1. Scientific News: Markus Büttiker’s group, Geneva: Electron waiting times in mesoscopic conductors: Electron transport in mesoscopic conductors has traditionally involved investigations of the mean current and the fluctuations of the current. A complementary view on charge transport is provided by the distribution of waiting times between charge carriers, but a proper theoretical framework for coherent electronic systems has so far been lacking. We have now developed a quantum theory of electron waiting times in mesoscopic conductors expressed by a compact determinant formula. We illustrate our methodology by calculating the waiting time distribution for a quantum point contact and find a crossover from WignerDyson statistics at full transmission to Poisson statistics close to pinchoff. Even when the lowfrequency transport is noiseless, the electrons are not equally spaced in time due to their inherent wave nature. We discuss the implications for renewal theory in mesoscopic systems and point out several analogies with level spacing statistics and random matrix theory. The paper was chosen as an Editor’s Suggestion in Phys. Rev. Lett. and highlighted with a synopsis in Physics. Mathias Albert, Géraldine Haack, Christian Flindt, and Markus Büttiker, Phys. Rev. Lett. 108, 186806 (2012): http://prl.aps.org/abstract/PRL/v108/i18/e18680 6 Physics synopsis: "Electrons Arrive By and By": http://physics.aps.org/synopsis for/10.1103/PhysRevLett.108.186806 Le Journal de l'Unige, page 14: "Des électrons comme sur une autoroute": http://www.unige.ch/communication/lejournal/ar chives/65.pdf Klaus Ensslin’s group, Zurich: Electron flow in splitgated bilayer graphene: Transport measurements are performed on a bilayer graphene sheet with a homogeneous back gate and a split top gate. The electronic transport data indicate the capability of directing electron flow through bilayer graphene nanostructures purely defined by electrostatic gating. Comparing the transconductance data recorded for different top gate geometries—continuous barrier and split gate—the observed transport features for the split gate can be attributed to interference effects inside the narrow opening. S. Dröscher, C. Barraud, K. Watanabe, T. Taniguchi, T. Ihn and K. Ensslin; New. J. Phys. 14, 103007 (2012) http://iopscience.iop.org/1367 2630/14/10/103007 Tilman Esslinger’s group, Zurich: Conduction of Ultracold Fermions Through a Mesoscopic Channel: In a mesoscopic conductor, electric resistance is detected even if the device is defectfree. JeanPhilippe Brantut and his colleagues engineered and studied a coldatom analog of a mesoscopic conductor. It consisted of a narrow channel connecting two macroscopic reservoirs of fermions that can be switched from ballistic to E F +eV E F T QPC ¯ τ τ Newsletter October 2012
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QSIT Newsletter QSIT Newsletter
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1. Scientific News:
Markus Büttiker’s group, Geneva: Electron waiting times in mesoscopic conductors: Electron transport in mesoscopic conductors has traditionally involved investigations of the mean current and the fluctuations of the current. A complementary view on charge transport is provided by the distribution of waiting times between charge carriers, but a proper theoretical framework for coherent electronic systems has so far been lacking. We have now developed a quantum theory of electron waiting times in mesoscopic conductors expressed by a compact determinant formula. We illustrate our methodology by calculating the waiting time distribution for a quantum point contact and find a crossover from Wigner-‐Dyson statistics at full transmission to Poisson statistics close to pinch-‐off. Even when the low-‐frequency transport is noiseless, the electrons are not equally spaced in time due to their inherent wave nature. We discuss the implications for renewal theory in mesoscopic systems and point out several analogies with level spacing statistics and random matrix theory. The
paper was chosen as an Editor’s Suggestion in Phys. Rev. Lett. and highlighted with a synopsis in Physics.
