Atomic, Mesoscopic and Optical Physics

AMOP Group

Studying at Cambridge

# Quantum Colloquium

## Cavendish Quantum Colloquium

This series of colloquia in the Cavendish laboratory aims to cover all aspects of modern quantum many-body physics. It is broadly aligned with our research themes  of Emergent Quantum Phenomena (EQP) and Quantum Devices and Measurements (QDM) and as such will feature talks on both fundamental many-body physics as well as their exploitation in devices. It therefore covers all aspects of quantum phenomena in condensed matter and synthetic many-body systems, and their theoretical description.

As a consequence, the aim for these colloquia is to be accessible to a wider audience compared to a typical group seminar.

## 18.01.19 Dr. Helen Walker -  *14:30 Rayleigh and JJ Thompson Seminar room, Maxwell Building* please note change of location this term

Merlin Instrument Scientist, STFC · Rutherford Appleton Laboratory

### Playing with perovskites - tuning physics through chemical control

The discovery of a CaTiO3 mineral by Gustav Rose in the Urals in 1839, might have been regarded as unremarkable at the time, but since then the perovskite structure has proved to be extremely versatile. It is currently the subject of intense research by geologists, environmental and materials scientists exploring hybrid organic-inorganic solar cell materials and alternative catalysts, and physicists, who look to exploit this versatility. I will present two very different examples of my research on perovskites. First I will look at the possibility of a square lattice double perovskite being the host to a spin liquid state, and second I will present results pushing the flexibility of the structure in the form of a family of metal organic frameworks, investigating how the magnetism can be tuned, with the aim of crystal engineering new multiferroic materials.

## 01.02.19 Prof. Andreas Wallraff -  *15:00 Small lecture theatre* please note change of time and location

ETH Zurich

### Quantum Information Processing with Superconducting Circuits

Superconducting circuits are a prime contender for realizing universal quantum computation and solving noisy intermediate-scale quantum (NISQ) problems on fault-tolerant or non-error-corrected quantum processors, respectively. In this talk, I will present elements of an architecture which allows for fast, high-fidelity, single shot qubit read-out [1], for unconditional reset [2], and can be multiplexed [3]. Integrating multiple qubits in a single device, we evaluate performance metrics such as the single and two-qubit gate fidelity and the qubit readout fidelity. We also test the performance of the architecture in parity measurements with real-time feedback, which is a basic element of a error correcting code. To provide a potential avenue for extending monolithic chip-based architectures for quantum information processing, we employ the circuit elements of our architecture to implement a deterministic state transfer and entanglement generation protocol [1]. Our protocol is based on an all-microwave process, which entangles or transfers the state of a superconducting qubit with a time-symmetric itinerant single photon exchanged between individually packaged chips connected by a transmission line. We transfer qubit states at rates of 50 kHz, absorb photons at the receiving node with near unit probability, and achieve transfer process fidelities and on demand remote entanglement state fidelities of about 80 %. We also show that time bin encoding can be used to further improve these quantum communication metrics [5]. Sharing information coherently between physically separated chips in a network of quantum computing modules may be an essential element for realizing a viable extensible quantum information processing system.

[1] T. Walter et al., Phys. Rev. Applied 7, 054020 (2017)
[2]P. Magnard et al., Phys. Rev. Lett. 121, 060502 (2018)
[3] J. Heinsoo et al., Phys. Rev. Applied 10, 034040 (2018)
[4]P. Kurpiers et al., Nature 558, 264-267 (2018)
[5]P. Kurpiers et al., arXiv:1811.07604 (2018)

## 15.02.19 Prof. David Lucas - *14:30 Rayleigh and JJ Thompson Seminar room, Maxwell Building*

University of Oxford

## 01.03.19 Dr. Hannah Price - *14:30 Rayleigh and JJ Thompson Seminar room, Maxwell Building*

Royal Society University Research Fellow and Birmingham Fellow in Theoretical Physics, University of Birmingham

### Simulating higher spatial dimensions with atoms and photons

Spatial dimensionality deeply affects the physical phenomena which can emerge in a system. Our physical world has three spatial dimensions, and so condensed matter physicists have long asked what happens when quantum particles are effectively confined to move in 1D, 2D or 3D geometries. However, recent advances in ultracold atomic and photonics systems are now providing ways to go beyond these limitations, in order to simulate the physics associated with four or more effective spatial dimensions. As I will present, the development of techniques such as “synthetic dimensions” and “topological pumping” with photons or atoms can allow us to explore higher-dimensional phenomena, such as the 4D topological quantum Hall effect, for the first time.

