Macroscopic Quantum Research Articles

On Black Holes as Macroscopic Quantum Objects

The relative flow of the Schwarzschild vs. the proper time during the classical evolution of a collapsing shell in the Schwarzschild coordinates practically forces us to interpret black hole formation as a highly non-local quantum process in which a shell/anti-shell pair is created within the incipient horizon, thus canceling out the original collapsing shell exactly at the horizon. By studying quantum fields in the black hole background, we reveal similar non-local effects. Among other things, the outgoing member of the Hawking pair very quickly becomes entangled with the black hole geometry (and not its partner), which is in contrast with the usual assumption that the Hawking pair is maximally entangled according to the local geometry near the horizon. Also, an infalling wave affects the black hole geometry even before it crosses the horizon. Finally, we find that a particle takes a finite amount of time to tunnel in and out of the black hole horizon, and thus avoids infinite blue and redshift in processes happening exactly at the horizon. These findings strongly support the picture of a black hole as a macroscopic quantum object.

Anomalous metals: From “failed superconductor” to “failed insulator”

Resistivity saturation is found on both superconducting and insulating sides of an "avoided" magnetic-field-tuned superconductor-to-insulator transition (H-SIT) in a two-dimensional In/InOx composite, where the anomalous metallic behavior cuts off conductivity or resistivity divergence in the zero-temperature limit. The granular morphology of the material implies a system of Josephson junctions (JJs) with a broad distribution of Josephson coupling EJ and charging energy EC, with an H-SIT determined by the competition between EJ and EC. By virtue of self-duality across the true H-SIT, we invoke macroscopic quantum tunneling effects to explain the temperature-independent resistance where the "failed superconductor" side is a consequence of phase fluctuations and the "failed insulator" side results from charge fluctuations. While true self-duality is lost in the avoided transition, its vestiges are argued to persist, owing to the incipient duality of the percolative nature of the dissipative path in the underlying random JJ system.

Signatures of Majorana bound states and parity effects in two-dimensional chiral p -wave Josephson junctions

We characterize topological features of Josephson junctions formed by coupled mesoscopic chiral $p$-wave superconducting islands. Through analytic and numerical studies of the low-lying BdG (Bogoliubov-de Gennes) spectrum, we identify localized MBS (Majorana bound states) nucleated in Josephson vortices by the application of a perpendicular magnetic field. Additionally, we demonstrate the existence of an extended MBS that is delocalized around the outer perimeter of the coupled islands, which has measurable consequences on the Josephson supercurrent and phase dynamics of the junction. In particular, we predict a change in the critical current diffraction pattern in which the odd integer-flux nodes are lifted in a fermion parity-dependent fashion. We model the competing stochastic effects of thermal noise and macroscopic quantum tunneling within the RCSJ framework and show the emergence of a bimodal critical current distribution. We demonstrate that increasing the parity transition rate suppresses the bimodal nature of the distribution, thus strongly emphasizing the non-trivial parity dependent nature of the many-body ground state. Finally, we consider a trijunction geometry with three islands and discuss possible schemes to braid Majorana bound states by moving the Josephson vortices to which they are bound.

Interaction Between Macroscopic Quantum Systems and Gravity

We review experiments and theoretical models about the possible mutual interplay between the gravitational field and materials in the superconducting state or other macroscopic quantum states. More generally, we focus on the possibility for quantum macrosystems in a coherent state to produce local alterations of the gravitational field in which they are immersed. This fully interdisciplinary research field has witnessed a conspicuous progress in the last decades, with hundreds of published papers, and yet several questions are still completely open.

Controlling quantum coherence and entanglement in cavity magnomechanical systems

We investigate theoretically macroscopic quantum coherence and tripartite entanglement in a three-mode magnomechanical system with a yttrium iron garnet sphere, which is simultaneously driven by a magnetic pump and a microwave field pump to establish an effective magnomechanical coupling. We discuss mainly the dependence of quantum properties on the driving power, the cavity field detuning and decay, the magnomechanical coupling strength, the bias magnetic field, and the temperature of an environment. It is found that the quantum coherence of the system can be attained easily in a large range of parameters. In contrast, tripartite photon-magnon-phonon entanglement can only be realized in a relatively small range of optical and magnetic parameters. In particular, the quantum coherence of the mechanical mode can be larger than that of the photon and magnon modes, which means that the environmental incoherence of the mechanical mode in the cavity magnomechanical system can be suppressed significantly. These results may help manipulate simultaneously the macroscopic quantum coherence and the tripartite entanglement of massive objects in coupled magnomechanical system, which may have potential applications for realizing highly tunable information processing through the flexible control of quantum properties.

