Special Seminar

Nuclear magnetic resonance is a key analytical tool for physics, chemistry and medicine. It relies upon applying a combination of a static magnetic field to split the nuclear energy levels, and an oscillating magnetic field to induce transitions between them. Alternatively, for nuclei with quantum number I > 1/2 placed in crystals of low symmetry, the static energy splitting can be provided the nuclear quadrupole interaction, but transitions between the levels are always induced by oscillating magnetic fields.

Some recent experiments have shown electrical control of nuclear spins by modulating the electron-nuclear hyperfine tensor, thus using the electron as a transducer from the external electric field to the microscopic (magnetic) hyperfine field. Therefore, they represent a form of electrically-driven magnetic resonance.

Is it possible to induce nuclear transition using only pure electric fields?

I will present the experimental demonstration of coherent electrical control of a single ¹²³Sb nuclear spin in silicon. The microscopic mechanism that underpins this demonstration is the coherent version of what was known since the 1960s as the Linear Quadrupole Stark Effect. It consists of a time-dependent modulation of the quadrupole coupling tensor, caused by the effect of the electric field on the bond orbitals around the atom. The bond orbital distortion, in turn, results in a local modulation of the electric field gradient at the nuclear site, which affects the quadrupole coupling and induces nuclear transitions. We observed coherent transitions for both 1 and 2 quanta of angular momentum, and demonstrated the ability to shift the nuclear resonance with static electric fields. A microscopic density functional theory provides quantitative agreement with the data, and insights into the delicate interplay between strain, electric fields and bond orbitals around the nucleus.

Newtonian and Scr{\"o}dinger dynamics can be formulated in a physically meaningful way within the same Hilbert space framework. The resulting unexpected relation between classical and quantum motions goes beyond the results provided by the Ehrenfest theorem. The normal probability distribution and the Born rule turn out to be related. A dynamical mechanism responsible for the latter formula will be proposed and applied to measurements of macroscopic and microscopic systems.

We demonstrate light-induced formation of coherence in a cold atomic gas system that utilizes the suppression of a competing density wave (DW) order. The condensed atoms are placed in an optical cavity and pumped by an external optical standing wave, which induces a long-range interaction, mediated by photon scattering, and DW order above a critical pump strength. We show that light induced modulation of the pump wave can suppress this DW order and restore coherence. This establishes a foundational principle of dynamical control of competing orders analogous to a hypothesized mechanism for light induced superconductivity in high-Tc cuprates.

 In recent years, efforts to observe helical edge states in materials with large spin-orbit coupling have accelerated. These material, once in proximity to an s-wave superconductor may, under certain conditions, manifest a topological superconductive (TS) phase [1]. Consequently, Majorana fermions, allusive quasiparticles with a non-Abelian exchange statistic, are expected to emerge. Even more interesting are the fractional helical states, previously not observed, which manifest the more exotic para-fermion anyons. Though evidence for the presence of Majorana fermions accumulates, observations of helical edge transport, being a prerequisite for the formation of Majorana quasiparticles, are scarce. Encouraged by proposals that induce topological superconductivity in 2DEG at the quantum Hall effect (QHE) regime [2, 3], we succeeded to form such, the sought after, robust chiral helical edge modes in GaAs-AlGaAs heterostructures.In order to have two adjacent counter-propagating edge modes with opposite spin, the 2DEG is embedded in a unique quantum well structure, which hosts two weakly interacting electronic sub-bands. Gating the 2DEG with two half-plain gates, enable a scenario where two different filling factors are applied to the lower and upper sub-bands. Landau levels of different sub-bands cross at the interface between the two gates; thus forming overlapping, counter-propagating, chiral edge modes. Two counter-propagating edges with opposite spins, both in the integer and fractional regime were observed, propagating for more than 300 microns without mixing. In addition, spin protected tunneling was observed depending on spin orientation.

[1] L. Fu and C. Kane, PRL 100, 096407 (2008).

[2] N.H. Lindner, E. Berg, G. Refael and A. Stern, PRX 2, 041002 (2012).

[3] D.J. Clarke, J. Alicea and K. Shtengel, Nature. Communication 4, 1348 (2013).

This will be a general lecture about the role that the scalar curvature plays in geometric problems. In particular, we will highlight connections to the Einstein equations and questions which arise from the physical side. We will also describe recent results which resolve some old issues in the subject.

