Publications

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On the generality of symmetry breaking and dissipative freezing in quantum trajectories

Joseph Tindall, Dieter Jaksch, Carlos Sánchez Muñoz

SciPost Phys. Core 6, 004 (2023) · published 25 January 2023 | · pdf

Recently, several studies involving open quantum systems which possess a strong symmetry have observed that every individual trajectory in the Monte Carlo unravelling of the master equation will dynamically select a specific symmetry sector to 'freeze' into in the long-time limit. This phenomenon has been termed 'dissipative freezing', and in this paper we argue, by presenting several simple mathematical perspectives on the problem, that it is a general consequence of the presence of a strong symmetry in an open system with only a few exceptions. Using a number of example systems we illustrate these arguments, uncovering an explicit relationship between the spectral properties of the Liouvillian in off-diagonal symmetry sectors and the time it takes for freezing to occur. In the limiting case that eigenmodes with purely imaginary eigenvalues are manifest in these sectors, freezing fails to occur. Such modes indicate the preservation of information and coherences between symmetry sectors of the system and can lead to phenomena such as non-stationarity and synchronisation. The absence of freezing at the level of a single quantum trajectory provides a simple, computationally efficient way of identifying these traceless modes.

2 citations
Nevanlinna.jl: A Julia implementation of Nevanlinna analytic continuation

Kosuke Nogaki, Jiani Fei, Emanuel Gull, Hiroshi Shinaoka

SciPost Phys. Codebases 19 (2023) · published 13 November 2023 | · pdf

We introduce a Julia implementation of the recently proposed Nevanlinna analytic continuation method. The method is based on Nevanlinna interpolants and inherently preserves the causality of a response function due to its construction. For theoretical calculations without statistical noise, this continuation method is a powerful tool to extract real-frequency information from numerical input data on the Matsubara axis. This method has been applied to first-principles calculations of correlated materials. This paper presents its efficient and full-featured open-source implementation of the method including the Hamburger moment problem and smoothing.

2 citations
Introduction to Monte Carlo for matrix models

Raghav G. Jha

SciPost Phys. Lect. Notes 46 (2022) · published 4 April 2022 | · pdf

We consider a wide range of matrix models and study them using the Monte Carlo technique in the large N limit. The results we obtain agree with exact analytic expres- sions and recent numerical bootstrap methods for models with one and two matrices. We then present new results for several unsolved multi-matrix models where no other tool is yet available. In order to encourage an exchange of ideas between different numerical approaches to matrix models, we provide programs in Python that can be easily modified to study potentials other than the ones discussed. These programs were tested on a laptop and took between a few minutes to several hours to finish depending on the model, N, and the required precision.

2 citations
Adiabatic state distribution using anti-ferromagnetic spin systems

Koen Groenland

SciPost Phys. 6, 011 (2019) · published 23 January 2019 | · pdf

Transporting quantum information is an important prerequisite for quantum computers. We study how this can be done in Heisenberg-coupled spin networks using adiabatic control over the coupling strengths. We find that qudits can be transferred and entangled pairs can be created between distant sites of bipartite graphs with a certain balance between the maximum spin of both parts, extending previous results that were limited to linear chains. The transfer fidelity in a small star-shaped network is numerically analysed, and possible experimental implementations are discussed.

2 citations
Spontaneous symmetry breaking in free theories with boundary potentials

Vladimir Procházka, Alexander Söderberg

SciPost Phys. 11, 035 (2021) · published 19 August 2021 | · pdf

Patterns of symmetry breaking induced by potentials at the boundary of free $O(N)$-models in $d=3- \epsilon$ dimensions are studied. We show that the spontaneous symmetry breaking in these theories leads to a boundary RG flow ending with $N - 1$ Neumann modes in the IR. The possibility of fluctuation-induced symmetry breaking is examined and we derive a general formula for computing one-loop effective potentials at the boundary. Using the $\epsilon$-expansion we test these ideas in an $O(N)\oplus O(N)$-model with boundary interactions. We determine the RG flow diagram of this theory and find that it has an IR-stable critical point satisfying conformal boundary conditions. The leading correction to the effective potential is computed and we argue the existence of a phase boundary separating the region flowing to the symmetric fixed point from the region flowing to a symmetry-broken phase with a combination of Neumann and Dirchlet boundary conditions.