-‐ Mathias Albert, Géraldine Haack, Christian Flindt, and Markus Büttiker, Phys. Rev. Lett. 108, 186806 (2012): http://prl.aps.org/abstract/PRL/v108/i18/e186806 -‐ Physics synopsis: "Electrons Arrive By and By": http://physics.aps.org/synopsis-‐for/10.1103/PhysRevLett.108.186806 -‐ Le Journal de l'Unige, page 14: "Des électrons comme sur une autoroute": http://www.unige.ch/communication/lejournal/archives/65.pdf
Klaus Ensslin’s group, Zurich:Electron flow in split-‐gated bilayer graphene: Transport measurements are performed on a bilayer graphene sheet with a homogeneous back gate and a split top gate. The electronic transport
data indicate the capability of directing electron flow through bilayer graphene nanostructures purely defined by electrostatic gating. Comparing the transconductance data recorded for different top gate geometries—continuous barrier and split gate—the observed transport features for the split gate can be attributed to interference effects inside the narrow opening. S. Dröscher, C. Barraud, K. Watanabe, T. Taniguchi, T. Ihn and K. Ensslin; New. J. Phys. 14, 103007 (2012) http://iopscience.iop.org/1367-‐2630/14/10/103007
Tilman Esslinger’s group, Zurich: Conduction of Ultracold Fermions Through a Mesoscopic Channel: In a mesoscopic conductor, electric resistance is detected even if the device is defect-‐free. Jean-‐Philippe Brantut and his colleagues
engineered and studied a cold-‐atom analog of a mesoscopic conductor. It consisted of a narrow channel connecting two macroscopic reservoirs of fermions that can be switched from ballistic to
diffusive. A current through the channel was induced and ohmic conduction was found, even for a ballistic channel. An in situ measurement of the density variations resulting from the presence of a current showed, that the density remained uniform and constant inside the ballistic channel. In contrast, for the diffusive case with disorder, a density gradient extending through the channel was observed. The approach opens the way towards quantum simulation of mesoscopic devices with quantum gases.
J. -‐ P. Brantut, J. Meineke, D. Stadler, S. Krinner, and T. Esslinger; Science 337, 1069-‐1071 (2012) http://www.sciencemag.org/content/337/6098/1069.full
Roton-‐type mode softening in a quantum gas with cavity-‐mediated long-‐range interactions: Long-‐range interactions in quantum gases are predicted to give rise to an excitation spectrum of roton character, similar to that observed in superfluid helium. Rafael Mottl and colleagues investigated the excitation spectrum of a Bose-‐Einstein condensate with cavity-‐mediated long-‐range interactions, which couple all particles to each other. Increasing the strength of the interaction lead to a softening of an excitation mode at a finite momentum, preceding a superfluid-‐to-‐supersolid phase transition. A variant of Bragg spectroscopy was used to study the mode softening across the phase transition. The measured spectrum was in very good agreement with ab initio calculations and, at the phase transition, a diverging susceptibility was observed. The work paves the way toward quantum simulation of long-‐range interacting many-‐body systems. R. Mottl, F. Brennecke, K. Baumann, R. Landig, T. Donner, and T. Esslinger: Science 336, 1570-‐1573 (2012) http://www.sciencemag.org/content/336/6088/1570.full
Nicolas Gisin, Geneva:Nicolas Gisin has written a book entitled "L'impensable Hasard -‐ non-‐localité, téléportation et autres merveilles quantiques". It appeared on September 6th in the Edition Odile Jacob. In contrast to the ordinary literature on the subject, this little book does not attempt to circumvent the real logical
difficulties imposed by quantum physics. From the "Jeu de Bell" towards experiments in quantum entanglement, it leads to a solid understanding of one of the most fascinating areas of contemporary physics. http://www.odilejacob.fr/catalogue/sciences/physique-‐chimie/impensable-‐hasard_9782738128317.php
Nicolas Gisin’s Group, Geneva:Quantum non-‐locality based on finite-‐speed causal influences leads to superluminal signaling: The experimental violation of Bell inequalities using space-‐like separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, any such experimental violation could always be explained in principle through models based on hidden influences propagating at a finite speed v>c, provided v is large enough. Here, we show that for any finite speed v with c < v < ∞, such models predict correlations that can be exploited for faster-‐
than-‐light communi-‐cation. This super-‐luminal communi-‐cation does not require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-‐day descript-‐tion of quantum experiments. Hence,
assuming the impossibility of using non-‐local correlations for superluminal communication, we exclude any possible explanation of quantum correlations in terms of influences propagating at any finite speed. Our result uncovers a new aspect of the complex relationship between multipartite quantum non-‐locality and the impossibility of signaling.
J-‐D. Bancal, S. Pironio, A. Acín, Y-‐C. Liang, V. Scarani & N. Gisin; Nature Physics (2012);Published online 28.10.12 2012 http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2460.html#ref4
Tobias Kippenberg’s group, Lausanne:An optomechanical transducer for detection of very small forces was realized by coupling a nanomechanical beam to an optical disk resonator via the near-‐field of the confined light. Using an active feedback control of the mechanical oscillator, it was possible to significantly decrease the measurement time needed to detect a small incoherent force. With this technique it was possible to retrieve a minute external force greatly buried in the background in less than a minute of averaging time.
