Charge Transfer Excitations, Pair Density Waves, and Superconductivity in Moiré Materials

abstract
Transition metal dichalcogenide heterobilayers are a new class of tunable Moiré systems attracting interest as quantum simulators of strongly-interacting electrons in two dimensions. In this talk, I will argue that the correlated insulator recently observed in WSe2/WS2 at half filling is a charge-transfer insulator similar to cuprates, in which hole doping induces a charge from anion-like to cation-like orbitals at different locations in the Moiré unit cell [1]. Interestingly, at small doping, tightly-bound charge-2e "trimers" consisting of two holes bound to a charge-transfer exciton can form to lower the total electrostatic repulsion [2]. When the bandwidth of doped holes is small, trimers crystallize into insulating pair density waves at a sequence of commensurate doping levels. When the bandwidth becomes comparable to the pair binding energy, itinerant holes and charge-2e trimers interact resonantly, leading to unconventional superconductivity similar to superfluidity in an ultracold Fermi gas near Feshbach resonance.

Charles Marcus, Center for Quantum Devices and Microsoft Quantum Lab, Copenhagen

Zero-field Topological Superconductivity in Ferromagnetic Hybrid Nanowires

abstract
We report transport measurements and tunneling spectroscopy in hybrid nanowires with epitaxial layers of superconducting Al and the ferromagnetic insulator EuS, grown on semiconducting InAs nanowires. In devices where the Al and EuS covered facets overlap, we infer a remanent effective Zeeman field of order 1 T, and observe stable zero-bias conductance peaks in tunneling spectroscopy into the end of the nanowire, consistent with topological superconductivity at zero applied field. Hysteretic features in critical current and tunneling spectra as a function of applied magnetic field support this picture. Nanowires with non-overlapping Al and EuS covered facets do not show comparable features. Topological superconductivity in zero applied field allows new device geometries and types of control.

Vidya Madhavan, Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign

Microscopic evidence for a chiral superconducting order parameter in the heavy fermion superconductor \(\text{UTe}_2\)

abstract
Spin-triplet superconductivity is a condensate of electron pairs with spin-1 and an odd-parity wavefunction. A particularly interesting manifestation of triplet pairing is a chiral p-wave state which is topologically non-trivial and a natural platform for realizing Majorana edge modes. Triplet pairing is however rare in solid state systems. The best-known example of chiral spin-triplet paring is the superfluid 3He-A phase and over the last few decades, there has been an intensive search for potential spin-triplet superconductors in solid-state systems. Since pairing is most naturally mediated by ferromagnetic spin fluctuations, uranium based heavy fermion systems containing f-electron elements that can harbor both strong correlations and magnetism are considered ideal candidate spin-triplet superconductors. In this work I will present scanning tunneling microscopy (STM) data on the newly discovered heavy fermion superconductor, $\textrm{UTe}_2$ with a $T_{\textrm{SC}}$ of 1.6K. I will show signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unit-cell. STM spectroscopy at step edges show signatures of chiral in-gap states, predicted to exist at the boundaries of a topological superconductor. Combined with existing data indicating triplet pairing, the presence of chiral edge states suggests that UTe2 is a strong candidate material for chiral-triplet topological superconductivity.

Nicola Spaldin, ETH Zürich

From Materials to Cosmology: Studying the early universe under the microscope

abstract
Uncovering the behavior of the early universe just after the Big Bang is an intriguing fundamental activity that is extraordinarily difficult because of insurmountable issues associated with replaying the Big Bang in the laboratory. One route to the answer -- which lies at the intersection between cosmology and materials science -- is to use laboratory materials to test the laws proposed for the formation of defects such as cosmic strings in the early universe. Here I will show that a popular multiferroic material, hexagonal yttrium manganite -- with its coexisting magnetic, ferroelectric and structural phase transitions -- generates the crystallographic equivalent of cosmic strings. I will describe how straightforward solution of the Schroedinger equation for yttrium manganite allows the important features of its behavior to be identified and quantified, and present experimental results of what seem to be the first unambiguous demonstration of the expected cosmological scaling laws in a real material. I will end with a plea for help with imaging the multiferroic "cosmic strings", and show some recent data suggesting that things might be less unambiguous than they seem.

Klaus Mølmer, Aarhus University

Quantum interactions with pulses of radiation

abstract
How does a quantum system interact with a travelling pulse of quantum radiation, prepared for example in a number state or a coherent state of light? While crucial for multiple effects in quantum optics and for the entire concept of flying and stationary qubits, quantum optics textbooks do not provide a formal description of this foundational and elementary interaction process. In this lecture, I shall introduce a new (and simple) master equation that describes the joint quantum state of travelling pulses of quantized radiation and local quantum systems. Applications of the theory will be presented for recent experiments with qubits and non-linear resonators interacting with pulses of optical, microwave and acoustic radiation.

