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QFarm

QFarm Quantum Seminar Series

Q-Farm RELATED: SITP Seminar "Statistical mechanics of near extremal black holes"

Topic: 
Statistical mechanics of near extremal black holes
Abstract / Description: 

I will argue that 2D Jackiw-Teitelboim (JT) gravity is a good approximation for some observables related to near extremal black holes in higher dimensions, beyond the semiclassical limit. In the first part of the talk I will apply this to the calculation of the density of states of charged black holes in 4D and give a resolution of the thermodynamic "mass gap" puzzle. In the second part, I will apply this to the calculation of the partition function and IR limit of boundary correlators of near extremal rotating BTZ in 3D. An independent boundary 2D CFT bootstrap argument shows that the emergence of a JT gravity sector is universal.

Date and Time: 
Monday, March 30, 2020 - 2:00pm
Venue: 
Zoom id: 165-492-015

Q-FARM presents "Atom arrays of ultracold strontium: new tools for metrology and many-body physics"

Topic: 
Atom arrays of ultracold strontium: new tools for metrology and many-body physics
Abstract / Description: 

The development of microscopic detection of ensembles of neutral atoms has transformed our ability to study complex many-body systems. Techniques like quantum gas microscopy and optical tweezer arrays grant a unique single-particle-resolved perspective on solid-state analogs and idealized quantum spin models, as well as novel detection capabilities for quantities like entanglement. In this talk, I will describe our progress towards developing these tools for a new atomic species, strontium. In doing so, we establish new prospects enabled by the rich internal degrees-of-freedom associated with alkaline-earth atoms. I will report on our recent results in which we apply our platform to optical atomic clocks, a new application of optical tweezer arrays which indicates a number of strengths for metrology. In particular, I will describe our strategies for reaching arrays with hundreds of tweezers with sub-Hz atom-optical coherence, 41 seconds of atomic coherence, and atomic stability on par with the state-of-the-art. I will then describe our parallel progress towards engineering entanglement on an optical clock transition, as well as new scaling strategies involving atom-by-atom assembly in optical lattice potentials.

Date and Time: 
Wednesday, April 1, 2020 - 12:00pm
Venue: 
Zoom link: stanford.zoom.us/j/987676025

SITP Seminar presents "Conformal field theories with exceptional symmetry"

Topic: 
Conformal field theories with exceptional symmetry
Abstract / Description: 

In hopes of re-starting our seminar series, we will have our first virtual seminar via zoom on Monday, March 23 at 2pm. Before the seminar, we will begin with a virtual "town meeting" (again via zoom) from 1:30 to 2pm to just check with people on how everybody is doing during this uncertain period. Hope to see you there!

Zoom link: https://stanford.zoom.us/j/784962786


Despite being some of the first and most basic objects one encounters in the study of algebra, finite simple groups are surprisingly rich, and participate in a variety of mysterious connections to other areas in math and physics. The classification of finite simple groups is a remarkable theorem of pure mathematics which brings their mystique to bear. Indeed, much as in the more familiar classification of simple Lie groups, there are exceptional cases --- the so-called "sporadic groups" -- which are relatively poorly understood. Since group theory is the abstraction of the study of symmetry, it is natural to ask what structures exactly they act on by symmetries. Generalizing the observations of monstrous moonshine, we will argue that many of the sporadic groups naturally arise as the global symmetries of distinguished conformal field theories in two dimensions. In the process, we will be able to compute the torus partition functions of these theories by solving a kind of highly-constrained modular bootstrap problem using methods, old and new, from the theory of modular forms.

Date and Time: 
Monday, March 23, 2020 - 2:00pm
Venue: 
Zoom link: stanford.zoom.us/j/784962786

Q-Farm Quantum Seminar Series presents "Quantum Gravity in the Lab"

Topic: 
Quantum Gravity in the Lab
Abstract / Description: 
The trend of theoretical advances in AdS/CFT suggests that quantum gravity is broadly applicable as an effective description of chaotic many-body physics. Experimental realizations of such systems are now coming online, with more progress expected in the next few years. We can and should use tools of quantum gravity to describe the physics of these experiments. I will sketch one possible experiment exhibiting nontrivial behavior which, though perfectly understandable in hindsight using conventional methods, is motivated entirely by the physics of wormholes.

