arXiv subjects

James Williams

Publications and source records attributed to James Williams.

28 records · Page 2Linked to original sources

arXiv2021

Topological Dissipation in a Time-Multiplexed Photonic Resonator Network

Topological phases feature robust edge states that are protected against the effects of defects and disorder. The robustness of these states presents opportunities to design technologies that are tolerant to fabrication errors and resilient to environmental fluctuations. While most topological phases rely on conservative, or Hermitian, couplings, recent theoretical efforts have combined conservative and dissipative couplings to propose new topological phases for ultracold atoms and for photonics. However, the topological phases that arise due to purely dissipative couplings remain largely unexplored. Here we realize dissipatively coupled versions of two prominent topological models, the Su-Schrieffer-Heeger (SSH) model and the Harper-Hofstadter (HH) model, in the synthetic dimensions of a time-multiplexed photonic resonator network. We observe the topological edge state of the SSH and HH models, measure the SSH model's band structure, and induce a topological phase transition between the SSH model's trivial and topological phases. In stark contrast with conservatively coupled topological phases, the topological phases of our network arise from bands of dissipation rates that possess nontrivial topological invariants, and the edge states of these topological phases exhibit isolated dissipation rates that occur in the gaps between the bulk dissipation bands. Our results showcase the ability of dissipative couplings to enable time-reversal symmetry broken topological phases with nonzero Chern numbers, which have proven challenging to realize in the optical domain. Moreover, our time-multiplexed network, with its ability to implement multiple synthetic dimensions, dynamic and inhomogeneous couplings, and time-reversal symmetry breaking synthetic gauge fields, offers a flexible and scalable architecture for future work in synthetic dimensions.

physics.optics↗

arXiv2021

On finite $p$-groups with powerful subgroups

In this paper we investigate the structure of finite $p$-groups with the property that every subgroup of index $p^i$ is powerful for some $i$. For odd primes $p$, we show that under certain conditions these groups must be potent. Then, motivated by a question of Mann, we investigate in detail the case when all maximal subgroups are powerful. We show that for odd $p$ any finite $p$-group $G$ with all maximal subgroups powerful has a regular power structure - with precisely one exceptional case which is a $3$-group of maximal class and order $81$. To show this counterexample is unique we use a computational approach. We briefly discuss the case $p=2$ and some generalisations.

math.GR↗

arXiv2020

Remote capacitive sensing in two-dimension quantum-dot arrays

We investigate gate-defined quantum dots in silicon on insulator nanowire field-effect transistors fabricated using a foundry-compatible fully-depleted silicon-on-insulator (FD-SOI) process. A series of split gates wrapped over the silicon nanowire naturally produces a $2\times n$ bilinear array of quantum dots along a single nanowire. We begin by studying the capacitive coupling of quantum dots within such a 2$\times$2 array, and then show how such couplings can be extended across two parallel silicon nanowires coupled together by shared, electrically isolated, 'floating' electrodes. With one quantum dot operating as a single-electron-box sensor, the floating gate serves to enhance the charge sensitivity range, enabling it to detect charge state transitions in a separate silicon nanowire. By comparing measurements from multiple devices we illustrate the impact of the floating gate by quantifying both the charge sensitivity decay as a function of dot-sensor separation and configuration within the dual-nanowire structure.

cond-mat.mes-hall↗

arXiv2020

On the regular power structure of $p$-groups and applications

In this paper, we give elementary proofs of the Restricted Burnside Problem and the Hughes Conjecture for finite $p$-groups with Hall's regular power structure property. Moreover, in this setting we determine an explicit bound on the order of a finite $d$-generator $p$-group of fixed exponent. Further applications of $p$-groups with regular power structure are presented. For example, we give a short new proof of an important property of powerful $p$-groups; namely, that the minimal number of generators of a subgroup of such a group $G$ is at most the number needed to generate $G$.

