# Upcoming Seminars

UMass Boston Physics Colloquia

Talks are on Thursdays, 1:00 pm, at S(cience)-3-126, unless stated otherwise

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**Thursday, January 26, 1:00pm**

**Efi Shahmoon
Harvard U**

__Cooperative resonances in light scattering from atomic metasurfaces__

We consider light scattering off a two-dimensional (2D) dipolar array and show how it can be tailored by properly choosing the lattice constant of the order of the incident wavelength. In particular, we demonstrate that such arrays can operate as a nearly perfect mirror for a wide range of incident angles and frequencies, and shape the emission pattern from an emitter into a well-defined, collimated beam. These results can be understood in terms of the cooperative resonances of the surface modes supported by the 2D array. Experimental realizations are discussed, using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials, as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.

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**Thursday, Feb 2, 1:00pm**

**NO SEMINAR **

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**Thursday, Feb 9, 1:00pm**

**NO SEMINAR **

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**Thursday, February 16, 1:00pm**

**Tigran Sedrakyan
UMass Amherst**

__Quantum spin-liquid with ulrtacold bosons: the smoking gun of statistical transmutation __

Optical lattices are remarkable for their capacity to host rich physics. Examples include lattices having moat-like band structures, i.e., a band with infinitely degenerate energy minima attained along a closed line in the Brillouin zone. It is entirely the effect of competing correlations which lifts this degeneracy and leads to an amazing variety of completely new quantum many-body states. If such a lattice is populated with bosons, the degeneracy prevents their condensation. Such degeneracy of the kinetic energy favors fermionic quasiparticles, leading to statistical transmutation. At hard-core repulsion, the system is equivalent to the spin-1/2 XY model, while the absence of condensation translates into the absence of magnetic order in the XY plane. In this talk I will show that the frustration in such lattices stabilizes a variety of novel quantum spin liquid phases including a composite fermion state and a chiral spin liquid. These are topologically ordered states, which may be viewed as states of fermions subject to Chern-Simons gauge fields. They have a bulk gap and chiral gapless edge excitations. The talk includes a suggestion for the chiral spin liquid realizations in experiments with cold atoms. The velocity distribution of the released bosons is a sensitive probe of the statistical transmutation and a chiral spin-liquid state.

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**Thursday, February 23, 1:00pm **

**Adolfo del Campo
UMass Boston **

__Engineering Quantum Thermal Machines __

Quantum thermodynamics has emerged as am interdisciplinary research field in quantum science and technology with widespread applications. Yet, the identification of scenarios characterized by quantum supremacy -a performance without match in the classical world- remains challenging. In this talk I shall review recent advances in the engineering and optimization of quantum thermal machines. I will show that nonadiabatic many-particle effects can give rise to quantum supremacy in finite-time thermodynamics [1]. Tailoring such nonadiabatic effects by making use of shortcuts to adiabaticity, quantum heat engines can be operated at maximum efficiency and arbitrarily high output power [2]. A thermodynamic cost of these shortcuts will be elucidated by analyzing the full work distribution function and introducing a novel kind of work-energy uncertainty relation [3]. I shall close by discussing the identification of scenarios with a quantum-enhanced performance in thermal machines run over many cycles [4].

Bibliography: [1] J. Jaramillo, M. Beau, A. del Campo, New J. Phys. 18, 075019 (2016).

[2] M. Beau, J. Jaramillo, A. del Campo, Entropy 18, 168 (2016).

[3] K. Funo, J.-N. Zhang, C. Chatou, K. Kim, M. Ueda and A. del Campo, Phys. Rev. Lett. (accepted); arXiv:1609.08889 (2016).

[4] G. Watanabe, B. P. Venkatesh, P. Talkner and A. del Campo, Phys. Rev. Lett. 118, 050601 (2017).

