@article{PattynPerichonDurandetal.2013, author = {Pattyn, Frank and Perichon, Laura and Durand, Gael and Favier, Lionel and Gagliardini, Olivier and Hindmarsh, Richard C. A. and Zwinger, Thomas and Albrecht, Torsten and Cornford, Stephen and Docquier, David and Furst, Johannes J. and Goldberg, Daniel and Gudmundsson, Gudmundur Hilmar and Humbert, Angelika and Huetten, Moritz and Huybrechts, Philippe and Jouvet, Guillaume and Kleiner, Thomas and Larour, Eric and Martin, Daniel and Morlighem, Mathieu and Payne, Anthony J. and Pollard, David and Rueckamp, Martin and Rybak, Oleg and Seroussi, Helene and Thoma, Malte and Wilkens, Nina}, title = {Grounding-line migration in plan-view marine ice-sheet models: results of the ice2sea MISMIP3d intercomparison}, series = {Journal of glaciology}, volume = {59}, journal = {Journal of glaciology}, number = {215}, publisher = {International Glaciological Society}, address = {Cambridge}, issn = {0022-1430}, doi = {10.3189/2013JoG12J129}, pages = {410 -- 422}, year = {2013}, abstract = {Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.}, language = {en} } @misc{RaederEisertWilkensetal.2005, author = {R{\"a}der, Andy and Eisert, Jens and Wilkens, Martin and Schmidt, Robert and Micka, Bettina and Ostermeyer, Martin and Zill, R{\"u}diger and Baur, J{\"u}rgen and Schmidt, Renate and Leppin, Karin and Slotowski, Agnes and Resch-Esser, Ursula}, title = {Portal = Albert Einsteins Erbe: Uni-Physiker forschen weiter}, number = {01-03/2005}, organization = {Universit{\"a}t Potsdam, Referat f{\"u}r Presse- und {\"O}ffentlichkeitsarbeit}, issn = {1618-6893}, doi = {10.25932/publishup-43984}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-439846}, pages = {55}, year = {2005}, abstract = {Aus dem Inhalt: - Albert Einsteins Erbe: Uni-Physiker forschen weiter - Uni-Studiengang im europ{\"a}ischen Exzellenzprogramm - Sternenstaubsammler - Mit Einfallsreichtum gegen den Trend}, language = {de} } @misc{VolkMarkertRiejoketal.2006, author = {Volk, Benno and Markert, Doreen and Riejok, Henriette and Dittberner, J{\"u}rgen and Wanka, Johanna and Wilkens, Martin and G{\"o}rtemaker, Manfred and Regierer, Babette and Steup, Martin and M{\"u}ller-R{\"o}ber, Bernd and Wernicke, Matthias and Altenberger, Uwe and St{\"o}lting, Erhard and Fer{\´y}, Carolin and Egenter, Peter and Lenz, Claudia and Jakubowski, Zuzanna and Kl{\"o}tzer, Sylvia and Krause, Michael and Dietsch, Ulrich}, title = {Portal = Vor der Pr{\"a}sidenten-Wahl: Erwartungen, W{\"u}nsche, Vorschl{\"a}ge}, number = {04-05/2006}, organization = {Universit{\"a}t Potsdam, Referat f{\"u}r Presse- und {\"O}ffentlichkeitsarbeit}, issn = {1618-6893}, doi = {10.25932/publishup-44000}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-440005}, pages = {50}, year = {2006}, abstract = {Aus dem Inhalt: - Vor der Pr{\"a}sidenten-Wahl: Erwartungen, W{\"u}nsche, Vorschl{\"a}ge - Der AStA in der Krise? - {\"U}ber Satire und Macht in der DDR - Vom F{\"u}nf-Sterne-Koch zum Mensaleiter}, language = {de} } @misc{NeumannHorstkemperKruegeretal.2002, author = {Neumann, Bernhard and Horstkemper, Marianne and Kr{\"u}ger, Wolfgang and Wilkens, Martin and Bohlen, Andreas and Fr{\"u}bing, Peter and Wegener, Michael and Scheff, Ullrich and Neher, Dieter and Brehmer, Ludwig and Kleinpeter, Erich and Wolf, Gunter and Koetz, Joachim and Peter, Martin G. and Senkbeil, Sigrid and Meyer, Till}, title = {Portal = Im Trend: Physiker und Chemiker erforschen "Soft Matter"}, volume = {2002}, issn = {1618-6893}, doi = {10.25932/publishup-50144}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-501441}, pages = {40}, year = {2002}, abstract = {Aus dem Inhalt: Im Trend: Physiker und Chemiker erforschen „Soft Matter" -Brandenburger Netzwerk f{\"u}r Existenzgr{\"u}nder erh{\"a}lt F{\"o}rderung -Universit{\"a}t leistet Beitrag zum Romantik-Jahr -Musiksender MTV und Bryan Adams auf dem Campus}, language = {de} } @article{RaetzelWilkensMenzel2017, author = {R{\"a}tzel, Dennis and Wilkens, Martin and Menzel, Ralf}, title = {Effect of polarization entanglement in photon-photon scattering}, series = {Physical review : A, Atomic, molecular, and optical physics}, volume = {95}, journal = {Physical review : A, Atomic, molecular, and optical physics}, number = {1}, publisher = {American Physical Society}, address = {College Park}, issn = {2469-9926}, doi = {10.1103/PhysRevA.95.012101}, pages = {6}, year = {2017}, abstract = {It is found that the differential cross section of photon-photon scattering is a function of the degree of polarization entanglement of the two-photon state. A reduced general expression for the differential cross section of photon-photon scattering is derived by applying simple symmetry arguments. An explicit expression is obtained for the example of photon-photon scattering due to virtual electron-positron pairs in quantum electrodynamics. It is shown how the effect in this explicit example can be explained as an effect of quantum interference and that it fits with the idea of distance-dependent forces.