Bistable Quantum Research Articles

Quantum bistability in the hyperfine ground state of atoms

Quantum bistability in the hyperfine ground state of atoms

Unidirectional mode selection in bistable quantum cascade ring lasers.

Ideal ring resonators are characterized by travelling-wave counter-propagating modes, but in practice travelling waves can only be realized under unidirectional operation, which has proved elusive. Here, we have designed and fabricated a monolithic quantum cascade ring laser coupled to an active waveguide that allows for robust, deterministic and controllable unidirectional operation. Spontaneous emission injection through the active waveguide enables dynamical switching between the clockwise and counterclockwise states of the ring laser with as little as 1.6% modulation of the electrical input. We show that this behavior stems from a perturbation in the bistable dynamics of the ring laser. In addition to switching and bistability, our novel coupler design for quantum cascade ring lasers offers an efficient mechanism for outcoupling and light detection.

Bistability and chaos-assisted tunneling in dissipative quantum systems.

We revisit the problem of quantum bi- and multistability by considering the dissipative double resonance model. For a large driving frequency, this system has a simpler phase structure than the driven dissipative nonlinear oscillator, the paradigm model for classical and quantum bistability. This allows us to obtain an analytical estimate for the lifetime of quantum limit cycles. On the other hand, for a small driving frequency, the system is much richer than the nonlinear oscillator. This allows us to address a novel phenomenon of dissipation- and chaos-assisted tunneling between quantum limits cycles.

Qubit decoherence and symmetry restoration through real-time instantons

A parametrically driven quantum oscillator, stabilized by a nonlinear dissipation, exhibits a spontaneous breaking of the parity symmetry. It results in the quantum bi-stability, corresponding to a Bloch sphere of dark states. This makes such a driven-dissipative system an attractive candidate for a qubit. The parity symmetry breaking is exact both on the classical level and within the quantum mechanical perturbation theory. Here we show that non-perturbative quantum effects lead to the symmetry restoration and result in exponentially small but finite qubit decoherence rate. Technically the symmetry restoration is due to real time instanton trajectories of the Keldysh path integral, which represents the Lindbladian evolution of the driven-dissipative oscillator.

Bistability of squeezing and entanglement in cavity magnonics

We demonstrate how magnon Kerr nonlinearity creates bistability of quantum states when the magnon mode is strongly driven by a microwave field in a nonlinear cavity magnonic system. Numerical simulation results with experimentally feasible parameters show that the method where the system is driven far from equilibrium is a reliable way to achieve squeezed states for magnons and photons and to carry out magnon-photon entanglement, revealing mysterious phenomena of bistability. In addition, the Kittel mode can jump from one state to another one near two switching points, thus achieving the hysteresis loop phenomenon. Our results indicate that bistable quantum states could provide a way to study macroscopic quantum bistable phenomena in nonlinear systems and also can be found in broad applications in magnetic spintronics.Received 30 October 2020Accepted 19 April 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023126Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum opticsAtomic, Molecular & Optical

Enhancing Metastability by Dissipation and Driving in an Asymmetric Bistable Quantum System.

The stabilizing effect of quantum fluctuations on the escape process and the relaxation dynamics from a quantum metastable state are investigated. Specifically, the quantum dynamics of a multilevel bistable system coupled to a bosonic Ohmic thermal bath in strong dissipation regime is analyzed. The study is performed by a non-perturbative method based on the real-time path integral approach of the Feynman-Vernon influence functional. We consider a strongly asymmetric double well potential with and without a monochromatic external driving, and with an out-of-equilibrium initial condition. In the absence of driving we observe a nonmonotonic behavior of the escape time from the metastable region, as a function both of the system-bath coupling coefficient and the temperature. This indicates a stabilizing effect of the quantum fluctuations. In the presence of driving our findings indicate that, as the coupling coefficient increases, the escape time, initially controlled by the external driving, shows resonant peaks and dips, becoming frequency-independent for higher values. Moreover, the escape time from the metastable state displays a nonmonotonic behavior as a function of the temperature, the frequency of the driving, and the thermal-bath coupling, which indicates the presence of a quantum noise enhanced stability phenomenon. Finally, we investigate the role of different spectral densities, both in sub-Ohmic and super-Ohmic dissipation regime and for different cutoff frequencies, on the relaxation dynamics from the quantum metastable state. The results obtained indicate that, in the crossover dynamical regime characterized by damped intrawell oscillations and incoherent tunneling, the spectral properties of the thermal bath influence non-trivially the short time behavior and the time scales of the relaxation dynamics from the metastable state.

