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Videos of Past Q-FARM Seminars

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Q-FARM Seminars are available on our YouTube channel, Q-FARM Stanford. Subscribe to receive notification of new recordings. Please note: not all seminars are recorded. 

May 31, 2023: Jeongwan Haah (Microsoft Research)

Talk Title: “Measurement QCA”

We investigate the evolution of quantum information under Pauli measurement circuits. We focus on the case of 1+1 and 2+1-dimensional systems, which are relevant to the recently introduced Floquet topological codes. We define local reversibility in context of measurement circuits, which allows us to treat finite depth measurement circuits on a similar footing to finite depth unitary circuits. In contrast to the unitary case, a finite depth locally reversible measurement sequence can implement a translation in one dimension. A locally reversible measurement sequence in two dimensions may also induce a flow of logical information along the boundary. We introduce “measurement quantum cellular automata” which unifies these ideas and define an index in one dimension to characterize the flow of logical operators. We find a Z_2 bulk invariant for Floquet topological codes which indicates an obstruction to having a trivial boundary. We prove that the Hastings–Haah honeycomb code belong to a class with such obstruction, which means that any boundary must have either non-local dynamics, period doubled, or admits boundary flow of quantum information. [Aasen, H., Li, Mong, 2304.01277]


May 10, 2023: Ani Krishna and Tibor Rakovszky (Stanford)

Krishna: "Why the buzz around quantum LDPC codes?"; Rakovszky: "Gauge dualities for (good) LDPC codes"

Ani Krishna: Quantum LDPC codes have attracted a lot of attention recently. In this talk, I will discuss why these codes are being studied from the perspective of fault-tolerant quantum computation. I will first discuss asymptotic guarantees—we expect that these codes will offer an efficient way to construct scalable quantum computers. This efficiency might not be available to all architectures—I shall discuss what your architecture needs to be able to do for you to be able to build these codes. I will then discuss some desiderata to translate asymptotic results to real-world applications.

Research interests: Quantum error correction and fault-tolerant quantum computation.

Tibor Rakovszky: This talk will discuss various recent ideas and constructions in (quantum) computer science from a physics perspective. I will introduce quantum LDPC codes, examples of which include the familiar toric code, fracton models, and more exotic systems that live on so-called expander graphs, and explain how all of these can be understood as generalized versions of Z2 gauge theories, familiar from high energy and condensed matter physics. I will use this perspective to relate properties of quantum and classical codes, using a form of generalized gauge duality; in particular to explore the relationship between the code distance of the quantum code and a property of classical codes called "local testability", which can be understood in terms of the scaling of energy barriers. Along the way, I will introduce various product constructions that can be used to systematically generate new models with interesting properties out of simpler ones.

Research interests: condensed matter theory, quantum many-body dynamics and (more recently) quantum error correction. 

May 17, 2023: Ignacio Cirac (MPQ)

"Quantum Simulation: from many to few body problems"

Many-body quantum systems are very difficult to simulate with classical computers, as the computational resources (time and memory) usually grow exponentially with the size of the system. However, quantum computers and analog quantum simulators can perform that task much more efficiently. In this talk, I will first review some of the quantum algorithms that have been proposed to simulate dynamics, prepare ground states, or compute physical properties at finite temperatures. I will then focus on analog quantum simulation with cold atoms in optical lattices and describe methods for tackling physics and chemistry problems with such a system.

Research Interests: Quantum Information Theory, Quantum Optics, Tensor Networks

April 26, 2023: Romain Vasseur (UMass)

"Learning global charges from local measurements"

Monitored random quantum circuits (MRCs) exhibit a measurement-induced phase transition between area-law and volume-law entanglement scaling. In this talk, I will review the physics of such entanglement transitions, and argue that MRCs with a conserved charge additionally exhibit two distinct volume-law entangled phases that cannot be characterized by equilibrium notions of symmetry-breaking or topological order, but rather by the non-equilibrium dynamics and steady-state distribution of charge fluctuations. These include a charge-fuzzy phase in which charge information is rapidly scrambled leading to slowly decaying spatial fluctuations of charge in the steady state, and a charge-sharp phase in which measurements collapse quantum fluctuations of charge without destroying the volume-law entanglement of neutral degrees of freedom. I will present some statistical mechanics description of such charge-sharpening transitions, and relate them to the efficiency of classical decoders to “learn” the global charge of quantum systems from local measurements.

