Applied Physics / Physics Colloquium

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Of the wide variety of sophisticated techniques employed in optical microscopy, of special interest to physicists are schemes which use quantum correlations to increase sensitivity beyond the classical limit. Such technology would be especially applicable in transmission electron microscopy (TEM) since the image resolution of samples of critical interest (e.g. proteins, polymers, and battery materials) is limited by beam damage. However, in contrast to the fantastic diversity and modularity of light optics, electron optics are significantly constrained. I will describe our project of developing new electron optics to enable dose-efficient TEM. While quantum metrology is generally associated with an entangled probe (which has not yet been demonstrated with freespace electrons), it is also possible to perform quantum-optimal measurements with a single particle using sequential measurements [1]. In fact, it is possible to gain significant information about absorbing samples using only damage-free counterfactual measurements [2]. More typically, TEM samples are phase objects. We have shown that an approach called Multi-Pass TEM (MPTEM) can reduce damage by an order of magnitude for realistic samples [3]. The key new electron optics of the MPTEM are the switchable mirrors, which trap electrons in a cavity where the sample is re-imaged multiple times. We are currently building a 10 keV MPTEM [4] as a proof of concept. [1] Quantum Metrology, Vittorio Giovannetti, Seth Lloyd, and Lorenzo Maccone (2006). [2] Designs for a Quantum Electron Microscope, P. Kruit et al, Ultramicroscopy (2016). [3] Multi-Pass Transmission Electron Microscopy, T. Juffmann et al, Scientific Reports (2017). [4] Design for a 10 keV Multi-Pass Transmission Electron Microscope, S. A. Koppell, Ultramicroscopy (2019).

High energy particle physics has the ambitious goals of uncovering the most fundamental constituents of reality and deciphering the rules by which those constituents interact, both today and in the first instants of the Big Bang. Our ability to construct higher and higher energy particle accelerators does not scale well with these ambitions, so progress here will increasingly depend on global collaboration and being smarter with the data that we have in hand.
Beyond colliders, the future of this field will increasingly rely on three other approaches that I will describe:
— Transformational advances in the sensitivities and capabilities of sensors to detect feebly interacting particles such as dark matter and neutrinos.
— Increasing our access to extreme environments provided by Nature, such as supernovae, black hole mergers, and the echoes of the Big Bang.
— Using theory to map fundamental questions seemingly out of experimental reach, e.g. the nature of quantum gravity and spacetime, into quantum systems and simulations that can be created and studied in the laboratory.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani

Location: Hewlett Teaching Center, Rm. 200

One of the most striking predictions of the general theory of relativity is the formation of black hole and cosmic horizons sequestering different regions of spacetime. In this talk we will overview recent theoretical and observational developments in this area. At the classical and quantum level, radiation plays an important role in observations and thought experiments. Hawking's result that black holes radiate raises serious puzzles, while its analogue in early universe cosmology yields a successful quantum theory of the origin of structure. The pursuit of a complete theory of quantum gravity has led to qualitatively new lessons about emergent spacetime structure in the presence of horizons. Turning to observations, the proposition that black holes with masses from three to ten billion times that of the sun are quite common in the universe grew from a conjecture to a conviction. Recent observations using the Fermi Gamma-ray Space Telescope, LIGO/VIRGO and the Event Horizon Telescope have validated general relativity and demonstrated how black holes are born, how they affect their surroundings and how they power the most luminous cosmic sources.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200

The normal state of high-temperature superconductors exhibits anomalous transport and spectral properties that are poorly understood. Cold atoms in optical lattices have been used to realize the celebrated Fermi-Hubbard model, widely believed to capture the essential physics of these materials. The recent development of fermionic quantum gas microscopes has enabled studying the normal state of Hubbard systems with single-site resolution. I will start by introducing the atomic platform and reviewing experiments that have been done on measuring spin and density correlations in half-filled systems [1]. Next, I will describe the development of a technique to measure microscopic diffusion, and hence resistivity, in doped Mott insulators. We have found that this resistivity exhibits a linear dependence on temperature and violates the Mott-Ioffe-Regel limit, two signatures of strange metallic behavior [2]. Finally, I will describe the development of angle-resolved photoemission spectroscopy (ARPES) for Hubbard systems and its application to studying pseudogap physics in an attractive Hubbard system, setting the stage for future studies of the pseudogap regime in repulsive Hubbard systems [3].