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**Thursday, March 2, 1:00pm **

**TBA
TBA **

__TBA __

TBA

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**Thursday, March 9, 1:00pm
joint QST Seminar + Physics colloquium event **

**Eric Lutz
University of Erlangen-Nuremberg **

__A single atom heat engine __

We review the miniaturization of heat engines towards the nanoscale. We discuss in detail the experimental realization of a single atom engine using an ultracold trapped ion coupled to engineered reservoirs. We further address the question of how to enter the quantum regime and exploit quantum effects to enhance the performance of the machine.

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**Thursday, March 16, 1:00pm**

**NO SEMINAR, SPRING BREAK**

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**Thursday, March 23, 1:00pm **

**Unai Alvarez
University of the Basque Country, Bilbao, Spain **

__Quantum Biomimetics __

In this talk I will cover the main ideas in Quantum Biomimetics, a research line oriented to the design of quantum protocols that emulate processes or properties of living systems. In particular we have directed our work towards two main goals, the replication of a natural selection scenario (Quantum Artificial Life) and the creation of a quantum learning environment (Quantum Artificial Intelligence). In parallel, we have developed simulation techniques for introducing memory in the dynamics, defined as an explicit nonlocal time dependence in the evolution equations. This feature is an important ingredient in the design of more autonomous and useful Quantum Biomimetic protocols.

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**Wednesday, March 29, 3:00pm, ISC-1200 (UNUSUAL DAT/TIME/LOCATION) **

**Giuseppe Pucci
MIT **

__Hydrodynamic Analogs __

In the last decade, a number of new hydrodynamic analogs of physical systems have come to light. I will ﬁrst present an example of self-adaptation and self-propulsion that arises when surface waves are triggered on a liquid drop ﬂoating on a vibrating bath [1] (a). The system behaviour is rationalized in terms of competition between the wave radiation pressure and the capillary response of the drop boundary, and the role of radiation pressure is similar to that in optical cavities.

Droplets may bounce on the surface of a vibrating liquid bath, self-propelling through a resonant interaction with their own wave ﬁelds [2] (b). These “walkers” represent a macroscopic realization of de Broglie’s early pilot-wave system and exhibit several features reminiscent of quantum particles [3], including quantized orbits and orbital-level splitting, tunneling and wavelike statistics. Early experi-ments suggested that walkers exhibit behaviour akin to single-particle diﬀraction and interference [4]. I revisit these single- and double-slit experiments with a reﬁned setup, showing the importance of experimental precision in walker-boundary interactions [5] (c). I explore the non-specular reﬂection of walkers from a planar wall [6] and their refraction across a discrete change in ﬂuid depth, in which case they obey a form of Snell’s law.

Future directions will be addressed, including hydrodynamic analogs of Kapitza-Dirac diﬀraction, spin lattices, electron bubbles and trapping with the Talbot eﬀect.

[1] Pucci, G., Fort, E., Ben Amar, M., and Couder, Y. Phys. Rev. Lett. 106(2), 024503 (2011).

[2] Couder, Y., Protière, S., Fort, E., and Boudaoud, A. Nature 437, 208 (2005).

[3] Bush, J. W. M. Phys. Today 68(8), 47 (2015).

[4] Couder, Y. and Fort, E. Phys. Rev. Lett. 97, 154101 (2006).

[5] Pucci, G., Harris, D. M., Faria, L. M., and Bush, J. W. M. submitted to the J. Fluid Mech. (2017).

[6] Pucci, G., Sáenz, P. J., Faria, L. M., and Bush, J. W. M. J. Fluid Mech. 804, R3 (2016).