Ali Yazdani, Princeton University

Strongly interacting electronic states of a moiré superlattice

abstract
Interactions among electrons or the topology of their energy bands can create novel quantum phases of matter. The discovery of electronic bands with flat energy dispersion in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new correlated and topological electronic phases. From the initial discovery of correlated insulators, superconductivity, and more recent reports of topological phases in various transport studies it is clear that this is a very rich electronic system. To understand the underlying mechanisms for formation of various phases and their properties, we have carried out a series of scanning tunneling microscopy (STM) and spectroscopy (STS) studies of MATBG while tuning its carrier concentration with a gate. Such density-tuned STS studies can reveal a variety of key information that are impossible to obtain from other techniques. For example, in our first study, we showed that while the properties of the flat bands in MATBG can be understood with a weakly interacting picture when they are fully occupied or empty, at partial filling such models fails to understand their spectroscopic properties [1]. In a subsequent study, we showed that within this highly interacting states there are a cascade of transitions occurring as a function of carrier density, at each integer filling—where insulating phases form at low temperatures [2]. We understand these transitions as a direct consequence of Coulomb interactions and spin/valley quantum degeneracy of this systems splitting the degenerate flat bands into Hubbard sub-bands. I will describe these studies as well as our ongoing efforts to probe the nature of insulating, topological, and superconducting states In MATBG using millikelvin STM measurements.

Nurit Avraham, Weizmann Institute

Multiple topological facets of Bismuth

abstract
Bismuth, due to its large spin-orbit coupling, plays a fundamental role in
many topological materials. Yet the topological classification of pure Bismuth has
remained, thus far, rather ambiguous. While some theoretical models indicate its trivial
topological nature, other theoretical and experimental studies suggest non-trivial
topological classifications, such as a strong or a higher order topological insulator. I will
explain the origin for this ambiguity and present scanning tunneling microscopy data in
which we resolve the topological classification of Bismuth, as a strong topological
insulator with weak indices, by spectroscopically mapping the response of its boundary
modes to a topological defect in the form of a screw dislocation [1]. Next, I will present
our work on Bi₂TeI, which consists of a stack of Bi-bilayers. Our data shows that in this
form the topological nature is of a dual topological insulator [2]. Bi 2 TeI hosts a weak topological insulator surface state on its ‘side’ facets and a topological crystalline
insulator surface state protected by mirror symmetry on its top and bottom facets. We
visualize the topological crystalline surface states and show their sensitivity to mirror
symmetry-breaking as well as the one dimensional channels, derived from the 2D weak
topological insulator states, which run along step-edges. We studied the coexistence of
the two types of states on step-edges, where both facets join. Our measurements reveal
that the two types of states remain well decoupled from one another due to separation
in momentum space and in energy. We show, however, that this protection is
susceptible to strong disorder.

Nathalie de Leon, Princeton University

Correlating surface spectroscopy with qubit measurements to systematically eliminate sources of noise

abstract
The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. I will describe our recent efforts to use X-ray and photoelectron spectroscopies, diffraction techniques, and morphology characterization to understand sources of noise at the diamond surface. By correlating this spectroscopic data with single spin measurements, we have been able to devise new surface processing and termination techniques to stabilize shallow NV centers within 5 nm of the surface with coherence times exceeding 100 μs (PRX 9, 031052, https://doi.org/10.1103/PhysRevX.9.03...). Specifically, we are able to demonstrate reversible and reproducible control over the top layer of atoms. These highly coherent, shallow NV centers will provide a platform for sensing and imaging down to the scale of single atoms.
In fact, many platforms for quantum technologies are limited by noise and loss arising from uncontrolled defects at surfaces and interfaces, including superconducting qubits, trapped ions, and semiconductor quantum dots. Our approach for correlating surface spectroscopy techniques with single qubit measurements to realize directed improvements is generally applicable to many systems, and I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. We have fabricated two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device (arXiv:2003.00024, https://arxiv.org/abs/2003.00024). We have observed increased lifetimes for many devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.

