# 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 **

**TBA
TBA **

__TBA __

TBA

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

**NO SEMINAR, SPRING BREAK**

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

**TBA
TBA **

__TBA __

TBA

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

**Partha Chowdhury
UMass Lowell **

__TBA __

TBA

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

**NO SEMINAR **

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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 **

**TBA
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__TBA __

TBA

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

**TBA
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__TBA __

TBA

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

**TBA
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__TBA __

TBA

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