Theory Seminar

Guest speaker: David Reeb (TU Munich, Germany)

Landauer's Principle fundamentally connects information theory and thermodynamics as it enforces a certain amount of heat dissipation during information erasure. We review this connection and prove an improved version of Landauer's Principle, formulated in information-theoretic terms. We use this formulation to explicitly sharpen the usual Landauer bound in cases where the assisting reservoir is of finite size, a situation that may be relevant for small computing devices. (Joint work with Michael Wolf.)

MPQ Theory group

Everybody gives a three-minutes presentation on some of his/her recent work.

Guest speaker: Horacio M. Pastawski (Universidad Nacional de Córdoba, Argentina)

The Loschmidt Echo [LE] is the amount of signal recovered from a spreading excitation after an imperfect time reversal procedure [1]. In NMR it has allowed us to quantify the decoherence induced by uncontrolled environment [2]. Notably complex many-body dynamics makes the system particularly sensitive to environmental disturbances beyond a small threshold [3]. In this talk, I will summarize, at a tutorial level, the experimental and theoretical results of our group that fueled the field of dynamical quantum chaos [4]. This also will lead us to discuss situations where swiping the environment strength induces non-analytic changes in the dynamical behavior, i.e. quantum dynamical phase transition [5]. More recently we have shown that, by filtering trivial parts in the dynamics [6], the numerical evaluation of the LE can be a useful tool to asses thermalization in interacting many-body systems.

[1] Environment-independent decoherence rate in classically chaotic systems. R. Jalabert and HMP, Phys. Rev. Lett. 86, 2490 (2001)
[2] Attenuation of polarization echoes in NMR: A test for the emergence of Dynamical Irreversibility in Many-Body Quantum Systems. P.R. Levstein, G. Usaj, HMP, J. Chem. Phys. 108,, 2718 (1998)
[3] A Nuclear Magnetic Resonance answer to the Boltzmann-Loschmidt controversy? HMP, G. Usaj , P. R. Levstein, J. Raya and J. Hirschinger. Physica A 283 166 (2000)
[4] Loschmidt Echo A. Gousev, R.A. Jalabert, HMP and D.A. Wisniacki. Scholarpedia 7, 11687 (2012) http://www.scholarpedia.org/article/Loschmidt_echo
[5] Environmentally induced quantum dynamical phase transition in the spin swap operation, G.A.Álvarez, E.P.Danieli, P.R. Levstein, and HMP, J. Chem.Phys. 124, 1 (2006)
[6] Loschmidt echo as a robust decoherence quantifier for many-body systems P.R. Zangara, A.D. Dente, P.R. Levstein, and HMP, Phys. Rev. A 86, 012322 (2012)

Thorsten Wahl

Since the prediction of the Quantum Spin Hall Effect in 2005, topological insulators became one of the most important subjects in modern Condensed Matter Physics. Commonly, they are described as phases resembling a doughnut, which cannot be transformed by a continuous map into the topologically trivial phase corresponding to an apple. In this talk, I want to go beyond the description by doughnuts and apples and introduce some of the basic mathematical concepts characterizing their topological structure. Most importantly, I will introduce the notion of homotopy groups and elucidate their relevance for the classification of free Fermionic systems.

Eliška Greplová

The upcoming Wednesday Seminar will be about teleportation, from the basic principles to recent generalizations. I will give an introductory comparison of the teleportation and dense coding, show how to use teleportation for performing unitaries and compare the original Bennet et al. teleportation protocol with some recently proposed teleportation schemes.

Maita Schade

Motivated by a somewhat recent paper by Bravyi & König (arXiv:1206.1609), I'll be giving an introduction to topological stabilizer codes in QIP. In particular, the presentation will focus on their topological nature (where is the donut?) and their inherent limitation when it comes to implementing quantum algorithms. At the end, I hope we will all understand why B&K's result is relevant, but not too surprising!