Titus Neupert, University of Zürich

Exceptional Topological Insulators

abstract
Since their discovery, three-dimensional topological insulators have become the focal point for research on topological quantum matter. Their key feature are conducting surface states resembling a single species of gapless Dirac electrons. Transcending the realm of quantum matter, the topological insulator phase has since been realized in many different settings including metamaterials, such as photonic and phononic crystals.
Most of such metamaterial platforms are accidentally or tunably lossy, such that their effective Hamiltonian description involves non-Hermitian terms due to the lack of energy conservation. The same holds for interacting electronic quantum systems in which quasiparticles attain a finite lifetime, for instance. Starting from the initial classification of topological matter based on Hermitian Hamiltonians, the study of systems with non-negligible loss and gain calls for an extension to non-Hermitian topological matter.
In this colloquium, I will introduce the exceptional topological insulator (ETI), a three-dimensional non-Hermitian topological state of matter that features exotic non-Hermitian surface states which can only exist within the three-dimensional topological bulk embedding. I will show how this phase can evolve from a Weyl semimetal or Hermitian three-dimensional topological insulator close to criticality when quasiparticles acquire a finite lifetime. The ETI does not require any symmetry to be stabilized. It is characterized by a bulk energy point gap, and exhibits robust surface states that cover the bulk gap as a single sheet of complex eigenvalues or with a single surface exceptional point. The ETI can be induced in genuine solid-state and metamaterial systems, thereby setting a paradigm for non-Hermitian topological matter.

Mohammad Hafezi, University of Maryland and JQI

Quantum optics meets correlated electrons

abstract
One of the key challenges in the development of quantum technologies is the control of light-matter interaction at the quantum level where individual excitations matter. During the past couple of decades, there has been tremendous progress in controlling individual photons and other excitations such as spin, excitonic, phononic in solid-state systems. Such efforts have been motivated to develop quantum technologies such as quantum memories, quantum transducers, quantum networks, and quantum sensing. While these efforts have been mainly focused on control and manipulation of individual excitations (i.e., single-particle physics), both desired and undesired many-body effects have become important. Therefore, it is intriguing to explore whether these quantum optical control techniques could pave a radically new avenue to prepare, manipulate, and detect non-local and correlated electronic states, such as topological ones.
We present several examples of such ideas: (1) Optically driven fractional quantum Hall states: While in Floquet band engineering, the focus is on the control of the single-particle Hamiltonian, here the optical drive can effectively engineer the interaction terms, which could lead to the preparation of model Hamiltonians and exotic topological states. (2) Enhancing superconductivity with an optical drive: we propose a new approach for the enhancement of superconductivity by the targeted destruction of the competing charge/bond density waves (BDW) order. By investigating the optical coupling of gapless, collective fluctuations of the BDWs, we argue that the resonant excitation of these modes can melt the underlying BDW order parameter. We propose an experimental setup to implement such an optical coupling using 2D plasmon-polariton hybrid systems. (3) We also discuss how the coupling of an empty cavity can enhance the superconducting transition temperature, in a quantum analogy to the Eliasberg effect. In the end, we discuss how by driving a semi-conductor and creating a population inversion, one could achieve s-wave and p-wave superconducting pairing.

Michel Devoret, Yale University

An error-corrected logical quantum bit encoded in grid states of a superconducting cavity

abstract
The accuracy of logical operations on quantum bits (qubits) must be improved for quantum computers to surpass classical ones in useful tasks. To do so, quantum information must be robust to noise that affects the underlying physical system. Rather than suppressing noise, Quantum Error Correction (QEC) aims at preventing it from causing logical errors. This approach derives from the reasonable assumption that noise is local: it does not act in a coordinated way on different parts of the physical system. Therefore, if a logical qubit is encoded non-locally, we can, during a limited time, detect and correct noise-induced evolution before it corrupts the encoded information. In 2001, Gottesman, Kitaev and Preskill (GKP) proposed a hardware-efficient instance of such a non-local qubit, which is based on superpositions of position states in a single oscillator. However, implementing measurements that reveal error syndromes of the oscillator while preserving the encoded information was considered experimentally challenging: the only realization so far relied on post-selection, which is incompatible with quantum error correction. The novelty of our experiment [1] is precisely that it implements these non-destructive error-syndrome measurements, the oscillator role being played by a superconducting microwave cavity. Moreover, we have designed and implemented an original feedback protocol that incorporates such measurements to prepare square and hexagonal GKP code states. We then demonstrated QEC of an encoded qubit with the unprecedented suppression of all logical errors, in quantitative agreement with a theoretical estimate based on the measured imperfections of the experiment. Our protocol is applicable to other continuous variable systems and can mitigate logical errors generated by a wide variety of noise processes.
[1] Campagne-Ibarcq et al., arXiv:1907.12487, Nature 584, 368–372 (2020).

Pedram Roushan, Google AI Quantum

Tuning quantum information scrambling in two-dimensional systems

abstract
The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. In 2019, we reported the use of a processor with programmable superconducting qubits to create quantum states on 53 qubits, corresponding to a computational state space of dimension 253 (about 1016). Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks indicate that the equivalent task for a classical supercomputer would take approximately 10,000 years. Established quantum supremacy, we now take a closer look at how quantum information scrambling takes place and computational complexity grows. We demonstrate that the complexity of quantum circuits is directly revealed through measurements of out-of-time-order correlators (OTOCs), which capture the spatial-temporal spread of local perturbations. We implement a variety of quantum circuits ranging from simple integrable circuits such as XY model in 1D to fully ergotic circuits such as 2D random circuits. Our protocol effectively separates scrambling from gate-error induced noise, allowing us to distinguish the complexity of these circuits. We image the dispersion of the scrambling wavefront as it changes from diffusive to ballistic propagation, resulting from changing the entangling gates. By tuning away from the Clifford gate set, we break integrability and dial-in ergodicity and distinguish these complexity classes from their fluctuation signatures. Our work establishes OTOC as a tool to visualize scrambling and diagnose complexity in time and size scales that are challenging to access classically.