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**Thursday, March 30, 1:00pm **

**Alexey Tonyushkin
UMass Boston **

__Magnetic Particle Imaging: Introduction to Imaging and Instrumentation __

Magnetic Particle Imaging (MPI) is a new tomographic imaging modality that offers high spatial and temporal resolution. Compared to the other imaging modalities such as MRI/CT/PET, MPI is non-toxic, more sensitive, and fully quantitative technique. To date a few small-bore MPI systems were developed, however, human-size MPI scanner has yet to be built. The major challenge of scaling up of MPI is in high power consumption that is associated with the traditional approach to designing the scanner. MPI signal relies on the nonlinear magnetization response from the presence of the injected tracer of iron oxide nanoparticles. Apart from biomedical imaging, MPI-based spectrometer or relaxometer is also a very useful tool for the material property studies such as the size, distribution, and magnetic properties of superparamagnetic nanoparticles. In my talk, I will start from the basics of MPI, specifically, the physics and instrumentation that includes two fundamental types of MPI topologies: field-free-point and field-free-line. Then I will talk about my approach to designing MPI scanner and also will show how traditional MPI can be blended with AMO physics to incorporate an atomic magnetometer as a very sensitive way to detect the signal.

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**Tuesday, April 4, 3:30pm, ISC-1200 (UNUSUAL DAT/TIME/LOCATION) **

**Kun-Ta Wu
Brandeis U **

__Scale-invariant transitionfrom turbulent to coherent flows in 3D confined active fluids __

Transport of fluid through a pipe is essential for the operation of macroscale machines and microfluidic devices. Conventional fluids only flow in response to external pressure. We demonstrate that an active isotropic fluid, comprised of microtubules and molecular motors, autonomously flows through meter-long three-dimensional channels. We establish control over the magnitude, velocity profile and direction of the self-organized flows, and correlate these to the structure of the extensile microtubule bundles. The inherently three-dimensional transition from bulk-turbulent to confined-coherent flows occurs concomitantly with a transition in the bundle orientational order near the surface, and is controlled by a scale-invariant criterion related to the channel profile. The non-equilibrium transition of confined isotropic active fluids can be used to engineer self-organized soft machines.

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**Wednesday, April 5, 2:00pm, ISC-1200 (UNUSUAL DAT/TIME/LOCATION) **

**Alexander Klotz
MIT **

__Studying Polymer Physics with DNA Molecules: Complex Geometry and Complex Topology __

In addition to containing our genetic information, DNA molecules also serve as a model system to study the physics of polymers, the chain-like molecules that make up many modern materials. The recent development of experimental and theoretical tools for DNA polymer physics have lead to the development of modern genetic sequencing technologies. Polymer physics is dominated by the simple tendency of entropy to increase, such that when a polymer is stretched, it relaxes into more compact conformation to increase entropy (explaining the elasticity of rubber). The thermodynamics and statistical properties of polymers can be studied by modifying in some way its maximum entropy state. I will present my work studying the statistical physics of DNA by constraining molecules to self-assemble into "Tetris"-like configurations using nano-fluidic confinement, to learn about how molecules behave when prevented from reaching their maximum entropy state. I will also discuss my more recent work studying the behavior of knots in DNA to understand the mechanics of long-lived topological entanglements.

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**Thursday, April 6, 1:00pm **

**Olga Goulko
UMass Amherst **

__Controlled results in quantum many-body physics: from a dream to reality __

Many interesting systems in modern condensed matter physics, such as strongly correlated fermions, are too complex to be dealt with by purely analytical methods because of the lack of small parameters. In this talk, I will discuss the power of the diagrammatic technique, which allows us to address problems in many-body physics (and indeed many other areas of physics!) directly from first principles. I will show how physical quantities can be obtained from the diagrammatic expansion in a controlled and systematically improvable manner using numerical algorithms, such as quantum Monte Carlo. In this sense, simulations can be viewed as "numerical experiments" probing the system and making quantitative predictions about its properties. In combination with analytical methods they also offer an exciting new way to address fundamental questions. We can test previously uncontrolled assumptions and explicitly check which properties of a model are necessary to produce certain features, and which are irrelevant. I will illustrate the successes and challenges of diagrammatic Quantum Monte Carlo on several examples from atomic physics.

