Theory Seminar

Christine Muschik

The topic of today's Wednesday seminar will be, how the potential outcome of a quantum computation can be inferred, even if the comupter is not run, and how quantum interrogation techniques can be improved using the zeno effect --> Nature 439, 949 (2006).

Dominic Kohler

In the talk, the concept of stochastic master equations to describe continuous measurements of quantum systems is introduced briefly [1]. Then the idea of (linearly) feeding back the measurement results is added in this formalism, and an interpretation of this feedback as conditioning the system's state on the measurement results is given with the aid of an example about squeezing [2]. To apply the theory, an approach to achieve spin squeezing of an atomic ensemble with feedback is presented [3,4].

[1] H. M. Wiseman, G. J. Milburn, Quantum Measurement and Control, Cambridge University Press, Cambridge, 2010 [2] H. M. Wiseman, G. J. Milburn, Squeezing via Feedback, PRA, 49, 1350 (1994) [3] L. K. Thomsen, S. Mancini, H. M. Wiseman, Spin Squeezing via Quantum Feedback, PRA, 65, 061801(R) (2002) [4] JM Geremia, J. K. Stockton, H. Mabuchi, Real-Time Quantum Feedback Control of Atomic Spin-Squeezing, Science, 304, 270-273 (2004)

Robert Raussendorf

We demonstrate that the two-dimensional AKLT state on a honeycomb lattice is a universal resource for measurement-based quantum computation. Our argument proceeds by reduction of the AKLT state to a 2D cluster state, which is already known to be universal, and consists of two steps. First, we devise a local POVM by which the AKLT state is mapped to a random planar graph state. Second, we show numerically that the connectivity properties of these random graphs are governed by percolation, and that typical graphs are in the connected phase. The corresponding graph states can then be transformed to 2D cluster states by standard techniques. Joint work with Tzu-Chieh Wei and Ian Affleck

Anika Pflanzer

There has been an immense effort during the past decades for the direct detection of gravitational waves, motivated by the possibility to test general relativity and to open up a new window in the observation of space.

I will give a very basic introduction and sketch how gravitational waves can be derived as a solution of Einstein's field equation in the weak-field limit, also describing their effect when passing an object. Starting from this, I will present methods for gravitational wave detection, such as LIGO, the most advanced interferometer at present.

Michael Lubasch

In February 2008 Hideo Hosono and co-workers reported their surprising discovery of high-temperature superconductivity in a new class of iron-based materials. A recent review (J. Paglione and R. L. Greene, Nature Physics 6, 645 (September 2010)) summarizes the progress made in the past two years. I will give an introduction into the topic.

'Empty Slot'

Dmitry Yudin

In this talk we’ll investigate some Bethe ansatz integrable models. We’ll analyze the Heisenberg spin ½ chain starting from basic principles. Another model we’ll consider is the Gaudin magnet (central spin model) which describes an electronic spin in a quantum dot coupled to the surrounding nuclear spins. As a conclusion, we’ll review algebraic Bethe ansatz.

Fernando Pastawski

The adiabatic theorem is one of the oldest and most widely used tools in quantum mechanics. The theorem states that sufficiently slow variations of the Hamiltonian preserve the relative probability of instantaneous eigenstates. The realization that the adiabatic approximation could be used as the basis for quantum computation has generated a resurgence of interest as well as controversy in the last decade. I will present some of the rigorous proofs and estimates and go onto to discuss some of the uses of the adiabatic theorem for quantum information processing such as adiabatic quantum computation and adiabatic gate teleportation.

Birger Horstmann

Graphene is a form of carbon. As a material it is completely new – not only the thinnest ever but also the strongest. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it. Carbon, the basis of all known life on earth, has surprised us once again.

Geim and Novoselov extracted the graphene from a piece of graphite such as is found in ordinary pencils. Using regular adhesive tape they managed to obtain a flake of carbon with a thickness of just one atom. This at a time when many believed it was impossible for such thin crystalline materials to be stable.

Leonardo Mazza

Although the phenomenon of Bose–Einstein condensation (BEC) is a purely statistical effect that also appears in an ideal gas, the physics of BEC is considerably enriched by the presence of interactions among the atoms. In usual experiments with BECs, the only relevant interaction is the isotropic and short-range contact interaction, which is described by a single parameter, the scattering length a. In contrast, the dipole–dipole interaction between particles possessing an electric or magnetic dipole moment is of long-range character and anisotropic, which gives rise to new phenomena. In will talk I will give a general and non-technical survey on the theoretical and experimental achievements in this subject, from the stability issues of dipolar gases to the quest for a BEC of polar molecules.