Michael Lubasch

The contraction of PEPS tensor networks constitutes a major contribution to the computational cost of a PEPS algorithm. Recently, an alternative scheme has been proposed [1] that reduces this cost significantly. I demonstrate the limitations of this contraction scheme, and explain them via the PEPS's boundary approximation. I propose a generalization where the boundary is approximated by a purification and give numerical evidence that this solves the problems of the previous contraction scheme.

[1] I. Pizorn, L. Wang, and F. Verstraete, PRA 83, 052321 (2011).

Guest speaker: Courtney Brell (University of Sydney, Australia)

Topological quantum computation offers a natural setting for fault-tolerant implementation of quantum information processing and storage. Typically the computational power of a topologically ordered system is measured by the braiding statistics of the anyonic excitations it allows. However, we show that by making use of topological defects (specifically twist defects related to symmetries of the model), a system whose anyons are classically simulatable can be used for universal topological quantum computation.

Guest speaker: Leonardo Mazza (Scuola Normale Superiore, Pisa, Italy)

We analyze the robustness of a quantum memory based on Majorana modes in a Kitaev chain. We identify the optimal recovery operation acting on the memory in the presence of perturbations and evaluate its fidelity in different scenarios. We show that for time-dependent Hamiltonian perturbations that preserve the topological features, the memory is robust even if the perturbation contains frequencies that lie well above the gap. We identify the condition that is responsible for this feature. At the same time we find that the memory is unstable with respect to particle losses.

References: arXiv:1212.4778

Guest speaker: Alexander Holevo (Steklov Mathematical Institute, Moscow, Russia)

The protocol of entanglement-assisted classical communication was introduced by Bennett, Shor, Smolin and Thapliyal as a generalization of superdense coding to noisy quantum channels. An interesting observation was: entanglement-assisted communication may be advantageous even for entanglement-breaking channels. We provide further results in this direction by considering two distinguished classes of entanglement-breaking channels, namely measurements and preparations. For both, we compare the (unassisted) classical capacity C and the entanglement-assisted classical capacity C_{ea}. Our conclusions:

1. Measurement channels give the most spectacular examples of the gain of entanglement-assistance: C_{ea}/C>>1.
2. On the contrary, preparation channels are essentially characterized by no gain of entanglement-assistance: C_{ea}/C=1.
3. The input constraints (e.g. energy constraint in continuous variable system) restores the gain for the preparation channels.

Martin Schütz

Majorana fermions are particles identical to their own antiparticles. One of the most promising proposals to engineer them employs a spin-orbit coupled nanowire subjected to a magnetic field and proximate to a s-wave superconductor. Based on this setup, the first experimental signatures of Majorana fermions have recently been reported. I will try to give a pedagogical introduction to this setup, show how to detect these mysterious particles and report on the experimental findings. Finally, it is shown why the race for the unambiguous detection of these elusive particles is still on.

Guest speaker: Jeff Shapiro (MIT, Boston, USA)

Entanglement is essential to many quantum information applications, but it is easily destroyed by quantum decoherence arising from interaction with the environment. We describe an entanglement-based protocol that is resilient to loss and noise which destroy entanglement, and then report its first experimental demonstration. Despite channel noise in our experiment that is more than 8.35 dB stronger than the threshold for entanglement breaking, eavesdropping-immune communication is achieved between the legitimate parties (Alice and Bob) when they employ an entangled source, but no such immunity is obtainable when their source is classical.

Alejandro Gonzalez-Tudela

I will present our recently developed theory of frequency-filtered and time-resolved N-photon correlations or N-photon spectra [1]. They are an extension of the standard N-th order temporal correlation functions, that measure the probability of emission of N photons at times T_1, ...T_N into the frequency domain. Heisenberg uncertainty principle requires the introduction of the detector/filter frequency resolution in the theory.

