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QFarm Quantum Seminar Series

Quantum Pathways for All Students

Topic: 
Undergraduate and graduate students can learn about available resources
Abstract / Description: 

Join Q-FARM and the Stanford Quantum Computing Association hosting Quantum Pathways, an event where undergraduate and graduate students can learn about the research, academic resources and career opportunities at Stanford related to Quantum Pathways.

Register to receive information on the Zoom link and password!

Date and Time: 
Tuesday, January 11, 2022 - 3:30pm

Q-FARM Seminar: A tutorial on: Search for Non-Abelian Majorana particles as a route to topological quantum computation

Topic: 
A tutorial on: Search for Non-Abelian Majorana particles as a route to topological quantum computation
Abstract / Description: 

Majorana zero modes are fermion-like excitations that were originally proposed in particle physics by Ettore Majorana
and are characterized as being their own anti-particle. In condensed matter systems Majorana zero modes occur as fractionalized excitations with topologically protected degeneracy associated with such excitations. For over a decade, the only candidate systems for observing Majorana zero modes were the non-Abelian fractional quantum Hall state and chiral p-wave superconductors. In this tutorial, I will start by explaining the basic ideas of topological quantum computation using Majorana zero modes. This will be followed by a status update on transport experiments on potential Majorana systems. I will then provide a more detailed explanation of braiding, Majorana operators and the associated topological degeneracy. I will end with my outlook on the challenges and future directions.

Date and Time: 
Tuesday, November 23, 2021 - 12:00pm

Q-FARM Seminar: Quantum criticality in transition metal dichalcogenides

Topic: 
Quantum criticality in transition metal dichalcogenides
Abstract / Description: 

I will discuss low temperature transport measurements on twisted bilayers of WSe2, where we see evidence for an electron-correlation driven insulating phase at half filling of the lowest moiré subband. I will then discuss how metal-insulator transitions can be driven in this system using gate voltages that tune doping and bandwidth. I will show evidence for the presence of a ring of quantum critical points in this system via the behavior of electron transport in the vicinity of the metal-insulator transitions. I will also describe generally the various types of disorder in moiré materials that we can image by microscopic measurements.

Date and Time: 
Wednesday, November 17, 2021 - 12:00pm
Venue: 
Physics & Astrophysics Building, Room 102/103

Q-FARM Seminar presents a double feature

Topic: 
An optical lattice with sound; Programmable Interactions and Emergent Geometry in an Array of Atomic Ensembles
Abstract / Description: 

ABSTRACT #1: Quantized sound waves – phonons – govern the elastic response of crystalline materials, and play an integral part in determining their thermodynamic properties and electrical response. The physics of lattice phonons and elasticity is absent in simulators of quantum solids constructed of neutral atoms in periodic light potentials: unlike real solids, traditional optical lattices are silent because they are infinitely stiff. Here we present the realization of optical lattices with sound, using a Bose-Einstein condensate coupled to a confocal optical resonator. Optical photons in this multimode cavity both image the phonons and mediate short-range atom-atom interactions causing a crystallization transition that supports these phonons. Dynamical susceptibility measurements reveal the phonon dispersion relation, showing that these collective excitations exhibit a sound speed dependent on the BEC-photon coupling strength. These results pave the way for exploring the rich physics of elasticity in quantum solids.

ABSTRACT #2: Tunable interactions are an essential component of flexible platforms for quantum simulation and computation. While most physical systems rely on local interactions dictated by the physical proximity of constituents, we now realize programmable nonlocal interactions by using an array of atomic ensembles within an optical cavity. Here photons carry information between atomic spins and the spectrum of a drive field sets the distance dependence of interactions. By programming the interactions, we access effective geometries where the dimensionality, topology, and metric are entirely distinct from the physical geometry of the array. As examples, we engineer an antiferromagnetic triangular ladder, a Moebius strip with sign-changing interactions, and a treelike geometry inspired by concepts of quantum gravity. Our work opens broader prospects for simulating frustrated magnets and topological phases, investigating quantum optimization paradigms, and engineering entangled resource states for sensing and computation.

