# Videos of Past Q-FARM Seminars

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.

## “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.

## “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.

## “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.

## 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.

## 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?

## 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.

## “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.

## “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.

## "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.

## “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.

## “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.

## "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.

## "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.

## "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.

## "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

## 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.

## 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.

## 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.

## Tutorial: Search for Non-Abelian Majorana particles as a route to topological quantum computation

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.

## 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.

## 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...

## 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.

## Emergent quantum randomness and its application for quantum device benchmarking

In this talk, we describe a novel, universal phenomenon that occurs in strongly interacting many-body quantum dynamics beyond the conventional thermalization.

## 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.

## 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 ...

## 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.

## 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.

## 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.

## 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.

## 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.

## Coupling diamond defects to high-finesse optical microcavities

Defect centers in diamond can offer atomic-like optical transitions and long-lived spin degrees of freedom.

## Direct laser cooling of polyatomic molecules

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

## Lattice atom interferometry in an optical cavity

Atom interferometers are powerful tools for both measurements in fundamental physics and inertial sensing applications.

## 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.