May 03, 2023: Liang Jiang (University of Chicago)

"Hardware-Aware Quantum Error Correction"

To effectively suppress practical imperfections, we aim to design quantum error correction schemes that can effectively suppress dominant errors and enhance performance for specific hardware. In this talk, I will discuss the design of quantum error correcting codes that can optimally suppress practically-relevant errors. Additionally, I will provide examples of custom-designed quantum error correction schemes that can be applied in quantum computing, communication, simulation, and sensing applications.

Research interests: Quantum optics, AMO physics, condensed matter physics, quantum control, quantum error correction

April 19, 2023: Kang-Kuen Ni (Harvard)

"Arrays of Individually-Controlled Molecules for Quantum Science"

Advances in quantum manipulation of molecules bring unique opportunities: the use of molecules to search for new physics; exploring chemical reactions in the ultra-low temperature regime; and harnessing molecular resources for quantum simulation and computation. I will introduce our approaches to building individual ultracold molecules in optical tweezer arrays with full quantum state control. This work expands the usual paradigm of chemical reactions that proceed via stochastic encounters between reactants, to a single controlled reaction of exactly two atoms. The new technique allows us to isolate two molecular rotational states as two-level systems for qubits. In order to preserve coherence of the qubits, we develop magic-ellipticity polarization trapping. Finally, we are taking advantage of the resonant dipolar interaction of molecules to entangle them with single site addressability. In combination, these ingredients will allow the molecular quantum system to be fully programmable.

Apr. 05, 2023: Tanya Zelevinsky (Columbia)

"Ultracold Molecule Lattice Clocks"

Ultracold atom technologies have transformed our ability to perform high-precision spectroscopy and apply it to time and frequency metrology. Many of the highest-performing atomic clocks are based on laser-cooled atoms trapped in optical lattices. These clocks can be applied to fundamental questions, for example to improve our understanding of gravity and general relativity. In this talk, I will discuss using lattice-trapped ultracold diatomic molecules, rather than atoms, as a reference for clocks. Molecules have more internal quantum states and therefore are relatively challenging to control. On the other hand, their vibrational modes offer a large number of prospective clock transitions, and can help us probe alternative aspects of new physical interactions. I will discuss the current precision limit of molecular metrology and possible paths forward.

Mar. 29, 2023: Aram Harrow (MIT)

Quantum Walks on Hierarchical Graphs

There are few known exponential speedups for quantum algorithms and these tend to fall into even fewer families. One speedup that has mostly resisted generalization is the use of quantum walks to traverse the welded-tree graph, due to Childs, Cleve, Deotto, Farhi, Gutmann, and Spielman. We show how to generalize this to a large class of hierarchical graphs in which the vertices are grouped into a d-dimensional lattice of "supervertices". Supervertices can have different sizes, and edges between supervertices correspond to random connections between their constituent vertices. The hitting times of quantum walks on these graphs is mapped to the localization properties of zero modes in certain disordered tight binding Hamiltonians. The speedups range from superpolynomial to exponential, depending on the underlying dimension and the random graph model.