Date and Time: 
Wednesday, October 27, 2021 - 12:00pm
Venue: 
Physics & Astrophysics Building, Room 102/103 + Zoom

SystemX Alliance presents "Scaling photonic quantum computers"

Topic: 
Scaling photonic quantum computers
Abstract / Description: 

Photonics is emerging as one of the leading contenders for building useful quantum computers. I will discuss our recent efforts in building and cloud-deploying photonic quantum computing devices, and our roadmap for scaling up and achieving fault tolerance.

Date and Time: 
Thursday, October 7, 2021 - 5:30pm
Venue: 
Huang 018 + Zoom

Q-FARM welcomes Norman Yao, "Time Crystals in Open Systems"

Topic: 
Time Crystals in Open Systems
Abstract / Description: 

Periodically driven (Floquet) systems provide an intriguing landscape for studying novel phases of non-equilibrium matter. In this talk, I will describe recent advances, surrounding the idea of time translation symmetry breaking --- the resulting discrete time crystal exhibits collective subharmonic oscillations. Particular care will be taken to contextualize modern results on discrete time crystals with related phenomena in non-linear dynamical systems. The focus of the talk will be on showing that locally-interacting, Floquet Hamiltonian dynamics coupled to a Langevin bath support finite-temperature discrete time crystals with an infinite auto-correlation time. Our approach utilizes a general mapping from probabilistic cellular automata (PCA) to open classical Floquet systems. Applying this mapping to a variant of the Toom cellular automata leads to a 2D Floquet Hamiltonian with a finite-temperature period-doubling phase transition. Finally, I will discuss how general results from the field of probabilistic cellular automata imply the existence of discrete time crystals in all dimensions, d > 0.


Boxed lunches will be served at 11:15am - 12:00pm outdoors in the Varian courtyard. No food is allowed inside the seminar room. RSVP required.

 

Meeting ID: 914 8268 2357; Password available after registration.

 

Date and Time: 
Wednesday, October 6, 2021 - 12:00pm
Venue: 
Physics & Astrophysics Building, Room 102/103 + Zoom

Q-FARM Seminar: Cross-platform comparison of Arbitrary Quantum Computations

Topic: 
Cross-platform comparison of Arbitrary Quantum Computations
Abstract / Description: 

In the era of quantum advantage, when quantum computers can no longer be simulated directly by classical computers, we must find other ways to validate their performance. While algorithms such as number factoring or oracular algorithms can be easily verified, these approaches only provide pass/fail information for a single system. In this talk, I present a comparison between different quantum computers in their ability to create a given arbitrary quantum state. Looking for agreement between computers is akin to evaluating metrological standards such as disparate atomic clocks. We use randomized and correlated measurements to perform shadow tomography and compare several different quantum computers with both trapped ions and superconducting platforms included, resulting in a wealth of information on the systems. We compare several quantum states, defined by quantum circuits, and analyze the cross-platform fidelities.

Reference: arXiv:2107.11387

Date and Time: 
Thursday, September 23, 2021 - 12:15pm
Venue: 
Physics & Astrophysics Building, Room 102/103

Q-FARM presents "Emergent quantum randomness and its application for quantum device benchmarking"

Topic: 
Emergent quantum randomness and its application for quantum device benchmarking
Abstract / Description: 

In this talk, we describe a novel, universal phenomenon that occurs in strongly interacting many-body quantum dynamics beyond the conventional thermalization. The observed universality leads to the development of a novel benchmarking method applicable for a wide variety of near-term quantum devices. More specifically, we point out that a single many-body wavefunction can encode an ensemble of a large number of pure states defined on a subsystem. For a wide class of many-body wavefunctions, we show that the ensembles encoded in them display universal statistical properties by using a notion in quantum information theory, called quantum state k-designs. The special case (k=1) reduces to the conventional quantum thermalization. The universality is corroborated by two theorems, solvable models, extensive numerical simulations of Hamiltonian dynamics, and recent experimental observations based on a Rydberg quantum simulator. Our results offer a new approach for studying quantum chaos and provide a practical method for sampling pseudorandom quantum states. As an example of practical utility, I will explain how our results allow us to develop a novel sample-efficient benchmarking protocol, which has been already demonstrated in an experiment.

