Harvard University Department of Physics

Efficient Topological Materials Discovery using Symmetry Indicators

pie chart of topological materials statistics

Researchers have now shown that the computation of symmetry indicators for any crystalline symmetry setting can readily be integrated into standard first-principle calculations...

Harvard Science Book Talk: David Reich (Feb 27 @6:00PM)

Photo of Prof. David Reich and cover of the book "Who We Are and How We Got Here"

In this lecture, Prof. Reich will discuss his new book, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past.

Congratulations to David Morin and Jacob Barandes!

David Morin and Jacob Barandes named Co-Directors of Undergraduate and Graduate Studies, respectively...

UPCOMING EVENTS

Monday, February 25th

Monika Schleier-Smith - Stanford University
Start: 04:15pm - End: 05:15pm
- Choreographing Quantum Spin Dynamics with Light
  
  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 necessarily local. I will report on experiments in which we engineer effectively non-local interactions among cold atoms by letting photons serve as messengers conveying information between them. 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.
- 04:15pm
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Tuesday, February 26th

Alex Ruichao Ma, University of Chicago
Start: 09:00am - End: 10:00am
- "Exploring Synthetic Quantum Matter in Superconducting Circuits"
  
  abstract: Understanding strongly correlated materials is an outstanding challenge in modern physics. Synthetic quantum matter, tailor-made in the lab as clean, tunable and easy to probe quantum simulators, can help us gain insights into such strongly correlated matter, and explore new parameter regimes with no natural counterpart.
  
  In this talk, I will show how we create and explore quantum materials made of light, specifically microwave photons trapped in superconducting circuits. We develop a new approach for preparing many-body phases, where engineered dissipation is used as a resource to protect the fragile quantum states against intrinsic losses. We apply it to our system, a strongly interacting Bose-Hubbard lattice, and realize a dissipatively stabilized Mott insulator of photons. The dynamics of thermalization towards the Mott phase is probed using lattice-site- and time- resolved microscopy.
  
  Our experiments establish superconducting circuits as a powerful platform for studying quantum materials, and open many future possibilities including the exploration of strongly interacting topological phases, quantum thermodynamics in coherent or driven-dissipative settings, and hybrid quantum systems with exotic couplings.
- 09:00am
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MATH: Fei Wei (Harvard), “Entropy of arithmetic functions and arithmetic compactifications”
Start: 03:00pm - End: 03:00pm
- A new notion of entropy of arithmetic functions is introduced. It is shown that all zero entropy arithmetic functions form a C*-algebra. This entropy also behaves nicely with respect to limits similar to Kolomogorov’s dynamical entropy and has many properties similar to Shannon’s entropy.
  
  Organizer: Mathematical Picture Language Seminar
- 03:00pm
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CUA Seminar: Prof. Ben Lev (Stanford University)
Start: 04:00pm - End: 05:30pm
- Many-body cavity QED with multimode resonatorsWe will present our first experiments involving multimode cavity QED, wherein the cavity-mediated atom-atom interaction is dramatically modified compared to that in a single-mode cavity. We demonstrate the engineering of strong, tunable-range, and sign-changing photon-mediated interactions between atoms and between atomic spins. A mechanism for photon-mediated interactions based on the Gouy phases of the cavity modes is introduced and demonstrated. Such interactions can be exploited for the creation of unusual quantum many-body phases. Other results include the observation of a ‘spinor density-wave polariton’ condensate and cavity-assisted dynamical spin-orbit coupling. Together, these results pave the way for future experimental access to nontrivial quantum phases and transitions in driven-dissipative quantum systems, synthetic dynamical gauge fields, and the ability to study nonequilibrium spin glasses and quantum neural networks.
- 04:00pm
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Wednesday, February 27th

CMP Special Seminar: Eun Gook Moon (KAIST), hosted by Subir Sachdev
Start: 09:30am - End: 10:30am
- Title: Exotic Thermal Phase Transitions associated with Deconfined Phases
  
  Abstract: Fathoming deconfined phases is one of the key issues in modern condensed matter since striking many-body effects including massive quantum entanglement and coherence may be realized, as manifested in quantum spin liquids and topological orders. Here, we show that deconfined phases even host exotic thermal phase transitions. Constructing Z2 lattice gauge models with interactions between Z2 gauge fluxes, we prove the existence of a deconfined-confined thermal phase transition in two spatial dimensions in sharp contrast to its absence in the Wegner model. We also show that symmetry breaking transitions in deconfined phases may be unconventional. Z2 and U(1) symmetry breaking transitions in three spatial dimensions may be in the same universality class, which is impossible under the conventional Landau-Ginzburg-Wilson paradigm. Characteristic signatures of the transitions in experiments and candidate strongly correlated systems such as Kitaev materials are also discussed.
- 09:30am
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Joint Quantum Seminar - Ignacio Cirac (MPQ)
Start: 04:00pm - End: 06:00pm
- 04:00pm
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Joint Quantum Seminar: Prof. Ignacio Cirac (Max-Planck-Institut für Quantenoptik)
Start: 04:00pm - End: 05:30pm
- “Quantum algorithms and simulation for quantum many-body systems”Quantum many-body systems are very hard to simulate, as computationalresources (time and memory) typically grow exponentially with system size. However, quantum computers or analog quantum simulators may perform that task in a much more efficient way. In this talk, I will first review some of the quantum algorithms that have been proposed for this task and then explain the advantages and disadvantages of analog quantum simulators. I will also describe a theoretical proposal to solve quantum chemistry problems with such devices.
- 04:00pm
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Melissa Rinaldin - Leiden University
Start: 06:00pm - End: 07:30pm
- 06:00pm
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Thursday, February 28th