Obtaining such cross correlations has been a common practice experimentally for years now, proving useful in many contexts such as lasers [2], cavity-QED [3] or resonance fluorescence [4,5]. However, their computation has remained a theoretical challenge due to their cumbersome integral expressions, only manageable for N = 2 and trivial Systems [2,6,7]. We show that they correspond exactly to the intensity-intensity correlations between two-level "sensors" (with the desired frequencies) in the limit of their vanishing coupling to the system. The sensor's decay rate Gamma, plays the role of the frequency resolution in a natural self-consistent way.

With this formalism, we are able to study the two-photon spectra of a variety of systems of increasing complexity: single mode emitters and the various coupling combinations. We consider both the linear and nonlinear regimes under incoherent and coherent excitation [8]. We find that even the simplest systems, such as laser light scattered by a qubit (resonance fluorescence) display a rich dynamics of emission, not accessible by the Standard single photon spectroscopy. In more sophisticated systems, such as the strong light-matter coupling achieved in cavity-QED experiments, two-photon Emission processes involving virtual states are revealed, that can be exploited for two-photon state generation [9].

[1] del Valle, Gonzalez-Tudela et al PRL 109, 183601 (2012)
[2] Centeno Neelen et al., Opt. Com. 100, 289 (1993)
[3] Hennessy et al, Nature 445, 896 (2007)
[4] Aspect et al, PRL 45, 617 (1980)
[5] Ulhaq et al, Nature Photonics 6, 238 (2012)
[6] Joosten & Nienhuis, J. Opt. B 2, 158 (2000)
[7] Bel & Brown, PRL 102, 018303 (2009)
[8] Gonzalez-Tudela et al, NJP 15, 033036 (2013)

Guest speaker: Stephanie Wehner (Centre for Quantum Technologies, Singapore)

A natural measure for the amount of quantum information that a physical system E holds about another system A = A_1,...,A_n is given by the min-entropy Hmin(A|E). Specifically, the min-entropy measures the amount of entanglement between E and A, and is the relevant measure when analyzing a wide variety of problems ranging from randomness extraction in quantum cryptography, decoupling used in channel coding to physical processes such as thermalization or the thermodynamic work cost (or gain) of erasing a quantum system. As such, it is a central question how the min-entropy changes after some process M transforms A to M(A). Here we introduce a powerful tool relating the resulting min-entropy to the original one that has numerous applications.

A prime example of such a process is the one of entanglement sampling, where we select some subset S of the systems A_1,...,A_n, and ask about the entanglement that E has with the selected systems A_S, i.e., Hmin(A_S|ES). This has two applications by itself in that it provides us with the first local quantum-to-classical randomness extractors for use in quantum cryptography, as well as decoupling operations acting on only a small fraction A_S of the input A.

Guest Speaker: Sae Woo Nam (NIST, Boulder, USA)

There is increasing interest in using superconducting optical photon detectors in a variety of applications. These applications require detectors that have extremely low dark count rates, high count rates, and high quantum efficiency. I will describe our work on two types of superconducting detectors, the Superconducting Nanowire Single Photon Detector (SNSPD or nSSPD) and superconducting Transition-Edge Sensor (TES). An SNSPD is an ultra-thin, ultra-narrow (nm scale) superconducting meander that is current biased just below its critical current density. When one or more photon is absorbed, a hot spot is formed that causes the superconductor to develop a resistance and consequently a voltage pulse. At NIST and JPL, we have been developing nanowire detectors using an amorphous alloy of tungsten-silicide. For applications requiring photon number resolution, we have been using superconducting transistion-edge sensors (TES). By exploiting the sharp superconducting-to-normal resistive transition of tungsten at 100mK, TES detectors give an output signal that is proportional to the cumulative energy in an absorption event. This proportional pulse-height enables the determination of the energy absorbed by the TES and the direct conversion of sensor pulse-height into photon number. I will discuss our progress towards developing both types of detectors with quantum efficiencies approaching 100%.

Lucas Clemente

Tba