Macroscopic Quantum Phenomena Research Articles

Effect of laser phase noise on the steady-state field-mirror entanglement and ground-state cooling in a Laguerre-Gaussian optorotational system

Cavity optomechanical systems are considered as one of the best platforms for studying macroscopic quantum phenomena. In this paper, we studied the effect of laser phase noise on the steady-state entanglement between a cavity mode and a rotating mirror in a Laguerre-Gaussian (L-G) optorotational system. We found that the effect of laser phase noise was non-negligible on the field-mirror entanglement especially at a larger input power and a larger angular momentum. We also investigated the influence of laser phase noise on the ground-state cooling of the rotating mirror. In the presence of laser phase noise, the ground-state cooling of the rotating mirror can still be realized within a range of input powers.

Electric‐Field‐Induced in‐Plane Anisotropy of Superconducting Transition Temperature for FeSe0.5Te0.5 Thin Films

AbstractSuperconductor/ferroelectric heterostructures are very important for fundamental research and applications since superconductivity can be controlled by electric fields via ferroelectric polarization/piezostrain. Superconductivity is a macroscopic quantum phenomenon and it is commonly believed that the in‐plane superconducting transition temperature (Tc) should be isotropic for superconductors with crystal structures of stacking layers (quasi 2D). Here, the observation of piezostrain‐induced in‐plane anisotropy of Tc in FeSe0.5Te0.5 (FST) thin films grown on ferroelectric single crystals is reported. The as‐prepared FST shows a tetragonal phase with equal in‐plane lattice parameters and an absence of in‐plane anisotropy of Tc. Upon applying electric fields, piezostrain induces a difference in the in‐plane lattice parameters (a‐b) and in‐plane anisotropy of Tc. The in‐plane anisotropy of Tc correlates with a‐b and becomes more remarkable for larger values of a‐b. Some possible extrinsic effects are ruled out by experiments and analysis. A possible mechanism is proposed involving electronic nematicity and inhomogeneous superconductivity in FST to account for this unusual phenomenon. This work is significant for uncovering the exotic nature of unconventional superconductors, as well as applications.

Radiative loss of coherence in free electrons: a long-range quantum phenomenon

Quantum physics rules the dynamics of small objects as they interact over microscopic length scales. Nevertheless, quantum correlations involving macroscopic distances can be observed between entangled photons as well as in atomic gases and matter waves at low temperatures. The long-range nature of the electromagnetic coupling between charged particles and extended objects could also trigger quantum phenomena over large distances. Here, we reveal a manifestation of quantum mechanics that involves macroscopic distances and results in a nearly complete depletion of coherence associated with which-way free-electron interference produced by electron–radiation coupling in the presence of distant extended objects. This is a ubiquitous effect that we illustrate through a rigorous theoretical analysis of a two-path electron beam interacting with a semi-infinite metallic plate and find the inter-path coherence to vanish proportionally to the path separation at zero temperature and exponentially at finite temperature. The investigated regime of large distances originates in the coupling of the electron to radiative modes assisted by diffraction at material structures but without any involvement of material excitations. Besides the fundamental interest of this macroscopic quantum phenomenon, our results suggest an approach to measuring the vacuum temperature and nondestructively sensing the presence of distant objects.

Ground state of Rydberg-dressed Bose gas confined in a toroidal trap

The experimental realization of Rydberg dressing technology in ultracold atomic systems provides another superior platform for studying novel states of matter and macroscopic quantum phenomena. In this work, based on the mean-field theory, we have investigated the ground-state phases of a two-component Bose–Einstein condensate with Rydberg interaction and confined in a toroidal trap. The effects of the Rydberg interaction and external potential, especially the Rydberg blockade radius, on the ground-state structure of such a system have been investigated in full parameter space. Our results show that the Rydberg blockade radius, which can be regarded as another controllable parameter, can be used to obtain a variety of ground-state phases. More interestingly, it is found that for weak Rydberg interactions, the Rydberg blockade radius breaks the spontaneous rotational symmetry of the system, leading to the formation of a discrete unit cell structure. For strongly interacting cases, it can be used to realize different orders of discrete rotational symmetry breaking.

