Transmon-based quantum computers from a many-body perspective

The quest for quantum computers is in full swing. Over the past decade, the frontiers of quantum computing have broadened from exploring few-qubit devices to developing viable multi-qubit processors. One of the protagonists of the present era is the superconducting transmon qubit. By harmoniously combining applied engineering with fundamental research in computer science and physics, transmon-based quantum processors have matured to a remarkable level. Their applications include the study of topological and nonequilibrium states of matter, and it is argued that they have already ushered us into the era of quantum advantage. Nevertheless, building a quantum computer that can solve problems of practical relevance remains a massive challenge. As the field progresses with unbridled panache, the question of whether we have a comprehensive picture of the potential dangers lurking in the wings acquires increasing urgency. In particular, it needs to be thoroughly clarified whether, with viable quantum computers of O(50) qubits at hand, new and hitherto unconsidered obstacles associated with the multi-qubit nature can emerge. For example, the high accuracy of quantum gates in small-scale devices is hard to obtain in larger processors. On the hardware side, the unique requirements posed by large quantum computers have already spawned new approaches to qubit design, control, and readout. This thesis introduces a novel, less applied perspective on multi-qubit processors. Specifically, we fuse the field of quantum engineering and many-body physics by applying concepts from the theories of localization and quantum chaos to multi-transmon arrays. From a many-body per- spective, transmon architectures are synthetic systems of interacting and disordered nonlinear quantum oscillators. While a certain amount of coupling between the transmons is indispensable for performing elementary gate operations, a delicate balancing with disorder—site-to-site varia- tions in the qubit frequencies—is required to prevent locally injected information from dispersing in extended many-body states. Transmon research has established different modalities to cope with this dilemma between inefficiency (slow gates due to small coupling or large disorder) and information loss (large couplings or too small disorder). We analyze small instances of transmon quantum computers in exact diagonalization studies, using contemporary quantum processors as blueprints. Scrutinizing the spectrum, many-body wave functions, and qubit-qubit correlations for experimentally relevant parameter regimes reveals that some of the prevalent transmon de- sign schemes operate close to a region of uncontrollable chaotic fluctuations. Furthermore, we establish a close link between the advent of chaos in the classical limit and the emergence of quantum chaotic signatures. Our concepts complement the traditional few-qubit picture that is commonly exploited to optimize device configurations on small scales. Destabilizing mecha- nisms beyond this local scale can be detected from our fresh perspective. This suggests that techniques developed in the field of many-body localization should become an integral part of future transmon processor engineering

Generic Field-Driven Phenomena in Kitaev Spin Liquids: Canted Magnetism and Proximate Spin Liquid Physics

Topological spin liquids in two spatial dimensions are stable phases in the presence of a small magnetic field, but may give way to field-induced phenomena at intermediate field strengths. Sandwiched between the low-field spin liquid physics and the high-field spin-polarized phase, the exploration of magnetic phenomena in this intermediate regime however often remains elusive to controlled analytical approaches. Here we numerically study such intermediate-field magnetic phenomena for two representative Kitaev models (on the square-octagon and decorated honeycomb lattice) that exhibit either Abelian or non-Abelian topological order in the low-field limit. Using a combination of exact diagonalization and density matrix renormalization group techniques, as well as linear spin-wave theory, we establish the generic features of Kitaev spin liquids in an external magnetic field. While ferromagnetic models typically exhibit a direct transition to the polarized state at a relatively low field strength, antiferromagnetic couplings not only substantially stabilizes the topological spin liquid phase, but generically lead to the emergence of a distinct field-induced intermediate regime, separated by a crossover from the high-field polarized regime. Our results suggest that, for most lattice geometries, this regime generically exhibits significant spin canting, antiferromagnetic spin-spin correlations, and an extended proximate spin liquid regime at finite temperatures. Notably, we identify a symmetry obstruction in the original honeycomb Kitaev model that prevents, at least for certain field directions, the formation of such canted magnetism without breaking symmetries -- consistent with the recent numerical observation of an extended gapless spin liquid in this case.Comment: 13 pages, 16 figures (Appendix: 7 pages, 7 figures