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**Wednesday, April 12, 2:00pm, ISC-1200 (UNUSUAL DAT/TIME/LOCATION) **

**Mohamed Amine Gharbi
McGill U and Brandeis U **

__Exploring imperfections in soft matter to direct assembly of biomaterials and functional nanomaterials __

The opportunities for guiding assembly using energy stored in biologically based soft materials are wide open. The emerging scientific frontiers in this field show an exceptional promise for significant new applications. Since soft materials can be readily reconfigured, there are unplumbed opportunities to make responsive devices with tunable properties. Liquid crystals (LCs) constitute a fascinating class of matter characterized by the counterintuitive combination of fluidity and long-range order. These materials are known for their exceptionally successful applications in displays, smart windows, and biosensing applications. When the order of molecules in some local regions of LCs is not well defined, topological defects form. These defects are strong trapping sites for colloidal inclusions and have been widely used to control their assembly. In this talk, I present few examples where I use defects in LCs as a platform to directed the assembly of colloidal inclusions. In the first system, defects created at distinct locations in nematic films and emulsions are explored to guide the assembly of microparticles into rich 2D structures. In the second system, defects in a smectic liquid crystal, known as the focal conic domains (FCDs), are created on curved interfaces to self-assemble into a highly ordered structure named the flower texture. These flowers are capable of focusing light and act as micro-lenses similar to an insect’s compound eye. I will present recent progress in understanding the mechanisms that govern the formation of theses assemblies and will report how defects can inspire strategies for making a new generation of bioinspired devices.

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**Thursday, April 13, 1:00pm **

**Boris A. Malomed
Tel Aviv U, Israel **

__Stable two-dimensional solitons in spin-orbit-coupled Bose-Einstein condensates and optical waveguides in bosonic gases __

The quantum-mechanical collapse (alias "fall onto the center" of particles attracted by potential ~ -V/r^2, with V > 0) is a well-known issue in the quantum theory. This work demonstrates that, in a rarefied gas of quantum particles attracted by the above-mentioned potential, the repulsive nonlinearity induced by collisions between the particles prevents the collapse, recreating the missing ground state, in the framework of the mean-field approximation [1]. Further, a phase transition in the ground state is found at a critical value of V. The setting may be realized in the 3D gas of dipolar bosons attracted by a central charge, including the case of the binary gas [2]. The addition of the harmonic-oscillator trapping potential gives rise to a tristability, in the case when the Schroedinger equation still does not lead to the collapse. In the 2D setting, the cubic nonlinearity is not strong enough to prevent the collapse; however, the quintic term does it. The analysis was also extended to the 3D anisotropic setting, with the dipoles polarized by an external uniform field [3].

Finally, an exact numerical analysis of the same model in the framework of the multi-body quantum theory [4] demonstrates that, although the complete suppression of the quantum collapse does not occur, the ground state predicted by the mean-field approximation corresponds to a metastable state which emerges in the framework of the multi-body system.

[1] H. Sakaguchi and B. A. Malomed, Suppression of the quantum-mechanical collapse by repulsive interactions in a quantum gas, Phys. Rev. A 83, 013907 (2011).

[2] H. Sakaguchi and B. A. Malomed, Suppression of the quantum collapse in binary bosonic gases, Phys. Rev. A 88, 043638 (2013).

[3] H. Sakaguchi and B. A. Malomed, Suppression of the quantum collapse in an anisotropic gas of dipolar bosons, Phys. Rev. A 84, 033616 (2011).

[4] G. E. Astrakharchik and B. A. Malomed, Quantum versus mean-field collapse in a many-body system, Phys. Rev. A 92, 043632 (2015).

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**Thursday, April 20, 1:00pm **

**Partha Chowdhury
UMass Lowell **

__TBA __

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**Thursday, April 27, 1:00pm **

**TBA
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**Thursday, May 4, 1:00pm **

**TBA
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