High sensitivity of semimetal photodetection via Bose–Einstein condensation

AbstractThe discovery of semiconductor has witnessed remarkable strides toward high performance of photodetectors attributed to its excellent carrier properties. However, semimetal, owning to the high carrier concentration and low carrier mobility compared to those of semiconductor, is generally considered unsuitable for photodetection. Herein, we demonstrate an outstanding photodetection in a layered semimetal titanium diselenide (TiSe2) in Bose–Einstein condensation (BEC) state. High sensitivity of semimetal photodetector is realized in the range of visible, infrared and terahertz bands. The noise equivalent power (NEP) has threefold improvement at the visible and infrared wavebands, and significant decrease by one order of magnitude in the terahertz frequencies via BEC phenomenon, attributed to the electrical parameter variation after condensation. The best NEP value in the terahertz frequency is comparable to that of commercial Si photodetector. Our results show another recipe to fabricate high performance of photodetection via semimetal except for semiconductor and pave the way to exploit macroscopic quantum phenomena for optoelectronics.image

Entanglement and Symmetry Structure of N(= 3) Quantum Oscillators with Disparate Coupling Strengths in a Common Quantum Field Bath

In this paper, we study the entanglement structure of a system of N quantum oscillators with distinctive coupling strengths, all linearly coupled to a common massless scalar quantum field. This study is helpful in characterizing the notion of an entanglement domain and its symmetry features, which is useful for understanding the interplay between different levels of structure in many-body quantum systems. The effect of the quantum field on the system is derived via the influence functional and the correlation functions are obtained from the solutions of the evolutionary operator of the reduced density matrix. They are then used to construct the covariance matrix, which forms the basis for our analysis of the structure of quantum entanglement in this open system. To make the physical features explicit, we consider a system of three quantum coupled oscillators placed at the vertices of an equilateral triangle with disparate pairwise couplings. We analyze the entanglement between one oscillator and the other two with equal (symmetric) and unequal (asymmetric) coupling strengths. As a physical illustration, we apply the results for these two different configurations to address some basic issues in macroscopic quantum phenomena from the quantum entanglement perspective.

Vortex-lattice structures in rotating Bose-Fermi superfluid mixtures

The system of Bose-Fermi superfluid mixture offers a playground to explore rich macroscopic quantum phenomena. In a recent experiment of Yao et al. (2016) [35], 41K-6Li superfluid mixture is implemented. Coupled quantized vortices are generated via rotating the superfluid mixture, and a few unconventional behaviors on the formations of vortex numbers are observed, which can be traced to boson-fermion interactions. Here we provide a theoretical insight into the unconventional behaviors observed in the experiment. To this end, the orbital-free density functional theory is hired, and its utility is validated by making comparison of the numerical results and a microscopic theory for vortex lattices in strongly interacting Fermi superfluids alone. We also predict interesting phenomena which can be readily explored experimentally, including the novel structures of bosonic-fermionic vortices, and attractive interactions between the bosonic and fermionic vortices.

Scrutinizing the feasibility of macroscopic quantum coherence in the brain: a field-theoretical model of cortical dynamics

The neural activity patterns associated with advanced cognitive processes are characterized by a high degree of collective organization, which raises the question of whether macroscopic quantum phenomena play a significant role in cortical dynamics. In order to pursue this question and scrutinize the feasibility of macroscopic quantum coherence in the brain, a model is developed regarding the functioning of microcolumns, which are the basic functional units of the cortex. This model assumes that the operating principle of a microcolumn relies on the interaction of a pool of neurotransmitter (glutamate) molecules with the vacuum fluctuations of the electromagnetic field, termed zero-point field (ZPF). Quantitative calculations reveal that the coupling strength of the glutamate pool to the resonant ZPF modes lies in the critical regime in which the criterion for the initiation of a phase transition is fulfilled, driving the ensemble of initially independent molecules toward a coherent state and resulting in the formation of a coherence domain that extends across the full width of a microcolumn. The formation of a coherence domain turns out to be an energetically favored state shielded by a considerable energy gap that protects the collective state against thermal perturbations and entails decoherence being greatly slowed down. These findings suggest that under the special conditions encountered in cortical microcolumns the emergence of macroscopic quantum phenomena is feasible. This conclusion is further corroborated by the insight that the presence of a coherence domain gives rise to downstream effects which may be crucial for the cortical communication and the formation of large-scale activity patterns. Taken together, the presented model sheds new light on the fundamental mechanism underlying cortical dynamics and suggests that long-range synchronization in the brain results from a bottom-up orchestration process involving the ZPF.

