Fakultät für Physik - Veranstaltungen

Dag Hanstorp (Sweden): Optical levitation - Studies of collision between droplets and a single drop Millikan´s experiment

Thu, 09 Nov 2017 16:30:00 +0100

Optical levitation was first demonstrated by Arthur Ashkin in 1971 when he trapped transparent particles in air using a single vertically aligned, focused laser beam. I will in this seminar present an optical levitation system that is used to study collisions between colliding droplets. Our goal is to study under what conditions two colliding water droplets coalesce. The results of the experiments are of importance for our understanding of the growth of rain droplets in the atmosphere. The collisions are observed using high speed cameras (up to 640 000 frames/sec) when droplets are released from two traps, sediment and then collide.
Second, a single drop Millikan´s experiment will be presented. The charge of a trapped droplet can be determined by measuring its oscillatory motion when an AC electric field is applied. The number of excess electrons on the droplet is thereafter changed by illuminating it with UV light. We have currently reached a resolution of ± 3 elementary charges. Different means to improve this in order to observe the quantization of the charge will be discussed.
In addition, it will be demonstrated how isotropic particles can be made to rotate and spin by trapping them using optical vortices and how fluorescence can be used to detect collisions between droplets.

Caslav Brukner (Vienna): Bell's theorem for temporal order

Thu, 09 Nov 2017 13:30:00 +0100

In general relativity causal relations between any pair of events is uniquely determined by locally predefined variables – the distribution of matter‐energy degrees of freedom in the events’ past light‐cone. Under the assumption of locally predefined causal order, agents performing freely chosen local operations on an initially local quantum state cannot violate Bell inequalities. However, superposition of massive objects can effectively lead to “entanglement” in the temporal order between groups of local operations, enabling the violation of the inequalities. This shows that temporal orders between events can be “indefinite” in non‐classical space‐times.

Alexia Auffèves (France): Rebuilding quantum thermodynamics on quantum measurement

Thu, 09 Nov 2017 11:30:00 +0100

Thermodynamics relies on randomness. In classical thermodynamics, the coupling to a thermal bath induces stochastic fluctuations on the system considered: Thermodynamic irreversibility stems from such fluctuations [1], which also provide the fuel of thermal engines. Quantum theory has revealed the existence of an ultimate source of randomness: Quantum measurement through the well-known measurement postulate [2].
In this talk I will present recent attempts to rebuild quantum thermodynamics on quantum measurement, from quantum irreversibility to quantum engines extracting work from quantum fluctuations [3,4].
[1] A. Auffèves, Viewpoint : Nuclear spin points out the arrow of time, Physics 8, 106 (2015)
[2] A. Auffèves, P. Grangier, Recovering the quantum formalism from physically realist axioms, Scientific Reports 43365 (2017)
[3] C. Elouard, D. Herrera-Marti, M. Clusel, A. Auffèves, The role of quantum measurement in stochastic thermodynamics, npj QI 10.1038 (2017)
[4] C. Elouard, D. Herrera-Marti, B. Huard, A. Auffèves, Extracting work from quantum measurement in Maxwell’s demon engines, Phys. Rev. Lett. 118, 260603 (2017), featured in Phys.org and Nature Research Highlights

Aditya Pathak (Vienna): Small‐x Resummation from Effective Field Theory

Tue, 07 Nov 2017 16:15:00 +0100

Barak Dayan (Israel): Passive and deterministic photon-atom gates

Tue, 07 Nov 2017 14:00:00 +0100

Single-photon Raman interaction (SPRINT) is a passive, interference-based effect that creates a deterministic coupling between single photons and a 3-level quantum emitter such as a single atom. As recently demonstrated [1,2], SPRINT enables controlling the state of a single atom deterministically by a single photon. It requires no control-fields, takes place 'automatically' at the timescale of the cavity-enhanced spontaneous emission, and can be harnessed to perform a variety of photon-atom and photon-photon quantum gates [3,4].
I will describe our recent demonstration of a SPRINT-based quantum SWAP gate between a flying photonic qubit (encoded in the two possible input modes) and a stationary atomic qubit.
Our realisation relies on a nanofiber-coupled microsphere resonator coupled to single Rb atoms. We apply the SWAP gate twice to demonstrate a passive quantum memory, i.e. to map the state of a photon onto the atom and then back onto a readout photon, achieving non-classical fidelities (0.74-0.77) in both directions at ~70% efficiency, with ~ 50 ns gate time. Requiring no control fields and applicable to any waveguide-coupled material Lambda system, this scheme can potentially provide a versatile building block for quantum networks and for the manipulation of single photons and photonic states.
Refs:
[1] Science 345, 903 (2014)
[2] Nature Photonics 10, 19 (2016)
[3] Phys. Rev. A. 82, 10301 (2010)
[4] Phys. Rev. A. 95, 0333814 (2017)

Harold Steinacker (Vienna): Quantized cosmological space‐times from Yang‐Mills matrix models

Tue, 07 Nov 2017 14:00:00 +0100

We present simple solutions of IKKT‐type matrix models describing quantized homogeneous and isotropic cosmologies with finite density of microstates and a resolved Big Bang (BB). The BB arises from a signature change of the effective metric on a fuzzy brane embedded in Lorentzian target space, in the presence ofa quantized 4‐volume form. The Hubble parameter is singular at the BB, and becomes small at late times. There is no singularity from the target space point of view. One solution describes a linear coasting cosmology at late times, which is remarkably close to observation. That solution consists of two sheets with opposite intrinsic chiralities, which are connected in a Euclidean pre‐big bang era.