Fabio Mezzacapo

I will discuss the relation between structural and superfluid properties of small parahydrogen clusters comprising N ~ 30 molecules. After a review of results obtained via Path integral Monte Carlo simulation at finite temperature I will show some new findings obtained at a temperature (T) as low as 0.0625 K for clusters of 25, 26 and 27 molecules. These clusters are superfluid in the low T limit but display markedly different physical behavior. For N=25 and 27, superfluidity at low temperature arises as clusters melt, i.e., become progressively liquid-like as a result of quantum effects. On the other hand, for N = 26 the cluster remains rigid and solid-like. The N=26 cluster can therefore be regarded as a ``supersolid". This physical picture is supported by results of simulations in which a single parahydrogen molecule in the cluster is substituted by a dopant one.

Miguel Aguado

I will talk about quantum chromodynamics, and pose the problem of quark confinement following Greensite's beautiful review [1] (what was state-of-the-art in 2003, I'm afraid, is not so much different from what is state-of-the-art right now).

[1] J. Greensite, The confinement problem in lattice gauge theory, Prog. Part. Nucl. Phys. 51, 1 (2003), arXiv:hep-lat/0301023.

Eric Kessler

Motivated by a recent publication on nature (doi:10.1038/nphys1821) demonstrating information to energy conversion experimentally, I will review the history of Maxwell's Demon from the early thought experiments about the second law of thermodynamics to its connections to modern information theory.

Heike Schwager

Two Nobel prizes for the discovery of new carbon allotropes in the last 15 years: the strongest, stiffest and thinnest materials yet discovered have attracted a lot of attention. I will review the history of the discovery of buckyballs, carbon nanotubes and graphene, discuss their physical properties and talk about current and future applications. Still not interested? Ok, let's try this: How is all this connected to tennis rackets and Audi A4 cars? How is the NASA space telescope "Spitzer" involved? How fat can a cat be lying in a hypothetical graphene hammock so that it doesn't break? And last but not least: How could these materials be used for Quantum computation?

Oliver Buerschaper

This question has tantalized phycisists for almost three decades, yet experimentalists recently claimed to have seen the first signatures of non-Abelian statitistics in a system governed by the fractional quantum Hall effect. Can victory be declared?

Lucas Lamata

I will give a non-exhaustive introduction to the topic of two-qubit gates with trapped ions. I will first give an overview of the basic trapped-ion physics. Then, I will discuss some relevant examples of two-ion gates.

Bibliography:

D. Leibfried, R. Blatt, C. Monroe, D. Wineland, RMP 75, 281 (2003). J.I. Cirac, P. Zoller, PRL 74, 4091 (1995). A. Sorensen, K. Molmer, PRL 82, 1971 (1999). J.J. Garcia-Ripoll, P. Zoller, J.I. Cirac, PRL 91, 157901 (2003).

Matteo Rizzi

Can physicists complement the super-reductionist view of protein reactions in cells of biologists and chemists , and help them to settle down what is relevant? I.e., can we revert the paradigm 'protein --> function to function --> cell biochem.' ? A tentative (very partial) answer comes with the help of some intriguing examples about swimming mechanisms which I review.

Humberto Michinel

By spatially modulating the strength of atomic interactions in a trapped BEC, it is possible to extract in a controllable way from an ultracold reservoir, bursts of matter waves that form robust atomic soliton clouds. This effect and its potential applications will be discussed in the present seminar.

Geza Giedke

I will give an introductory review on bosonic gaussian states.

Rainer Kaltenbaek

While quantum concepts like superposition and entanglement are frequently being confirmed in the lab, these concepts seem to be in blatant contradiction to our everyday experience. There, objects are always in distinct states that can be verified without disturbing the system under investigation. Is there an essential difference between macroscopic and microscopic objects, and if there is, what brings about the transition between these two distinct realms, and when does it occur? This question lies at the heart of Schrödinger’s famous gedankenexperiment, where a cat is brought into a superposition of being dead and alive. According to quantum theory, this is possible in principle as long as the system in question is isolated completely from its environment such that nobody could possibly know whether the system is in one state or the other except by performing a measurement on the system itself. Several theoretical models have been put forward that propose to modify the laws of quantum theory to introduce a collapse of the wavefunction for complex and/or massive objects or objects that are distributed over large distances. We call these models macrorealistic. Here, we propose a space-based experiment that aims at testing the predictions of quantum theory against those of macrorealistic models. Using this setup, a dielectric nanosphere is optically trapped and then cooled to the ground state. Then the particle is released, and its wavefunction will expand. After a certain time, a UV pulse is sent through the center of the wavefunction. If the UV light is not scattered, the wavefunction will be divided in two regions that will interfere. In order to parametrize the decoherence predicted by quantum theory or by other theoretical models, we introduce the Coherent-Expansion Distance; that is the distance between the two parts of the wavefunction that we can achieve and still see good interference afterwards. We will describe the proposed experimental layout, the technical prerequisites, and we will describe the conditions under which the proposed experiment should be able to distinguish between quantum theory and several distinct macrorealistic models like the K-Model, the Penrose Model, and the GRW Model.
See the longer version of the abstract

Hong-Hao Tu

The seminar is intended to give a brief introduction to Abelian bosonization, which is a popular technique for treating interacting fermions in one spatial dimension. I will review how bosonic description of 1D fermion problems emerges and derive the standard bosonization identities. By using these results, I will show how to obtain the exact solution of the Luttinger model, which is a relativistic interacting fermion problem.

