Exotic vortices in superfluids and matrix product states for quantum optimization and machine learning
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The interest in vortices and vortex lattices was sparked by the prediction of quantisation of circulation by Onsager in the 1940s. The field has since developed dramatically and attracted a lot of interest across the physics community. In this dissertation we study vortices in two different systems: a rotating, Rabi-coupled, two-component Bose--Einstein condensate (BEC) and a rotating spinor-BEC, in two spatial dimensions.
Vortex molecules can form in a two-component superfluid when a Rabi field drives transitions between the
two components. We study the ground state of an infinite system of vortex molecules in two dimensions, using
a numerical scheme which makes no use of the lowest Landau level approximation.
We find the ground state lattice geometry for different values of intercomponent interactions and strength of the Rabi field. In the limit of large field, when molecules are tightly bound, we develop a complementary analytical description. The energy
governing the alignment of molecules on a triangular lattice is found to correspond to that of an infinite system of
two-dimensional quadrupoles, which may be written in terms of an elliptic function
In the
Matrix product state (MPS) based methods are currently regarded as one of the most powerful tools to study the low-energy physics of one-dimensional many-body quantum systems. In this work we find a connection between MPS in the left canonical form and the Stiefel manifold. This relation allows us to constrain the optimisation to this subspace of the otherwise larger MPS manifold. We find that our method suffers from two undesirable features. First, the need of a large unit cell to achieve machine precision. Second, because of the presence of the power method in the variational energy expression, it is possible for the convergence process to get stuck in regions of the Stiefel manifold where the modulus of the second largest eigenvalue of the transfer matrix is very close to one.
Since the foundation of the field of