# 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.

## 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]

## 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.

## "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.

## "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.

## 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.

## "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.

## 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

## “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.

## “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.

## “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: “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?

## “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.

## "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.

## “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.

## "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.

## "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

## 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.

## 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.

## 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...

## 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.

## 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 ...

## 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.

## 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.

## 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.

## Lattice atom interferometry in an optical cavity

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