E. Gavartin, P. Verlot & T. J. Kippenberg Nature Nanotechnology 7, 509–514 (2012) http://www.nature.com/nnano/journal/v7/n8/full/nnano.2012.97.html
Daniel Loss’ group, Basel High threshold error correction for the surface code: The surface codes are a spin lattice based proposal for quantum computation with a high error threshold of 18.9% when each spin is subject to depolarizing noise. However, this value is of little use if no efficient means of performing such error correction can be found. As such, we have developed an algorithm that assesses the syndrome of the surface codes, a set of observables used to determine as much about the spin errors as possible without disturbing the stored qubits. It then runs a Markov chain Monte Carlo process to determine
and correct the most likely logical qubit errors to have been caused. Our algorithm has been shown to correct errors up to 18.5%, though this can be increased all the way to the limit of 18.9%by setting stricter convergence conditions for the process. Our next challenge is to analyze the case of noisy syndrome measurements. Current methods can reach error thresholds at the 1% level, and the use of our algorithm can only make this increase. -‐ James R. Wootton and Daniel Loss; accepted in Phsical Review Letters, http://arxiv.org/abs/1202.4316
Christian Schönenberger’s group, Basel: Near-‐Unity Cooper Pair Splitting Efficiency: The two electrons of a Cooper pair in a conventional superconductor form a spin singlet and therefore a maximally entangled state. In Christian Schönenberger’s group it was demonstrated that the two particles can be extracted from the superconductor into two spatially separated contacts via two quantum dots in a process called Cooper pair splitting (CPS). Competing transport processes, however, limit the efficiency of this process. They demonstrate efficiencies up to 90%, significantly larger than required to demonstrate interaction-‐dominated CPS, and on the right order to test Bell’s inequality with electrons. They compare the CPS currents through both quantum dots, for which large apparent discrepancies are possible. The latter is explained intuitively and in a
semi-‐classical master equation model. Large efficiencies are required to detect electron entanglement and for prospective electronics-‐based quantum information technologies.
J. Schindele, A. Baumgartner, and C. Schönenberger Phys. Rev. Lett. 109, 157002 (2012), http://prl.aps.org/abstract/PRL/v109/i15/e157002
Dominik Zumbühl’s group, Basel: Metallic Coulomb blockade thermometry down to 10 mK and below: We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronic experiments, and use it to investigate metallic Coulomb blockade thermometers (CBTs) with various resistances R. The high-‐R devices cool to slightly lower T, consistent with better isolation from the noise environment, and exhibit electron-‐phonon cooling ∝ T^5 and a residual heat-‐leak of 40 aW. In contrast, the low-‐R CBTs display cooling with a clearly weaker T-‐dependence, deviating from the electron-‐
phonon mechanism. The CBTs agree excellently with the refrigerator temperature above 20 mK and reach a minimum-‐T of 7.5 ± 0.2 mK. L. Casparis, M. Meschke, D. Maradan, A. C. Clark, C. P. Scheller, K. K. Schwarzwälder, J. P. Pekola, and D. M. Zumbühl Rev. Sci. Instrum. 83, 083903 (2012) http://rsi.aip.org/resource/1/rsinak/v83/i8/p083903_s1
Dominik Zumbühl’s group and IBM group:Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs: We present nuclear spin relaxation measurements in GaAs epilayers using a new pump-‐probe technique in all-‐electrical, lateral spin-‐valve devices. The measured T1 times agree very well with NMR data available for T > 1 K. However, the nuclear spin relaxation rate clearly deviates from the well-‐established Korringa law expected in metallic samples and follows a
sublinear temperature dependence T1^-‐1 ∝ T^0.6 for 0.1 ≤ T ≤ 10 K. Further, we investigate nuclear spin inhomogeneities. D. Kölbl and D.M. Zumbühl, A. Fuhrer, G.Salis, and S. F. Alvarado Phys. Rev. Lett. 109, 086601 (2012). http://prl.aps.org/abstract/PRL/v109/i8/e086601
2. First award of QSIT Tech Transfer Grant: The QSIT Tech Transfer Grant is a newly established instrument to promote application-‐oriented projects within QSIT. This year it was awarded to two projects within the groups of Tilman Esslinger and Andreas Wallraff. In a cooperation with the startup company QUBIG of ETH alumni Anton Oettl, prototypes of electro-‐optic amplitude modulators are being optimized in the Esslinger group. The Wallraff group is exploring the options of making their expertise in design, fabrication and operation of low-‐noise amplifiers for detection and generation of frequency radiation available to the scientific community. The 2nd generation of QSIT Tech Transfer Grants will be given out in spring 2013. For more information, please visit the QSIT Tech Transfer homepage:
http://www.nccr-‐qsit.ethz.ch/ktt/index Custom-‐made modulator using a matched pair of lithium niobate crystals developed by Qubig in the frame of the project.