Brian Skinner, Ohio State University

Measurement-induced Phase Transitions in the Dynamics of Quantum Entanglement

abstract
When a quantum system evolves under unitary dynamics, as produced by either a Hamiltonian or by a sequence of quantum gates, its various component parts tend to become more entangled with each other. Making measurements, on the other hand, tends to reduce this entanglement by collapsing some of the system's degrees of freedom. In this talk we explore what happens to the entanglement when a quantum many-body system undergoes both unitary evolution and sporadic measurements. We show that the competition between these two effects leads to a new kind of dynamical phase transition, such that when the measurement rate is lower than a critical value the dynamics is "entangling", while a higher-than-critical measurement rate leads to a "disentangling" phase. We study this transition both in one-dimensional spin chains and in "all-to-all" coupled systems, for which unitary operators can directly couple any two degrees of freedom. In both cases the qualitative features of the transition can be understood by mapping to a problem of classical percolation, and in the all-to-all case some features of the transition can be understood exactly.

Natalia Ares, Oxford University

Measuring and tuning quantum devices faster than human experts

abstract
Machine learning has been the enabler of well-known breakthroughs in computer science, such as the victory of Alpha Go over a Go world champion and superhuman face recognition. I will show you how we can direct this great potential to the characterisation and tuning of quantum devices in real time.
As in Go, where a player must carefully balance short and long-term goals and devise actions accordingly, we have demonstrated a deep reinforcement learning algorithm that devises efficient policies to find desired measurement features. Our algorithm divides the parameter space of a semiconductor quantum device in blocks, and finds target measurement features by performing a minimum number of block measurements. In this way, we reduce the long characterisation times required due to device variability.
We have also developed an algorithm that measures bias triangles, important features for qubit operation, and gives them a score. The device parameters are then updated to optimise this score in real-time. The algorithm, using a disentangling variational auto-encoder, proves capable of fine-tuning several device parameters at once. Finally, I will show you an algorithm that can tune a ‘virgin’ double quantum dot device to operation conditions in real time faster than human experts and without the need of specifying a device architecture.
These approaches are widely applicable, opening the way to a completely automatic and efficient route to quantum device measurement and tuning, and thus taking a crucial step towards the scalability of quantum circuits.

Jennifer Cano, Stony Brook University

Higher magic angles in twisted bilayer graphene and topological twistronics

abstract
We present recent analytical and numerical results on the chiral model of twisted bilayer graphene and introduce a new platform for twistronic devices on the surface of a topological insulator. In the first part of the talk, we study the flat band wavefunctions of chiral twisted bilayer graphene at higher magic angles. We show that at higher magic angles, the wavefunctions exhibit an increasing number of zeros, resembling quantum Hall wavefunctions at higher Landau levels. Zeros of the same chirality cluster near the center of the moire unit cell, causing an enhanced phase winding and circulating current. The wavefunctions at higher magic angles have signatures in scanning tunneling microscopy and orbital magnetization experiments. In the second half of the talk, we investigate the fate of the surface Dirac cone of a three-dimensional topological insulator subject to a superlattice potential. Using a combination of diagrammatic perturbation theory, lattice model simulations, and ab initio calculations, we report a dramatic renormalization of the surface Dirac cone velocity and the formation of gapless satellite Dirac cones. The latter can produce very flat bands that may be a fruitful place to searching for interaction-driven physics.

Deji Akinwande, University of Texas at Austin

2D materials: From atoms to applications

abstract
This talk will present our latest research adventures on 2D nanomaterials towards greater scientific understanding and advanced engineering applications. In particular, the talk will highlight our work on flexible electronics, zero-power devices, monolayer memory (atomristors), non-volatile RF switches, and wearable tattoo sensors. Non-volatile memory devices based on 2D materials represent an application of defects and are a rapidly advancing field with rich physics that can be attributed to sulfur vacancies or metal diffusion. Atomistic modeling and atomic-resolution imaging are contemporary tools used to elucidate the memory phenomena in these systems. Likewise, from a practical point of view, electronic tattoos based on graphene have ushered a new material platform that has highly desirable practical attributes including optical transparency, mechanical imperceptibility, and is the thinnest conductive electrode sensor that can be integrated on skin for physiological measurements.