15 research outputs found
Quantum Knots and Monopoles
No full textBose–Einstein condensation is a quantum statistical phase transition that occurs in a system consisting of bosons when a single-particle quantum state becomes macroscopically occupied. This peculiar state of matter was first predicted in 1925 and finally realized seventy years later in vapours of weakly-interacting alkali-metal atoms. Since then, Bose–Einstein condensates have been one of the most fascinating research fields in modern physics.
The gaseous condensates offer a robust platform to accurately study interacting many-particle systems from the first principles. Experimentally, the possibility to precisely control a condensate with external fields and directly image its order parameter provides unforeseen opportunities to obtain deep insight into phenomena across different subfields of physics. In particular, gaseous condensates can emulate complicated models that arise in atomic, condensed-matter, and even particle physics, allowing realizations of exotic phenomena that are elusive in their original contexts. An outstanding example is the existence of various topological defects in ultacold quantum gases with internal degrees of freedom.
In this thesis, we investigate the creation, stability, and dynamical properties of various topological defects in spinor Bose–Einstein condensates. The majority of the theoretical results is obtained by numerically solving the dynamics using Gross–Pitaevskii equations for spin-1 condensates. Many theoretical predictions are confirmed by the very good agreement with experiments.
The first experimental observations of a topological point defect in an order parameter describing the quantum gas are presented. Such a point defect is reminiscent to the magnetic monopole particle appearing in grand unified theories. Therefore, the discovery of monopoles in quantum gases further encourages the quest for finding magnetic monopoles in natural electromagnetic fields, a search largely initiated by Paul Dirac almost a century ago.
Subsequently, the fine structure and decay dynamics of the point defect are studied numerically and verified in experiments. The created monopole is gradually destroyed during the polar-to-ferromagnetic quantum phase transition, which results in the spontaneous emergence of a Dirac monopole in synthetic magnetic field. In addition to the singular point defect, the first experimental realization of a knot soliton in the context of quantum field is reported. This thesis lays the foundation for studies of the dynamics and stability of three-dimensional topological structures in quantum systems
Fidelity measurement of a multiqubit cluster state with minimal effort
No full textThe size of the Hilbert space for a multiqubit state scales exponentially
with the number of constituent qubits. Often this leads to a similar
exponential scaling of the experimental resources required to characterize the
state. Contrary to this, we propose a physically-motivated method for
experimentally assessing the fidelity of an important class of entangled states
known as cluster states. The proposed method always yields a lower bound of the
fidelity with a number of measurement settings scaling only linearly with the
system size, and is tailored to correctly account for the errors most likely to
occur in experiments. For one-dimensional cluster states, the constructed
fidelity measure is tight to lowest order in the error probability for
experimentally realistic noise sources and thus closely matches the true
fidelity. Furthermore, it is tight for the majority of higher-order errors,
except for a small subset of certain non-local multiqubit errors irrelevant in
typical experimental situations. The scheme also performs very well for
higher-dimensional cluster states, including correctly the majority of
experimentally relevant errors
Creation of a Dirac monopole-antimonopole pair in a spin-1 Bose-Einstein condensate
No full text| openaire: EC/H2020/681311/EU//QUESSWe theoretically demonstrate that a pair of Dirac monopoles with opposite synthetic charges can be created within a single spin-1 Bose-Einstein condensate by steering the spin degrees of freedom by external magnetic fields. Although the net synthetic magnetic charge of this configuration vanishes, both the monopole and the antimonopole are accompanied by vortex filaments carrying opposite angular momenta. Such a Dirac dipole can be realized experimentally by imprinting a spin texture with a nonlinear magnetic field generated by a pair of coils in a modified Helmholtz configuration. We also investigate the case where the initial state for the dipole-creation procedure is pierced by a quantized vortex line with a winding number κ. It is shown that if κ=-1, the resulting monopole and antimonopole lie along the core of a singly quantized vortex whose sign is reversed at the locations of the monopoles. For κ=-2, the monopole and antimonopole are connected by a vortex line segment carrying two quanta of angular momentum, and hence the dipole as a whole is an isolated configuration. In addition, we simulate the long-time evolution of the dipoles in the magnetic field used to create them. For κ=0, each of the semi-infinite doubly quantized vortices splits into two singly quantized vortices, as in the case of a single Dirac monopole. For κ=-1 and κ=-2, the initial vortices deform into a vortex with a kink and a vortex ring, respectively.Peer reviewe
