In this paper, we present a systematic approach to building useful time-dependent effective Hamiltonians in molecular quantum electrodynamics. The method is based on considering part of the system as ...an open quantum system and choosing a convenient unitary transformation based on the evolution operator. We illustrate our formalism by obtaining four Hamiltonians, each suitable to a different class of applications. We show that we may treat several effects of molecular quantum electrodynamics with a direct first-order perturbation theory. In addition, our effective Hamiltonians shed light on interesting physical aspects that are not explicit when employing more standard approaches. As applications, we discuss three examples: two-photon spontaneous emission, resonance energy transfer, and dispersion interactions.
We report coherent coupling between two macroscopically separated nitrogen-vacancy electron spin ensembles in a cavity quantum electrodynamics system. The coherent interaction between the distant ...ensembles is directly detected in the cavity transmission spectrum by observing bright and dark collective multiensemble states and an increase of the coupling strength to the cavity mode. Additionally, in the dispersive limit we show transverse ensemble-ensemble coupling via virtual photons.
In high-intensity (>1021 Wcm−2) laser–matter interactions gamma-ray photon emission by the electrons can strongly affect the electronʼs dynamics and copious numbers of electron–positron pairs can be ...produced by the emitted photons. We show how these processes can be included in simulations by coupling a Monte Carlo algorithm describing the emission to a particle-in-cell code. The Monte Carlo algorithm includes quantum corrections to the photon emission, which we show must be included if the pair production rate is to be correctly determined. The accuracy, convergence and energy conservation properties of the Monte Carlo algorithm are analysed in simple test problems.
Self-bearing machines offer a highly integrated solution to achieve the rotor drive and magnetic levitation within a single structure. Although the trend is toward a diminution in the number of ...actively stabilized degrees of freedom to reduce the requirement for sensors, power electronics, and controllers, no self-bearing machine relying solely on passive phenomena has been successfully tested so far due to the restrictions highlighted by Earnshaw's theorem. Recent researches have demonstrated, through electromechanical models and in-depth experimental investigations, that electrodynamic thrust self-bearing machines (EDTSBMs) can gather, within a combined winding, both the rotor axial passive suspension and motor functions. Nonetheless, the test rigs implementing them still comprised external means to ensure the rotor radial and tilt support. In this context, this article takes the study of EDTSBMs to the final step by combining them with two permanent magnet centering bearings, yielding the first fully passively levitated self-bearing machines. A reaction wheel demonstrator based on the latter is designed and sized according to an optimal approach. Thorough experimental analyses dealing with the axial forces that support the rotor and its resulting axial position as well as the drive torque are carried out, notably proving that stable fully passive levitation can be achieved. Finally, the motor and suspension currents as well as the corresponding losses are investigated.
The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide ...attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field. There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits-'qubits', the basic building blocks of a quantum computer-based on molecular magnets. But strong decoherence must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/2 spins, leading to an S = 1/2 collective ground state. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 s (at liquid helium temperatures) are a realistic prospect.
PDF