Macro-mechanics controls quantum mechanics: mechanically controllable quantum conductance switching of an electrochemically fabricated atomic-scale point contact

Here, we present a silver atomic-scale device fabricated and operated by a combined technique of electrochemical control (EC) and mechanically controllable break junction (MCBJ). With this EC-MCBJ technique, we can perform mechanically controllable bistable quantum conductance switching of a silver quantum point contact (QPC) in an electrochemical environment at room temperature. Furthermore, the silver QPC of the device can be controlled both mechanically and electrochemically, and the operating mode can be changed from ‘electrochemical’ to ‘mechanical’, which expands the operating mode for controlling QPCs. These experimental results offer the perspective that a silver QPC may be used as a contact for a nanoelectromechanical relay.

Ultralong relaxation times in bistable hybrid quantum systems.

Nonlinear systems, whose outputs are not directly proportional to their inputs, are well known to exhibit many interesting and important phenomena that have profoundly changed our technological landscape over the last 50 years. Recently, the ability to engineer quantum metamaterials through hybridization has allowed us to explore these nonlinear effects in systems with no natural analog. We investigate amplitude bistability, which is one of the most fundamental nonlinear phenomena, in a hybrid system composed of a superconducting resonator inductively coupled to an ensemble of nitrogen-vacancy centers. One of the exciting properties of this spin system is its long spin lifetime, which is many orders of magnitude longer than other relevant time scales of the hybrid system. This allows us to dynamically explore this nonlinear regime of cavity quantum electrodynamics and demonstrate a critical slowing down of the cavity population on the order of several tens of thousands of seconds-a time scale much longer than observed so far for this effect. Our results provide a foundation for future quantum technologies based on nonlinear phenomena.

Probing the Critical Point of the Jaynes–Cummings Second-Order Dissipative Quantum Phase Transition

We highlight the importance of quantum fluctuations in organizing a dissipative quantum phase transition for the driven Jaynes-Cummings interaction with variable qubit-cavity detuning. The system response presents a substantial difference from the predictions of the semiclassical theory, the extent of which is revealed in the properties of quantum bistability, and visualized with the help of quasi-distribution functions for the cavity field, subject to an appropriate scale parameter. States anticipated by the neoclassical theory of radiation coexist in the quantum picture, following the occurrence of spontaneous dressed-state polarization and phase bistability at resonance.

Quantum phase transitions in the driven dissipative Jaynes-Cummings oscillator: From the dispersive regime to resonance

We follow the passage from complex amplitude bistability to phase bistability in the driven dissipative Jaynes-Cummings oscillator. Quasi-distribution functions in the steady state are employed, for varying qubit-cavity detuning and drive parameters, in order to track a first-order dissipative quantum phase transition up to the critical point marking a second-order transition and spontaneous symmetry breaking. We demonstrate the photon blockade breakdown in the dispersive regime, and find that the coexistence of cavity states in the regime of quantum bistability is accompanied by pronounced qubit-cavity entanglement. Focusing on the effect of quantum-activated switching for both coupled degrees of freedom, we move from a region of minimal entanglement in the dispersive regime, where we derive analytical perturbative results, to the threshold behaviour of spontaneous dressed-state polarization at resonance.

Interplay between Switching Driven by the Tunneling Current and Atomic Force of a Bistable Four-Atom Si Quantum Dot.

We assemble bistable silicon quantum dots consisting of four buckled atoms (Si4-QD) using atom manipulation. We demonstrate two competing atom switching mechanisms, downward switching induced by tunneling current of scanning tunneling microscopy (STM) and opposite upward switching induced by atomic force of atomic force microscopy (AFM). Simultaneous application of competing current and force allows us to tune switching direction continuously. Assembly of the few-atom Si-QDs and controlling their states using versatile combined AFM/STM will contribute to further miniaturization of nanodevices.

Intrinsic optical intersubband bistability in quantum well structures: Role of multiple reflections

We theoretically investigate the nonlinear intersubband response, in particular the intrinsic optical bistability (IOB), of quantum well (QW) structures embedded into layered systems. A semiclassical approach is employed based on the plane-wave approximation, the transfer-matrix formalism, the sheet model, and the density-matrix formalism developed to calculate the two-dimensional nonlinear intersubband electron conductivity and the IOB. A variety of presented numerical results demonstrate the importance of inhomogeneity of spatial distribution of radiation intensity induced by multiple radiation reflection for the IOB in layered structures. It is shown that knowing the radiation intensity distribution in the area occupied by QWs, in part standing-wave patterns, are crucial for understanding of IOB pictures. The IOB in systems of one, two, three, five, and ten QWs is investigated. The multistability phenomenon is addressed.