Entanglement entropy as an order parameter for strongly coupled nodal line semimetals

Topological semimetals are a class of many-body systems exhibiting novel macroscopic quantum phenomena at the interplay between high energy and condensed matter physics. They display a topological quantum phase transition (TQPT) which evades the standard Landau paradigm. In the case of Weyl semimetals, the anomalous Hall effect is a good non-local order parameter for the TQPT, as it is proportional to the separation between the Weyl nodes in momentum space. On the contrary, for nodal line semimetals (NLSM), the quest for an order parameter is still open. By taking advantage of a recently proposed holographic model for strongly-coupled NLSM, we explicitly show that entanglement entropy (EE) provides an optimal probe for nodal topology. We propose a generalized c-function, constructed from the EE, as an order parameter for the TQPT. Moreover, we find that the derivative of the renormalized EE with respect to the external coupling driving the TQPT diverges at the critical point, signaling the rise of non-local quantum correlations. Finally, we show that these quantum information quantities are able to characterize not only the critical point but also features of the quantum critical region at finite temperature.

Layer-dependent correlated phases in WSe2/MoS2 moiré superlattice.

Electron correlation plays an essential role in the macroscopic quantum phenomena in the moiré heterostructure, such as antiferromagnetism and correlated insulating phases. Unlike the phenomena where the interaction involves only electrons in one layer, the interaction of distinct phases in two or more layers represents a new horizon forward, such as the one in the Kondo lattice model. Here, using interlayer excitons as a probe, we show that the interlayer interactions in heterobilayers of tungsten diselenide and molybdenum disulfide (WSe2/MoS2) can be electrically switched on and off, resulting in a layer-dependent correlated phase diagram, including single-layer, layer-selective, excitonic-insulator and layer-hybridized regions. We demonstrate that these correlated phases affect the interlayer exciton non-radiative decay pathways. These results reveal the role of strong correlation on interlayer exciton dynamics and pave the way for studying the layer-resolved strong correlation behaviour in moiré heterostructures.

Nanophotonics of microcavity exciton–polaritons

The past 30 years have witnessed remarkable developments of microcavity exciton–polaritons, which have made a great impact on photonics and optoelectronics from fundamental physics to device applications. New materials and optical structures have been developed for novel polariton lasers for the sake of room temperature operation, flexible mode engineering, and high power efficiency. More powerful spectroscopic techniques have also promoted the understanding of polariton dynamics, coherence, nonlinearity, and topology. In this review, we start with a brief introduction to the picture of polaritons, and various polariton systems based on different microcavity structures and semiconductor materials. Then, we present several important spectroscopic techniques and numerical tools for characterizing polaritons experimentally and theoretically. Next, we address the macroscopic quantum phenomena observed in the polariton systems and review the physics and applications of polariton nonlinearity. Moreover, we highlight the new emerging fields of topological and non-Hermitian polaritons. In the end, we conclude with the future perspectives of microcavity exciton–polaritons.

Resolution of 100 photons and quantum generation of unbiased random numbers

Macroscopic quantum phenomena, such as observed in superfluids and superconductors, have led to promising technological advancements and some of the most important tests of fundamental physics. At present, quantum detection of light is mostly relegated to the microscale, where avalanche photodiodes are very sensitive to distinguishing single-photon events from vacuum but cannot differentiate between larger photon-number events. Beyond this, the ability to perform measurements to resolve photon numbers is highly desirable for a variety of quantum information applications, including computation, sensing and cryptography. True photon-number resolving detectors do exist, but they are currently limited to the ability to resolve on the order of 10 photons, which is too small for several quantum-state generation methods based on heralded detection. Here we extend photon measurement into the mesoscopic regime by implementing a detection scheme based on multiplexing highly quantum-efficient transition-edge sensors to accurately resolve photon numbers between 0 and 100. We then demonstrate the use of our system by implementing a quantum random-number generator with no inherent bias. This method is based on sampling a coherent state in the photon-number basis and is robust against environmental noise, phase and amplitude fluctuations in the laser, loss and detector inefficiency as well as eavesdropping. Beyond true random-number generation, our detection scheme serves as a means to implement quantum measurement and engineering techniques valuable for photonic quantum information processing. A spatially multiplexed detection system of three transition-edge sensors is developed to resolve photon numbers up to 100 in a single laser pulse. Using the detector to measure parity of a coherent state allows for the extraction of quantum random numbers.