Entanglement Between Two Mechanical Oscillators in a Dual-Coupling Optomechanical System

In this study, we investigated the properties of entanglement of an optomechanical system including two mechanical oscillators, where two mechanical oscillators are coupled with the cavity field in different coupling modes: the interaction between one mechanical oscillator and cavity field is first-order coupling, and the interaction between the other and the cavity field is quadratic coupling. On the one hand, the entanglement between two mechanical oscillators is studied. The influence of the frequency of the cavity field, the mass of mechanical oscillators, and the temperature of mechanical oscillators on the entanglement between two mechanical oscillators are investigated. On the other hand, for the given parameters of the system, we also obtained the entanglement between the cavity field and the mechanical oscillator that has a quadratic coupling. Compared with the entanglement between microscopic particles, the stable macroscopic quantum entanglement between two mechanical oscillators has the characteristics of long existence life and can be reused, and it also plays an important role in quantum information processing and quantum network construction.

Slowing quantum decoherence of oscillators by hybrid processing

Quantum information encoded into the superposition of coherent states is an illustrative representation of practical applications of macroscopic quantum coherence possessing. However, these states are very sensitive to energy loss, losing their non-classical aspects of coherence very rapidly. An available deterministic strategy to slow down this decoherence process is to apply a Gaussian squeezing transformation prior to the loss as a protective step. Here, we propose a deterministic hybrid protection scheme utilizing strong but feasible interactions with two-level ancillas immune to spontaneous emission. We verify the robustness of the scheme against the dephasing of qubit ancilla. Our scheme is applicable to complex superpositions of coherent states in many oscillators, and remarkably, the robustness to loss is enhanced with the amplitude of the coherent states. This scheme can be realized in experiments with atoms, solid-state systems, and superconducting circuits.

Nonlinear two-level dynamics of quantum time crystals

A time crystal is a macroscopic quantum system in periodic motion in its ground state. In our experiments, two coupled time crystals consisting of spin-wave quasiparticles (magnons) form a macroscopic two-level system. The two levels evolve in time as determined intrinsically by a nonlinear feedback, allowing us to construct spontaneous two-level dynamics. In the course of a level crossing, magnons move from the ground level to the excited level driven by the Landau-Zener effect, combined with Rabi population oscillations. We demonstrate that magnon time crystals allow access to every aspect and detail of quantum-coherent interactions in a single run of the experiment. Our work opens an outlook for the detection of surface-bound Majorana fermions in the underlying superfluid system, and invites technological exploitation of coherent magnon phenomena – potentially even at room temperature.

Extraordinary optical transmission from a thin microcavity by macroscopic quantum effect

The macroscopic quantum effect is revealed to elaborate the extraordinary optical transmission (EOT) from a subwavelength thin microcavity based on the uncertainty property of the transmitted electromagnetic fields after the aperture. A critical radius is found in the thin microcavity under a certain incident electromagnetic wavelength. With the aperture radius varying, the transmitted field can be divided into three regimes: I) the macroscopic quantum regime when the aperture radius is less than the critical radius, in which the field edge effect occurs and the EOT phenomenon is perfectly manifested; II) the wave-particle duality regime in the vicinity of the critical radius, in which the edge effect and diffraction phenomenon exist simultaneously; III) the wave regime when the aperture radius is greater than the critical radius, in which the near-field diffraction emerges. In addition, the influences of incident wavelength and microcavity thickness on EOT are also investigated. Our research has potential applications in advanced optical devices, such as light switch and optical manipulations.

Suppression of Macroscopic Quantum Effects in Arrays with Spiral Nanotubes Associated with the Dispersion of Their Characteristic Sizes

Suppression of Macroscopic Quantum Effects in Arrays with Spiral Nanotubes Associated with the Dispersion of Their Characteristic Sizes

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.