Transmon platform for quantum computing challenged by chaotic fluctuations

From the perspective of many body physics, the transmon qubit architectures currently developed for quantum computing are systems of coupled nonlinear quantum resonators. A significant amount of intentional frequency detuning (disorder) is required to protect individual qubit states against the destabilizing effects of nonlinear resonator coupling. Here we investigate the stability of this variant of a many-body localized (MBL) phase for system parameters relevant to current quantum processors of two different types, those using untunable qubits (IBM type) and those using tunable qubits (Delft/Google type). Applying three independent diagnostics of localization theory - a Kullback-Leibler analysis of spectral statistics, statistics of many-body wave functions (inverse participation ratios), and a Walsh transform of the many-body spectrum - we find that these computing platforms are dangerously close to a phase of uncontrollable chaotic fluctuations.Comment: 9 pages, 8 figure

Bulk-Processed Pd Nanocube-Poly(methyl methacrylate) Nanocomposites as Plasmonic Plastics for Hydrogen Sensing

Nanoplasmonic hydrogen sensors are predicted to play a key role in safety systems of the emerging hydrogen economy. Pd nanoparticles are the active material of choice for sensor prototype development due to their ability to form a hydride at ambient conditions, which creates the optical contrast. Here, we introduce plasmonic hydrogen sensors made from a thermoplastic nanocomposite material, that is, a bulk material that can be molded with standard plastic processing techniques, such as extrusion and three-dimensional (3D) printing, while at the same time being functionalized at the nanoscale. Specifically, our plasmonic plastic is composed of hydrogensensitive and plasmonically active Pd nanocubes mixed with a poly(methyl methacrylate) matrix, and we optimize it by characterization from the atomic to the macroscopic level. We demonstrate meltprocessed deactivation-resistant plasmonic hydrogen sensors, which retain full functionality even after SO weeks. From a wider perspective, we advertise plasmonic plastic nanocomposite materials for application in a multitude of active plasmonic technologies since they provide efficient scalable processing and almost endless functional material design opportunities via tailored polymer- colloidal nanocrystal combinations

Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic Hydrogen Sensing

Hydrogen (H2) sensors that can be produced en masse with cost-effective manufacturing tools are critical for enabling safety in the emerging hydrogen economy. The use of melt-processed nanocomposites in this context would allow the combination of the advantages of plasmonic hydrogen detection with polymer technology; an approach which is held back by the slow diffusion of H2 through the polymer matrix. Here, we show that the use of an amorphous fluorinated polymer, compounded with colloidal Pd nanoparticles prepared by highly scalable continuous flow synthesis, results in nanocomposites that display a high H2 diffusion coefficient in the order of 10-5 cm2 s-1. As a result, plasmonic optical hydrogen detection with melt-pressed fluorinated polymer nanocomposites is no longer limited by the diffusion of the H2 analyte to the Pd nanoparticle transducer elements, despite a thickness of up to 100 μm, thereby enabling response times as short as 2.5 s at 100 mbar (10 vol. %) H2. Evidently, plasmonic sensors with a fast response time can be fabricated with thick, melt-processed nanocomposites, which paves the way for a new generation of robust H2 sensors

Optimal inference with suboptimal models:Addiction and active Bayesian inference

When casting behaviour as active (Bayesian) inference, optimal inference is defined with respect to an agent's beliefs - based on its generative model of the world. This contrasts with normative accounts of choice behaviour, in which optimal actions are considered in relation to the true structure of the environment - as opposed to the agent's beliefs about worldly states (or the task). This distinction shifts an understanding of suboptimal or pathological behaviour away from aberrant inference as such, to understanding the prior beliefs of a subject that cause them to behave less 'optimally' than our prior beliefs suggest they should behave. Put simply, suboptimal or pathological behaviour does not speak against understanding behaviour in terms of (Bayes optimal) inference, but rather calls for a more refined understanding of the subject's generative model upon which their (optimal) Bayesian inference is based. Here, we discuss this fundamental distinction and its implications for understanding optimality, bounded rationality and pathological (choice) behaviour. We illustrate our argument using addictive choice behaviour in a recently described 'limited offer' task. Our simulations of pathological choices and addictive behaviour also generate some clear hypotheses, which we hope to pursue in ongoing empirical work

Ataxin-3 Plays a Role in Mouse Myogenic Differentiation through Regulation of Integrin Subunit Levels