## 15.03.19 Prof. Antoine Browaeys - *14:30 Rayleigh and JJ Thompson Seminar room, Maxwell Building*

Institut d’Optique, Laboratoire Charles Fabry

### Many-body physics with arrays of individual Rydberg atoms

This talk will present our effort to control and use the dipole-dipole interactions between cold Rydberg atoms in order to implement spin Hamiltonians useful for quantum simulation of condensed matter situations. In our experiment, we trap individual atoms in arrays of optical tweezers separated by few micrometers and excite them to Rydberg states using lasers. The arrays are produced by a spatial light modulator, which shapes the dipole trap beam. We can create almost arbitrary geometries of the arrays with near unit filling in two and three dimensions up to about 70 atoms. We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipole-dipole interaction and its control by addressable lasers. This interaction realizes the XY spin model. We use this control to study elementary excitations in a di-merized spin chain featuring topological properties, thus implementing the Su-Schrieffer-Heeger model. We have observed the edge states in the topological condition and their hybridization by studying their dynamics. We explored the regime beyond the linear response by adding several excitations, which act as hard-core bosons. Using the van der Waals interaction between atoms, we have also implemented the quantum Ising model in one-dimensional chains with periodic boundary conditions and two-dimensional arrays containing up to about 50 atoms. We measure the dynamics of the excitation for various strengths of the interactions and compare the data to numerical simulations of this many-body system. This control of an ensemble of interacting Rydberg atoms demonstrates an interesting platform for quantum simulation using neutral atoms, complementary to the other platforms based on ions, magnetic atoms or dipolar molecules.

## 30.11.18 Prof. Sid Parameswaran -  *14:30* small lecture theatre

Associate Professor, Rudolf Peierls Centre for Theoretical Physics, University of Oxford

### Topology, symmetry, and anomalies: investigating domain wall physics in quantum Hall nematic states

I will present recent work on ferromagnetic quantum Hall states that form on (111) surfaces of elemental Bismuth in high magnetic fields. This unusual states of matter combine the topological features of quantum Hall states with orientational symmetry breaking characteristic of nematic order. Recent scanning tunneling microscopy measurements have directly visualized the spontaneous formation of boundary modes between distinct nematic domains and investigated their electronic structure [1]. I will demonstrate that these boundary modes belong to a new class of symmetry-protected’ Luttinger liquid that arise from the interplay of symmetry-breaking with quantum Hall physics, and that they provide a concrete realization of anomaly inflow’ [2]. The analysis reveals strikingly different behavior of domain wall transport at quantum Hall filling factor $\nu=1,2$, in striking agreement with the STM results. I will explore implications of these ideas for the global phase diagram of quantum Hall valley nematics [3].

[1] M.T. Randeria, K. Agarwal, B. E. Feldman, H. Ding , Huiwen Ji3 , R. J. Cava , S. L. Sondhi , S. A. Parameswaran, A. Yazdani, under review.
[2]K. Agarwal, M.T. Randeria, A. Yazdani, S.L. Sondhi, S.A. Parameswaran, arXiv:1807.10293.
[3] S.A. Parameswaran and B.E. Feldman, arXiv:1809.09616.

## 16.11.18 Dr. Andrew Jardine -  *14:30* small lecture theatre

Cavendish Laboratory, University of Cambridge

### Helium Spin-Echo: A Flexible Tool for Studying Nanoscale Processes at Surfaces

Many fundamental and technologically relevant surface processes take place over Angstrom to nanometre length scales and picosecond to nanosecond timescales. However, the combination is extremely challenging to study experimentally; microscopy cannot achieve the necessary time-resolution, whereas spectroscopic experiments generally have poor spatial resolution. In this talk, I will introduce the Helium Spin-Echo technique, a method pioneered at the Cavendish, which has enabled the regime to be accessed experimentally for the first time. The technique involves scattering helium atoms while simultaneously using nuclear spin-polarisation of the atoms to split and recombine the helium wavepackets, giving a reciprocal-space surface-correlation measurement, with sensitivity over the picosecond range. Helium Spin-Echo measurements can be applied to a wide range of surface processes and in many cases have revolutionised our understanding of underlying physical phenomena. I will discuss a series of examples from recent experiments including the rates and mechanisms of nanoscale diffusion, the behaviour arising from complex extended molecules compared to point particles, determination of interaction potentials and rate limiting energy barriers, the transition to quantum transport, and energy exchange rates / vibrational lifetimes giving subsequent insights into friction on the nanoscale.

## 02.11.18 Dr. Akshay Rao -  *14:30* small lecture theatre

EPSRC Early Career Fellow, Winton Advanced Research Fellow, Cavendish Laboratory

### Probing Non-Equilibrium Dynamics in Energy Materials with Extreme Spatio-Temporal Resolution

The interaction of light with matter is fundamental to the operation of a range of devices such as solar cells, LEDs, photocatalytic, plasmonic and many QI systems. For over 30 years, ultrafast spectroscopy has served as the key tool to understand the dynamics of the quasiparticles mediating light-matter interaction such as excitons, polarons and polaritons. However, to date, ultrafast spectroscopies were designed and largely applied to species dissolved in liquids or homogeneous bulk solids. The advent of nanoscience and thin films materials with nanoscale inhomogeneity and disorder has rendered these ensemble-based methods inadequate and they continue to be used largely because of a lack of alternatives, rather than being particularly suited to address the questions of interest. In this talk I will present first results from a new experimental platform combining unprecedented spatial, temporal, spectral and vibronic sensitivity. This allows us to follow the dynamics and motion of charges, excitons, polaritons and other quasiparticles down to 10nm length scales with 10fs time resolution, while providing few nm spectral resolution as well as information about the coupling between electronic and nuclear degrees of freedom in the system. I will highlight unexpected results including ultrafast long-range motion of photoexcitations in 2D semiconductors, organic molecules and hybrid organic-inorganic systems and how these may be harnessed in novel devices.

