Kvantinformation & Kvantoptik
Quantum Information & Quantum Optics

Erik Sjöqvist, Uppsala University (10th November, 2015)

ENTANGLEMENT-INDUCED TOPOLOGICAL PHASE FACTORS FOR QUBITS

Abstract: We consider local cyclic SU(2) evolution of multiqubit systems. We show that the resulting phase factors may in some cases have a topological character. This happens for certain entangled states of the qubits. The allowed values for these topological phases seem to be related to nonzero SLOCC-invariant polynomials of the qubits. We describe how the topological phases have been observed experimentally for qubit pairs and outline an experiment to demonstrate the three-qubit topological phases involving entangled photons.


Markus Hennrich, Stockholm University (13th october, 2015)

RYDBERG EXCITATION OF TRAPPED STRONTIUM IONS

Abstract: A long-standing aim of quantum information research Cold trapped ions are well isolated from the environment and can be coherently manipulated by laser light. This makes them a useful system for building a simple quantum computer. In this presentation, I will first introduce trapped ions as a quantum system and discuss their application for quantum information processing. Then, I will present our current focus of research, trapped Rydberg ions, a novel approach for quantum information processing [1,2]. This idea joins the advanced quantum control of trapped ions with the strong dipolar interaction between Rydberg atoms. For trapped ions this method promises to speed up entangling interactions [3] and to enable such operations in larger ion crystals [4]. Experimentally we have just recently realized trapped strontium Rydberg ions. The transitions we observed for Rydberg excitation are very narrow and the excitation can be performed repeatedly which indicates that the Rydberg ions are stable in the ion trap. Similar results have been recently reported on a single photon Rydberg excitation of trapped calcium ions [5]. The tunability of our lasers should enable us to excite our strontium ions to even higher Rydberg levels. Such highly excited levels are required to achieve a strong interaction between neighboring Rydberg ions in the trap as will be required for quantum gates using the Rydberg interaction.
References:
M. Müller, L. Liang, I. Lesanovsky, P. Zoller, New J. Phys. 10, 093009 (2008).
F. Schmidt-Kaler, et al., New J. Phys. 13, 075014 (2011).
W. Li, I. Lesanovsky, Appl. Phys. B 114, 37-44 (2014).
W. Li, A.W. Glaetzle, R. Nath, I. Lesanovsky, Phys. Rev. A 87, 052304 (2013).
T. Feldker, et al., arXiv:1506.05958.


Ingemar Bengtsson, Stockholm University (29th September, 2015)

GEOMETRIC PHASES FOR MIXED STATES OF THE KITAEV CHAIN

Abstract: I will give a vest-pocket introduction to the subject of topological quantum matter, why geometric phases can sometimes serve as order parameters for phase transitions, and why my collaborators and I concluded that Uhlmann's geometric phase does not serve this particular role.


Niklas Johansson, Linköping University (15th September, 2015)

Efficient classical algorithms of both the Deutsch-Jozsa and Simon's problem

Abstract: A long-standing aim of quantum information research is to understand what gives quantum computers their advantage. This requires separating problems that need genuinely quantum resources from those for which classical resources are enough. Two examples of problems which are believed to show quantum speed-up are the Deutsch-Jozsa and Simon's problem, both efficiently solvable on a quantum Turing machine, and both believed to lack efficient classical solutions. During the seminar I intend to show that there are classical simulations of these algorithms that performs equally efficiently as the quantum algorithms. This shows that the Deutsch-Jozsa and Simon's algorithm do not require any genuinely quantum resources, and that the quantum algorithms show no speed-up when compared with the corresponding classical simulation.


Gunnar Björk, KTH(1st September, 2015)

MAXIMALLY UNCLASSICAL POLARIZED LIGHT --- THE STARS OF THE QUANTUM UNIVERSE

Abstract: In classical optics, the degree of polarization is characterized by the length of the normalized Stokes vector. A state with a vanishing Stokes vector is assumed to be unpolarized. That is, it is assumed to have a completely random polarization. In a quantum description a different picture emerges, where there are pure and polarized states that yet have a vanishing Stokes vector. However, one can go further than that, because just as a quantum state is defined by the Glauber coherence functions (i.e., the expectation values of successive moments of the annihilation and creation operators) of all order (and knowing those, the state's density matrix can be calculated), a state's polarization is defined by all moments of the Stokes operators. However, it is possible to find pure quantum states that have vanishing Stokes operator moments to arbitrarily high order. This is in contrast to the classical states that have unit first order moments and the maximal aggregated moments to any order. The pure states that have vanishing lower order moments we call the "stars of the quantum universe" for reasons that will be clear during the talk. These states can be said to be the maximally, non-classical polarization states. We shall discuss them during the talk and discuss what application they may have.