Levitated optomechanics: From single to many-body physics

The levitated optomechanics, because of its ultra-high mechanical Q > 1010, is considered to be one of the best testbeds for macroscopic quantum superpostions. In this perspective, we give a brief review on the development of the levitated optomechanics, focusing on the macroscopic quantum phenomena, and the applications in quantum precision measurement. The levitated nanodiamond with built-in nitrogen-vacancy centers is discussed as an example. Finally, we discuss the future dirctions of the levtated optomechanics, such as the space-based experiments, the arrays of levitated optomechanics and applications in quantum simulation.

Entangling two cavity modes and squeezing magnon mode via parametric down-conversion

A scheme to entangle two cavity modes and squeeze magnon mode in a magnon–cavity QED system is presented, where the two microwave cavity modes are coupled to a massive yttrium iron garnet sphere. The nonlinearity used in our system originates from parametric down-conversion. By using the mean field approximation and employing experimentally feasible parameters, we indicate that the entanglement between squeezed cavity mode and magnon mode can be transferred to the other cavity mode and magnon mode, and then the two cavity modes get entangled. Meanwhile, the magnon mode is squeezed in our QED system. Furthermore, we show that it is a good way to enhance entanglement and squeezing by increasing the nonlinear gain. Our results denote that magnon–cavity QED system is a powerful platform for studying macroscopic quantum phenomena, which illustrates a new approach to photon–photon entanglement and magnon squeezing.

Multimode optomechanical cooling via general dark-mode control

The dark-mode effect is a stubborn obstacle for ground-state cooling of multiple degenerate mechanical modes optomechanically coupled to a common cavity-field mode. Here we propose an auxiliary-cavity-mode method for simultaneous ground-state cooling of two degenerate or near-degenerate mechanical modes by breaking the dark mode. We find that the introduction of the auxiliary cavity mode not only breaks the dark-mode effect, but also provides a new cooling channel to extract the thermal excitations stored in the dark mode. Moreover, we study the general physical-coupling configurations for breaking the dark mode in a generalized network-coupled four-mode optomechanical system consisting of two cavity modes and two mechanical modes. We find the analytical dark-mode-breaking condition in this system. This method is general and it can be generalized to break the dark-mode effect and to realize the simultaneous ground-state cooling in a multiple-mechanical-mode optomechanical system. We also demonstrate the physical mechanism behind the dark-mode breaking by studying the breaking of dark-state effect in the $N\text{\ensuremath{-}}\mathrm{type}$ four-level atomic system. Our results not only provide a general method to control various dark-mode and dark-state effects in physics, but also present an opportunity to the study of macroscopic quantum phenomena and applications in multiple-mechanical-resonator systems.

Magnon squeezing enhanced ground-state cooling in cavity magnomechanics

Cavity magnomechanics has recently become a new platform for studying macroscopic quantum phenomena. The magnetostriction induced vibration mode of a large-size ferromagnet or ferrimagnet reaching its ground state represents a genuine macroscopic quantum state. Here we study the ground-state cooling of the mechanical vibration mode in a cavity magnomechanical system, and focus on the role of magnon squeezing in improving the cooling efficiency. The magnon squeezing is obtained by exploiting the magnon self-Kerr nonlinearity. We find that the magnon squeezing can significantly and even completely suppress the magnomechanical Stokes scattering. It thus becomes particularly useful in realizing ground-state cooling in the unresolved-sideband regime, where the conventional sideband cooling protocols become inefficient. We also find that the coupling to the microwave cavity plays only an adverse effect in mechanical cooling. This makes essentially the two-mode magnomechanical system (without involving the microwave cavity) a preferred system for cooling the mechanical motion, in which the magnon mode is established by a uniform bias magnetic field and a microwave drive field.