Mar. 15, 2023: Shimon Kolkowitz (University of Wisconson-Madison)

"Testing relativity in the lab and other applications of multiplexed optical atomic clocks"

The remarkable precision of optical atomic clocks enables new clock applications, and offers sensitivity to new and exotic physics. In this talk I will explain the motivation and operating principles of a multiplexed strontium optical lattice clock, which consists of two or more atomic clocks in one vacuum chamber. This miniature clock network enables us to bypass the primary limitations to typical atomic clock comparisons and achieve new levels of precision. I will present recent experimental results in which we performed a novel, blinded, precision test of the gravitational redshift with an array of atomic ensembles spanning a total height difference of 1 cm. Finally, I will discuss the outlook and planned future experiments with our current apparatus, as well as plans for a second generation multiplexed clock with novel capabilities.

Research interests: Precision measurement; metrology; optical atomic clocks; quantum sensing; tests of fundamental physics.

Mar. 08, 2023: Chinmay Nirkhe (IBM)

"Why can’t we classically describe quantum systems?"

A central goal of physics is to understand the low-energy solutions of quantum interactions between particles. This talk will focus on the complexity of describing low-energy solutions; I will show that we can construct quantum systems for which the low-energy solutions are highly complex and unlikely to exhibit succinct classical descriptions. I will discuss the implications these results have for robust entanglement at constant temperature and the quantum PCP conjecture. En route, I will discuss our [Anshu, Breuckmann, and Nirkhe] positive resolution of the No Low-energy Trivial States (NLTS) conjecture on the existence of robust complex entanglement.

Mathematically, for an n-particle system, the low-energy states are the eigenvectors corresponding to small eigenvalues of an exp(n)-sized matrix called the Hamiltonian, which describes the interactions between the particles. Low-energy states are the quantum generalizations of approximate solutions to satisfiability problems such as 3-SAT. In this talk, I will discuss the theoretical computer science techniques used to prove circuit lower bounds for all low-energy states. This morally demonstrates the existence of Hamiltonian systems whose entire low-energy subspace is robustly entangled. I will also discuss stronger separations between ground-states of local Hamiltonians and the set of classically describable quantum states; these separations are provable [Natarajan and Nirkhe] in the distribution-testing oracle model.

Research Interests: Chinmay Nirkhe’s research interests are in theoretical computer science centered around quantum information and hardness of approximation. He is currently interesting in studying the quantum PCP conjecture and the complexity of quantum states.

Mar. 01, 2023: Christian Heide (Stanford PULSE Institute) & Sebastien Leger (Stanford University)

Heide's title "Wavetronics: Light-field-driven quantum electronics"; Leger's title: "Quantum simulator with a Josephson junction array."

Christian Heide's (partial) Abstract: Precisely controlling the light waveform allows us to manipulate and study processes on a sub-cycle timescale of the laser pulse. Such waveform control opens prospects for technological applications, especially for on-chip signal processing at speeds at optical clock rates.

Sebastien Leger's (partial) Abstract: Quantum impurity problems, that describe the interaction between a degree of freedom (DOF) and an environment, are at the heart of a very rich physics covering fields as diverse as   quantum optics and strongly correlated matter . In this work, we use the tools of circuit QED to address a  quantum impurity problem called Boundary Sine Gordon (BSG).

Feb. 22, 2023: Monika Aidelsburger (Ludwig-Maximilians-Universität München & Munich Center for Quantum Science and Technology)

Quantum simulation – Engineering & understanding quantum systems atomby- atom

The computational resources required to describe the full state of a quantum many-body system scale exponentially with the number of constituents. This severely limits our ability to explore and understand the fascinating phenomena of quantum systems using classical algorithms. Quantum simulation offers a potential route to overcome these limitations. The idea is to build a well-controlled quantum system in the lab, which represents the problem of interest and whose properties can be studied by performing measurements. In this talk I will introduce quantum simulators based on neutral atoms that are confined in optical arrays using laser beams. State-of-the-art experiments now generate arrays of several thousand particles, while maintaining control on the level of single atoms. I will show how these systems can be used to study the properties of topological phases of matter. In the end I will provide a brief outlook on new directions in the field based on the unique properties of alkaline-earth(-like) atoms.

