arXiv subjects

Mikko Möttönen

Publications and source records attributed to Mikko Möttönen.

At least 91 records · Page 5Linked to original sources

arXiv2013

Vortex pump for Bose-Einstein condensates utilizing a time-averaged orbiting potential trap

We show that topological vortex pumping can be implemented for a dilute Bose-Einstein condensate confined in a magnetic time-averaged orbiting potential trap with axial optical confinement. Contrary to earlier proposals for the vortex pump, we do not employ an additional optical potential to trap the condensate in the radial direction, but instead, the radial confinement is provided by the magnetic field throughout the pumping cycle. By performing numerical simulations based on the spin-1 Gross-Pitaevskii equation, we find that several pumping cycles can be carried out to produce a highly charged vortex before a majority of the particles escape from the trap or before the vortex splits into singly charged vortices. On the other hand, we observe that an additional, relatively weak optical plug potential is efficient in preventing splitting and reducing particle loss. With these results, we hope to bring the vortex pump closer to experimental realization.

cond-mat.quant-gas↗

arXiv2013

Geometric Phase Gates with Adiabatic Control in Electron Spin Resonance

High-fidelity quantum operations are a key requirement for fault-tolerant quantum information processing. In electron spin resonance, manipulation of the quantum spin is usually achieved with time-dependent microwave fields. In contrast to the conventional dynamic approach, adiabatic geometric phase operations are expected to be less sensitive to certain kinds of noise and field inhomogeneities. Here, we investigate such phase gates applied to electron spins both through simulations and experiments, showing that the adiabatic geometric phase gate is indeed inherently robust against inhomogeneity in the applied microwave field strength. While only little advantage is offered over error-correcting composite pulses for modest inhomogeneities <=10%, the adiabatic approach reveals its potential for situations where field inhomogeneities are unavoidably large.

quant-ph↗

arXiv2012

Stationary States of Trapped Spin-Orbit-Coupled Bose-Einstein Condensates

We numerically investigate low-energy stationary states of pseudospin-1 Bose-Einstein condensates in the presence of Rashba-Dresselhaus-type spin-orbit coupling. We show that for experimentally feasible parameters and strong spin-orbit coupling, the ground state is a square vortex lattice irrespective of the nature of the spin-dependent interactions. For weak spin-orbit coupling, the lowest-energy state may host a single vortex. Furthermore, we analytically derive constraints that explain why certain stationary states do not emerge as ground states. Importantly, we show that the distinct stationary states can be observed experimentally by standard time-of-flight spinindependent absorption imaging.

cond-mat.quant-gas↗

arXiv2012

Exotic vortex lattices in two-species Bose-Einstein condensates

We numerically investigate vortex lattices in rotating two-component Bose-Einstein condensates in which the two components have unequal atomic masses and interact attractively with each other. For sufficiently strong attraction, the system is found to exhibit exotic ground-state structures in a harmonic trap, such as lattices having a square geometry or consisting of two-quantum vortices. The obtained states satisfy the Feynman relation, and they can be realized with current experimental techniques.

cond-mat.quant-gas↗

arXiv2011

Superadiabatic theory for Cooper pair pumping under decoherence

We introduce a method where successive coordinate transformations are applied to decrease the error in the adiabatic master equation resulting from truncation in the local adiabatic parameter. Our method reduces the nonphysical behaviour stemming from the lack of complete positivity. The strong environment-induced relaxation at high Cooper pair pumping frequencies leads to adiabatic ground-state pumping only in the lowest-order approximation. We illustrate the robustness of the frequency where the adiabaticity breaks down using the high-order theory and show the emergence of an optimal environmental coupling strength, for which ideal pumping is preserved for the highest frequency. Finally, we study the effect of quantum interference on the pumped current and give an estimate for the relaxation rate of an experimentally measured system.