Qiong Ma, MIT
Start: 09:00am - End: 10:00am
- "Geometry and Topology in Quantum Materials"
  
  abstract: Close to large masses, the geometry of space-time warps, bending the light trajectory and leading to dramatic phenomena like black holes and gravitational waves. The quantum world of electrons likewise has a geometrical structure, constructed from electronic quantum wave functions. Such a quantum geometrical structure, studied as quantum metric and Berry curvature, can reshape an electron’s behaviors in nontrivial ways. Quantum geometry also has a deep relation with topological phases of matter, which have taken center stage in condensed matter over the past decade. Although many topological phases have been experimentally discovered in quantum materials, the understanding of how bulk quantum geometry is manifest in material properties has remained limited.
  
  In this talk, I will show that quantum geometry can strongly modify the electronic response to external electromagnetic waves, giving rise to a wide range of previously unexplored nonlinear responses. First, I will discuss our observation of the nonlinear Hall effect, in the 2D quantum ferroelectric semimetal, bilayer WTe2, which originates from the dipole distribution of Berry curvature in momentum space. Remarkably, this is a new electrical Hall effect realized in a nonmagnetic material and without external magnetic field. Second, I will demonstrate how we use infrared nonlinear photocurrent to probe the chirality of Weyl fermions in 3D topological Weyl semimetals, which manifests as source or drain of Berry curvature. Third, with the same nonlinear optoelectronic probe, I will show our detection and manipulation of a novel electronic instability, the gyrotropic order, in the correlated semimetal TiSe2, and discuss its theoretical relation with Berry curvature. Looking forward, the combination of advanced optoelectronic techniques and tunable material platforms both in 2D and 3D can enable ample exciting possibilities to discover quantum geometrical and topological phenomena beyond the current paradigm.
- 09:00am
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ITAMP Lunch Seminar
Start: 12:00pm - End: 01:00pm
- Speaker: Nicole Yunger Halpern
- 12:00pm
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CMP Seminar: Fabian Grusdt (Munich), hosted by Eugene Demler
Start: 03:00pm - End: 04:00pm
- Title: A microscopic perspective on correlated quantum matter
  
  Abstract: Strongly correlated quantum systems host a variety of intriguing physical phenomena, but in many cases the underlying physical mechanisms remain elusive. Quantum simulations with ultracold atoms provide a new microscopic perspective, enabled by access to individual quantum projective measurements. In the first part of this talk, I will discuss the recent progress that has been made in the study of doped 2D anti-ferromagnets. Specifically, I will show that a conceptually simple geometric string model, free of any fit parameters, can predict the doping dependence of local correlation functions of spins in the normal phase of the Fermi Hubbard model. I will also present a corresponding single-hole variational wavefunction and discuss the resulting ARPES spectrum of a single hole and its relation to the Fermi-arcs observed in cuprates. In the second part of the talk, I will briefly comment on the prospects of quantum simulation of Z2 lattice gauge theories with couplings to dynamical matter, which have attracted significant attention in connection with the rich phase diagram of strongly correlated electrons.
- 03:00pm
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Friday, March 1st

CMP Seminar: Fabio Boschini, UBC
Start: 02:00pm - End: 03:00pm
- Title: Study of the phase diagram of cuprates in an ultrafast fashion
  
  Abstract: The phase diagram of copper-oxides hosts intertwined phases as disparate as high-temperature superconductivity, charge order and the pseudogap, making the precise description of each separate phase challenging. In the last decade the development of time-resolved techniques has offered a novel perspective for investigating dynamical properties of quantum phases. In this regard, we recently demonstrated that time- and angle-resolved photoemission spectroscopy (TR-ARPES) can disentangle the dynamics of phase fluctuations and charge excitation, establishing the dominant role of phase coherence in the emergence of high-temperature superconductivity in Bi-based cuprates [1].
  In light of this, we employed this same dynamical approach to reveal the competition between charge-order and superconductivity in Y-based cuprates in the time-domain [2] and demonstrate the intimate relation between the well-known ≈70 meV electron-boson band renormalization (kink) and the superconducting gap [3]. In addition, time-resolved photoemission approach has allowed us to reveal unambiguously the relation of the pseudogap and short-range antiferromagnetic correlations in optimally-doped NCCO electron-doped cuprate [4].
  Finally, I will propose a way to redefine the paradigms of the TR-APRES analysis accessing the mode-projected electron-phonon matrix element in an ultrafast fashion [5].
  
  [1] Boschini et al., Nature Materials 17, 416-420 (2018)
  [2] Wandel, Boschini et al., in preparation (2019)
  [3] Razzoli*, Boschini* et al., in preparation (2019)
  [4] Boschini*, Zonno* et al., arXiv:1812.07583 (2018)
  [5] Na*, Mills*, Boschini et al., arXiv:1902.05572
- 02:00pm
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