Prof. Garnet Chan (Cornell University)

Mari Carmen Banuls

The theory of optimal control plays an important role in many engineering applications. These concepts can be also applied to quantum dynamics, giving raise to the "quantum optimal control theory". In this seminar I will review the basic ingredients of quantum optimal control, which are the established results, and some examples.

Empty Slot

Empty Slot

Oriol Romero-Isart

This talk is based on the review paper by A. J. Leggett (Rep. Prog. Phys. 71 022001 (2008)), from which I have taken the title. We will discuss the concept of realism and how it can be falsified by experiments. This includes a review on the fundamentals of EPR-Bell experiments as well as the less known temporal Bell inequalities (also called Leggett-Garg inequalities).

Empty Slot

Christine Muschik

A universal quantum simulator is a controlled quantum device that reproduces the dynamics of any other many-particle quantum system with short-range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open-system evolution. Here we propose that laser-excited Rydberg atoms in large-spacing optical or magnetic lattices provide an efficient implementation of a universal quantum simulator for spin models involving n-body interactions, including such of higher order. This would allow the simulation of Hamiltonians of exotic spin models involving n-particle constraints, such as the Kitaev toric code, colour code and lattice gauge theories with spin-liquid phases. In addition, our approach provides the ingredients for dissipative preparation of entangled states based on engineering n-particle reservoir couplings. The basic building blocks of our architecture are efficient and high-fidelity n-qubit entangling gates using auxiliary Rydberg atoms, including a possible dissipative time step through optical pumping. This enables mimicking the time evolution of the system by a sequence of fast, parallel and high-fidelity n-particle coherent and dissipative Rydberg gates. Reference: Hendrik Weimer, Markus Müller, Igor Lesanovsky, Peter Zoller and Hans Peter Büchler, Nature Physics 6, 382 - 388 (2010).

Kalle Vollbrecht

I will give a boring talk about an old purely mathematical concept in a completely non understandable way. ( arXiv:1105.2768, arXiv:1103.4032 )

Kai P. Schmidt (TU Dortmund)

Anne Nielsen

Quantum stochastic calculus provides a valuable mathematical framework to derive equations for the time evolution of a large number of open quantum systems, including systems that are probed continuously in time [1]. Leaving aside all technical issues, I will introduce the Hudson-Parthasarathy QSDE [2] through a simple example. I will briefly explain how to deal with quantum networks [3] and how to derive simplified models through adiabatic elimination [4]. An interesting application is a recently proposed network for continuous quantum error correction [5], which I will also discuss.

[1] L. Bouten, R. van Handel, and M. James, SIAM J. Control Optim. 46, 2199-2241 (2007). [2] R. L. Hudson and K. R. Parthasarathy, Commun. Math. Phys. 93, 301–323 (1984). [3] J. Gough and M. R. James, Commun. Math. Phys. 287, 1109-1132 (2009). [4] L. Bouten and A. Silberfarb, Commun. Math. Phys. 283, 491-505 (2008); L. Bouten, R. van Handel and A. Silberfarb, Journal of Functional Analysis 254, 3123-3147, (2008). [5] J. Kerckhoff, H. I. Nurdin, D. S. Pavlichin, and H. Mabuchi, Phys. Rev. Lett. 105, 040502 (2010).

Juan Bermejo

Once upon a time, and a very good time it was, people observed that "hard" problems in computer science can be studied as many-body spin systems. Since then, people have applied statistical mechanical tools to get a better understanding of computational hardness. Results show that the solutions of NP-complete problems live in ground states of Hamiltonians with very exotic energy-landscapes. I will review this topic and the main methods they use. I will focus on an classical problem known as "boolean satisfiability" or SAT.

1) C.R Laumann, R. Moessner. Statistical mechanics of classical and quantum computational complexity. Les Houches School on "Modern theories of correlated electron systems". Les Houches School on "Modern theories of correlated electron systems" 2009. http://arxiv.org/abs/1009.1635v1 2) Mézard, Montanari. Information, Physics and Computation, Oxford Graduate Texts, 2009. [Online] 3) Bogdanov and Trevisan. Average-Case Complexity [Do not use at home.]