cond-mat.supr-con↗

arXiv2011

Ground-State Dirac Monopole

We show theoretically that a monopole defect, analogous to the Dirac magnetic monopole, may exist as the ground state of a dilute spin-1 Bose-Einstein condensate. The ground-state monopole is not attached to a single semi-infinite Dirac string, but forms a point where the circulation of a single vortex line is reversed. Furthermore, the three-dimensional dynamics of this monopole defect are studied after the magnetic field pinning the monopole is removed and the emergence of antimonopoles is observed. Our scheme is experimentally realizable with the present-day state of the art.

cond-mat.quant-gas↗

arXiv2011

Information-to-energy conversion in an electron pump

We propose and analyze a possible implementation of Maxwell's demon based on a single-electron pump. We show that measurements of the charge states of the pump and feedback control of the gate voltages lead to a net flow of electrons against the bias voltage ideally with no work done on the system by the gate control. The information obtained in the measurements converts thermal fluctuations into free energy. We derive the conditions on the detector back-action and measurement time necessary for implementing this conversion.

cond-mat.mes-hall↗

arXiv2011

Phase transitions in dipolar spin-1 Bose gases

We study phase transitions in homogeneous spin-1 Bose gases in the presence of long-range magnetic dipole-dipole interactions (DDI). We concentrate on three-dimensional geometries and employ momentum shell renormalization group to study the possible instabilities caused by the dipole-dipole interaction. At the zero-temperature limit where quantum fluctuations prevail, we find the phase diagram to be unaffected by the dipole-dipole interaction. When the thermal fluctuations dominate, polar and ferromagnetic condensates with DDI become unstable and we discuss this crossover in detail. On the other hand, the spin-singlet condensate remains stable in the presence of dipole-dipole interactions.

cond-mat.quant-gas↗

arXiv2011

Entanglement generation between unstable optically active qubits without photodetectors

We propose a robust deterministic scheme to generate entanglement at high fidelity without the need of photodetectors even for quantum bits, qubits, with extremely poor optically active states. Our protocol employs stimulated Raman adiabatic passage for population transfer without actually exciting the system. Furthermore, it is found to be effective even if the environmental decoherence rate is of the same order of magnitude as the atom--photon coupling frequency. Our scheme holds potential to solve entanglement generation problems, e.g., in distributed quantum computing.

quant-ph↗

arXiv2011

Size and dynamics of vortex dipoles in dilute Bose-Einstein condensates

Recently, Freilich et al. [Science 329, 1182 (2010)] experimentally discovered stationary states of vortex dipoles, pairs of vortices of opposite circulation, in dilute Bose-Einstein condensates. To explain their observations, we perform simulations based on the Gross-Pitaevskii equation and obtain excellent quantitative agreement on the size of the stationary dipole. We also investigate how their imaging method, in which atoms are repeatedly extracted from a single condensate, affects the vortex dynamics. We find that it mainly induces isotropic size oscillations of the condensate without otherwise disturbing the vortex trajectories. Thus, the imaging technique appears to be a promising tool for studying real-time superfluid dynamics.

cond-mat.quant-gas↗

arXiv2010

Stabilization and pumping of giant vortices in dilute Bose-Einstein condensates

Recently, it was shown that giant vortices with arbitrarily large quantum numbers can possibly be created in dilute Bose-Einstein condensates by cyclically pumping vorticity into the condensate. However, multiply quantized vortices are typically dynamically unstable in harmonically trapped nonrotated condensates, which poses a serious challenge to the vortex pump procedure. In this theoretical study, we investigate how the giant vortices can be stabilized by the application of a Gaussian potential peak along the vortex core. We find that achieving dynamical stability is feasible up to high quantum numbers. To demonstrate the efficiency of the stabilization method, we simulate the adiabatic creation of an unsplit 20-quantum vortex with the vortex pump.