Ref:
arXiv:2103.03535,
arXiv:2103.03536

Date and Time: 
Wednesday, May 26, 2021 - 12:00pm to Thursday, May 27, 2021 - 11:55am

Q-FARM Colloquium - two talks -

Topic: 
"Memory and optimization with multimode cavity QED" and "Transverse-Field Ising Dynamics by Rydberg Dressing in a cold atomic gas"
Abstract / Description: 

Memory and optimization with multimode cavity QED

Quantum systems are driving a revolution in computing and information theory. Driven-dissipative quantum systems, which are both pumped by an external source and are open to environmental interactions, have not been explored as a computational resource as fully as their closed counterparts. In this talk, I will describe how a driven-dissipative system is realized by coupling ultracold atoms to a multimode optical cavity and how it can perform various computational tasks. Through a combination of unitary and dissipative dynamics, the system can learn and recognize arbitrary sets of patterns, an ability known as associative memory, and also functions as a heuristic solver for (NP hard) Ising optimization problems. These functionalities can be understood in terms of semiclassical theories that describe the transition to a superradiant ordered state, which encodes a learned pattern or a solution to an Ising problem. While a fully quantum description remains intractable, experimental progress will be discussed that demonstrates the required ingredients for near-term realization.


 

Transverse-Field Ising Dynamics by Rydberg Dressing in a cold atomic gas

With recent progress on building large and controllable quantum systems, we are on the cusp of harnessing these systems for quantum computation and metrology. Optically controlled interactions are a necessary tool to implement computation and metrology schemes in a system of cold atoms. In this talk, we will present a realization of long-range optically-controllable Ising interactions in a cold gas of cesium atoms by Rydberg dressing. By adding microwave coupling between the clock states we emulate the transverse-field Ising model and detect dynamical signatures of the paramagnetic-ferromagnetic phase transition. We will discuss current progress towards producing spin squeezing by using local and dynamical control of interactions. Finally, we will describe prospects of encoding a quantum algorithm in a hardware-efficient way in this system.

Date and Time: 
Wednesday, May 12, 2021 - 12:00pm to Thursday, May 13, 2021 - 11:55am

Q-FARM presents "Quantum probes of two-dimensional materials"

Topic: 
Quantum probes of two-dimensional materials
Abstract / Description: 

Spin qubits based on diamond NV centers can detect tiny magnetic fields; thin two-dimensional materials produce tiny magnetic fields. Do they make a good match? I will discuss two works that explored how NV magnetometry can uniquely probe the spins and currents in crystals that are one-atom thick.

Outline:
How we discovered, via local magnetic noise measurements, that graphene electrons at high bias undergo cherenkov radiation of phonons
How we performed the first NMR measurement on a single-atom thick crystal.
Some thoughts and outlook on the benefits and challenges of using spin qubits to measure condensed matter systems.
Preview of UC Irvine work on nanomanipulation of 2d material heterostructures

References:
"Electron-phonon instability in graphene revealed by global and local noise probes"
T. I. Andersen*, B. L. Dwyer*, J. D. Sanchez-Yamagishi*, J. F. Rodriguez-Nieva, K. Agarwal, K. Watanabe, T. Taniguchi, E. A. Demler, P. Kim, H. Park, M. D. Lukin
Science 364 ,6436 (2019) *equal contribution

"Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit"
I. Lovchinsky, J. D. Sanchez-Yamagishi, E. K. Urbach, S. Choi, S. Fang, T. Andersen, K. Watanabe, T. Taniguchi, A. Bylinskii, E. Kaxiras, P. Kim, H. Park, and M. D. Lukin
Science 355, 6324 (2017)

Date and Time: 
Wednesday, May 5, 2021 - 12:00pm to Thursday, May 6, 2021 - 11:55am

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