We theoretically investigate the enhancement of the charging power in a Dicke quantum battery which consists of an array of N two-level systems (TLS)coupled to a single mode of cavity photons. In the limit of small N, we analytically solve the time evolution for the full charging process. The eigenvectors of the driving Hamiltonian are found to be pseudo-Hermite polynomials and the evolution is thus interpreted as harmonic oscillator like behaviour. Then we demonstrate the average charging power using a collective protocol is (Formula presented.) times larger than that of the parallel protocol when transferring the same amount of energy. Unlike previous studies, we point out that such quantum advantage does not originate from entanglement but is due to the coherent cooperative interactions among the TLSs. Our results provide intuitive quantitative insight into the dynamic charging process of a Dicke battery and can be observed under realistic experimental conditions.

We propose a quantum-mechanical device consisting of transmon qubits connected to single-photon transmission lines that can be used to perform a boolean quantum private query on a database. Using numerical simulations verified by analytical calculations we analyse the dynamics of the query process in the proposed system by calculating the reflection and transmission probabilities of the photons encoding the questions. We show that the proposed device, which consists of experimentally available components, is capable of performing a quantum private query with high reliability and may form the basic building block of a lager-scale private search engine.

We theoretically investigate the effect of transverse magnetic anisotropy on spin-flip assisted tunneling through atomic-spin chains. Using a phenomenological approach and first-order perturbation theory, we analytically calculate the inelastic tunneling current, differential conductance, and atomic-spin transition rates. We predict the appearance of additional steps in the differential conductance and a pronounced increase in the spin-flip transition rate which at low voltages scales quadratically with the ratio of the transverse anisotropy energy and the sum of the longitudinal anisotropy energy and the exchange energy. Our results provide intuitive qualitative insight into the role played by transverse anisotropy in inelastic tunneling spectroscopy of atomic chains and can be observed under realistic experimental conditions.

We investigate inelastic cotunneling in a model system where the charging island is connected to the leads through molecules with energy-dependent transmission functions. To study this problem, we propose two different approaches. The first is a pragmatic approach that assumes Lorentzian-like transmission functions that determine the transmission probability to the island. Using this model, we calculate current versus voltage (IV) curves for increasing resonance level positions of the molecule. We find that shifting the resonance energy of the molecule away from the Fermi energy of the contacts leads to a decreased current at low bias, but as bias increases, this difference decreases and eventually inverses. This is markedly different from IV behavior outside the cotunneling regime. The second approach involves multiple cotunneling where also the molecules are considered to be in the Coulomb blockade regime. We find here that when Ec≫eV,kBT, the IV behavior approaches the original cotunneling behavior proposed by Averin and Nazarov [Phys. Rev. Lett. 65, 2446-2449 (1990)].

In this paper we propose a superadiabatic protocol where quantum state transfer can be achieved with arbitrarily high accuracy and minimal control across long spin chains with an odd number of spins. The quantum state transfer protocol only requires the control of the couplings between the qubits on the edge and the spin chain. We predict fidelities above 0.99 for an evolution of nanoseconds using typical spin-exchange coupling values of μeV. Furthermore, by building a superadiabatic formalism on top of this protocol, we propose an effective superadiabatic protocol that retains the minimal control over the spin chain and further improves the fidelity.

We propose an implementation of a quantum router for microwave photons in a superconducting qubit architecture consisting of a transmon qubit, SQUIDs and a nonlinear capacitor. We model and analyze the dynamics of operation of the quantum switch using quantum Langevin equations in a scattering approach and compute the photon reflection and transmission probabilities. For parameters corresponding to up-to-date experimental devices we predict successful operation of the router with probabilities above 94%.