3. Awards and grants: Christian Degen was awarded an ERC starting grant for his project entitled «NANOMRI -‐ Three-‐dimensional Magnetic Resonance Imaging at Molecular Resolution». The aim of the ERC project is to promote the combination of magnetic resonance imaging and atomic force microscopy. A new microscope that could represent complex biological objects such as single viruses or large protein molecules directly and in three dimensions, would open up entirely new opportunities in medicine and structural biology. Tobias Kippenberg’s Laboratory of Photonics and Quantum Measurements will coordinate a European consortium of academic and industry partners combining both research and training in the emerging field of Cavity Optomechanics. This Initial Training Network (ITN) is part of the European Marie Curie actions. The network of 10 partners was awarded a total of 5.7 million Euros.
Letica Tarruell and Daniel Greif (Esslinger group) won the Poster Prize at the 23rd International Conference on Atomic Physics ICAP 2012. Martino Poggio wins SSSTC funding: a joint research project between our group and the group of Prof. Jiangfeng Du of the University of Science and Technology of China (USTC) will begin. The project is funded by a 3-‐year grant given by the Sino Swiss Science and Technology Cooperation (SSSTC). Prix Latsis 2012 goes to Albert Schliesser (Kippenberg group) for the improvement of optomechanical cooling techniques, the discovery of optomechanically induced transparency and the demonstration of quantum coherent coupling of a micromechanical oscillator to an optical cavity mode. Vanessa Wood received an Intel Early Career Faculty Honor Program award in September.
Congratulations!
4. Recent Events
International Conference on Quantum Systems and Technology June 17 – 22, 2012 in Monte Verità, TI, SwitzerlandGianni Blatter and other colleagues from the NCCR QSIT have organized the International Conference on Quantum Systems and Technology, June 17 – 22, 2012 in Monte Verità, TI, Switzerland. The program covered topics at the edge of the international research in the field: quantum dots, spins, quantum Hall effect, cold atoms, trapped ions, interaction of light and matter, Majorana fermions and topological states, new semiconductor devices, superconducting qubits and quantum information theory. Talks were organized in 16 short sessions with 2 speakers each. Of the 89 lecturers and participants, 32 came from abroad. In an attempt to advance the career of young scientists speakers from ETH Zurich were selected among the junior members of the NCCR QSIT research groups.
Meeting of the Swiss Physical Society June 21 and 22, 2012, ETH Zurich During this year’s meeting of the SPS the NCCR organized one of the sessions. One highlight was the SPS prize ceremony. One former QSIT student, Dr. Johannes Güttinger, shared the SPS Award in General Physics, for his pioneering PhD work on graphene quantum dots. The QSIT talks took place on Friday and started with a plenary presentation by Prof. Kotthaus from the LMU Munich. A sequence of speakers from various groups participating in
NCCR QSIT reported about their results in quantum optics, noise-‐spectroscopy in self-‐assembled quantum dots, graphene connected to superconducting electrodes in the quantum Hall regime, graphene three-‐terminal dots, as well as the observation of the strong coupling regime in a two-‐dimensional electron gas coupled to photonic crystal structures with resonance frequencies below 10 meV. The poster session monitored a
variety of topics presented by the members of QSIT, other physics-‐NCCRs and numerous young Swiss physicists and gave plenty occasions for discussion between groups. SPG Mitteilungen 38, 7, Sept2012 and http://www.sps.ch/events/spg_jahrestagung_2012/programm/
NCCR QSIT Junior Meeting July 3 -‐ 6, 2012, Passug, GR, Switzerland This year’s Junior Meeting took place from July 3 to 6, 2012, again in Passugg GR. It was entirely organized by two young PhD students, Tobias Thiele (Wallraff group, ZH) and Andreas Kuhlmann (Warburton group, Basel). They received support for practical matters and financial support from the NCCR. Participation at this workshop is limited to PhD students and postdocs with the goal of enhancing the networking between young scientists from different sub-‐fields and of contributing to their education. There were 24 participants from the different partner institutions of the NCCR presenting talks and posters. http://www.nccr-‐qsit.ethz.ch/news/events/JuniorMeeting2012
