-Markovian dynamics of this strong-coupling hybrid system has been experimentally explored and theoretically investigated. We apply the renormalization theory of quantum thermodynamics to study the transient quantum heat and work in this strong-coupling hybrid system. We find that the dissipation and fluctuation dynamics of the system induce the transient quantum heat current which shows significant non-Markovian effects. On the other hand, the energy and driving field renormalization produces quantum work power. In particular, the driving-induced work power can be largely enhanced by non-Markovian dynamics through the cavity coupling strongly with the spin ensemble at the resonance. Our results show that non-Markovian dynamics makes faster energy conversion of the heat and work.