Medium vacuum feasible displacement calibration of an optically levitated Duffing nonlinear oscillator

Optical levitation in vacuum has been proved as a promising platform for precision measurement and research on microscopic thermodynamics and the macroscopic quantum system. The performance of these applications is highly determined by the detection of the trapped particle's displacement, which is linked with the detection signal through a calibration coefficient. However, it is still a challenge to get the calibration coefficient in medium vacuum. Here, we present a nonlinear method to obtain the calibration coefficient with high accuracy to fill the pressure gap between 5×10−3 and 50 mbar. By averaging the trajectory segments with the same initial displacement, we analyze the relationship between the nonlinearity induced frequency shift and the amplitude, where the calibration coefficient can be obtained without a priori knowledge about mass and temperature of the trapped particle or the assistance with external force. We further apply this method to detect the mass and density of the trapped nanoparticle and observe the mass variation under different pressures. Therefore, it will pave the way to study the physical processes in medium vacuum based on the precision measurement of nanoparticle's mass, density, and other properties.

The archetypal molecular patterns of conscious experience are quantum analogs

We define quantum analogs as vibrational excitations of quasiparticles coupled to electromagnetically-mediated resonance energy transfer in water (a crystal lattice). This paper addresses how neural magnetic resonance spectra of the brain’s magnetic field influence dipolar oscillation waves in crystal lattices of interfacial water molecules to produce correlates of phenomenal consciousness. We explore dipolar oscillation waves in hydrophobic protein cavities of aromatic amino acids as a conduit for coherent propagation of vibrational excitation and hydrogen bond distortion associated with phase coherence present in the magnetic field intensity oscillations at a frequency at which the energy switches from its trapped form as excited phonon states to free, cavity-mode magnetic field energy states. A quasi-polaritons that reflect “hydro-ionic waves” is a macroscopic quantum effect of crystal lattice vibrations, consisting of vibron polaritons coupled to ions across the neocortex, except the cerebellum, due to the absence of protein-protein interactions. They are quantum-like at the core and hence can exhibit quantum-like signaling properties when resonant energy is transferred as dipolar waves in hydrophobic protein cavities of aromatic amino acids. This is due to aromatic residue flexibility in molecular electromagnetic resonances. Finally, the archetypal molecular patterning of conscious experiences, which carries an inherent ambiguity necessary for non-contextually applying ‘meaning’ that encompasses cognitive signatures of conscious experience, satisfies the nature of quantum analogs and their transmutative properties.

Measurement-based preparation of multimode mechanical states.

Nanomechanical resonators are a key tool for future quantum technologies, such as quantum force sensors and interfaces, and for studies of macroscopic quantum physics. The ability to prepare room temperature nonclassical states is a major outstanding challenge. It has been suggested that this could be achieved using a fast continuous measurement to break the usual symmetry between position and momentum. Here, we demonstrate this symmetry breaking and use it to prepare a thermally squeezed mechanical state. Our experiments take advantage of collective measurements on multiple mechanical modes, which we show can increase the measurement speed and improve state preparation. Theoretically, we show that this result extends to the quantum regime, relaxing the requirements to generate nonclassical states. We predict that multimode conditioning can enable room temperature quantum squeezing with existing technology. Our work paves the way toward room temperature quantum nanomechanical devices and toward their application in quantum technology and fundamental science.

Polariton Bose-Einstein condensate from a bound state in the continuum.

Bound states in the continuum (BICs)1-3 are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone4. These BICs possess an invariant topological charge given by the winding number of the polarization vectors5, similar to vortices in quantum fluids such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not yet been fully exploited. Here we show non-equilibrium Bose-Einstein condensation of polaritons-hybrid light-matter excitations-occurring in a BIC thanks to its peculiar non-radiative nature, which favours polariton accumulation. The combination of the ultralong BIC lifetime and the tight confinement of the waveguide geometry enables the achievement of an extremely low threshold density for condensation, which is reachednot in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we reveal ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits.

