Macroscopic Quantum Research Articles

Cooling the mechanical resonator by Gaussian pulses

The optomechanical system has been widely used torealize the quantum ground state cooling of mechanical resonator, which is the way to create macroscopic quantum state. How to get the better cooling effect is evidently the central point. However, it has been demonstrated that there is a cooling limit if using only one single continuous-wave pump to drive the mechanical resonator, i.e., the cooling rate cannot be decreased by increasing the drive intensity or the mechanical frequency, so that the new dynamics should be developed. Here we present an approach to cool the mechanical resonator by the Gaussian pulses. For the pulsed optomechanical system, the common steady-state expansion approach is not available, since no steady state can be found in most cases. We use the new linearized approach to get the effective Hamiltonian and the dynamical evolution of the optomechanical system, with which the thermal phonon number of the mechanical resonator can be achieved directly. After investigating how the pulse profile, drive intensity and pulse duration affect the evolution of thermal phonon number in details, we show that the mechanical resonator can be well cooled down to the ground state by a single pulse, and can be well preserved by the multiple pulses, with the appreciated drive intensity, duration. Moreover, the character of drive intensity increasing and decreasing of the pulse profile, enables one to delay the squeezing effect which results in the heating of the mechanical resonator. As a result, the cooling limit of single continuous-wave driven optomechanical system can be broken by the pulsed optomechanical system. Even the half of the cooling limit of the continuous optomechanical system can be achieved by the short and strong Gaussian pulses, which is the main advantage of the ground state cooling by pulse drives.

Prethermalization and Nonreciprocal Phonon Transport in a Levitated Optomechanical Array

AbstractRecent advances in levitated optomechanics have brought new opportunities for the study of macroscopic quantum mechanics and precision measurement. Previous studies have mainly focused on the single levitated nanoparticle, the systems of multiple levitated nanoparticles were rarely involved. Here an array of optically levitated nanospheres in vacuum is considered and nontrivial phonon transport in this system is investigated. The levitated nanospheres are coupled by optical binding. Key parameters of this system, such as the interaction range, trapping frequencies, and mechanical dissipation, are highly tunable. Due to these advantages, counter‐intuitive phenomena such as prethermalization and nonreciprocal phonon transport can be achieved by tuning the spacing between neighboring spheres, the mechanical dissipation, and the trapping frequency of each sphere. The system provides a great platform to investigate novel phonon transport and thermal energy transfer.

Observation of exciton polariton condensation in a perovskite lattice at room temperature

Bose-Einstein condensation in strongly correlated lattices provides the possibility to coherently generate macroscopic quantum states, which have attracted tremendous attention as ideal platforms for quantum simulation. Ultracold atoms in optical lattices are one of such promising systems, where their realizations of different phases of matter exhibit promising applications in condensed matter physics, chemistry, and cosmology. Nevertheless, this is only accessible with ultralow temperatures in the nano to micro Kelvin scale set by the typical inverse mass of an atom. Alternative systems such as lattices of trapped ions and superconducting circuit arrays also rely on ultracold temperatures. Exciton polaritons with extremely light effective mass, are regarded as promising alternatives to realize Bose-Einstein condensation in lattices at higher temperatures. Along with the condensation, an efficient exciton polariton quantum simulator would require a strong lattice with robust trapping at each lattice site as well as strong inter-site coupling to allow coherent quantum motion of polaritons within the lattice. However, exciton polaritons in a strong lattice have only been shown to condense at liquid helium temperatures. Here, we report the observation of exciton polariton condensation in a one-dimensional strong lead halide perovskite lattice at room temperature. Modulated by deep periodic potentials, the strong lead halide perovskite lattice exhibits a large forbidden bandgap opening up to 13.3 meV and a lattice band up to 8.5 meV wide, which are at least 10 times larger than previous systems. Above a critical density, we observe exciton polariton condensation into py orbital states with long-range spatial coherence at room temperature. Our result opens the route to the implementation of polariton condensates in quantum simulators at room temperature.

