Harvard University Department of Physics

Einstein's Unfinished Revolution

Wednesday, November 13 @6:00PM, Science Center Hall C

In this talk, Lee Smolin will discuss his latest book Einstein's Unfinished Revolution, in which he argues that the current theory of quantum mechanics is fundamentally incomplete, ...

Riding the Quantum Computing 'Wave'

The computing world was abuzz last week after Google scientists announced they’d passed a key threshold in which an experimental quantum computer solved a problem in just minutes that a classical computer would take years —10,000 by Google’s count — to solve. The pronouncement, later disputed by scientists at rival IBM, was widely hailed as proof...

First Video of Viruses Assembling

For the first time, researchers have captured images of the formation of individual viruses, offering a real-time view into the kinetics of viral assembly. The research provides new insights into how to fight viruses and engineer self-assembling particles...

UPCOMING EVENTS

Today

CMP Special Seminar: Trithep Devakul (Princeton University), hosted by Bert Halperin
Start: 01:30pm - End: 02:00pm
- Title: Subsystem symmetry-protected topological phases in two and three dimensions
  
  Abstract: Beyond the global symmetries usually dealt with in condensed matter physics (such as the spin-flip symmetry of the Ising model), it is also possible to have symmetries which act on only a subextensive subsystem. These subsystem symmetries act along rigid subsystems, such as: along a line in 2D, a plane in 3D, or even subsystems of fractal dimension.
  It was recently appreciated that such symmetries could also protect non-trivial symmetry-protected topological (SPT) phases. Subsystem SPT phases have gained interest recently due to their capability to act as a resource for universal measurement-based quantum computation (MBQC) in 2D, and due to their connection (by duality) to fracton topological order in 3D.
  I review this and recent progress in subsystem SPT phases, with a particular focus on their classification and its implication on fracton phases.
- 01:30pm
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Yichen Huang, MIT “Renyi entanglement entropy in quantum many-body systems”
Start: 03:30pm - End: 04:30pm
- We present two concrete examples where the Renyi rather than just the von Neumann entanglement entropy is necessary in order to obtain certain insights into quantum many-body systems.In the first example, we consider systems supporting ballistic information propagation and diffusive transport. It is well known that the linear-in-time growth of the von Neumann entanglement entropy (starting from a product state) is a probe of the former. Perhaps surprisingly, we show that the Renyi entanglement entropy (with Renyi index greater than 1) grows diffusively (i.e., as a square root of time) and is consequently a probe of the latter.In the second example, we study the problem of approximating local properties of a quantum many-body state using matrix product and projected entangled pair representations in one and two dimensions, respectively. We prove that area laws for the Renyi entanglement entropy (with Renyi index less than 1) lead to nontrivial upper bounds on the bond dimension. The bounds only depend on the accuracy of the desired approximation but not the system size.References:https://arxiv.org/abs/1902.00977 https://arxiv.org/abs/1903.10048
- 03:30pm
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CUA Seminar: Monika Aidelsburger (University of Munich)
Start: 04:00pm - End: 05:30pm
- Floquet topological phases with ultracold atoms in periodically-driven latticesTopological phases of matter exhibit remarkable electronic properties that offer unique possibilities for applications. A prominent example is the robust quantization of the Hall conductivity in quantum Hall insulators. A widespread technique for generating topological band structures in synthetic systems, such as ultracold atoms in optical lattices, is Floquet engineering [1]. This method relies on the periodic modulation of the system’s parameters to emulate the properties of a non-trivial static system and facilitated the realization of two paradigmatic topological lattice models: the Hofstadter and the Haldane model. Moreover, it inspired ideas for implementing complete lattice gauge theories [2].The rich properties of Floquet systems, however, transcend those of their static counterparts [3]. The associated quasienergy spectrum can exhibit a non-trivial winding number, which leads to the appearance of anomalous chiral edge modes even in situations where the bulk bands are topologically trivial, hence, altering the well-known bulk-edge correspondence. A full classification of Floquet phases requires a new set of topological invariants. Here, I will report on recent results, where we have studied the rich Floquet phase diagram of a periodically-modulated honeycomb lattice using bosonic atoms. The novel properties of anomalous Floquet phases mentioned above open the door to exciting new non-equilibrium phases without any static analogue.[1] A. Eckardt, Phys. Mod. Phys. 89, 311 (2017)[2] C. Schweizer et al., Nat. Phys. (2019); L. Barbiero et al., Science Advances 5, eaav744 (2019)[3] T. Kitagawa et al., Phys. Rev. B 82, 235114 (2010)
- 04:00pm
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CUA Seminar: Monika Aidelsburger, Univ. of Munich
Start: 04:00pm - End: 05:30pm
- 04:00pm
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Tomorrow