NanoMRI Conference July 22 -‐ 27, 2012, Monte VeritàProf. Christian Degen (ETH), Prof. Poggio, and Prof. Beat Meier (ETH) have organized the 4th nanoMRI Conference at the Centro Stefano Franscini in Monte Verità, Ascona, Switzerland. The conference aimed to bring together scientists and engineers interested in the field of ultrasensitive spin detection and nanoscale magnetic resonance imaging. A particular
focus was on magnetic resonance force microscopy, nanomechanical resonators, single spins in diamond, and on the experimental and theoretical challenges that go with pushing the limits of measurements. http://www.spin.ethz.ch/NanoMRI/index
31st International Conference on the Physics of Semiconductors 29.7. -‐ 3.8.2012, Zurich Another noteworthy event was the “International Conference on the Physics of Semiconductors” organized by K. Ensslin, J. Faist, A. Fontcuberta, T. Ihn, A. Imamoglu, G. Salis, R. Warburton and W. Wegscheider. The largest semiconductor physics worldwide conference hosts about 1200 attendees. It is the first time that this prestigious conference has been held in Switzerland. The traditional Nobel Prize Session at the beginning of the ICPS meeting included the Nobel Laureates Klaus von Klitzing (Nobel Prize in Physics 1985), Albert Fert (Nobel Prize in Physics together with Peter Grünberg
2007), Konstantin Novoselov (Nobel Prize in Physics together with André Geim, 2010) and Richard Ernst (Nobel Prize in Chemistry 1991). A
significant fraction of the science discussed at this meeting was QSIT-‐related. See: http://www.icps2012.ethz.ch/
5. Mini-sabbaticals
The Mini-‐sabbaticals have started to take place between the groups in Basel, Geneva, Lausanne, and Zurich. Future internships will hopefully be envisaged during the next months. http://www.nccr-‐qsit.ethz.ch/education/minisabbaticals
6. Agenda
NCCR QSIT Site Visit
NCCR QSIT Student School Jan 28 – Jan 30, 2013 and
Tutorial-‐style lectures will be given covering the topics Quantum Repeater, Quantum Information, Trapped Ions, Cold Atoms, Spins in Semiconductors, and Opto-‐Mechanics. The goal is help QSIT researchers, in particular the young researchers, follow the talks in the General Meeting and contribute to our discussions. There will be space for near 50 researchers at the Student School.
NCCR QSIT General Meeting Jan 30 – Feb 1, 2013 Waldhotel National, Arosa Organizer: NCCR QSIT
7. New collaborators Daniela Frauchinger joined Renato Renner’s group on October 1st. She will work on the NCCR Transfer Project “Cryptographic Randomness Extractor (CREx) in cooper-‐ation with IDquantuique in Geneva.
Renate Landig is a PhD student in Tilman Esslinger's quantum optics group at ETH Zürich. She works in the field of quantum gases with long-‐range interactions and cavity-‐optomechanics, where she is currently studying quantum fluctuations at the Dicke phase-‐transition.
Each PhD student and post-‐doc associated with this NCCR has the opportunity to work one week per year in another NCCR group of his/her choice. This step will promote collaboration and exchange between the younger researchers and will also serve the purpose of general education. These research stages will be centrally financed by the NCCR and are open for all young researchers working on NCCR-‐related projects in the participating research groups, even if the salary of these researchers is not directly provided by NCCR resources. Please contact your supervisor or the NCCR office for further information.
November 29 -‐ 30, 2012, ETH Science City, HIT Building. Detailed information will be communicated in due time.
October 2012
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Moonjoo Lee is a PostDoc in Tilman Esslinger's quantum optics group at ETH Zürich. He works in the field of quantum gases and cavity QED, where he is currently setting up a new experiment to investigate novel quantum phases in atom-‐light many-‐body systems.
Julian Leonard is a PhD student in Tilman Esslinger's quantum optics group at ETH Zürich. He works in the field of quantum gases and cavity QED, where he is currently setting up a new experiment to investigate novel quantum phases in atom-‐light many-‐body systems.
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