Research Interests: Ultracold Atoms in Optical Lattices, Topology, Out-of-equilibrium dynamics, Lattice Gauge Theories

Feb. 15, 2023: Jun Ye (JILA)

"Quantum matter and clock: from emergent phenomena to fundamental physics"

Precise quantum state engineering, many-body physics, and innovative laser technology are revolutionizing the performance of atomic clocks and metrology, providing opportunities to explore emerging phenomena and probe fundamental physics. Recent advances include measurement of gravitation time dilation across a few hundred micrometers, and employment of quantum entanglement for clock comparison.  

Feb. 1, 2023: Zoe Yan (Princeton University)

“Quantum many-body physics with ultracold molecules”

A central challenge of modern physics is understanding the behavior of strongly correlated matter.   Current knowledge of such systems is limited on multiple fronts: experimentally, these materials are often difficult to fabricate in laboratory settings, and numerical simulations become intractable as the number of particles approaches meaningful values.  In the spirit of Feynman, physicists can model diverse phenomena, from high-temperature superconductivity to quantum spin liquids, using analog quantum simulation.  My research explores emergent quantum phenomena in pristine systems made of atoms, molecules, and electromagnetic fields.  In particular, ultracold molecules are a promising platform due to their tunable long-range interactions and large set of internal states. However, this nascent platform requires new experimental techniques to create, control, and probe molecular systems.

Jan. 25, 2023: Soonwon Choi (MIT)

“Toolbox for Analog Quantum Simulators”

Analog quantum simulation is one of the most promising applications of existing quantum technologies. A defining characteristic of analog quantum simulators is that they often lack the ability to control individual constituent particles in arbitrary ways. In this talk, we will present novel methods for improving the utilization of present-day quantum simulators such as Rydberg atom arrays or quantum gas microscopes. These methods include high-precision benchmarking and advanced measurement techniques. We will briefly discuss our benchmarking protocol for estimating the many-body fidelities of small or intermediate-size quantum systems and present experimental demonstrations of the technique. Then, we will introduce a simple, universal measurement protocol for extracting arbitrary physical properties of quantum states obtained from experiments. Our protocol leverages the information scrambling that occurs in natural quench dynamics of generic quantum systems, providing a scalable and efficient solution for measuring observables that are otherwise not directly accessible. In an ideal limit, our approach performs on par with state-of-the-art techniques such as classical shadow tomography, yet it does not require sophisticated gate operations. We will illustrate the power of our approach with several examples.

Dec 7, 2022: Cindy Regal (JILA)

“Time-of-Flight Quantum Tomography of an Atom in an Optical Tweezer”

I will discuss experiments with atoms in optical tweezers in which we use time-of-flight imaging to demonstrate full tomography of a non-classical motional state. By combining time-of-flight imaging with coherent evolution of an atom in the optical tweezer, we are able to access arbitrary quadratures in phase space without relying on coupling to a spin degree of freedom. To create non-classical motional states, we using tunneling in the potential landscape of optical tweezers, and our tomography both demonstrates Wigner function negativity and assesses coherence of non-stationary states. We are motivated to explore this tomography method for its applicability to other neutral particles, such as large-mass dielectric spheres. I will also provide a brief description of our broader optical tweezer work focused on studying light-assisted collisions and on extending atom lifetimes with a new cryogenic optical tweezer array apparatus.

Dec. 14, 2022: Matteo Ippoliti (Stanford University)

“Universal randomness beyond thermalization in quantum dynamics”

The advent of quantum simulators has made it possible to probe quantum many-body systems with unprecedented resolution. Microscopic read-out of individual degrees of freedom gives access to a far more detailed picture of quantum dynamics than what has been traditionally available in condensed matter physics, and motivates the search for novel universal phenomena. In this talk, I will discuss one such example: "deep thermalization", a recently proposed framework for the emergence of universal randomness in quantum dynamics, based on the statistics of conditional wavefunctions obtained after measuring part of a system. I will present recent results on deep thermalization in tractable quantum circuit models of dynamics, leveraging connections to monitored dynamics and random-matrix theory.