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor/ferromagnet/superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-pi transition in tunnel-ferromagnetic spin-filter JJs could enhance the capabilities of SFS JJs also as active elements. Here, we provide a self-consistent electrodynamic characterization of NbN/GdN/NbN spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the IV characteristics at 4.2K and at 300mK by using the Tunnel Junction Microscopic model allows us to determine the subgap resistance Rsg, the quality factor Q and the junction capacitance C. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel ferromagnetic JJs as active elements in classical and quantum circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.

Long-Range Order in the Dislocation Structure of Martensite Crystals

Optimal estimation of the diffraction observations over the object reliably detects periodicity in the dislocation structure of martensitic transformation as an exhibition of its wave nature. The period along normal to the slip planes is comparable with the radius of dislocation loops in crystals. The measured degree of one-dimensional long-range order in the arrangement of the loops is close to the upper limit equal to unity. Subject to the theory of metals, the observed structure could be generated by quantum lattice vibrations, which actuate a jump-like phase transition. A simple explanation exists: after a sharp fall in temperature, the excess energy of conduction electrons causes the crystal to expand instantly with the transformation of translational symmetry. Internal shifts of the crystal lattice caused by electron-phonon interactions concurrently trigger the wave process of formation of thin martensitic plates in the surrounding matrix, which are observed in metallography. Based on an in-depth analysis of the dislocation structure of martensite crystals, a physically founded concept is advanced in which the martensitic transformation is a macroscopic quantum phenomenon connected with the symmetry properties of a crystal system in metals.

Bipartite and tripartite entanglement caused by squeezed drive in magnetic-cavity quantum electrodynamics system

Utilizing optical nonlinearity for generating the entanglement is still a most widely used approach due to its quality and simplicity. Here in this paper, we propose a theoretical scheme to generate bipartite and tripartite entanglement in a cavity quantum electrodynamics (QED) system with one Yttrium iron garnet (YIG) sphere by using a squeezed drive. In such a system, the parametric down-conversion process is used to generate the nonlinearity and further increase the coupling between cavity and YIG. Thus, the enhanced coupling between the microwave cavity photons and the ferromagnetic resonance (FMR) mode/magnetostatic (MS) mode results in bipartite entanglements. By using the mean field theory, we show that the bipartite entanglements strongly depend on the detuning of the cavity and magnon mode. When the driving field is tuned to be resonant with the FMR mode, but the MS mode is far off-resonant, the entanglement between photons and the FMR mode reaches its maximum. However, when the driving field is tuned to be resonant with the MS mode, but the FMR mode is detuned very well, the entanglement between photons and the MS mode reaches its maximum. We show that the dissipation of the FMR/MS mode affects the entanglement greatly, and the bipartite entanglement decreases as the dissipation rate of the FMR/MS mode increases. Under the steady-state approximation, we also show that the tripartite entanglement can be generated, and the minimum residual contangle increases with the enhancement of the nonlinear gain coefficient. With the nonlinearity induced by the parametric down conversion process, the interaction between the driving field and the magnetic-cavity QED system leads to the tripartite entanglement involving the cavity photons, FMR mode and the MS mode. Likewise, we show that the tripartite entanglement also strongly depends on the dissipation rate of MS mode, and the minimum residual contangle increases as the dissipation rate of the MS mode decreases. We also show that the squeezed field induced tripartite entanglement is insensitive to the temperature and has good robustness. Our results suggest that the magnetic-cavity QED system could provide a promising platform for studying the macroscopic quantum phenomena, and the squeezing field opens a new method of generating the entanglement.

