Quantentechnologie

Invited speaker: Prof. Martin Holthaus
Affiliation: Universität Oldenburg
Title: Macroscopic Wave Functions Without Spontaneous Symmetry Breaking
Time and room: 17:15 lecture hall IAP
Abstract: The time-dependent Gross-Pitaevskii equation (GPE) equation still is the workhorse for computing condensate dynamics. Essentially, this is a nonlinear evolution equation for the order parameter, that is, for the macroscopic wave function. The introduction of this concept often involves the idea of "spontaneously broken gauge symmetry", implying wave functions which amount to superpositions of states with different particle numbers. However, as emphasized by Legget [1], there are no circumstances in which such superposition states are the physically correct description of the system. Thus, it would be desirable to derive the GPE without invoking spontaneous symmetry breaking. Obviously, the description of a many-body system in terms of a macroscopic wave function enforces a drastic loss of information, unless the exact many-body wave function is in some sense exceptionally simple. But what precisely does that mean?

In this talk I will approach this question by discussing a strongly simplified, yet meaningful model system which allows one, on the one hand, to compute the exact N-particle dynamics numerically for reasonably large N, but still contains generic, nontrivial features on the other.    

[1] Anthony J. Leggett, Rev. Mod. Phys. 73, 307 (2001)

Invited speaker: Prof. Otfried Guehne
Affiliation: Universität Siegen
Title: Analyzing Multiparticle Quantum States: Problems And Some Solutions
Time and room: 17:15 lecture hall IAP
Abstract: Many experiments nowadays aim at the observation of quantum phenomena with several particles, such as trapped ions or polarized photons. In my talk I present results on three problems concerning the characterization of multiparticle quantum states. First, in many experiments one measures certain observables in order to determine the quantum state completely. The resulting state, however, has often unphysical properties (such as negative eigenvalues). This can be due to systematic errors, such as a misalignment of the measurement directions, or due to statistical fluctuations coming from the finite number of experiments. I will introduce a method to distinguish such statistical errors from systematic errors and apply the method to data obtained in a ion-trap experiment [1].
Second, if measurement data without systematic errors are given, the task remains to reconstruct a density matrix. I will show that the frequently used methods of maximum likelihood reconstruction and free least squares optimization lead to a systematic bias, underestimating the fidelity and overestimating the entanglement. This is shown to be a fundamental problem for any state reconstruction scheme that results always in valid density operators [2].
Finally, if in an experiment the quantum state has been reconstructed properly, the question remains how to characterize its correlations. I will introduce a method based on exponential families, which leads to a natural extension of the concept of multiparticle entanglement. This approach can, for instance, be used to verify the presence of complex interactions in experiments for quantum simulation.

[1] T. Moroder et al., Phys. Rev. Lett. 110, 180401 (2013).
[2] C. Schwemmer et al., arXiv:1310.8465.
[3] S. Niekamp et al., J. Phys. A: Math. Theor. 46, 125301 (2013).

Invited speaker: Dr. Leon Karpa
Affiliation: Universität Freiburg
Title: Hybrid And All-Optical Trapping Of Atoms And Ions
Time and room: 17:15 lecture hall IAP
Abstract: The ground-breaking demonstration of laser cooling of atoms to ultra-low temperatures and the observation of the Bose-Einstein Condensate (BEC) at the end of the last century have led to a tremendous breakthrough in the field of atomic physics with a vast number of applications across different disciplines. More recently, the exploration of collisions between laser-cooled neutral atoms and atomic ions has drawn increasing attention due to its potential significance in various fields ranging from quantum information processing, sympathetic cooling of atoms and molecules lacking closed optical transitions to the formation of mesoscopic molecules. First experiments utilizing a combination of conventional techniques to trap and manipulate atoms and ions have led to sympathetic cooling of ions to temperatures in the mK range and unveiled many interesting insights [1]. At the same time, the implementation of cooling to ultralow temperatures typically observed in BECs remains desirable e. g. due to the prospect of observing ultracold ion-atom collisions in the quantum mechanically dominated regime.
A novel approach towards this goal is based on optical trapping, a technique that is expected to avoid temperature limiting heating mechanisms inherent to Paul traps interacting with atoms that stem from driven motion induced by the rotating potentials [2]. While it is known to work very well for confining and manipulating neutral atoms, its implementation in ion traps is very challenging because even weak electric forces from residual fields usually exceed the dipole forces of an optical trap. Here, this concept and its perspectives are discussed in the context of recent experiments demonstrating all-optical ion trapping [3] and robust subwavelength localization of ions in a hybrid trap utilizing a one-dimensional optical standing wave [4].

[1] see e. g. : A. T. Grier, M. Cetina, F. Oručević, and V. Vuletić, Phys. Rev. Lett. 102, 223201 (2009); W. W. Smith, O. P. Makarov, and J. Lin, J. Mod. Opt. 52, 2253 (2005); C. Zipkes, S. Palzer, C. Sias, and M. Köhl, Nature 464, 388 (2010).
[2] M. Cetina, A. T. Grier, and V. Vuletić, Phys. Rev. Lett. 109, 253201 (2012).
[3] Ch. Schneider, M. Enderlein, T. Huber und T. Schaetz, Nat. Photonics 4, 772–775 (2010).
[4] L. Karpa, A. Bylinskii, D. Gangloff, M. Cetina und V. Vuletić, Phys. Rev. Lett. 111, 163002 (2013).
 

Invited speaker: Prof. Klaus Sengstock
Affiliation: Universität Hamburg
Title: Magnetism With And Without Magnetism: Different Regimes Of Magnetic
Interactions In Quantum Gases
Time and room: 17:15 lecture hall IAP
Abstract: The talk will address different regimes of magnetic interactions in
ultracold quantum gases. The spin dependent contact interaction of
ultracold atoms can lead to surprising collective behavior of e.g.
fermionic atoms in optical lattices and in bulk [1,2]. Recently
artifcial gauge fields even allowed to study magnetic like interactions
for completely non-magnetic atoms [3] and to simulate strong external
magnetic fields which eventually allow to realize high-B-field physics
like the Hofstadter butterfly.
[1] Krauser et al., Nature Physics 8, 813 (2012)
[2] Krauser et al., Science 343, 157 (2014)
[3] Struck et al., Science, 333, 996 (2011)

Invited speaker: Dr. Stephan Ritter
Affiliation: MPI für Quantenoptik, Garching
Title: Quantum Networks made of Single Atoms and Photons
Time and room: 17:15 lecture hall IAP
Abstract: Quantum networks form the basis of distributed quantum computing architectures and quantum communication. Single atoms in optical cavities are ideally suited as universal network nodes capable of sending, storing, retrieving and even processing quantum information. We demonstrate this by presenting an elementary version of a quantum
network based on two identical nodes in remote, independent laboratories. The reversible exchange of quantum information and the creation of remote entanglement are achieved by exchange of a single photon. A heralded alternative to the direct state transfer is provided
by teleportation, which we implement using a time-resolved photonic Bell-state measurement. Quantum control over all degrees of freedom of the single atoms also allows for the nondestructive detection of flying photons. Upon reflection from the cavity, a single photon flips the phase of the atomic state, which unambiguously allows us to trace the photon. Our cavity-based approach to quantum networking paves the way towards hybrid quantum gates and offers a clear perspective for scalability.