arXiv subjects

M. Möttönen

Publications and source records attributed to M. Möttönen.

47 records · Page 3Linked to original sources

arXiv2011

Measurement scheme for the Lamb shift in a superconducting circuit with broadband environment

Motivated by recent experiments on quantum mechanical charge pumping in a Cooper pair sluice, we present a measurement scheme for observing shifts of transition frequencies in two-level quantum systems induced by broadband environmental fluctuations. In contrast to quantum optical and related set-ups based on cavities, the impact of a thermal phase reservoir is considered. A thorough analysis of Lamb and Stark shifts within weak-coupling master equations is complemented by non-perturbative results for the model of an exactly solvable harmonic system. The experimental protocol to measure the Lamb shift in experimentally feasible superconducting circuits is analysed in detail and supported by numerical simulations.

cond-mat.mes-hall↗

arXiv2011

Cooper pair current in the presence of flux noise

We study the effect of the flux noise on the Cooper pair current and pumping. We generalize the definition of the current in order to take into account the contribution induced by the environment. It turns out that this dissipative current vanishes for charge noise but it is finite in general for noise operators that do not commute with the charge operator. We discuss in a generic framework the effect of flux noise and present a way to engineer it by coupling the system to an additional external circuit. We calculate numerically the pumped charge through the device by solving the master equation for the reduced density matrix of the system and show how it depends on the coupling to the artificial environment.

quant-ph↗

arXiv2011

Coherent superconducting quantum pump

We demonstrate non-adiabatic charge pumping utilizing a sequence of coherent oscillations between a superconducting island and two reservoirs. Our method, based on pulsed quantum state manipulations, allows to speedup charge pumping to a rate which is limited by the coupling between the island and the reservoirs given by the Josephson energy. Our experimental and theoretical studies also demonstrate that relaxation can be employed to reset the pump and avoid accumulation of errors due to non-ideal control pulses.

cond-mat.mes-hall↗

arXiv2011

Single-photon heat conduction in electrical circuits

We study photonic heat conduction between two resistors coupled weakly to a single superconducting microwave cavity. At low enough temperature, the dominating part of the heat exchanged between the resistors is transmitted by single-photon excitations of the fundamental mode of the cavity. This manifestation of single-photon heat conduction should be experimentally observable with the current state of the art. Our scheme can possibly be utilized in remote interference-free temperature control of electric components and environment engineering for superconducting qubits coupled to cavities.

quant-ph↗

arXiv2011

Geometric quantum gates with superconducting qubits

We suggest a scheme to implement a universal set of non-Abelian geometric transformations for a single logical qubit composed of three superconducting transmon qubits coupled to a single cavity. The scheme utilizes an adiabatic evolution in a rotating frame induced by the effective tripod Hamiltonian which is achieved by longitudinal driving of the transmons. The proposal is experimentally feasible with the current state of the art and could serve as a first proof of principle for geometric quantum computing.

quant-ph↗

arXiv2011

Ground-state geometric quantum computing in superconducting systems

We present a theoretical proposal for the implementation of geometric quantum computing based on a Hamiltonian which has a doubly degenerate ground state. Thus the system which is steered adiabatically, remains in the ground-state. The proposed physical implementation relies on a superconducting circuit composed of three SQUIDs and two superconducting islands with the charge states encoding the logical states. We obtain a universal set of single-qubit gates and implement a non-trivial two-qubit gate exploiting the mutual inductance between two neighboring circuits, allowing us to realize a fully geometric ground-state quantum computing. The introduced paradigm for the implementation of geometric quantum computing is expected to be robust against environmental effects.

quant-ph↗

arXiv2011

Decoherence of adiabatically steered quantum systems

We study the effect of Markovian environmental noise on the dynamics of a two-level quantum system which is steered adiabatically by an external driving field. We express the master equation taking consistently into account all the contributions to the lowest non-vanishing order in the coupling to the Markovian environment. We study the master equation numerically and analytically and we find that, in the adiabatic limit, a zero-temperature environment does not affect the ground state evolution. As a physical application, we discuss extensively how the environment affects Cooper pair pumping. The adiabatic ground state pumping appears to be robust against environmental noise. In fact, the relaxation due to the environment is required to avoid the accumulation of small errors from each pumping cycle. We show that neglecting the non-secular terms in the master equation leads to unphysical results, such as charge non-conservation. We discuss also a possible way to control the environmental noise in a realistic physical setup and its influence on the pumping process.

quant-ph↗

arXiv2011

Capacitively Enhanced Thermal Escape in Underdamped Josephson Junctions

We have studied experimentally the escape dynamics in underdamped capacitively shunted and unshunted Josephson junctions with submicroampere critical currents below 0.5 K temperatures. In the shunted junctions, thermal activation process was preserved up to the highest temperature where the escape in the unshunted junctions exhibits the phase diffusion. Our observations in the shunted junctions are in good agreement with the standard thermal activation escape, unlike the results in the unshunted junctions.

cond-mat.mes-hall↗

arXiv2010

Non-Abelian geometric phases in ground state Josephson devices

We present a superconducting circuit in which non-Abelian geometric transformations can be realized using an adiabatic parameter cycle. In contrast to previous proposals, we employ quantum evolution in the ground state. We propose an experiment in which the transition from non-Abelian to Abelian cycles can be observed by measuring the pumped charge as a function of the period of the cycle. Alternatively, the non-Abelian phase can be detected using a single-electron transistor working as a charge sensor.

cond-mat.supr-con↗

arXiv2006

Dynamically stable multiply quantized vortices in dilute Bose-Einstein condensates

Multiquantum vortices in dilute atomic Bose-Einstein condensates confined in long cigar-shaped traps are known to be both energetically and dynamically unstable. They tend to split into single-quantum vortices even in the ultralow temperature limit with vanishingly weak dissipation, which has also been confirmed in the recent experiments [Y. Shin et al., Phys. Rev. Lett. 93, 160406 (2004)] utilizing the so-called topological phase engineering method to create multiquantum vortices. We study the stability properties of multiquantum vortices in different trap geometries by solving the Bogoliubov excitation spectra for such states. We find that there are regions in the trap asymmetry and condensate interaction strength plane in which the splitting instability of multiquantum vortices is suppressed, and hence they are dynamically stable. For example, the doubly quantized vortex can be made dynamically stable even in spherical traps within a wide range of interaction strength values. We expect that this suppression of vortex-splitting instability can be experimentally verified.

cond-mat.stat-mech↗

arXiv2006

Splitting times of doubly quantized vortices in dilute Bose-Einstein condensates

Recently, the splitting of a topologically created doubly quantized vortex into two singly quantized vortices was experimentally investigated in dilute atomic cigar-shaped Bose-Einstein condensates [Y. Shin et al., Phys. Rev. Lett. 93, 160406 (2004)]. In particular, the dependency of the splitting time on the peak particle density was studied. We present results of theoretical simulations which closely mimic the experimental set-up. Contrary to previous theoretical studies, claiming that thermal excitations are the essential mechanism in initiating the splitting, we show that the combination of gravitational sag and time dependency of the trapping potential alone suffices to split the doubly quantized vortex in time scales which are in good agreement with the experiments. We also study the dynamics of the resulting singly quantized vortices which typically intertwine--especially, a peculiar vortex chain structure appears for certain parameter values.

quant-ph↗