Live Broadcast and Recorded (no in-person meeting) - Zoom Link: https://stanford.zoom.us/j/987676025
Date and Time: 
Wednesday, March 18, 2020 - 12:00pm
Venue: 
Remote only

Q-Farm Quantum Seminar Series presents "Ultrafast Spectroscopy of Quantum Materials" and "Quantum and classical information spreading in many-body systems"

Topic: 
Ultrafast Spectroscopy of Quantum Materials and Quantum and classical information spreading in many-body systems
Abstract / Description: 

Ultrafast Spectroscopy of Quantum Materials: Quantum materials such as topological insulators, Weyl-semimetals, and atomically thin two-dimensional crystals have intriguing electronic properties, which makes them promising candidates for their potential applications in next-generation technology. Therefore, it is desirable to investigate the electronic properties of these materials through various methods such that their full potential, as well as the limitations, can be identified. In this talk, I will introduce a novel spectroscopic approach based on the use of strong ultrafast laser pulses and the generation of high-order harmonics. Analysis of temporal and spectral properties of high-order harmonics from these materials reveals their electronic band-structure, topology, as well as driven electron dynamics in the natural time scales of electrons. The advantages of this approach over conventional methods include the use of the all-optical setting, no physical contacts to samples, and much of the measurements that can be performed in ambient conditions. More importantly, the non-perturbative interactions between materials and strong laser fields could generate transient novel quantum phases, which can be subsequently probed by analyzing high-order harmonics.


 

Quantum and classical information spreading in many-body systems: I describe a new theoretical result that constrains how quantum information can spread through large systems. When a small subsystem interacts with a large environment, which information about the subsystem can we detect locally in the environment? In great generality, we show there exists at most an O(1)-sized region of the environment where quantum information about the subsystem can be locally detected, whereas in the rest of the environment, any locally detectible information about the subsystem must be classical information, and moreover this classical information can be modeled by measuring the subsystem in a fixed basis. We will discuss implications for many-body physics. This work builds on earlier work by Brandao et al. in arXiv:1310.8640.

Date and Time: 
Wednesday, March 4, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Long-lived interacting phases of matter protected by multiple time-translation symmetries in quasiperiodically driven systems"

Topic: 
Long-lived interacting phases of matter protected by multiple time-translation symmetries in quasiperiodically driven systems
Abstract / Description: 

The discrete time-translation symmetry of a periodically-driven (Floquet) system allows for the existence of novel, nonequilibrium interacting phases of matter. A well-known example is the discrete time crystal, a phase characterized by the spontaneous breaking of this time-translation symmetry. In this talk, I will show that the presence of *multiple* time-translational symmetries, realized by quasiperiodically driving a system with two or more incommensurate frequencies,  leads to a panoply of novel non-equilibrium phases of matter, both spontaneous symmetry breaking ("discrete time quasi-crystals") and topological.

In order to stabilize such phases, I will outline rigorous mathematical results establishing slow heating of systems driven quasiperiodically at high frequencies. As a byproduct, I will introduce the notion of many-body localization (MBL) in quasiperiodically driven systems. 

arXiv reference: 1910.03584

   

 

Date and Time: 
Wednesday, February 26, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Learning Adaptive Quantum State Tomography with Neural Networks and Differentiable Programming"

Topic: 
Learning Adaptive Quantum State Tomography with Neural Networks and Differentiable Programming
Abstract / Description: 

Quantum State Tomography is the task of determining an unknown quantum state by making measurements on identical copies of the state. Current algorithms are costly both on the experimental front -- requiring vast numbers of measurements -- as well as in terms of the computational time to analyze those measurements. In this talk, we address the problem of analysis speed and flexibility, introducing Neural Adaptive Quantum State Tomography (NA-QST), a machine learning based algorithm for quantum state tomography that adapts measurements and provides orders of magnitude faster processing while retaining state-of-the-art reconstruction accuracy. Our algorithm is inspired by particle swarm optimization and Bayesian particle-filter based adaptive methods, which we extend and enhance using neural networks. The resampling step, in which a bank of candidate solutions -- particles -- is refined, is in our case learned directly from data, removing the computational bottleneck of standard methods. We successfully replace the Bayesian calculation that requires computational time of O(poly(n)) with a learned heuristic whose time complexity empirically scales as O(log(n)) with the number of copies measured n, while retaining very comparable reconstruction accuracy. This corresponds to a factor of a million speedup for 10^7 copies measured. We demonstrate that our algorithm learns to work with basis, symmetric informationally complete (SIC), as well as other types of POVMs. We discuss the value of measurement adaptivity for each POVM type, demonstrating that its effect is significant only for basis POVMs. Our algorithm can be retrained within hours on a single laptop for a two-qubit situation, which suggests a feasible time-cost when extended to larger systems. It can also adapt to a subset of possible states, a choice of the type of measurement, and other experimental details.