МАТЕМАТИЧНА МОДЕЛЬ ПЕРЕХІДНИХ ПРОЦЕСІВ У ДЖОЗЕФСОНІВСЬКИХ ЕЛЕМЕНТАХ ПАМЯТІ

The goal of this work is to find ways of enhancing the speed of computer memory cells by using structures that employ operating principles other than those of traditional semiconductors’ schemes. One of the applications of the unique properties of Josephson structures is their usage in novel superfast computer memory cells. Thanks to their high working characteristic frequencies close to 1 THz, the Josephson structures are most promising candidates to be used in petaflop computers. Moreover, both Josephson cryotrons and Josephson SQUIDs can be used in qubits, which are basic units in quantum computers, and also for describing a macroscopic quantum behavior, for example, during read-out processes in quantum computations. In the present work, we have created a mathematical model of transition processes in Josephson cryotrons during direct, “1” → ”0”, as well as inverse, “0” → “1”, logical transitions. We have considered controlling the logical state of Josephson memory cells based on Josephson tunneling junctions of the S-I-S type via external current pulses. By means of mathematical modelling, we have studied transition processes in cryotrons during the change of their logical state and calculated their transition characteristics for working temperatures T1 = 11.6 K and T2 = 81.2 K, which ale close to the boiling temperatures of helium and nitrogen, respectively. It has been shown that such memory cells can effectively operate at the working temperature T2 = 81.2 K. We have determined commutation times for both the direct “0” → “1” and inverse “0” → “1” transitions. We have also identified peculiar behaviors of the Josephson cryotrons based memory cells and studied the stability of their operation.

Macroscopic quantum electrodynamics and density functional theory approaches to dispersion interactions between fullerenes.

The processing and material properties of commercial organic semiconductors, for e.g. fullerenes is largely controlled by their precise arrangements, specially intermolecular symmetries, distances and orientations, more specifically, molecular polarisabilities. These supramolecular parameters heavily influence their electronic structure, thereby determining molecular photophysics and therefore dictating their usability as n-type semiconductors. In this article we evaluate van der Waals potentials of a fullerene dimer model system using two approaches: (a) Density Functional Theory and, (b) Macroscopic Quantum Electrodynamics, which is particularly suited for describing long-range van der Waals interactions. Essentially, we determine and explain the model symmetry, distance and rotational dependencies on binding energies and spectral changes. The resultant spectral tuning is compared using both methods showing correspondence within the constraints placed by the different model assumptions. We envision that the application of macroscopic methods and structure/property relationships laid forward in this article will find use in fundamental supramolecular electronics.

Emergence of singularities from decoherence: Quantum catastrophes

We use a master equation to study the dynamics of two coupled macroscopic quantum systems (e.g.\ a Josephson junction made of two Bose-Einstein condensates or two spin states of an ensemble of trapped ions) subject to a weak continuous measurement. If the coupling between the two systems is suddenly switched on the resulting dynamics leads to caustics (fold and cusp catastrophes) in the number-difference probability distribution, and at the same time the measurement gradually induces a quantum-to-classical transition. Decoherence is often invoked to help resolve paradoxes associated with macroscopic quantum mechanics, but here, on the contrary, caustics are well-behaved in the quantum (many-particle) theory and divergent in the classical (mean-field) theory. Caustics thus represent a breakdown of the classical theory towards which decoherence seems to inevitably lead. We find that measurement backaction plays a crucial role in softening the resulting singularities and calculate the modification to the Arnol'd index which governs the scaling of the caustic's amplitude with the number of atoms. The Arnol'd index acts as a critical exponent for the formation of singularities during quantum dynamics and its modification by the open nature of the system is analogous to the modification of the critical exponents of phase transitions occurring in open systems.

Enhancement of long-distance Casimir-Polder interaction between an excited atom and a cavity made of metamaterials.

Within the framework of macroscopic quantum electrodynamics, we investigate both the radiation force and the potential of Casimir-Polder type acting on an excited cold two-level atom in a cavity made of left-handed materials and topological insulators. As the time-reversal symmetry is broken on the surface of the topological insulators, the spontaneous emission of the atom placed near the focus point(s) exhibits anisotropic properties. While the potential wells are normally shallow for topological trivial dielectric, they may be amplified in the presence of topological magnetoelectric effect. We find that when there exists only one focus point in the cavity, it is possible to boost the forces or the potential wells by up to one order of magnitude. Meanwhile, the lifetime of the atom could be prolonged owing to the focus effect of the left-handed materials, where the emitted photons can trace back to the atom and reabsorbed by itself. Our results indicate the possibility in forming long-lived potential wells, which may have potential applications in trapping and guiding cold atoms far away from the surface.

Direct determination of mode-projected electron-phonon coupling in the time domain.

Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of nonthermal electrons, a material's dominant scattering processes can be revealed. Here, we present a method for extracting the electron-phonon coupling strength in the time domain, using time- and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the dynamics of photoinjected electrons at the [Formula: see text] point, detecting quantized energy-loss processes that correspond to the emission of strongly coupled optical phonons. We show that the observed characteristic time scale for spectral weight transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements for specific modes.

A timely look into electron-phonon coupling

Solid-State Physics The coupling between electrons and phonons—lattice vibrations in solids—is responsible for macroscopic quantum phenomena such as superconductivity. Yet, experimentally measuring this coupling as a function of momentum and for a particular phonon mode is tricky. Na et al. used time- and angle-resolved photoemission spectroscopy to excite electrons in graphite and monitor their decay, which was accompanied by the release of phonons. The time constants of these decay processes provided direct information on electron-phonon couplings in this system. Science , this issue p. [1231][1] [1]: /lookup/doi/10.1126/science.aaw1662

Generating macroscopic quantum superposition and a phonon laser in a hybrid optomechanical system

The generation of a superposition state of a single-mode or two-mode coherent state is always an attractive topic. We propose schemes to generate the Schrödinger cat state and entangled coherent state of a mechanical oscillator in a hybrid optomechanical system. By introducing time-dependent coupling between the atom and photon, an effective Hamiltonian is deduced, where a tripartite interaction with time-dependent coupling is achieved. The effect of relatively high free energy can be cancelled so that we can obtain a relatively large average phonon number. Including dissipation, we derive an analytic solution of the system and further discuss the decoherence and disentanglement. In addition, a steady phonon laser can be reached.

Floquet dynamics in the quantum measurement of mechanical motion

The radiation-pressure interaction between one or more laser fields and a mechanical oscillator gives rise to a wide range of phenomena: from sideband cooling and backaction-evading measurements to pondermotive and mechanical squeezing to entanglement and motional sideband asymmetry. In many protocols, such as dissipative mechanical squeezing, multiple lasers are utilized, giving rise to periodically driven optomechanical systems. Here we show that in this case, Floquet dynamics can arise due to presence of Kerr-type nonlinearities, which are ubiqitious in optomechanical systems. Specifically, employing multiple probe tones, we perform sideband asymmetry measurements, a macroscopic quantum effect, on a silicon optomechanical crystal sideband-cooled to 40% ground-state occupation. We show that the Floquet dynamics, resulting from the presence of multiple pump tones, gives rise to an artificially modified motional sideband asymmetry by redistributing thermal and quantum fluctuations among the initially independently scattered thermomechanical sidebands. For pump tones exhibiting large frequency separation, the dynamics is suppressed and accurate quantum noise thermometry demonstrated. We develop a theoretical model based on Floquet theory that accurately describes our observations. The resulting dynamics can be understood as resulting from a synthetic gauge field among the Fourier modes, which is created by the phase lag of the Kerr-type response. This novel phenomenon has wide-ranging implications for schemes utilizing several pumping tones, as commonly employed in backaction-evading measurements, dissipative optical squeezing, dissipative mechanical squeezing and quantum noise thermometry. Our observation may equally well be used for optomechanical Floquet engineering, e.g. generation of topological phases of sound by periodic time-modulation.

Macroscopic quantum coherent effects induced by electric current in dynamics of lanthanide based single molecule toroics

The dynamics of a quantum system with a large toroidal moment under the influence of a time-dependent electric current coupled with the toroidal moment is considered. The possibility of observing the quasi-anyonic excitations, the Bloch type oscillations in the precession motion of the toroidal moment, the Stark-type resonances, and the tunnel transitions between different precession modes are discussed.

Large spatial Schrödinger cat state using a levitated ferrimagnetic nanoparticle

The superposition principle is one of the main tenets of quantum mechanics. Despite its counter-intuitiveness, it has been experimentally verified using electrons, photons, atoms, and molecules. However, a similar experimental demonstration using a nano or a micro particle is non-existent. Here in this article, exploiting macroscopic quantum coherence and quantum tunneling, we propose an experiment using a levitated magnetic nanoparticle to demonstrate such an effect. It is shown that the spatial separation between the delocalized wavepackets of a 20 nm ferrimagnetic yttrium iron garnet (YIG) nanoparticle can be as large as 5 μm. We argue that, in addition to using for testing one of the most fundamental aspects of quantum mechanics, this scheme can simultaneously be used to test different modifications, such as wavefunction collapse models, to the standard quantum mechanics. Furthermore, we show that the spatial superposition of a core–shell structure, a YIG core and a non-magnetic silica shell, can be used to probe quantum gravity.