Marcus Appleby, University of Sydney (18th August)

ENTANGLEMENT AND DESIGNS

Yoon-Ho Kim, POSTECH, Korea (24th February, 2015)

TIME-BIN ENTANGLED PHOTON PAIRS FROM SPONTANEOUS PARAMETRIC DOWN-CONVERSION PUMPED BY A CW MULTIMODE DIODE LASER

Abstract: The Franson interferometer consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and the idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit. It is often used to prepare time-bin entanglement of two photons in the two SPDC pumping regimes: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally.


Pawel Horodecki, University of Gdansk (27th January, 2015)

FROM QUANTUM CORRELATIONS TO EMERGENCE OF OBJECTIVITY FROM QUANTA

Mohamed Bourennane, SU (16th September, 2014)

MULTIPHOTON ENTANGLEMENT

Abstract: Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. I show that a class of entangled 6-photon states can be directly created by parametric down-conversion. These states exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. Therefore these states are well suited for new types of quantum communication. Bound entanglement is one of the most puzzling forms of entanglement. Being a peculiar form of entanglement, bound entanglement emerges in certain mixed quantum states. This form of entanglement is not distillable by local operators and classical communication. Bound entangled states are different from both the free entangled (distillable) and separable states. I will speak on the experimental evidence of the existence of bound entangled state, the so-called Smolin state. I will report on recent results on measurement-device independent entanglement detection. In the end of my talk, I will give a brief account on going experiments on multipartite entanglement.

Adan Cabello, Sevilla (26th August, 2014)

THE GRAPH APPROACH TO QUANTUM CORRELATIONS

Dardo Goyeneche, Concepcion (26th August, 2014)

GENUINELY MULTIPARTITE ENTANGLED STATES AND ORTHOGONAL ARRAYS
Reference: D. Goyeneche, K. Zyczkowski, arXiv:1404.3586v2

Jonas Larson, SU (20th May, 2014)

THERMALIZATION IN CLOSED QUANTUM SYSTEMS

Abstract: Predicting the fate of a given quantum state evolving under some Hamiltonian is computationally very hard, especially when the degrees-of-freedom become large. However, often are we only interested in local measurable properties of the state. Such a local measurement can only access information about the corresponding reduced density operator and not the full system state, and, quiet generally, this reduced density operator turns out to be in a thermal state. If this is true for any reduced state the system is said to have thermalized. With ever refined experimental techniques, this field of research has become very active within the last years. In this talk, I will give an introduction to the topic, discussing properties like quantum chaos, integrability, and localization.

Ana Predojevic, Innsbruck (29th April, 2014)

SINGLE QUANTUM DOTS AS PHOTON PAIR EMITTERS

Andreas Rydh, SU (22nd April, 2014)

POSSIBILITES WITH DIFFERENTIAL MEMBRANE-BASED NANOCALORIMETRY

Saroosh Shabbir, KTH (8th April, 2014)

THERMODYNAMIC IMPLICATIONS OF INFORMATION ERASURE

Ingemar Bengtsson, SU (4th March, 2014)

WHY ARE SICS INTERESTING?

Ole Andersson, SU (18th February, 2014)

ON TIME BOUNDS FOR QUANTUM STATE TRANSFORMATION

Toshiki Kobayashi, Osaka (4th February, 2014)

WAVELENGTH CONVERSION FOR QUANTUM REPEATER AND TRANSFORMATION OF MULTIPARTITE ENTANGLED DICKE STATES BY MINIMAL ACCESS

Fredrik Laurell, KTH (21st January, 2014)

TAILORED FERRO-ELECTRICS FOR NONLINEAR OPTICAL APPLICATIONS