Jin Wang

Information can improve heat engine performance, but the underlying principles are still not so clear. Here we introduce a Carnot information machine (CIE) and obtain a quantitative relationship between the engine performance and information. We demonstrate that the presence of information changes allows the CIE to operate as a heat engine in the regime where the standard Carnot cycle is prohibited, ensures that the efficiency of the CIE is greater than or equal to the standard Carnot efficiency, and significantly enables it to achieve 100\% efficiency with positive work extraction for arbitrary two-level systems. We explicitly demonstrate these features using a spin-1/2 system and propose an experimental implementation scheme based on a trapped $^{40}\mathrm{Ca}^+$ ion.

The interplay of strong single-ion anisotropy and magnetic interactions often give rise to novel magnetic behavior and can provide additional routes for controlling magnetization dynamics. However, novel effects arising from interactions between lanthanide and transition-metal ions are nowadays rarely observed. Herein, a {$Dy^{III}Cu^{II}_5$} 3d-4f single-molecule magnet (SMM) is constructed as a rigid and planar [15-MC-5] metallacrown (MC), where the $Dy^{III}$ ion is trapped in the central pseudo-$D_{5h}$ pocket. A strong axial crystal field (CF) imbues the $Dy^{III}$ ion with large Ising-type magnetic anisotropy, and we are able to observe and model the magnetic interactions between the $Cu^{II}-Cu^{II}$ and $Dy^{III}-Cu^{II}$ pairs. Butterfly-shaped magnetic hysteresis shows clear steps at $\pm$0.4 T, coincident with level crossings in our model exchange Hamiltonian between the {$Cu^{II}_5$} and $Dy^{III}$ spin systems. Most intriguingly, this air-stable SMM exhibits three distinct regimes in its magnetic relaxation dynamics, all clearly displaying an exponential dependence on temperature.

The random magnetic field produced by nuclear spins has long been viewed as the dominating source of decoherence in the quantum-dot based spins. Here we obtain in both exact and analytical manner the dynamics of spin qubits coupled to nuclear spin environments via the hyperfine interaction, going beyond the weak system-bath interaction and Markovian approximation. We predict that the detailed-balance breaking produced by chemical potential gradient in nuclear baths leads to the rapid oscillations of populations, quantum coherence and entanglement, which are absent in the conventional case (i.e., Overhauser noise). This is attributed to the nonequilibrium feature of the system as shown in the relation between the oscillation period and the chemical potential imbalance. Our results reveal the essentiality of nonequilibriumness with detailed-balance breaking for enhancing the dynamical coherence and entanglement of spin qubits. Moreover, our exact solution explicitly demonstrates that the non-Markovian bath comprised by nuclear spins can preserve the collective quantum state, due to the recovery of coherence. Finally, we propose an experiment using ultracold trapped ions to observe these nonequilibrium and memory effects.