Nov. 16, 2022: Ehud Altman (UC Berkeley)

“Measurement induced criticality in many-body states”

A strange aspect of quantum mechanics is what Einstein called “spooky action at a distance”: measuring the spin of one particle of an EPR pair leads to wavefunction collapse that instantaneously changes the correlation between the two particles regardless of how far they are separated. In this talk I will discuss how this effect is generalized to entangled states of many particles. In particular I will show that local measurements of a critical quantum ground state can induce a phase transition that instantaneously modifies the power-law decay of correlations at arbitrary long distances. I will explain how this transition can be analyzed through a mapping to a statistical field theory with boundary criticality and discuss a realistic scheme for observing these phenomena in experiments. 

Nov. 9, 2022: Oskar Painter (Caltech)

Talk title: “Quantum dynamics of a superconducting-circuit quantum simulator with metamaterial quantum bus”

While the majority of engineerable many-body systems, or quantum simulators, consist of particles on a lattice with local interactions, quantum systems featuring long-range interactions are particularly challenging to model and interesting to study due to the rapid spatio-temporal growth of quantum entanglement and correlations. In my talk I will present a scalable quantum simulator architecture based on a linear array of superconducting qubits locally connected to an extensible photonic-bandgap metamaterial. The metamaterial acts both as a quantum bus mediating qubit-qubit interactions, and as a readout channel for multiplexed qubit-state measurement. As an initial demonstration, we realize a 10-qubit simulator of the one-dimensional Bose-Hubbard model with in situ tunability of both the hopping range and the on-site interaction. 

Nov. 2, 2022: Hsin Yuan (Robert) Huang (Caltech)

Talk title: “Theory of learning in the quantum universe”

I will present recent progress in building a rigorous theory for understanding how scientists, machines, and future quantum computers could learn models of our inherently quantum universe. The talk will include mathematical results answering two fundamental questions at the intersection of machine learning and quantum physics: Can classical machines learn to solve challenging problems in quantum physics? Can quantum machines learn exponentially faster than classical machines?

Oct 26, 2022: Philipp Kunkel and Nick Hunter-Jones (Stanford University)

Philipp Kunkel - “Engineering Entanglement between Atomic Ensembles” Nick Hunter-Jones - “Complexity and randomness in quantum circuits”

Abstract (Philipp Kunkel):

Control over interactions form the basis for generating entanglement between quantum objects. In this talk, I will show how we use all-to-all interactions mediated by an optical cavity together with local spin rotations to engineer a wide variety of entanglement structures between ensembles of neutral atoms. The structure of these quantum correlations can then be tailored to a specific quantum enhanced task such as distributed quantum sensing and measurement-based quantum computation via cluster states.

Abstract (Nick Hunter-Jones):

Random quantum circuits (RQCs) are a solvable model of strongly-interacting quantum dynamics, efficient implementations of quantum pseudorandomness, and have been the central focus of recent demonstrations of quantum computational advantage. In this talk we’ll overview some techniques for studying properties of RQCs and their implications in both quantum many-body physics and near-term quantum computing.

Oct 19, 2022: Adam Kaufman (JILA)

“Quantum science with microscopically-controlled arrays of alkaline-earth atoms”

Quantum science with neutral atoms has seen great advances in the past two decades. Many of these advances follow from the development of new techniques for cooling, trapping, and controlling atomic samples. In this talk, I will describe ongoing work where we have explored a new type of atom - alkaline-earth(-like) atoms - for optical tweezer trapping, a technology which allows microscopic control of arrays of 100s to potentially 1000s of atoms. While their increased complexity leads to challenges, alkaline-earth atoms offer new scientific opportunities by virtue of their rich internal degrees of freedom. Combining features of these atoms with tweezer-based control has impacted multiple areas in quantum science, including quantum information processing, quantum simulation, and quantum metrology. 