}, language = {en} } @article{LewensteinCirauquiAngelGarciaMarchetal.2022, author = {Lewenstein, Maciej and Cirauqui, David and Angel Garcia-March, Miguel and Corominas, Guillem Guigo and Grzybowski, Przemyslaw and Saavedra, Jose R. M. and Wilkens, Martin and Wehr, Jan}, title = {Haake-Lewenstein-Wilkens approach to spin-glasses revisited}, series = {Journal of physics : A, Mathematical and theoretical}, volume = {55}, journal = {Journal of physics : A, Mathematical and theoretical}, number = {45}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {1751-8113}, doi = {10.1088/1751-8121/ac9d10}, pages = {20}, year = {2022}, abstract = {We revisit the Haake-Lewenstein-Wilkens approach to Edwards-Anderson (EA) model of Ising spin glass (SG) (Haake et al 1985 Phys. Rev. Lett. 55 2606). This approach consists in evaluation and analysis of the probability distribution of configurations of two replicas of the system, averaged over quenched disorder. This probability distribution generates squares of thermal copies of spin variables from the two copies of the systems, averaged over disorder, that is the terms that enter the standard definition of the original EA order parameter, qEA 0 0}, language = {en} } @misc{EnderleinZimmermannPeteretal.2011, author = {Enderlein, Hinrich and Zimmermann, Matthias and Peter, Andreas and Micka, Bettina and Wilkens, Martin and Lohnwasser, Roswitha and Horn-Conrad, Antje}, title = {Portal = Gesichter und Geschichten einer jungen Universit{\"a}t}, number = {02/2011}, organization = {Universit{\"a}t Potsdam, Referat f{\"u}r Presse- und {\"O}ffentlichkeitsarbeit}, issn = {1618-6893}, doi = {10.25932/publishup-44045}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-440456}, pages = {42}, year = {2011}, abstract = {Aus dem Inhalt: - 20 Jahre Universit{\"a}t Potsdam - Gesichter und Geschichten einer jungen Universit{\"a}t}, language = {de} } @misc{WilkensSuetterlinWelleretal.2014, author = {Wilkens, Martin and S{\"u}tterlin, Sabine and Weller, Nina and Horn-Conrad, Antje and Kampe, Heike and Eckardt, Barbara and G{\"o}rlich, Petra and J{\"a}ger, Sophie and Zimmermann, Matthias and Mitsch, Wolfgang}, title = {Portal Wissen = Zeit}, number = {02/2014}, organization = {Universit{\"a}t Potsdam, Referat f{\"u}r Presse- und {\"O}ffentlichkeitsarbeit}, issn = {2194-4237}, doi = {10.25932/publishup-44084}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-440842}, pages = {99}, year = {2014}, abstract = {„Was ist also 'Zeit'?" seufzt Augustinus von Hippo im 11. Buch seiner „Confessiones" melancholisch, und f{\"a}hrt fort „Wenn mich niemand danach fragt, weiß ich es; will ich einem Fragenden es erkl{\"a}ren, weiß ich es nicht." Auch heute, 1584 Jahre nach Augustinus, erscheint 'Zeit' immer noch r{\"a}tselhaft. Abhandlungen {\"u}ber das Wesen der Zeit f{\"u}llen Bibliotheken. Oder eben dieses Heft. Wesensfragen sind den modernen Wissenschaften allerdings fremd. Zeit ist - zumindest in der Physik - unproblematisch. „Time is defined so that Motion looks simple" erk{\"a}rt man kurz und trocken, und verabschiedet sich damit vom Augustinischen R{\"a}tsel oder der Newtonschen Vorstellung einer absoluten Zeit, deren mathematischen Fluss man durch irdische Instrumente eh immer nur n{\"a}herungsweise erfassen kann. In der Alltagssprache, selbst in den Wissenschaften, reden wir zwar weiterhin vom Fluss der Zeit, aber Zeit ist schon lange keine nat{\"u}rliche Gegebenheit mehr. Zeit ist vielmehr ein konventioneller Ordnungsparameter f{\"u}r {\"A}nderung und Bewegung. Geordnet werden Prozesse, indem eine Klasse von Prozessen als Z{\"a}hlsystem dient, um andere Prozesse mit ihnen zu vergleichen und anhand der tempor{\"a}ren Kategorien „vorher", „w{\"a}hrend" und „nachher" anzuordnen. Zu Galileis Zeiten galt der eigene Pulsschlag als Zeitstandard