Seminars this week

In a world where a single company like Google consumed more energy than a country with population of about 20 million [1] in 2019 and this is growing exponentially, it is essential to find energy efficient approaches to make our computing needs sustainable. One potential solution is the use of nanoscale magnetic computing devices. Towards this end, energy efficient approaches based on electrical field control of nanoscale magnetism are pursued in our group: (i) strain mediated switching of the magnetization of nanomagnets [2]; (ii) creation and annihilation of magnetic skyrmions using direct voltage control of magnetic anisotropy (VCMA) [3]; and more recently magnetoionic control [4]. Such nanoscale magnetic devices have application to non-volatile memory [3], hardware AI [4, 5, 6] and quantum control of spins [7,8,9]. We will discuss skyrmion mediated voltage control of nanoscale magnetization that has potential for extremely energy efficient non-volatile memory [3] and are robust to switching errors in the presence of thermal noise, material and device inhomogeneities, while scaling to lateral dimensions of 20 nm and below [3]. Furthermore, energy efficient AI hardware can be realized with nanomagnetic devices. Multi-state nanoscale domain wall racetracks can be used as highly quantized synapses in deep neural networks [5] and convolutional neural networks, with overall improvement in area, energy, and latency by 13.8, 9.6, and 3.5 times respectively [5] compared to purely CMOS implementations. Additionally, interacting nanomagnets can be used for analog [6] and digital reservoir computing [6] and long-term prediction of temporal data [6]. We will specifically discuss experimental implementation of reservoir computing with magnetoionic devices [4] that do not need conversion of signals to GHz unlike when Spin Torque Nano Oscillators (STNOs) are used. In quantum computing, in addition to energy efficiency, one significant problem is implementing qubits in a scalable manner at temperatures of a few Kelvin. We argue that ensemble spin qubits may offer such a possibility [7]. Furthermore, by driving the magnetization of nanomagnets electrically, highly confined microwaves can be generated at the Larmor precession frequency of proximally located spins [8]. This can implement single-qubit quantum gates with fidelities approaching state-of-the-art in a scalable manner. Further confinement of microwaves using convergent-divergent skyrmion devices can implement even more localized and low footprint quantum control of spins [8]. New experimental and simulation results in these directions will be discussed [9]. References [1] FORBES Editor’s pick, Oct 21, 2020,04:26pm EDT [2] Nano Letters, 16, 1069, 2016; Nano Lett., 16, 5681, 2016; Appl. Phys. Lett. 121, 252401, 2022; https://arxiv.org/abs/2501.00980 [3] Nature Electronics 3, 539, 2020; Scientific Reports,11, 20914, 2021; Scientific Reports, 14, 17199, 2024 [4] https://arxiv.org/abs/2412.06964 [5] Nanotech. 31 145201, 2020, IEEE Access, 10, 84946, 2022; IEEE Trans. on Neural Networks and Learning Sys, 36, 4996, 2024. [6] Appl. Phys. Lett. 121, 102402, 2022; Comm. Phys. 6, 215, 2023; Neuromorph. Comput. Eng. 2 044011; IEEE Access, 11,124725, 2023 [7] https://arxiv.org/abs/2503.12071 [8] Communication Physics 5, 284, 2022; Physical Phys. Rev. Applied 22, 06407, 2024. [9] https://arxiv.org/abs/2407.14018

In this talk, we derive the coupled dynamics between the bubble wall and the plasma from first principles using nonequilibrium quantum field theory. The commonly used equation of motion of the bubble wall in the kinetic approach is shown to be incomplete. In the language of the two-particle-irreducible effective action, the conventional equation misses higher-loop terms generated by the condensate-particle type vertices (e.g.,~ φϕχ2, where φ is the background field describing the bubble wall, ϕ the corresponding particle excitation and χ another particle species in the plasma). From the missing terms, we identify an additional dissipative friction which is contributed by particle production processes from the condensate-particle type vertices. We also show how other transmission processes beyond the 1-to-1 elementary transmission studied in the literature for ultrarelativistic bubble walls, e.g., 1-to-1 mixing and 1-to-2 transition radiation, can be understood from the kinetic approach.

Centuries after Newton described a gravitational force, Einstein revolutionised our understanding of space, time and gravity in his general theory of relativity. Another 100 years later marked the beginning of a new era of astronomy, in which spacetime oscillations (a.k.a. gravitational waves) are used to detect the inspiral and merger of the most compact objects in the Universe. In my talk, I will review the methods and results of 10 years of gravitational-wave astronomy that saw already about 300 detections, most of them of black-hole mergers. I will discuss some insights and open questions that emerged from those observations and illustrate that Einstein's picture still holds for these most violent cosmic collisions.

In this talk, I will review two device concepts based on nonlinear dissipative magnetic dynamics. First, we revisit the problem of spin superfluidity, which has been predicted to facilitate coherent spin transport. We propose to both exhibit and exploit this elusive transport phenomenon via the "spin-superfluid quantum interference device" (spin SQUID) — inspired by its superconducting (rf SQUID) and superfluid helium (SHeQUID) counterparts. In particular, we discuss its potential electric-field sensing functionality based on the microwave response of the simplest pertinent structure: a magnetic ring with a single weak link. In the second part of the talk, we systematically address the pseudo-Hermitian physics of dynamically-coupled magnetic macrospins, with a focus on non-Hermitian mode hybridization and its potential utility as a scalable building block for dynamic Ising machines.

As high-performant sources of single photons, epitaxial quantum dots can be considered as a semiconductor launchpad for quantum photonic technologies. There is still a variety of challenges to tackle on the road to an ideal source of single or entangled photons for quantum photonic applications. Here, we present an overview of recent developments in our group on the engineering of single-photon sources for quantum photonic applications made from III-V semiconductor quantum dots grown by molecular beam epitaxy. By integrating InAs/InP quantum dots into circular Bragg grating resonators, Purcell-enhanced single-photon emission with a Purcell factor of ≈7 in the telecom C-band was achieved. Low multi-photon emission probabilities are obtained, and Hong-Ou-Mandel two-photon interference is demonstrated. By further improving the epitaxial growth, photonic design and excitation schemes, we were able to obtain record two photon interference visibilities. We furthermore report on recent progress made in our group towards deterministic generation of one-dimensional photonic cluster states directly in the telecom C-Band as a resource for quantum repeaters and quantum computers.