Paper link: https://arxiv.org/abs/1812.06693

Date and Time: 
Wednesday, February 19, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Quantum technology with fiber Fabry-Perot cavities"

Topic: 
Quantum technology with fiber Fabry-Perot cavities
Abstract / Description: 

Atom-Photon interaction is everywhere in quantum information, and it is never good enough. Optical cavities with small mode cross-section and lowest loss therefore play a key role, enhancing the interaction in quantum interfaces, but also providing effective coupling that creates entanglement between particles. Fiber Fabry-Perot cavities (FFPs) with laser- machined, ultralow roughness micromirrors are one cavity type that is being used quite successfully with an increasing range of emitters, ranging from ultracold atoms to diamond NV centers and carbon nanotubes. I will give an overview of this cavity technology and its latest developments, and describe two experiments where FFP cavities are used to generate many-particle entanglement. In the first, the cavity measurement effectively blocks part of the qubits' Hilbert space, and entanglement emerges when the coherently driven qubits approach this blocked bart. The second experiment is a trapped-atom clock on an atom chip, where the FFP cavity has allowed us to generate long-lived spin squeezed states in a metrological environment.

Date and Time: 
Wednesday, February 5, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Surprises from Time Crystals"

Topic: 
Surprises from Time Crystals
Abstract / Description: 

Time crystals are new states of matter that only exist in an out-of-equilibrium setting. I will review the state of this rapidly evolving field, focusing in particular on some of the remarkable properties of this phase, and the surprises coming out of its study. I will provide a detailed overview of existing experiments, with a view towards identifying the ingredients needed for an unambiguous observation of this phase in the future.

Date and Time: 
Wednesday, January 29, 2020 - 12:00pm
Venue: 
Hansen Physics & Astrophysics Building, 102/103

Q-Farm & Geballe Laboratory for Advanced Materials (GLAM) special seminar: "Generative Modelling of Quantum Systems via Quantum Neural Networks"

Topic: 
Generative Modelling of Quantum Systems via Quantum Neural Networks
Abstract / Description: 

In this talk I will first provide some context on the state of the field of quantum machine learning and recent successes of quantum deep learning. Quantum deep learning consists of learning representations of data using composite networks of parameterized quantum transformations, also known as parameterized quantum circuits or quantum neural networks. Following this introduction, I will introduce a new class of generative quantum-neural-network-based models called Quantum Hamiltonian-Based Models (QHBMs). These models provide a novel paradigmatic approach for quantum-probabilistic hybrid variational learning, where one efficiently decomposes the tasks of learning classical and quantum correlations in a way which maximizes the utility of both classical and quantum processors, thereby combining the capabilities of classical probabilistic deep learning and quantum deep learning. Following this will be the introduction of the Variational Quantum Thermalizer (VQT) algorithm for generating the thermal state of a given Hamiltonian and target temperature, and an explanation of how QHBM's are naturally suited for this task. The VQT can be seen as a generalization of the Variational Quantum Eigensolver (VQE) to thermal states. As another application of QHBM's, we will define the tasks of Quantum Modular Hamiltonian learning, where one learns to generatively model mixed quantum states as the thermal state of a learned Hamiltonian. I will provide numerical results demonstrating the efficacy of these techniques in illustrative examples. Namely, using QHBMs for modelling Heisenberg spin systems, to learn entanglement Hamiltonians and compression codes in simulated free Bosonic systems, and to create thermal states for quantum simulation of Fermionic systems.

Date and Time: 
Thursday, November 21, 2019 - 1:30pm
Venue: 
McCullough Building Room 130

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