Lyapunov control of squeezed noise quantum trajectory

The quantum trajectory renders the optimal estimation of quantum state. It is a classical Itô stochastic differential equation. The Lyapunov global stabilization problem is solved for squeezed noise quantum trajectory. Lyapunov control stabilizes the quantum system toward one eigenstate. A two-level bistable quantum system is simulated as an example.

Successive bi-stable quantum phases in HMTSF-TCNQ induced by field-sweep

With the interest of ground state near CDW at low temperature, the electronic properties under high pressure, at low temperature and in high magnetic field of HMTSF-TCNQ are examined. At 1.5–1.6 GPa, but not at P=0, a novel hysteretic behavior irrespective of magnetic field orientation and probable quantum oscillations in a specific orientation of magnetic field in magnetoresistance are found.

Dynamics of a bistable Mott insulator to superfluid phase transition in cavity optomechanics

We study the dynamics of the many-body state of ultracold bosons trapped in a bistable optical lattice in an optomechanical resonator controlled by a time-dependent input field. We focus on the dynamics of the many-body system following discontinuous jumps of the intracavity field. We identify experimentally realizable parameters for the bistable quantum phase transition between Mott insulator and superfluid.

Quantum bistability and spin current shot noise of a single quantum dot coupled to an optical microcavity

Here we explore the quantum-mechanical limit of bistability that appears in the spin-dependent transport through a single quantum dot coupled to an optical microcavity. The spin current is generated by electron tunneling between a single doped reservoir and the dot combined with intradot spin-flip transitions induced by a quantized cavity mode. Earlier research has shown that in the classical limit where a large number of such dots interacts with the cavity so that quantum effects were ignorable, the average spin current exhibits bistability as a function of the laser that drives the cavity. We show that in the limit of a single quantum dot this bistability continues to be present in the intracavity photon statistics. Signatures of the bistable photon statistics manifest themselves in the frequency-dependent shot noise of the spin current despite the fact that the quantum-mechanical average spin current no longer exhibits bistability. Besides having significance for future quantum-dot-based optoelectronic devices, our results shed light on the relation between bistability, which is traditionally viewed as a classical effect, and quantum mechanics.

Analysis of kinetic isotope effects for nonadiabatic reactions

Factors influencing the rates of quantum mechanical particle transfer reactions in many-body systems are discussed. The investigations are carried out on a simple model for a proton transfer reaction that captures generic features seen in more realistic models of condensed phase systems. The model involves a bistable quantum oscillator coupled to a one-dimensional double-well reaction coordinate, which is in turn coupled to a bath of harmonic oscillators. Reactive-flux correlation functions that involve quantum-classical Liouville dynamics for chemical species operators and quantum equilibrium sampling are used to estimate the reaction rates. Approximate analytical expressions for the quantum equilibrium structure are derived. Reaction rates are shown to be influenced significantly by both the quantum equilibrium structure and nonadiabatic dynamics. Nonadiabatic dynamical effects are found to play the major role in determining the magnitude of the kinetic isotope effect for the model transfer reaction.

Pure dephasing due to damped bistable quantum impurities

We study the dynamics of a spin (qubit) coupled to a bistable quantum impurity interacting with a Gaussian bath modeled by a ohmic spin boson model. For white noise the complete dynamics in the four-dimensional Hilbert space is analyzed within a Lindblad formalism. For ohmic damping at finite temperatures we resort to a functional integral approach. We show how different dynamical regimes and crossover of the nonlinear spin boson system are probed by the qubit dynamics.

Coherent destruction of tunneling in waveguide directional couplers

It is theoretically shown that the phenomenon of coherent destruction of tunneling in bistable quantum systems induced by an external driving field can be realized in a periodically curved optical waveguide coupler, the periodic bending of the coupler simulating the effect of the external field in the quantum system.

Relaxation oscillations in a bistable quantum Hall system

Quantum Hall systems can show a bistability in the hysteresis of the breakdown. We realized a relaxation oscillator based on a quantum-Hall device with Corbino geometry. Investigations of the performance of such an oscillator revealed an increase of the hysteresis of the current–voltage curve of the device in comparison with the width of hysteresis at dc voltages. By direct measurements of the hysteresis of the current–voltage curve at different frequencies, we found a marked increase of the breakdown hysteresis at low frequencies of some Hz. We explain the observed dynamical enhancement of the breakdown hysteresis applying an electron heating model with a background (delocalization-related) component of conductivity, which decreases with increasing frequency, indicating a more effective localization already at low frequencies.