First-principles prediction of outstanding mechanical and thermodynamic properties of YX2Si2 (X = Pd and Rh) superconductors

Superconductivity is a macroscopic quantum phenomenon with unprecedented applications in high-efficient power transmission, electric motors, generators, transportation, particle accelerators, and computing. Although the superconducting capabilities of YX2Si2 (X = Pd, and Rh) compounds have been experimentally realized, the physical aspects of these two superconductors are yet to be understood. Herein, employing first-principle-based Density Functional Theory (DFT) calculations, we realized the excellent mechanical strength of the YRh2Si2 compound. Firstly, the estimated X-ray diffraction, as well as unit cell parameters of both superconductors, show excellent conformity with prior experimental data. Also, mechanical parameters which are most fundamental, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, anisotropy factor, ductility, and Vickers hardness factors of both superconductors have been evaluated. Despite the brittle nature, YRh2Si2 shows excellent mechanical strength compared to that of YPd2Si2. Besides, the assessment of the density of states, Mulliken atomic populations, and charge density map reveals a complex chemical bonding in both superconductors including ionic, covalent, and metallic features. Furthermore, charge density data suggest that Si-Si, Si-Pd, and Si-Rh bonds are indeed covalent while Pd-Pd and Rh-Rh demonstrate metallic features. Afterward, different optical parameters such as dielectric functions, reflectivity, absorption coefficient, refractive index, photoconductivity, and loss function of both superconductors have been anticipated. Notably, it has been unveiled that both superconductors show excellent absorption and photoconductivity in the ultraviolet region of the electromagnetic spectrum. Additionally, sound velocity, Debye temperature, melting point, and minimum thermal conductivity of both superconductors have been estimated from DFT computation. Interestingly, most of the thermodynamic parameters of YRh2Si2 are notably higher compared to those of the YPd2Si2 compound. Remarkably, the DFT results suggest that YRh2Si2 exhibits excellent mechanical as well as thermodynamic properties compared to other reported superconductors of identical classes.

Improving the Stochastic Feedback Cooling of a Mechanical Oscillator Using a Degenerate Parametric Amplifier

Cooling of a macroscopic mechanical resonator to extremely low temperatures is a necessary condition to observe a variety of macroscopic quantum phenomena. Here, we study the stochastic feedback cooling of a mechanical resonator in an optomechanical system with a degenerate optical parametric amplifier (OPA). In the bad-cavity limit, we find that the OPA can enhance the cooling of the movable mirror in the stochastic feedback cooling scheme. The movable mirror can be cooled from 132 mK to 0.033 mK, which is lower than that without the OPA by a factor of about 5.

Spinor Bose-Einstein condensates

Bose-Einstein (BE) condensation is a phenomenon in which a macroscopic number of particles occupy a single-particle state. Once a system undergoes BE condensation, a significant fraction of particles behave in lockstep, leading to a number of spectacular macroscopic quantum phenomena. This article delineates fundamental aspects of BE condensates with an emphasis on spin degrees of freedom. Here the gauge and spin degrees of freedom are coupled to produce a rich variety of spinor superfluidity and topological excitations.

Quantum simulation of interacting fermions

Fermions are basic building blocks in the standard model. Interactions among these elementary particles determine how they assemble and consequently form various states of matter in our nature. Simulating fermionic degrees of freedom is also a central problem in condensed matter physics and quantum chemistry, which is crucial to understanding high-temperature superconductivity, quantum magnetism and molecular structure and functionality. However, simulating interacting fermions by classical computing generically face the minus sign problem, encountering the exponential computation complexity. Ultracold atoms provide an ideal experimental platform for quantum simulation of interacting fermions. This highly-controllable system enables the realizing of nontrivial fermionic models, by which the physical properties of the models can be obtained by measurements in experiment. This deepens our understanding of related physical mechanisms and helps determine the key parameters. In recent years, there have been versatile experimental studies on quantum ground state physics, finite temperature thermal equilibrium, and quantum many-body dynamics, in fermionic quantum simulation systems. Quantum simulation offers an access to the physical problems that are intractable on the classical computer, including studying macroscopic quantum phenomena and microscopic physical mechanisms, which demonstrates the quantum advantages of controllable quantum systems. This paper briefly introduces the model of interacting fermions describing the quantum states of matter in such a system. Then we discuss various states of matter, which can arise in interacting fermionic quantum systems, including Cooper pair superfluids and density-wave orders. These exotic quantum states play important roles in describing high-temperature superconductivity and quantum magnetism, but their simulations on the classical computers have exponentially computational cost. Related researches on quantum simulation of interacting fermions in determining the phase diagrams and equation of states reflect the quantum advantage of such systems.