Strong Angular-Momentum Optomechanical Coupling for Macroscopic Quantum Control

Optomechanical systems offer unique opportunities to explore macroscopic quantum state and related fundamental problems in quantum mechanics. Here, we propose a quantum optomechanical system involving exchange interaction between spin angular momentum of light and a torsional oscillator. We demonstrate that this system allows coherent control of the torsional quantum state of a torsional oscillator on the single-photon level, which facilitates efficient cooling and squeezing of the torsional oscillator. Furthermore, the torsional oscillator with a macroscopic length scale can be prepared in Schr\"odinger catlike state. Our work provides a platform to verify the validity of quantum mechanics in macroscopic systems on the micrometer and even centimeter scale.

Duality, decay rates, and local-field models in macroscopic QED

Any treatment of magnetic interactions between atoms, molecules, and optical media must start at the form of the interaction energy. This forms the base on which predictions about any number of magnetic atom-light properties stands, spontaneous decay rates and forces included. As is well known, the Heaviside-Larmor duality symmetry of Maxwell's equations, where electric and magnetic quantities are exchanged, is broken by the usual form of the magnetic interaction energy. We argue that this symmetry can be restored by including general local-field effects and that local fields should be treated as a necessity for correctly translating between the microscopic world of the dipole and the macroscopic world of the measured fields. This may additionally aid in resolving a long-standing debate over the form of the force on a dipole in a medium. Finally, we compute the magnetic dipole decay rate in a magnetodielectric with local-field effects taken into account and show that macroscopic quantum electrodynamics can be made to be dual symmetric at an operator level, instead of only for expectation values.

Room-temperature polariton lasing in GaN microrods with large Rabi splitting.

Room-temperature polariton lasing is achieved in GaN microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting (Ω = 2g0) up to 162 meV, exceeding the results from both the state-of-the-art nitride-based planar microcavities and previously reported GaN microrods. An ultra-low threshold of 1.8 kW/cm2 is observed by power-dependent photoluminescence spectra, with the linewidth down to 1.31 meV and the blue shift up to 17.8 meV. This large Rabi splitting distinguishes our coherent light emission from a conventional photon lasing, which strongly supports the preparation of coherent light sources in integrated optical circuits and the study of exciting phenomena in macroscopic quantum states.

Exponential Clustering of Bipartite Quantum Entanglement at Arbitrary Temperatures

Macroscopic quantum effects play central roles in the appearance of inexplicable phenomena in low-temperature quantum many-body physics. Such macroscopic quantumness is often evaluated using long-range entanglement, i.e., entanglement in the macroscopic length scale. The long-range entanglement not only characterizes the novel quantum phases but also serves as a critical resource for quantum computation. Thus, the problem that arises is under which conditions can the long-range entanglement be stable even at room temperatures. Here, we show that bi-partite long-range entanglement is unstable at arbitrary temperatures and exponentially decays with distance. Our theorem provides a no-go theorem on the existence of the long-range entanglement. The obtained results are consistent with the existing observations that long-range entanglement at non-zero temperatures can exist in topologically ordered phases, where tripartite correlations are dominant. In the derivation of our result, we introduce a quantum correlation defined by the convex roof of the standard correlation function. We establish an exponential clustering theorem for generic quantum many-body systems for such a quantum correlation at arbitrary temperatures, which yields our main result by relating quantum correlation to quantum entanglement. As a simple application of our analytical techniques, we derived a general limit on the Wigner-Yanase-Dyson skew information and the quantum Fisher information, which will attract significant attention in the field of quantum metrology. Our work reveals novel general aspects of low-temperature quantum physics and sheds light on the characterization of long-range entanglement.

Universality of Dicke superradiance in arrays of quantum emitters

Dicke superradiance is an example of emergence of macroscopic quantum coherence via correlated dissipation. Starting from an initially incoherent state, a collection of excited atoms synchronizes as they decay, generating a macroscopic dipole moment and emitting a short and intense pulse of light. While well understood in cavities, superradiance remains an open problem in extended systems due to the exponential growth of complexity with atom number. Here we show that Dicke superradiance is a universal phenomenon in ordered arrays. We present a theoretical framework – which circumvents the exponential complexity of the problem – that allows us to predict the critical distance beyond which Dicke superradiance disappears. This critical distance is highly dependent on the dimensionality and atom number. Our predictions can be tested in state of the art experiments with arrays of neutral atoms, molecules, and solid-state emitters and pave the way towards understanding the role of many-body decay in quantum simulation, metrology, and lasing.