## 19.10.18 Prof. Manish Chhowalla -  *14:30* small lecture theatre

Goldsmiths' Professor of Materials Science, University of Cambridge

### Electrical contacts between three-dimensional metals and two-dimensional semiconductors

As the dimensions of semiconducting channels in field effect transistors (FETs) decrease, the contact resistance of metal-semiconductor interface at the source and drain electrodes dominates the performance. Two dimensional (2D) transitional metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semi-conductors for ultra-thin FETs. However, unusually high contact resistance has been observed across the metal-2D TMD interface. We have shown that it is possible to reduce the contact resistance by forming lateral junctions between metallic and semiconducting phases of 2D materials. Recent studies have shown that van der Waals (vdW) contacts formed by graphene on 2D TMDs provide lowest contact resistance. However, vdW contacts between evaporated three-dimensional metal and 2D TMDs have yet to be demonstrated. Here, we report the realization of ultra-clean vdW contacts between 3D metals and single layer MoS2. Using scanning transmission electron microscopy (STEM) imaging, we show that the 3D metal and 2D MoS2 interface is atomically sharp with no detectable chemical interaction, suggesting van-der-Waals-type bonding between the metal and MoS2. We show that the contact resistance of indium electrodes is ~ 800 Ω-μm – amongst the lowest observed for metal electrodes on MoS2 and is translated into high performance FETs with mobility in excess of 160 cm2-V-s-1 at room temperature without encapsulation. We also demonstrate low contact resistance of 220 Ω-μm on 2D NbS2 and near ideal band offsets, indicative of defect free interfaces, in WS2 and WSe2. I will introduce 2D TMDs and their properties and then describe our efforts on making good contacts on 2D semiconductors.

## 4.10.18 Prof. Ali Yazdani  -  15:30 small lecture theatre

Princeton University, Princeton, US

### Spotting the elusive Majorana under the microscope

Ettore Majorana famously considered that there may be fermions in nature that are their own antiparticle — and then he mysteriously disappeared just after proposing the idea in 1938. In recent years, following pioneering theoretical work of Kitaev and others, we have learned how to engineer materials that harbor quasiparticles that behave similar to fermions Majorana had envisioned. In particular, there has been a focus on one-dimensional topological superconductor that harbor Majorana zero modes (MZM) that can potentially be used to make fault-tolerant topological quantum computation possible.

Recently, we have proposed and implemented a platform for realization of topological superconductivity and MZM in chains of magnetic atoms on the surface of a superconductor [1,2]. In this talk, I will describe this platform and the series of experiments we have performed to establish the presence of these exotic quasi-particle using spectroscopic mapping with the scanning tunneling microscope (STM). [2-4] These include study of the unique spin signature of MZM.[4] Finally, if there is time I will discuss our most recent work on realization of MZM in a platform based on chiral quantum spin Hall edge states. Overall these experiments, illustrate how the power of spectroscopic imaging with the STM can be used to visualize novel quantum states of matter and their exotic quasi-particles.

[1] S. Nadj-Perge et al. PRB 88, 020407 (2013).
[2] S. Nadj-Perge et al. Science 346, 6209 (2014).
[3] B. E. Feldman et al. Nature Physics 13, 286 (2016).
[4] S. Jeon et al. Science 358, 772 (2017).

## Past talks -- Joint Seminar on Emergent Quantum Phenomena

### Lent term 2017

#### 2.2.18 Dr. Christian Gross

Max Planck Institute for Quantum Optics, Garching, Germany

### 16.2.18 Prof. Achim Rosch

Institute for Theoretical Physics, University of Cologne, Germany

### 9.3.18 Prof. Chris Ford & Prof. Charles Smith

SP Group, Cavendish laboratory, Cambridge

### 16.3.18 Prof. Christos Panagopoulos

Nanyang Technological University, Singapore

### Michaelmas term 2017

Fridays, 2pm,  new location: small lecture theater, Bragg building

• Friday October 13th:

Cold atoms vs solids: Differences and opportunities

Dr. Ulrich Schneider AMOP group, Cavendish Laboratory, Cambridge.

• Friday October 27th:

Concepts and techniques in Magnetism

Dr. Adrian Ionescu TFM group, Cavendish Laboratory, Cambridge.

• Friday November 10th:

Introduction to topological phases

Dr. Benjamin Beri Cavendish & DAMTP, Cambridge.

• Friday October 13th:

Concepts and techniques in Quantum Matter research at the Cavendish

Dr. F. Malte Grosch QM group, Cavendish Laboratory, Cambridge.

Mar 20, 2019
Mar 04, 2019
Jul 05, 2018
May 16, 2018
Jan 19, 2018
Nov 17, 2017