Kishan Dholakia UK): Let Nothing slow you down: new directions in optical manipulation

Mon, 06 Nov 2017 16:30:00 +0100

In science fiction, one is quite familiar with the idea of moving objects using laser beams, evoking concepts such as a “tractor beam”. In the laboratory science fiction turns into science fact: a powerful technique known as “optical tweezers” (OT) shows that micrometre-sized particles (and even biological material and atoms) can be grabbed, moved and generally manipulated without any physical contact using optical forces. This is a powerful demonstration of the optical dipole or gradient force in action. Such “optical tweezers”, based primarily on Newton’s laws and fundamental optics have enabled unprecedented insight about biological molecules such as DNA and molecular motors. In the microscopic world of optical tweezers, researchers are now harnessing these systems to study a host of science: this includes advanced colloidal interactions, dynamics of particles in various potentials (with strong analogues to atomic systems), insights into superconductivity, optically bound matter, studies of the optical angular momentum of light, thermodynamics, microfluidics and motor protein transport. The list is ever growing and now includes potential studies in quantum physics.
This talk will give a perspective of emergent studies in manipulation using materials science particularly for studies at the classical-quantum interface. This can include the rotation of particles in liquid and vacuum using vaterite [1] and nanovaterite particles [2]. These particles exhibit a birefringence that allows them to spin when using circularly polarised trapping beams. Such studies can lead to very high rotation rates and exhibit new features that link to optomechanical cooling of the particle motion and potential future studies of quantum friction. This work may be extended to study the rotation of two particles in vacuum in co- and counter-rotating geometries [3]. The use of these latter types of particles can lead to new studies in optomechanics [4].

[1] Y. Arita, M. Mazilu, and K. Dholakia, Nat Commun 4, 2374 (2013)
[2] J. Yoshihiko Arita, Joseph M. Richards, Michael Mazilu, Gabriel C. Spalding, Susan E. Skelton Spesyvtseva, Derek Craig, and Kishan Dholakia, ACS Nano, 2016, 10 (12), 11505 (2016)
[3] Yoshihiko Arita, Michael Mazilu, Tom Vettenburg, Ewan M. Wright, and Kishan Dholakia, Optics Letters 40(20), 4751-4754 (2015).
[4] Susan E. Skelton Spesyvtseva and Kishan Dholakia, ACS Photonics 3(5), 719-736 (2016)

Thomas Heuser (VBCF) und Piotr T. Chrusciel (Vienna): Vorträge zum Chemie und Physik-Nobelpreis 2017

Tue, 31 Oct 2017 17:30:00 +0100

Vorträge im Rahmen der Chemisch Physikalischen Gesellschaft

Chemie-Nobelpreis 2017

Dieses Jahr erhalten drei Pioniere der Kryo-Elektronenmikroskopie den Chemie Nobelpreis: Jacques Dubochet, Joachim Frank und Richard Henderson. Die Schwedische Akademie der Wissenschaften würdigt hiermit die Entwicklung einer neuen Methode, die es erlaubt die 3D Struktur wichtiger Biomoleküle in ihrer nativen Form in (fast) atomarer Auflösung zu visualisieren

Physik-Nobelpreis 2017

This year's Nobel Prize was awarded to Rainer Weiss, Barry Barish und Kip Thorne for their contributions to the first direct observation of gravitational waves. I will describe the experiment, present the Nobel Prize recipients together with the main remaining contributors, and shortly discuss the future perspectives of gravitational waves astronomy

Christopher T. Sachrajda (Southampton): Electromagnetic Corrections in Lattice Simulations

Tue, 31 Oct 2017 16:15:00 +0100

4. Vorlesung im Rahmen der "Schrödinger-Gastprofessur 2017"

Sougato Bose (London): From Macroscopic Superpositions to Quantum Gravity

Mon, 30 Oct 2017 16:30:00 +0100

We will start by justifying the importance of creating ever more macroscopic quantum superpositions and the great progress already made in this arena (much of which have been made in Vienna). I will then highlight the convenience of a method were a bonafide ancillary quantum system is coupled to a much more macroscopic system to achieve the above. In particular, I will describe how Ramsey interferometry on a spin degree of freedom can be used to create and verify such a superposition and how the scale can be enhanced by using freely propagating (untrapped) objects. Finally, we will show how two such interferometers interacting purely gravitationally can enable one to test whether gravity is fundamentally a "quantum" entity.