f{\"u}r den Flug von Kanonenkugeln. Mit zunehmender Verfeinerung der Untersuchungsmethoden erschien das zu unpraktisch: Die Weg-Zeit-Diagramme frei fliegender Kanonenkugeln erweisen sich in diesem Standard ziemlich verwackelt, schlecht reproduzierbar, und keineswegs „simpel". Heutzutage greift man zu C{\"a}sium-Atomen. Demnach dauert ein Prozess eine Sekunde, wenn ein 133Cs-Atom genau 9 192 631 770 Schwingungen zwischen zwei sogenannten Hyperfeinzust{\"a}nden des Grundzustands vollf{\"u}hrt hat. Und ein Meter ist die Entfernung, die Licht im Vakuum in exakt 1/299 792 458 Sekunden zur{\"u}cklegt. Gl{\"u}cklicherweise sind diese Daten im General Positioning System GPS hart kodiert, so dass der Nutzer sie nicht jedes Mal aufs Neue eingeben muss, wenn er wissen will, wo er ist. Aber schon morgen muss er sich vielleicht ein Applet runterladen, weil der Zeitstandard durch raffinierte {\"U}berg{\"a}nge in Ytterbium ersetzt wurde. Der konventionelle Charakter des Zeitbegriffs sollte nicht dazu verf{\"u}hren zu glauben, alles sei irgendwie relativ und daher willk{\"u}rlich. Die Beziehung eines Pulsschlags zu einer Atomuhr ist absolut, und genauso real, wie die Beziehung einer Sanduhr zum Lauf der Sonne. Die exakten Wissenschaften sind Beziehungswissenschaften. Sie handeln nicht vom Ding an sich, was Newton und Kant noch getr{\"a}umt haben, sondern von Beziehungen - worauf schon Leibniz und sp{\"a}ter Mach hingewiesen haben. Kein Wunder, dass sich f{\"u}r andere Wissenschaften der physikalische Zeit-Standard als ziemlich unpraktisch erweist. Der Psychologie der Zeitwahrnehmung entnehmen wir - und jeder wird das best{\"a}tigen k{\"o}nnen - dass das gef{\"u}hlte Alter durchaus verschieden ist vom physikalischen Alter. Je {\"a}lter man ist, desto k{\"u}rzer erscheinen einem die Jahre. Unter der einfachen Annahme, dass die gef{\"u}hlte Dauer umgekehrt proportional zum physikalischen Alter ist, und man als Zwanzigj{\"a}hriger ein physikalisches Jahr auch psychologisch als ein Jahr empfindet, ergibt sich der erstaunliche Befund, dass man mit 90 Jahren 90 Jahre ist. Und - bei einer angenommenen Lebenserwartung von 90 Jahren - mit 20 (bzw. 40) physikalischen Jahren bereits 67 (bzw. 82) Prozent seiner gef{\"u}hlten Lebenszeit hinter sich hat. Bevor man angesichts der „Relativit{\"a}t von Zeit" selbst in Melancholie versinkt, vielleicht die Fortsetzung des Eingangszitats von Augustinus: „Aber zuversichtlich behaupte ich zu wissen, dass es vergangene Zeit nicht g{\"a}be, wenn nichts verginge, und nicht k{\"u}nftige Zeit, wenn nichts herank{\"a}me, und nicht gegenw{\"a}rtige Zeit wenn nichts seiend w{\"a}re." Tja - oder mit Bob Dylan „The times they're a changing". Ich w{\"u}nsche Ihnen eine spannende Zeit bei der Lekt{\"u}re dieser Ausgabe. Prof. Dr. Martin Wilkens Professor f{\"u}r Quantenoptik}, language = {de} } @misc{WilkensSuetterlinKampeetal.2014, author = {Wilkens, Martin and S{\"u}tterlin, Sabine and Kampe, Heike and Eckardt, Barbara and J{\"a}ger, Sophie and Zimmermann, Matthias}, title = {Portal Wissen = Time}, number = {02/2014}, organization = {University of Potsdam, Press and Public Relations Department}, issn = {2198-9974}, doi = {10.25932/publishup-44149}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-441497}, pages = {55}, year = {2014}, abstract = {"What then is time?", Augustine of Hippo sighs melancholically in Book XI of "Confessions" and continues, "If no one asks me, I know; if I want to explain it to a questioner, I don't know." Even today, 1584 years after Augustine, time still appears mysterious. Treatises about the essence of time fill whole libraries - and this magazine. However, questions of essence are alien to modern sciences. Time is - at least in physics - unproblematic: "Time is defined so that motion looks simple", briefly and prosaically phrased, waves goodbye to Augustine's riddle and to the Newtonian concept of absolute time, whose mathematical flow can only be approximately recorded with earthly instruments anyway. In our everyday language and even in science we still speak of the flow of time but time has not been a natural condition for quite a while now. It is rather a conventional order parameter for change and movement. Processes are arranged by using a class of processes as a counting system in order to compare other processes and to organize them with the help of the temporary categories "before", "during", and "after". During