cond-mat.quant-gas↗

arXiv2010

Splitting dynamics of giant vortices in dilute Bose-Einstein condensates

We study the splitting of multiply quantized vortices with large quantum numbers in dilute nonrotated Bose-Einstein condensates in the zero-temperature limit. The splitting is observed to result in vortex-free condensate fragments which are separated by vortex sheets. The number of these fragments is found to be equal to the angular-momentum quantum number of the Bogoliubov excitation mode responsible for the splitting, although the formulation of the fragments cannot be described by small-amplitude excitations. Thus, the realization of an isolated giant vortex and the observation of its splitting would provide a means to directly relate the experimental data to discrete theoretical quantities.

cond-mat.quant-gas↗

arXiv2010

Finite-temperature phase transitions in quasi-two-dimensional spin-1 Bose gases

Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D systems and find that the crossover temperature is shifted upwards if skyrmions are allowed. Above the defect binding temperatures we observe transitions corresponding to the onset of a coherent condensate and a quasi-condensate and discuss the absence of a fragmented condensate.

cond-mat.quant-gas↗

arXiv2010

Core sizes and dynamical instabilities of giant vortices in dilute Bose-Einstein condensates

Motivated by a recent demonstration of cyclic addition of quantized vorticity into a Bose-Einstein condensate, the vortex pump, we study dynamical instabilities and core sizes of giant vortices. The core size is found to increase roughly as a square-root function of the quantum number of the vortex, whereas the strength of the dynamical instability either saturates to a fairly low value or increases extremely slowly for large quantum numbers. Our studies suggest that giant vortices of very high angular momenta may be achieved by gradually increasing the operation frequency of the vortex pump.

cond-mat.quant-gas↗

arXiv2010

Transport Spectroscopy of Single Phosphorus Donors in a Silicon Nanoscale Transistor

We have developed nano-scale double-gated field-effect-transistors for the study of electron states and transport properties of single deliberately-implanted phosphorus donors. The devices provide a high-level of control of key parameters required for potential applications in nanoelectronics. For the donors, we resolve transitions corresponding to two charge states successively occupied by spin down and spin up electrons. The charging energies and the Lande g-factors are consistent with expectations for donors in gated nanostructures.

cond-mat.mes-hall↗

arXiv2009

Quantized current of a hybrid single-electron transistor with superconducting leads and a normal-metal island

We discuss the operation of the superconductor - insulator - normal-metal - insulator - superconductor (SINIS) turnstile. This voltage-biased hybrid single-electron transistor (SET) provides current quantization even with only one radio-frequency (rf) control parameter, namely the gate voltage of the single island. We give an overview of the main error mechanisms of the turnstile and consider its feasibility as a quantum current standard. We also present experimental results of pumping with the SINIS structure which show decreased leakage current compared to earlier measurements with the opposite NISIN structure.

cond-mat.mes-hall↗

arXiv2009

Creation of Dirac monopoles in spinor Bose-Einstein condensates

We demonstrate theoretically that using standard external magnetic fields, one can imprint point-like topological defects to the spin texture of a dilute Bose-Einstein condensate. Symmetries of the condensate order parameter render this topological defect to be accompanied with a vortex filament corresponding to the Dirac string of a magnetic monopole. The vorticity in the condensate coincides with the magnetic field of a magnetic monopole, providing an ideal analogue to the monopole studied by Dirac.

cond-mat.other↗

arXiv2009

Non-Abelian magnetic monopole in a Bose-Einstein condensate

Recently, an effective non-Abelian magnetic field with a topology of a monopole was shown to emerge from the adiabatic motion of multilevel atoms in spatially varying laser fields [J. Ruseckas et al., Phys. Rev. Lett. 95, 010404 (2005)]. We study this monopole in a Bose-Einstein condensate (BEC) of degenerate dressed states and find that the topological charge of the pseudospin cancels the monopole charge resulting in a vanishing gauge invariant charge. As a function of the laser wavelength, different stationary states are classified in terms of their effect to the monopole part of the magnetic field and a cross-over to vortex ground state is observed.

cond-mat.other↗