math.GR↗

arXiv2020

Powerfully nilpotent groups of rank 2 or small order

In this paper we continue the study of powerfully nilpotent groups. These are powerful $p$-groups possessing a central series of a special kind. To each such group one can attach a powerful nilpotency class that leads naturally to the notion of a powerful coclass and classification in terms of an ancestry tree. In this paper we will give a full classification of powerfully nilpotent groups of rank $2$. The classification will then be used to arrive at a precise formula for the number of powerfully nilpotent groups of rank $2$ and order $p^{n}$. We will also give a detailed analysis of the ancestry tree for these groups. The second part of the paper is then devoted to a full classification of powerfully nilpotent groups of order up to $p^{6}$.

math.GR↗

arXiv2019

Quasi-powerful $p$-groups

In this paper we introduce the notion of a quasi-powerful $p$-group for odd primes $p$. These are the finite $p$-groups $G$ such that $G/Z(G)$ is powerful in the sense of Lubotzky and Mann. We show that this large family of groups shares many of the same properties as powerful $p$-groups. For example, we show that they have a regular power structure, and we generalise a result of Fernández-Alcober on the order of commutators in powerful $p$-groups to this larger family of groups. We also obtain a bound on the number of generators of a subgroup of a quasi-powerful $p$-group, expressed in terms of the number of generators of the group. We give an infinite family of examples which demonstrates this bound is close to best possible.

math.GR↗

arXiv2019

Normal Subgroups of Powerful $p$ -groups

In this note we show that if $p$ is an odd prime and $G$ is a powerful $p$-group with $N\leq G^{p}$ and $N$ normal in $G$, then $N$ is powerfully nilpotent. An analogous result is proved for $p=2$ when $N\leq G^{4}$.

math.GR↗

arXiv2019

Omegas of Agemos in Powerful Groups

In this note we show that for any powerful $p$-group $G$, the subgroup $Ω_{i}(G^{p^{j}})$ is powerfully nilpotent for all $i,j\geq1$ when $p$ is an odd prime, and $i\geq1$, $j\geq2$ when $p=2$. We provide an example to show why this modification is needed in the case $p=2$. Furthermore we obtain a bound on the powerful nilpotency class of $Ω_{i}(G^{p^{j}})$. We give an example to show that powerfully nilpotent characteristic subgroups of powerful $p$-groups need not be strongly powerful.

math.GR↗

arXiv2018

Powerfully nilpotent groups

We introduce a special class of powerful $p$-groups that we call powerfully nilpotent groups that are finite $p$-groups that possess a central series of a special kind. To these we can attach the notion of a powerful nilpotence class that leads naturally to a classification in terms of an `ancestry tree' and powerful coclass. We show that there are finitely many powerfully nilpotent $p$-groups of each given powerful coclass and develop some general theory for this class of groups. We also determine the growth of powerfully nilpotent groups of exponent $p^{2}$ and order $p^{n}$ where $p$ is odd. The number of these is $f(n)=p^{αn^{3}+o(n^{3})}$ where $α=\frac{9+4\sqrt{2}}{394}$. For the larger class of all powerful groups of exponent $p^{2}$ and order $p^{n}$, where $p$ is odd, the number is $p^{\frac{2}{27}n^{3}+o(n^{3})}$. Thus here the class of powerfully nilpotent $p$-groups is large while sparse within the larger class of powerful $p$-groups.

math.GR↗

arXiv2016

Spin correlation tensor for measurement of quantum entanglement in electron-electron scattering

We consider the problem of correct measurement of a quantum entanglement in the two-body electron-electron scattering. An expression is derived for a spin correlation tensor of a pure two-electron state. A geometrical measure of a quantum entanglement as the distance between two forms of this tensor in entangled and separable cases is presented. We prove that this measure satisfies properties of a valid entanglement measure: nonnegativity, discriminance, normalization, non-growth under local operations and classical communication. This measure is calculated for a problem of electron-electron scattering. We prove that it does not depend on the azimuthal rotation angle of the second electron spin relative to the first electron spin before scattering. Finally, we specify how to find a spin correlation tensor and the related measure of a quantum entanglement in an experiment with electron-electron scattering.

quant-ph↗