2 citations
Study of deuteron-proton backward elastic scattering at intermediate energies

Nadezhda Ladygina

SciPost Phys. Proc. 3, 053 (2020) · published 27 February 2020 | · pdf

We study deuteron- proton elastic scattering in the deuteron energy range between 500 MeV and 2~GeV at the cms scattering angle $\theta^*\ge 140^\circ$. The reaction is considered in the relativistic multiple scattering expansion framework. The four reaction mechanisms are included into consideration: one-nucleon exchange, single scattering, double scattering, and the term corresponding to the delta excitation in the intermediate state. The model is applied to describe the angular dependence of the differential cross section at the deuteron energies of between 880 and 1300 MeV. Also the energy dependence of the differential cross section and polarisation observables such as tensor analyzing power $T_{20}$ and polarization transfer from the deuteron to proton $\varkappa$ are considered at the scattering angle equal to 180$^\circ$. Contributions of the different reaction mechanisms into the reaction amplitude are demonstrated in comparison with the existing experimental data.

2 citations
On the quantum simulation of complex networks

Duarte Magano, João Moutinho, Bruno Coutinho

SciPost Phys. Core 6, 058 (2023) · published 24 August 2023 | · pdf

Quantum walks provide a natural framework to approach graph problems with quantum computers, exhibiting speedups over their classical counterparts for tasks such as the search for marked nodes or the prediction of missing links. Continuous-time quantum walk algorithms assume that we can simulate the dynamics of quantum systems where the Hamiltonian is given by the adjacency matrix of the graph. It is known that such can be simulated efficiently if the underlying graph is row-sparse and efficiently row-computable. While this is sufficient for many applications, it limits the applicability for this class of algorithms to study real world complex networks, which, among other properties, are characterized by the existence of a few densely connected nodes, called hubs. In other words, complex networks are typically not row-sparse, even though the average connectivity over all nodes can be very small. In this work, we extend the state-of-the-art results on quantum simulation to graphs that contain a small number of hubs, but that are otherwise sparse. Hopefully, our results may lead to new applications of quantum computing to network science.

2 citations
Interplay between chiral media and perfect electromagnetic conductor plates: Repulsive vs. attractive Casimir force transitions

Thomas Oosthuyse, David Dudal

SciPost Phys. 15, 213 (2023) · published 28 November 2023 | · pdf

We determine the Casimir energies and forces in a variety of potentially experimentally viable setups, consisting of parallel plates made of perfect electromagnetic conductors (PEMCs), which generalize perfect electric conductors (PECs) and perfect magnetic conductors (PMCs), and Weyl semimetals (WSMs). Where comparison is possible, our results agree with the Casimir forces calculated elsewhere in the literature, albeit with different methods. We find a multitude of known but also new cases where repulsive Casimir forces are in principle possible, but restricting the setup to PECs combined with the aforementioned WSM geometry, results in purely attractive Casimir forces.

2 citations
A time-dependent Hartree-Fock study of triple-alpha dynamics

Paul D. Stevenson, J. L. Willerton

SciPost Phys. Proc. 3, 047 (2020) · published 27 February 2020 | · pdf

Time-dependent Hartree-Fock calculations have been performed for fusion reactions of He-4 + He-4 -> Be*-8, followed by He-4 + Be*-8 . Depending on the orientation of the initial state, a linear chain vibrational state or a triangular vibration is found in 12C, with transitions between these states observed. The vibrations of the linear chain state and the triangular state occur at ~9 and 4 MeV respectively.

2 citations
Lucky planets: how circum-binary planets survive the supernova in one of the inner-binary components

Fedde Fagginger Auer, Simon Portegies Zwart

SciPost Astro. 2, 002 (2022) · published 1 April 2022 | · pdf

A planet hardly ever survives the supernova of the host star in a bound orbit, because mass loss in the supernova and the natal kick imparted to the newly formed compact object cause the planet to be ejected. A planet in orbit around a binary has a considerably higher probability to survive the supernova explosion of one of the inner binary stars. In those cases, the planet most likely remains bound to the companion of the exploding star, whereas the compact object is ejected. We estimate this to happen to $\sim 1/33$ the circum-binary planetary systems. These planetary orbits tend to be highly eccentric ($e \ {\raise-.5ex\hbox{$\buildrel>\over\sim$}}\ 0.9$), and $\sim 20$% of these planets have retrograde orbits compared to their former binary. The probability that the planet as well as the binary (now with a compact object) remains bound is about ten times smaller ($\sim 3\cdot 10^{-3}$). We then expect the Milky way Galaxy to host $\ {\raise-.5ex\hbox{$\buildrel<\over\sim$}}\ 10$ x-ray binaries that are still orbited by a planet, and $\ {\raise-.5ex\hbox{$\buildrel<\over\sim$}}\ 150$ planets that survived in orbit around the compact object's companion. These numbers should be convolved with the fraction of massive binaries that is orbited by a planet.

2 citations

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