Wednesdays at 4:00 PM
Jefferson 250

Unless otherwise noted, all events will take place in Jefferson 250

Student presentation begins at 4:00 PM
Refreshments are served from 4:10-4:30 PM
Guest presentation begins at 4:30 PM

Wednesday, May 2, 2018 – Prof. Peter Barker (University College London)

Quantum optomechanics with levitated nanoscale oscillators

Nanoscale oscillators levitated by optical, electric or magnetic fields in high vacuum offer a completely new arena for studies of foundational science and applications. The drastic suppression of decoherence potentially allows observation of non-classical states of motion, while the creation of long-lived macroscopic quantum states enable demonstrations of quantum behaviour on very large mass scales. This includes the possibility of creating large macroscopic superpositions, as well as tests of proposed mechanisms of wavefunction collapse at large length scales.

An important requirement for these studies is the development of methods to manipulate and cool the oscillator’s centre-of-mass motion as well as controlling the internal temperature.  In this talk I will describe our recent work which has demonstrated cavity cooling of levitated silica spheres (200 nm) to milliKelvin temperatures, as well as internal cooling of optically levitated nanocrystals (Yb: YLF)  using anti-stokes laser refrigeration.  Given time, I will also describe the development of an optical accelerometer that has resulted from this work.

Wednesday, April 18, 2018 – Prof. Harold Baranger (Duke University)

Waveguide QED: Photon Correlations, Capture, and Production

Strong coupling between a local quantum system (qubit) and one-dimensional bosonic states has recently become experimentally feasible in a variety of plasmonic, photonic, circuit-QED, and cold-atom contexts. This has opened up a new field dubbed “waveguide QED”. The key ingredient in the many new effects in this area is inelastic scattering into the one-dimensional continuum. Using such inelastic scattering as a unifying theme, I shall discuss our results on (i) characterization of photon correlations using the waiting-time distribution, (ii) capture of a photon into a bound state in the continuum, and (iii) photon production when the coupling is ultrastrong. In the ultrastrong example, we find surprisingly that the off-resonant inelastic emission is dominated by broadband photon production, coming from contributions in which the number of excitations is not conserved.

Wednesday, March 21st, 2018 – Prof. Markus Aspelmeyer (University of Vienna)

Quantum optomechanics with levitated solids: sensing, simulation and the gravity-quantum interface

I will discuss our current experiments to achieve quantum optical control over motional states of levitated solids. This includes dielectric nanospheres coupled to Fabry-Perot cavities and nanophotonic structures, as well as micron-sized superconductors and magnets that will eventually be coupled to superconducting circuits. I will provide an overview of the status and challenges, and of the perspectives for such experiments for sensing, simulation and for novel tests of the interface between quantum physics and gravity.

Wednesday, December 13th, 2017 – Prof. Andrea Cavalleri (Max Planck Institute for the Structure and Dynamics of Matter, Hamburg GERMANY Department of Physics, University of Oxford)

Nonlinear THz optics as probe of quantum solids

In  this  lecture,  I  will  discuss  how  coherent  electromagnetic  radiation  at  infrared  and  TeraHertz  frequencies  can  be  used  to  collective  excitations like  phonons  in  solids.  I  will  emphasize  experiments  in  which  hidden  features the superconducting order parameter can be revealed. I will also discuss advances in the nonlinear response of phonons, which I will show can be used as a probe of the interatomic potential of a solid.

Wednesday, November 29th, 2017 – Dr. Pierre Pillet (Laboratoire Aimé Cotton, CNRS, Univ Paris-­Sud, ENS Paris-­Saclay)

Interplay between two-­, few-­ and many-­body effects in a dense, cold and disordered gases in strong dipole-­dipole coupling

Dipole-­dipole long-­range interactions between two atoms in dense and cold atomic media play a crucial role in many configurations by considering ground state atoms as well as highly-­excited Rydberg atoms. They may alter the coherent scattering of light by a dense and cold atomic sample. They open also many opportunities for studying few-­body and many-­body physics, and hopefully for simulating many different quantum systems. A cold, disordered and dense cesium Rydberg gas in Förster resonant configurations is an interesting example of out-­of-­equilibrium quantum system.  The atoms are prepared in a state, np, exchanging internal energy by a resonant way +→+(+1) (the resonance is obtained by Stark shift of the p level). The reaction due dipole-­dipole interaction leads to a very efficient transfer of population from the p level to the s ones. The Förster coupling offer the possibility to isolate few-­body effects from two-­body ones and to characterize their features. A saturation regime can be reached, characterized by an amazing behavior corresponding to the “thermalization” of the atomic sample, meaning an equal-­distribution of the populations of the relevant levels of the resonant reaction. The dynamics of the thermalization seems to be the result of few-­body effects. The interplay between two-­, few-­ and many-­body regime in dipole coupling will be discussed, as so well as the role of the diffusion scattering of the products of the reaction.