BACKGROUND: During myogenesis several transcription factors and regulators of protein synthesis and assembly are rapidly degraded by the ubiquitin-proteasome system (UPS). Given the potential role of the deubiquitinating enzyme (DUB) ataxin-3 in the UPS, and the high expression of the murine ataxin-3 homolog in muscle during embryogenesis, we sought to define its role in muscle differentiation. METHODOLOGY/PRINCIPAL FINDINGS: Using immunofluorescence analysis, we found murine ataxin-3 (mATX3) to be highly expressed in the differentiated myotome of E9.5 mouse embryos. C2C12 myoblasts depleted of mATX3 by RNA interference exhibited a round morphology, cell misalignment, and a delay in differentiation following myogenesis induction. Interestingly, these cells showed a down-regulation of alpha5 and alpha7 integrin subunit levels both by immunoblotting and immunofluorescence. Mouse ATX3 was found to interact with alpha5 integrin subunit and to stabilize this protein by repressing its degradation through the UPS. Proteomic analysis of mATX3-depleted C2C12 cells revealed alteration of the levels of several proteins related to integrin signaling. CONCLUSIONS: Ataxin-3 is important for myogenesis through regulation of integrin subunit levels.This work was financed by the Fundacao para a Ciencia e a Tecnologia (FCT) (POCI/SAU-MMO/60412/2002) and by National Institutes of Health/National Institute of Neurological Disorders and Stroke (NIH/NINDS) grant RO1 NS038712 to HLP. MCC, FB, AJR, and RJT were supported by the FCT fellowships (SFRH/BD/9759/2003 and SFRH/BPD/28560/2006), (SFRH/BPD/17368/2004), (SFRH/BD/17066/2004), (SFRH/BD/29947/2006), respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

The role of proteomics in depression research

Depression is a severe neuropsychiatric disorder affecting approximately 10% of the world population. Despite this, the molecular mechanisms underlying the disorder are still not understood. Novel technologies such as proteomic-based platforms are beginning to offer new insights into this devastating illness, beyond those provided by the standard targeted methodologies. Here, we will show the potential of proteome analyses as a tool to elucidate the pathophysiological mechanisms of depression as well as the discovery of potential diagnostic, therapeutic and disease course biomarkers

Stability of the Weyl-semimetal phase on the pyrochlore lattice

Motivated by the proposal of a Weyl-semimetal phase in pyrochlore iridates, we consider a Hubbard-type model on the pyrochlore lattice. To shed light on the question as to why such a state has not been observed experimentally, its robustness is analyzed. On the one hand, we study the possible phases when the system is doped. Magnetic frustration favors several phases with magnetic and charge order that do not occur at half filling, including additional Weyl-semimetal states close to quarter filling. On the other hand, we search for density waves that break translational symmetry and destroy the Weyl-semimetal phase close to half filling. The uniform Weyl semimetal is found to be stable, which we attribute to the low density of states close to the Fermi energy

Field stability of Majorana spin liquids in antiferromagnetic Kitaev models

Magnetic fields can give rise to a plethora of phenomena in Kitaev spin systems, such as the formation of non-trivial spin liquids in two and three spatial dimensions. For the original honeycomb Kitaev model, it has recently been observed that the sign of the bond-directional exchange is of crucial relevance for the field-induced physics, with antiferromagnetic couplings giving rise to an intermediate spin liquid regime between the low-field gapped Kitaev spin liquid and the high-field polarized state, which is not present in the ferromagnetically coupled model. Here, by employing a Majorana mean-field approach for a magnetic field pointing along the [001] direction, we present a systematic study of field-induced spin liquid phases for a variety of two and three-dimensional lattice geometries. We find that antiferromagnetic couplings generically lead to (i) spin liquid phases that are considerably more stable in field than those for ferromagnetic couplings, and (ii) an intermediate spin liquid phase which arises from a change in the topology of the Majorana band structure. Close inspection of the mean-field parameters reveal that the intermediate phase occurs due to a field-driven sign change in an 'effective' $z$-bond energy parameter. Our results clearly demonstrate the richness of the Majorana physics of the antiferromagnetic Kitaev models, in comparison to their ferromagnetic counterparts.Comment: 14 pages, 17 figures, Appendix: 6 pages, 4 figure