[1] Parsons et. al., Science 353, 1253 (2016), Boll et. al., Science 353, 1257 (2016), Cheuk et. al., Science 353, 1260 (2016), Brown et. al., 357, 1385 (2017).
[2] Brown et. al., Science 363, 379 (2019).
[3] Brown et. al., Nature Physics, in press, arxiv:1903.05678 (2019).

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani

Location: Hewlett Teaching Center, Rm. 200

In the past few years, a tension has emerged between the current expansion rate of the universe (H0) and the value predicted by early universe probes under the assumption of a standard LCDM cosmology. The tension is statistically significant: combinations of local probes are 4-6 sigma away from H0 as inferred by Planck, for example. Efforts to uncover systematic uncertainties are under way, but they have been unsuccessful so far. If the tension is real, most proposals to resolve it require changing the expansion history before recombination in a non-trivial way, possibly as a result of early dark energy, or sterile neutrinos. I will review measurements of H0, including the results of the well known methods based on the local distance ladder and on the cosmic microwave background. I will explain in detail the approach based on gravitational time delays, that have recently reached comparable precision, providing an independent verification of the tension. I will conclude by discussing the prospects of reaching sub-percent precision on multiple independent methods as a way to control systematics, and verify whether indeed this is the first hint of new physics beyond LCDM.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani

Location: Hewlett Teaching Center, Rm. 200

Studies of various types of Hall effects have led to great advances in solid state physics. In this talk, I will describe two novel Hall phenomena. The first is thermoelectric Hall effect that describes the generation of a transverse electrical current under a temperature gradient. Under a quantizing magnetic field, thermoelectric Hall conductivity is proportional to thermal entropy which is strongly enhanced by Landau level degeneracy. This leads to an unbounded growth of the thermopower in three- dimensional Dirac/Weyl semimetals, a parametrically large thermoelectric figure of merit in quantum Hall systems at low temperature, and a new experimental way to study neutral collective modes in the fractional quantum Hall liquids.

Second, I will discuss a nonlinear Hall effect in nonmagnetic materials at zero magnetic field, where the transverse current depends quadratically on the applied electric field. This effect arises from anomalous velocity in a current-carrying state, driven by Berry curvature in inversion-breaking systems.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200

'Circuit quantum electrodynamics' is the theory of non-linear quantum optics extended to the study of microwave photons strongly interacting with 'artificial atoms' (Josephson junction qubits) embedded in superconducting electrical circuits. Recent remarkable theoretical and experimental progress in our ability to measure and manipulate the quantum states of individual microwave photons is leading to novel applications ranging from accelerating dark matter searches to quantum error correction that, for the first time in any technology, has successfully extended the lifetime of quantum information. This talk will present an elementary introduction to the basic concepts underlying circuit QED and describe several recent novel experiments demonstrating these new found capabilities.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200

Quantum sensors based on optically addressable solid-state spins are powerful tools that offer high sensitivity, nanoscale spatial resolution, and quantitative field information. The nitrogen vacancy (NV) center in diamond is the most advanced such sensor because of its robust, room-temperature coherence and its high sensitivity to a variety of fields: magnetic, electric, thermal, and strain. Here I discuss an NV-based imaging platform where we have incorporated an NV center into a scanning probe microscope and used it to image skyrmions, nanoscale topological spin textures. I also discuss recent experiments that utilize the NV center's sensitivity to fluctuating magnetic fields to image conductivity with nanoscale spatial resolution. A grand challenge to improving the spatial resolution and magnetic sensitivity of the NV is mitigating surface-induced quantum decoherence, which I will discuss in the second part of this talk. Decoherence at interfaces is a universal problem that affects many quantum technologies, but the microscopic origins are as yet unclear. Our studies guide the ongoing development of quantum control and materials control, pushing towards the ultimate goal of NV-based single nuclear spin imaging.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200

The power of quantum information lies in its capacity to be non-local, encoded in correlations among two, three, or many entangled particles. Yet our ability to produce, understand, and exploit such correlations is hampered by the fact that the interactions between particles and ordinarily local. I will report on experiments in which we use light to induce long-range interactions among cold atoms, with photons acting either as messengers or as a means of coupling to highly polarizable Rydberg states. The combination of optically programmable interactions with imaging of spin dynamics opens new opportunities in areas ranging from quantum-enhanced sensing to quantum simulation. As an illustrative example, I will touch on prospects for building and probing toy models for information scrambling in black holes.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200