Wednesday, November 15th, 2017 – Dr. Tongcang Li, Purdue University


Levitated spin-optomechanics

Optomechanical systems, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) and atomic force microscopes, are very sensitive devices. Among different optomechanical systems, an optically levitated nanoparticle in vacuum can have a particularly high quality factor. In this talk, I will discuss how to couple the mechanical motion of an optically levitated nanoparticle to photon spins, electron spins, and the spin angular momentum of quantum vacuum fluctuations for sensing, quantum information science, and macroscopic quantum mechanics. Recently, we optically levitated a nanodiamond and demonstrated electron spin control of its built-in nitrogen-vacancy (NV) centers in vacuum. We have also driven a nanoparticle to rotate beyond 1GHz with a circularly polarized laser beam, and observed the torsional vibration of a nanoparticle with a linearly polarized laser beam. Based on our experimental results, we propose to achieve strong coupling between an NV electron spin and the torsional vibration of a levitated nanodiamond with a uniform magnetic field. We also propose to use our system to detect the Casimir torque due to angular momentum of quantum vacuum fluctuations, which has not been observed to date. At the end of the talk, I will briefly describe my other works in quantum optics.

Wednesday, November 1st, 2017 – Dr. Ronnie Kosloff, Hebrew University of Jerusalem


Thermodynamics of quantum devices

Quantum thermodynamics addresses the emergence of thermodynamical laws from quantum me- chanics. The viewpoint advocated is based on the intimate connection of quantum thermodynamics with the theory of open quantum systems. Quantum mechanics inserts dynamics into thermody- namics giving a sound foundation to finite-time-thermodynamics. The emergence of the 0-law I-law II-law and III-law of thermodynamics from quantum considerations will be presented through exam- ples. I will show that the 3-level laser is equivalent to Carnot engine. I will reverse the engine and obtain a quantum refrigerator. Different models of quantum refrigerators and their optimization will be discussed. A heat-driven refrigerator (absorption refrigerator) is compared to a power-driven refrigerator related to laser cooling. This will lead to a dynamical version of the III-law of thermo- dynamics limiting the rate of cooling when the absolute zero is approached. The thermodynamically equivalence of quantum engines in the quantum limit of small action will be discussed. I will ad- dress the question why we find heat exchangers and flywheels in quantum engines. I will present a molecular model of a heat rectifier and a heat pump in a non-Markovian and strong coupling regime. Finely the question of quantum supremacy in heat devices will be adressed.

Wednesday, October 18th, 2017 – Dr. Xiaodong Xu, University of Washington


Monolayer Magnets

Since the discovery of graphene, the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. However, until recently 2D crystals with intrinsic magnetism were still lacking. Such crystals would enable new ways to study 2D magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for magnetoelectronics and van der Waals engineering for novel interface phenomena. In this talk, I will describe our recent magneto-optical spectroscopy experiments on van der Waals magnets, chromium(III) iodide CrI3. I will first demonstrate the existence of isolated monolayer semiconductor with intrinsic Ising ferromagnetism. I will then show the layer number-dependent magnetic phases. The magnetic ground state evolves from being ferromagnetic in a monolayer, to antiferromagnetic in a bilayer, and back to ferromagnetic in a trilayer and thin bulk. Lastly, I will discuss the emerging spin phenomena in monolayer WSe2/CrI3 ferromagnetic semiconductor heterostructures, including ferromagnetic control of valley pseudospin in WSe2 via large magnetic exchange field, and optical analog of giant magnetoresistance effect.

CUA Seminar

Seminars on on Tuesdays at 4:30 PM, with student talk and cookies at 4:00 PM
Alternates between Harvard (J356) and MIT (26-214)

The HQOC does not provide direct support for CUA seminars, but organizers speakers in close collaboration with the CUA, as well as collaborates on special talks, and visitors.



Harvard Condensed Matter Physics seminar

Unless noted otherwise, seminars are held at noon on Thursdays
Lyman Laboratory, Room 425, Harvard University

The HQOC fully supports the CMT seminar.

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