Oct. 5, 2022: Jon Simon (Stanford University)

“Cavity QED from Manybody Physics to Transduction.”

In this talk, I will describe recent developments in the Simon/Schuster collaboration, where we are harnessing cavity quantum electrodynamics for both manybody physics and quantum information. I will begin with an overview of our photonic quantum materials efforts, highlighting the analogy between photons in a lattice of cavities (or family of cavity modes) and electrons in solids. I will then focus in on our explorations of Hubbard physics in a quantum circuit, where we have demonstrated reservoir engineering approaches to stabilizing incompressible solids, and more recently, disorder-assisted adiabatic approaches to preparation of compressible fluids and even cat states of fluids. Finally I will change gears and talk briefly about interfacing superconducting and optical cavities using Rydberg atoms, where we have just demonstrated a quantum limited mmwave-to-optical transducer with >50% transduction efficiency, 100’s of kHz of bandwidth, and less than one noise photon.

Mark van Raamsdonk (University of British Columbia)

"Negative energy, wormholes, and cosmology"

We discuss a framework for cosmological physics where the cosmological observables are related by analytic continuation to vacuum observables in a static asymptotically AdS Lorentzian wormhole geometry. The existence of these wormhole solutions appears to require states for quantum field theories on bounded regions with extremely large Casimir energies compared with those for standard boundary conditions. To check whether such states exist, we study free Dirac fermions on a bounded region via a lattice regularization, and find numerical evidence that for 3+1 dimensional Dirac fermions on a region of fixed size, there are states with uniform negative energy density of arbitrarily large magnitude.

Sept. 28, 2022: Ana Asenjo-Garcia (Columbia University)

“Many-body physics and self-organization with atoms and photons”

Dissipation and fluctuations are known to be sources of order in complex non-linear systems formed by many agents, as they lead to the generation of self-organized spatial or temporal structures. However, dissipation is considered to produce loss of coherence in open quantum systems, contributing to the inherent fragility of quantum states. Here, I will discuss how coherent behavior emerges in large quantum systems consisting of many atoms if dissipation is collective, in the form of correlated photon emission and absorption.  In particular, I will examine the many-body out-of-equilibrium physics of atomic arrays, and focus on the problem of Dicke superradiance, where a collection of excited atoms synchronizes as they decay, emitting a short and intense pulse of light. Superradiance remains an open problem in extended systems due to the exponential growth of complexity with atom number. I will show that superradiance is a universal phenomenon in ordered arrays. Our predictions can be tested in state of the art experiments with arrays of neutral atoms, molecules, and solid-state emitters and pave the way towards understanding the role of many-body decay in quantum simulation, metrology, and lasing.

May 19, 2022: Jonathan Home (ETH Zürich)

“Scalable approaches for ion trap quantum computing”

Quantum computing requires implementation of high fidelity control operations across an interconnected array of qubit systems. The requirements of quantum error correction put stringent limits on tolerable errors as well as introducing a larger overhead in the number of qubits. In this talk I will describe two approaches to the challenges of scaling trapped-ion quantum computers. The first is in the optical delivery, where we have recently demonstrated the first multi-qubit gates between ions using light delivered from trap-integrated waveguides. In further work, we have been investigating further possibilities arising from this technology, including the use of optical standing waves generated on-chip and protocols for entanglement generation. A second generation of photonic chips recently ordered from the foundry features modifications for blue light, tightly focused laser beams and better ion performance. I will then outline a new approach to implementing large scale quantum computing with trapped-ions based on micro fabricated Penning traps, also giving an insight into the physics of these systems and their advantages for scaling up.

May 18, 2022: Nathalie de Leon (Princeton University)

"Correlating materials analysis with qubit measurements to systematically eliminate sources of noise"

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. I will describe our recent efforts to correlate direct materials characterization with single spin measurements to devise methods to stabilize highly coherent NV centers within nanometers of the surface. We also deploy these shallow NV centers as a probe to study the dynamics of a disordered spin ensemble at the diamond surface and other sources of external noise.