Xiaotong Ni

From the beginning of quantum physics, entanglement has always been the focus, and now entanglement almost become the synonyms of quantumness. Today I'll going to talk about two papers which discuss the role of entanglement in the quantum computation. One of them has the counter-intuitive result that too much entanglement may be also not suitable for quantum computation.

1.On the role of entanglement in quantum computational speed-up Richard Jozsa, Noah Linden arXiv:quant-ph/0201143 2.Most state are too entangled to be useful as computational resources D. Gross, et al arXiv:0810.4331

Zhao Liu

When a two-dimensional electron system is exposed to a strong magnetic field, it forms a new quantum liquid called fractional quantum Hall liquid, which looks incredibly complex theoretically at first. However, it comes as a surprise that the understanding of it has been achieved for most of the filling factors observed in experiments. What is perhaps even more surprising is that much of our understanding follows from a single principle, namely composite fermion theory. Today, I will talk about the basic concept of composite fermion theory. I think everyone of you can see the simplicity of this theory.

Elisabeth Rieper

One of the central questions in the emerging field of quantum biology is under which conditions quantum mechanical properties are crucial to fully describe a biological system. In this talk I will highlight the measurement problem in biology. In living cells there is a flow of information, induced by molecules 'measuring' the identity of other molecules. A prominent example is DNA, where the genetic information is measured and copied along the chain of DNA, mRNA, tRNA and finally amino acids. Which physical mechanism is used in cells for molecules to recognize each other? More precisely, which physical degree of freedom participates in the measurement process? I will apply the concept of cq-states to molecules, dividing them into a classical part, which (for all practical purposes) cannot exist in superposition, and a quantum part. It is not only very difficult to determine exactly which degree of freedom of a molecule dominates the measurement process, but there is not even a simple criteria to decide whether it is quantum or classical. The fact that biological information processing extensively relies on chemical reactions, supplies good reason to believe that the quantum part plays a crucial role. In this talk I will discuss the concepts and consequences of information processing using quantum channels in biology systems. Examples will include the avian compass, the olfactory sense and replication of DNA

Thorsten Wahl

Matrix Product States (MPS) have turned out to describe a variety of systems efficiently, including gapped spin systems at zero temperature and systems at finite temperature (Gibbs states). My talk will first shed light on the generalization of MPS to the continuous case, where the particles are described in second quantization and take continuous positions. Thereafter, I will present examples of applications of continuous MPS. Those elucidate how the continuous MPS formalism can be used to determine efficiently the ground states of both non-relativistic and relativistic quantum fields.

Empty Slot

Empty Slot

Fabio Mezzacapo

I will review some recent studies based on different techniques on the ground-state phase diagram of the spin 1/2 J1-J2 model on the honeycomb lattice.

Alejandro Manjacavas

In this talk I will try to give a brief introduction to plasmon optics together with an overview of some of the work performed in the last years in the group of Prof. García de Abajo at CSIC.

Christine Muschik

Gemma de las Cuevas

I will talk about the paper written together with W. Dür, M. Van den Nest and M. A. Martin-Delgado: arxiv:1104.2517. To appear in NJP

Empty Slot

Geza Giedke

Hong-Hao Tu

This Wednesday seminar is intended to discuss the Majorana fermions in 1D quantum systems, a currently hot topic in condensed matter. Two representative candidates supporting Majorana edge states will be introduced, including Kitaev's Majorana chain (1D p-wave superconductor) and semiconducting wire with spin-orbit coupling in proximity to s-wave superconductors. I will also address the stability of Majorana fermions in the presence of interactions and the possibility of generating Majorana fermions by interactions.

Empty Slot

Empty Slot

Anne Nielsen

I will give an introduction to the quantum Hall effect and discuss some properties of the Laughlin wave function.

Empty Slot

Empty Slot

Fernando Pastawski

I will present the paper quant-ph > arXiv:1111.3328 by Matthew F. Pusey, Jonathan Barrett and Terry Rudolph.

Manuel Endres

Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body systems possesses a hidden non-local order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particle-hole pairs that lead to the emergence of a non-local string order in one dimension. Using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case.

Alexander Müller-Hermes

The talk will be about different approaches to do numerics in high-dimensional spaces and recent developments made by numerical mathematicians. These developments lead to MPS from a different point of view and may offer new insights and new numerical techniques using MPS.

Anika Pflanzer

Martin Schuetz

Raul Garcia-Patron Sanchez

Roman Orus

Lucas Clemente

Heike Schwager

Matteo Rizzi

Eric Kessler

Maarten van den Nest

Michael Lubasch

Mari Carmen Banuls

Gemma De las Cuevas

Oriol Romero-Isart