Joint Quantum Seminar -Michelle Simmons (Univ of New South Wales)
Start: 12:00pm - End: 01:00pm
- 12:00pm
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Thursday, November 14th

Richard Fletcher ITAMP Seminar
Start: 12:00pm - End: 01:00pm
- Geometric squeezing into the lowest Landau level
  
  One consequence of life in a rotating frame is that translations along orthogonal spatial directions no longer commute, and time-evolution under a scalar potential corresponds to a geometric transformation of the original spatial distribution. We demonstrate geometric squeezing of a rotating Bose-Einstein condensate down to the zero-point cyclotron motion in the lowest Landau level. The extent of squeezing is dramatic, with the condensate attaining a spatial aspect ratio exceeding 100 and an angular momentum of more than 1000 \hbar per particle. As the condensate enter the lowest Landau level, we directly observe the minimal cloud size set by the quantum fluctuations of the cyclotron motional ground state. This technique achieves filling fractions of 10, and offers a new route towards creating strongly correlated fluids.
- 12:00pm
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CMP Weekly Seminar: Roger Melko (University of Waterloo), hosted by Ashvin Vishwanath
Start: 01:30pm - End: 02:30pm
- Title: How hard is it to learn a quantum state?
  
  Abstract: The fundamental difficulties in simulating quantum physics is one of the core motivations for building a quantum computer. As we enter the current era of NISQ hardware, we are faced with a new paradigm: devices that are difficult to simulate, but which can be measured to produce an abundance of data. At the same time, powerful machine learning methods are being adapted to learn representations of quantum states directly from measurement data. It is therefore fair to wonder whether difficulties in simulating large quantum systems also translate into difficulties in learning their states from data. In this talk I will use recent strategies for quantum state reconstruction based on generative modelling with neural networks, to study the learnability scaling of prototypical groundstate wavefunctions in NISQ devices. The efficiency with which quantum states can be reconstructed in neural networks can be quantified numerically, and compared to expectations such as those given by tensor network theory. I will speculate on the implications that an answer to the question "how hard is it to learn a quantum state" would have, focusing on the current generation of experimental quantum simulators.
- 01:30pm
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Monday, November 18th

Adam Riess - STScI
Start: 04:30pm - End: 05:30pm
- The Expansion of the Universe, Faster Than We Thought
  One of the few ways we can learn about the composition, age and fate of the Universe is by measuring its expansion rate and its expansion history. Assuming that the Universe is homogeneous and isotropic on large scales and that it obeys General Relativity, we can derive an equation of motion for the Universe, the Friedmann equation, which describes how the scale factor of the Universe, a(t) changes due to the physics and composition of the Universe. Most of what we know about the Universe from this route comes from measuring just the first two derivatives of a(t), parameters known as the Hubble constant (a dot) and the deceleration parameter (a double-dot). These quantities can be measured by estimating distances to exploding and pulsating stars. In 1998, two teams (including the High-z Team and this author) used measurements of supernovae to show that the deceleration parameter was negative, that is, the expansion is now accelerating due to mysterious dark energy. Understanding the nature of dark energy remains one of the biggest goals of modern physics. More recently, the author has been working to improve measurements of the other parameter, the Hubble constant, which measures the present expansion rate. The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide <2% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. I will review recent and expected progress.
- 04:30pm
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Tuesday, November 19th

CUA Seminar: Klaus Sengstock (University of Hamburg)
Start: 04:00pm - End: 05:30pm
- 04:00pm
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CUA Seminar: Klaus Sengstock, Univ. of Hamburg
Start: 04:00pm - End: 05:30pm
- 04:00pm
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