Galileo's time one's own pulse was seen as the time standard for the flight of cannon balls. More sophisticated examination methods later made this seem too impractical. The distance-time diagrams of free-flying cannon balls turned out to be rather imprecise, difficult to replicate, and in no way "simple". Nowadays, we use cesium atoms. A process is said to take one second when a caesium-133 atom completes 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state. A meter is the length of the path travelled by light in a vacuum in exactly 1/299,792,458 of a second. Fortunately, these data are hard-coded in the Global Positioning System GPS so users do not have to reenter them each time they want to know where they are. In the future, however, they might have to download an app because the time standard has been replaced by sophisticated transitions to ytterbium. The conventional character of the time concept should not tempt us to believe that everything is somehow relative and, as a result, arbitrary. The relation of one's own pulse to an atomic clock is absolute and as real as the relation of an hourglass to the path of the sun. The exact sciences are relational sciences. They are not about the thing-initself as Newton and Kant dreamt, but rather about relations as Leibniz and, later, Mach pointed out. It is not surprising that the physical time standard turned out to be rather impractical for other scientists. The psychology of time perception tells us - and you will all agree - that the perceived age is quite different from the physical age. The older we get the shorter the years seem. If we simply assume that perceived duration is inversely related to physical age and that a 20-year old also perceives a physical year as a psychological one, we come to the surprising discovery that at 90 years we are 90 years old. With an assumed life expectancy of 90 years, 67\% (or 82\%) of your felt lifetime is behind you at the age of 20 (or 40) physical years. Before we start to wallow in melancholy in the face of the "relativity of time", let me again quote Augustine. "But at any rate this much I dare affirm I know: that if nothing passed there would be no past time; if nothing were approaching, there would be no future time; if nothing were, there would be no present time." Well, - or as Bob Dylan sings "The times they are a-changin". I wish you an exciting time reading this issue. Prof. Martin Wilkens Professor of Quantum Optics}, language = {en} } @article{AlbusGardinerIlluminatietal.2002, author = {Albus, Alexander P. and Gardiner, Simon A. and Illuminati, Fabrizio and Wilkens, Martin}, title = {Quantum field theory of dilute homogeneous Bose-Fermi-mixtures at zero temperature : general formalismand beyond mean-field corrections}, year = {2002}, abstract = {We consider a dilute homogeneous mixture of bosons and spin-polarized fermions at zero temperature. We first construct the formal scheme for carrying out systematic perturbation theory in terms of single particle Green's functions. We introduce a new relevant object, the renormalized boson-fermion T-matrix which we determine to second order in the boson-fermion s-wave scattering length. We also discuss how to incorporate the usual boson-boson T-matrix in mean-field approximation to obtain the total ground state properties of the system. The next order term beyond mean- field stems from the boson-fermion interaction and is proportional to \$a_{scriptsize BF}k_{scriptsize F}\$. The total ground-state energy-density reads \$E/V =epsilon_{scriptsize F} + epsilon_{scriptsize B} + (2pihbar^{2}a_{ m BF}n_{scriptsize B}n_{scriptsize F}/m) [1 + a_{scriptsize BF}k_{scriptsize F}f(delta)/pi]\$. The first term is the kinetic energy of the free fermions, the second term is the boson-boson mean-field interaction, the pre-factor to the additional term is the usual mean-field contribution to the boson-fermion interaction energy, and the second term in the square brackets is the second-order correction, where \$f(delta)\$ is a known function of \$delta= (m_{scriptsize B} - m_{scriptsize F})/(m_{scriptsize B} + m_{scriptsize F})\$. We discuss the relevance of this new term, how it can be incorporated into existing theories of boson-fermion mixtures, and its importance in various parameter regimes, in particular considering mixtures of \$^{6}\$Li and \$^{7}\$Li and of \$^{3}\$He and \$^{4}\$He.}, language = {en} }