May 6, 2022: Tobias Donner (ETH Zürich)

"Dissipative crystals of matter and light - from self-oscillating pumps to dissipation-stabilized phases"

The time evolution of a driven quantum system can be strongly affected by dissipation. Although this mainly implies that the system relaxes to a steady state, in some cases it can lead to the appearance of new phases and trigger emergent dynamics. I will report on experiments where we dispersively couple a quantum gas to an optical cavity. When the dissipation via cavity losses and the coherent timescales are comparable, we find a regime of persistent oscillations leading to a topological pumping of the atoms. Furthermore, I will report on the observation of a dissipation-stabilized phase in a system with tunable decay.

May 4, 2022: Immanuel Bloch (Max-Planck-Institute of Quantum Optics)

"From Kardar-Parisi-Zhang Superdiffusion in Heisenberg Quantum Magnets to Novel Quantum Optical Light Matter Interfaces with Subwavelength Atomic Arrays"

Quantum simulation with ultracold atoms has opened the avenue to probe non-equilibrium quantum many body dynamics in new parameters regimes and with completeley new detection techniques. In my talk, I will show how we utilize the high-resolution, single-spin sensitive detection afforded by a quantum gas microscope to track the out-of-equilibrium dynamics of Heisenberg quantum magnets in one and two dimension. Surprisingly, in 1D, the system exhibits a novel transport paradigm of anomalous superdiffusive transport compared to standard ballistic or diffusive transport scenarios. Additionally, by accessing the full counting statistics of transported spins, we find strong supporting evidence for the conjecture that transport in the XXZ chain at the Heisenberg point indeed falls in the so called Kardar-Parisi-Zhang universality class. I will explain the arguments for this conjecture and introduce the peculiar features of this anomalous transport regime.

April 6, 2022: Jack Harris (Yale University)

"Measuring the higher-order phonon-phonon coherences in a superfluid optomechanical device"

 I will describe measurements in which we detect the individual sideband photons produced by an optomechanical device consisting of a nanogram of superfluid helium confined in a cavity. We use the photon-counting data to probe the phonon-phonon correlations (up to fourth order) in a single acoustic mode of the superfluid. The data is consistent with the acoustic mode being in a thermal state with mean phonon number ~ 1. We also use sideband-photon counting to show that the acoustic mode can be driven to a coherent amplitude corresponding to tens of thousands of phonons without harming the state's purity. I will discuss applying these results to testing models of discrete spacetime, and to distributing entanglement over kilometer-scale optical fiber networks

March 23, 2022: Felipe H. da Jornada (Stanford University)

Understanding excited states in 2D and moiré materials for quantum applications

Low-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs), are marked by their spatial confinement, weak electronic screening, and large many-electron interactions. Such systems host a variety of multiparticle excitations – such as excitons, trions, biexcitons – often displaying large binding energies and long lifetimes even at room temperature. I will present new first-principles formalisms and calculations to understand the fingerprints of these excitations and their applicability for quantum science.

March 2, 2022: Rahul Trivedi (Max Planck Institute of Quantum Optics)

Non Markovian open quantum systems: Theoretical description and simulatability

Quantum systems arising in solid state physics, chemistry and biology invariably interact with their environment, and need to me modelled as open systems. While the theory of Markovian open quantum systems has been extensively developed, their non-Markovian generalization remains less well understood. In this talk, I will first review quantum stochastic calculus which provides a mathematically rigorous description of a unitary group generating Markovian sub-system dynamics. 

Feb. 2, 2022: Lorenzo Magrini (University of Vienna)

Quantum measurement and control of mechanical motion at room temperature

The Heisenberg uncertainty principle establishes the frontier to the quantum realm. The position of a particle, the spin of an atom, the energy of a photon can only be known with finite precision. Realizing measurements close to this limit requires high efficiency and good environmental isolation. 