Three-dimensional skyrmions in spin-2 Bose–Einstein condensates
No full textWe introduce topologically stable three-dimensional skyrmions in the cyclic and biaxial nematic
phases of a spin-2 Bose–Einstein condensate. These skyrmions exhibit exceptionally high mapping
degrees resulting from the versatile symmetries of the corresponding order parameters. We show how
these structures can be created in existing experimental setups and study their temporal evolution and
lifetime by numerically solving the three-dimensional Gross–Pitaevskii equations for realistic
parameter values. Although the biaxial nematic and cyclic phases are observed to be unstable against
transition towards the ferromagnetic phase, their lifetimes are long enough for the skyrmions to be
imprinted and detected experimentally.peerReviewe
Evolution of an isolated monopole in a spin-1 Bose-Einstein condensate
No full textWe simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number κ. It is shown to decay into a coreless Anderson-Toulouse vortex if κ=1 and into a singular vortex with an empty core if κ=2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.Peer reviewe
Correcting non-independent and non-identically distributed errors with surface codes
No full textA common approach to studying the performance of quantum error correcting
codes is to assume independent and identically distributed single-qubit errors.
However, the available experimental data shows that realistic errors in modern
multi-qubit devices are typically neither independent nor identical across
qubits. In this work, we develop and investigate the properties of topological
surface codes adapted to a known noise structure by Clifford conjugations. We
show that the surface code locally tailored to non-uniform single-qubit noise
in conjunction with a scalable matching decoder yields an increase in error
thresholds and exponential suppression of sub-threshold failure rates when
compared to the standard surface code. Furthermore, we study the behaviour of
the tailored surface code under local two-qubit noise and show the role that
code degeneracy plays in correcting such noise. The proposed methods do not
require additional overhead in terms of the number of qubits or gates and use a
standard matching decoder, hence come at no extra cost compared to the standard
surface-code error correction
Towards Spin-Multiphoton Entanglement using Quantum Dots with Asymmetric Waveguide Coupling
No full textWe demonstrate selectively enhanced dipoles of an InAs quantum dot embedded in a nanophotonic waveguide, thereby forming optical cycling transitions, a basic tool for scalable spin-multiphoton entanglement generation. (C) 2020 The Author(s)</p
Correcting non-independent and non-identically distributed errors with surface codes
No full textA common approach to studying the performance of quantum error correcting codes is to assume independent and identically distributed single-qubit errors. However, the available experimental data shows that realistic errors in modern multi-qubit devices are typically neither independent nor identical across qubits. In this work, we develop and investigate the properties of topological surface codes adapted to a known noise structure by Clifford conjugations. We show that the surface code locally tailored to non-uniform single-qubit noise in conjunction with a scalable matching decoder yields an increase in error thresholds and exponential suppression of sub-threshold failure rates when compared to the standard surface code. Furthermore, we study the behaviour of the tailored surface code under local two-qubit noise and show the role that code degeneracy plays in correcting such noise. The proposed methods do not require additional overhead in terms of the number of qubits or gates and use a standard matching decoder, hence come at no extra cost compared to the standard surface-code error correction
Experimental Realization of a Dirac Monopole through the Decay of an Isolated Monopole
No full textWe experimentally observe the decay dynamics of deterministically created isolated monopoles in spin-1 Bose-Einstein condensates. As the condensate undergoes a change between magnetic phases, the isolated monopole gradually evolves into a spin configuration hosting a Dirac monopole in its synthetic magnetic field. We characterize in detail the Dirac monopole by measuring the particle densities of the spin states projected along different quantization axes. Importantly, we observe the spontaneous emergence of nodal lines in the condensate density that accompany the Dirac monopole. We also demonstrate that the monopole decay accelerates in weaker magnetic field gradients.peerReviewe