A new approach to study the Zeno effect for a macroscopic quantum system under frequent interactions with a harmonic environment

Quantum Zeno and anti-Zeno behaviors of a two-level macroscopic quantum system in interaction with a harmonic environment are studied using the perturbation theory. The system-environment interactions are applied in a successive and step-by-step way. A new expression for the probability of surviving the macrosystem in its initial state, after the Nth interaction step, is derived through a different method of calculation. It is shown that multiple transitions between Zeno and anti-Zeno behaviors can be found in our approach. We have shown that in addition to the environmental parameters like the Ohmicity, the macroscopic trait of the system also has a notable effect on the decay rate Γ(τ). Moreover, we have investigated how the decay rate Γ(τ) varies as the parameter of the macroscopicity of the system tilde{{boldsymbol{h}}} changes continuously in a given domain.

Engineering of strong mechanical squeezing via the joint effect between Duffing nonlinearity and parametric pump driving

Previous works for achieving mechanical squeezing focused mainly on the sole squeezing manipulation method. Here we study how to construct strong steady-state mechanical squeezing via the joint effect between Duffing nonlinearity and parametric pump driving. We find that the 3 dB limit of strong mechanical squeezing can be easily overcome from the joint effect of two different below 3 dB squeezing components induced by Duffing nonlinearity and parametric pump driving, respectively, without the need of any extra technologies, such as quantum measurement or quantum feedback. We first demonstrate that, in the ideal mechanical bath, the joint squeezing effect just is the superposition of the two respective independent squeezing components. The mechanical squeezing constructed by the joint effect is fairly robust against the mechanical thermal noise. Moreover, different from previous mechanical squeezing detection schemes, which need to introduce an additional ancillary cavity mode, the joint mechanical squeezing effect in the present scheme can be directly measured by homodyning the output field of the cavity with an appropriate phase. The joint idea opens up a new approach to construct strong mechanical squeezing and can be generalized to realize other strong macroscopic quantum effects.

Spin-rotation mode in a quantum Hall ferromagnet

A spin-rotation mode emerging in a quantum Hall ferromagnet due to laser pulse excitation is studied. This state, macroscopically representing a rotation of the entire electron spin-system to a certain angle, is not microscopically equivalent to a coherent turn of all spins as a single-whole and is presented in the form of a combination of eigen quantum states corresponding to all possible Sz spin numbers. The motion of the macroscopic quantum state is studied microscopically by solving a non-stationary Schrödinger equation and by means of a kinetic approach where damping of the spin-rotation mode is related to an elementary process, namely, transformation of a ‘Goldstone spin exciton’ to a ‘spin-wave exciton’. The system exhibits a spin stochastization mechanism (determined by spatial fluctuations of the Landé factor) ensuring damping, transverse spin relaxation, but irrelevant to decay of spin-wave excitons and thus not involving longitudinal relaxation, i.e. recovery of the Sz number to its equilibrium value.

Protected cat states from kinetic driving of a boson gas

We investigate the behavior of a one-dimensional Bose-Hubbard gas in both a ring and a hard-wall box, whose kinetic energy is made to oscillate with zero time-average, which suppresses first-order particle hopping. For intermediate and large driving amplitudes the system in the ring has similarities to the Richardson model, but with a peculiar type of pairing and an attractive interaction in momentum space. This analogy permits an understanding of some key features of the interacting boson problem. The ground state is a macroscopic quantum superposition, or cat state, of two many-body states collectively occupying opposite momentum eigenstates. Interactions give rise to a reduction (or modified depletion) cloud that is common to both macroscopically distinct states. Symmetry arguments permit a precise identification of the two orthonormal macroscopic many-body branches which combine to yield the ground state. In the ring, the system is sensitive to variations of the effective flux but in such a way that the macroscopic superposition is preserved. We discuss other physical aspects that contribute to protect the cat-like nature of the ground state.