November 17, 2021 Speaker: Abhay Pasupathy (Columbia University)

Quantum criticality in transition metal dichalcogenides

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. 

October 27, 2021 Speakers: Stanford PhD Candidates Ronen Kroeze & Eric Cooper

Two Talks

Talk #1: Here we present the realization of optical lattices with sound, using a Bose-Einstein condensate coupled to a confocal optical resonator.  Talk #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...

October 06, 2021 Speaker: Prof. Norman Yao (UC Berkeley)

Time Crystals in Open Systems

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.

May 12, 2021 - Speaker: Brendan Marsh & Ognjen Marković (Stanford University)

Double Feature: Memory and optimization with multimode cavity QED; Transverse-Field Ising Dynamics by Rydberg Dressing in a cold atomic gas

In this first 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. 

In this second talk, we will present a realization of long-range optically-controllable Ising interactions in a cold gas of cesium atoms by Rydberg dressing.

April 21, 2021 - Speaker: Valentina Parigi (Laboratoire Kastler Brossel – Sorbonne Université, Paris)

Quantum probes of two-dimensional materials

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 ...

April 28, 2021 - Speaker: Ben Bartlett & Sunil Pai (Stanford University)

Double Feature: A photonic quantum computer design with only one controllable qubit; Towards MEMS-driven photonic computing

Talk #1: We describe a design for a photonic quantum computer which requires minimal quantum resources: a single coherently-controlled atom.

Talk #2: Programmable nanophotonic networks of Mach-Zehnder interferometers are energy-efficient circuits for matrix-vector multiplication that benefit a wide variety of applications such as artificial intelligence, quantum computing and cryptography.

April 21, 2021 - Speaker: Valentina Parigi (Laboratoire Kastler Brossel – Sorbonne Université, Paris)

Continuous variables quantum complex networks

Experimental procedures based on optical frequency combs and parametric processes produce quantum states of light involving large numbers of spectro-temporal modes that can be mapped and analyzed in terms of quantum complex networks.

April 14, 2021 - Speaker: Timothy P. McKenna (Stanford University) & Ryotatsu Yanagimoto (Stanford University)

Double Feature: Ultra-low-power second-order nonlinear optics on a chip; Quantum Dynamics of Ultrafast Nonlinear Photonics

Talk #1: Thin-film lithium niobate is a promising platform for integrated photonics because it can tightly confine light in small waveguides which allows for large interactions between light, microwaves, and mechanics.

Talk #2: Broadband optical pulses propagating in highly nonlinear nanophotonic waveguides can significantly leverage optical nonlinearity by tight temporal and spatial field confinements, promising a route towards all-optical quantum engineering and information with single-photon nonlinearities.

March 24, 2021 - Speaker: Avi Pe’er (Bar Ilan University)

Quantum sensing with unlimited optical bandwidth

Squeezed light is a major resource for quantum sensing, which has been already implemented in high-end interferometric sensing, such as gravitational wave detection. However, standard squeezed interferometry methods suffer from two severe limitations.

March 10, 2021 - Speaker: Natalia Berloff (University of Cambridge)

Unconventional computing with liquid light

The recent advances in the development of physical platforms for solving combinatorial optimisation problems reveal the future of high-performance computing for quantum and classical devices.

January 13, 2021 - Speaker: Debayan Mitra (Harvard University)

Direct laser cooling of polyatomic molecules

Laser cooling and evaporative cooling are the workhorse techniques that have revolutionized the control of atomic systems.

December 2, 2020 - Speaker: Kartik Srinivasan (University of Maryland/NIST Joint Quantum Institute)

Towards quantum and classical light sources and transducers at any wavelength using nonlinear nanophotonics

Nanophotonics provides the unprecedented opportunity to engineer nonlinear optical interactions through the nanometer-scale control of geometry provided by modern fabrication technology.