Ion implanted phosphorus (³¹P) atoms in isotopically pure silicon devices have driven a sequence of discoveries reporting exceptionally long coherence times for the ³¹P nuclear spin quantum bit [1-4] with coherence times longer than 30 s [2].  To make the devices used for this work, a small number of ³¹P atoms are implanted into a nano-scale construction site on a ²⁸Si substrate that is later surrounded by nanocircuitry for programming and read out of the qubit.   Post-implant donor selection by tuning gate electrodes is used to select a single ³¹P donor atom for the experiments.  However the next step requires achieving the key milestone of qubit entanglement in an ordered array of ³¹P dopant atoms placed with nanometer precision.  Deterministic implantation of atoms into such arrays is possible by counting the ion-implantation-induced transient pulse of electron-hole pairs created in the substrate that incorporates suitable detector electrodes.  The detector electrodes produce a gate pulse signalling the implantation of single ions that have a random arrival time.  Achieving the required spatial precision is difficult owing to straggling and channelling effects.  This can be addressed by low energy (<10 keV) implantation of ³¹P ions, but in this regime the signal from the generation of  electron-hole pairs in the substrate is close to the noise threshold of present charge-sensitive electronics.  This method could also be extended for the deterministic placement of colour centres in diamond provided the implanted-ion to colour-centre conversation problem is solved.  This presentation shows how these challenges are being addressed [5] both within CQC2T and in parallel developments in the UK, EU and USA.  Within CQC2T our approach will take this technology to the next stage by building deterministic arrays of single atoms with the goal of 6 nm positioning precision in new architectures that could form the building blocks of a future CMOS quantum computer fabricated with the standard tools of the semiconductor industry.

[1] Bell's inequality violation with spins in silicon, JP Dehollain, et al., Nature Nanotechnology 11 p242 (2016)

[2] Storing quantum information for 30 seconds in a nanoelectronic device, JT Muhonen, et al. Nature Nanotechnology 9, p986 (2014)

[3] High-fidelity readout and control of a nuclear spin qubit in silicon, JJ Pla, et al., Nature 496, p334 (2013)

[4] A single-atom electron spin qubit in silicon, JJ Pla, et al., Nature 489, p541 (2012)

Single-shot readout of an electron spin in silicon, A Morello, et al., Nature 467, p687 (2010)

[5] Single atom devices by ion implantation, JA van Donkelaar, et al., J. Phys. Cond. Mat. 27, 154204 (2015)

I will discuss the delocalization in a many-body system due to a power-law interaction. One experimentally relevant realization of this problem is the effect of Coulomb interaction in Anderson insulators. Particle-hole excitations built on localized electron states are viewed as two-level systems (“spins”) randomly distributed in space and energy and coupled due to electron-electron interaction. A small fraction of these states form resonant pairs that in turn build a complex network allowing for energy propagation. We identify the character of energy transport and evaluate the spin relaxation rate and the thermal conductivity. For physically relevant cases of two-dimensional and three-dimensional spin systems with 1/r^{3} dipole-dipole interaction (originating from the conventional 1/r Coulomb interaction between electrons), the found thermal conductivity κ scales with temperature as κ ∝ T^{3} and κ ∝ T^{4/3}, respectively. Our results are of relevance also to other realizations of random spin Hamiltonians with long-range interactions. We also determine the delocalization threshold for a finite-size system (“quantum dot” with localized single-particle states and power-law interaction). In this context, I will discuss a connection of this problem with Anderson localization on random regular graphs.  

It is well accepted that a measurement of an ion's mobility through an inert gas can be compared with calculated mobilities for trial geometries to obtain insight about the abundances and overall shapes of specific ions. In these studies, energy can be added to induce structural transitions and follow changes in the abundances of different conformations that are favored before and after activation. Here, we extend this idea as a means of following structural transitions in solution, examining the well-studied model systems: bradykinin and polyproline. Our approach is to vary the solution composition from which ions are electrosprayed. Overall, we conclude that in some cases it appears that the solution phase structures are more or less preserved in the gas phase; in other cases, new structures are favored in the gas phase –but, these can be mapped back in order to obtain insight about populations of states that were favored in solution. In the case of bradykinin, we find evidence for ~10 different solution phase states that vary in abundance as the solution composition is changed. These populations largely arise from variations in the cis- and trans-configurations of three proline residues in the nonapeptide sequence. The polyproline system provides a chance to study such transitions in detail. When in relatively non-polar solvents such as propanol, polyproline forms a compact type PPI helix; when placed in water the polymer undergoes a series of cis-trans interconversions to produce a type PPII helix, in which water molecules intercalate along the peptide backbone, stabilizing a much more extended structure in solution. We find that although the PPII helix collapses in the gas phase, IMS-MS techniques provide an ideal means of studying the step-by-step transitions that connect these different structures.

Bacteria are among the oldest and most abundant living species on Earth, and their activity influences the planet’s environmental dynamics in multiple ways. Bacteria often migrate en masse over large distances, moving in dense groups in a highly organized, collective fashion known as “swarming motility.” The flow dynamics of dense bacterial colonies can be very complex and, because of the interaction between the bacteria and the fluid, remarkably different from those predicted by conventional fluid models. In particular, turbulent swimming patterns often emerge, characterized by chaotic motions and the formation of vortices, even in situations where liquids should exhibit laminar flow. But these complex phenomena are difficult to characterize experimentally, and a predictive model that describes them has not emerged to date. Additional complications arise when the suspending medium is anisotropic exemplified by lyotropic liquid crystals. In my talk I will survey the most recent progress in experimental and theoretical studies of manipulation of bacterial swimming trajectories in liquid crystals^1,2 We also demonstrated that shear flow created by rotating magnetic particle leads to surprisingly rapid depletion of bacterial concentration near the particle³. Our observation highlights that the expulsion of bacteria is not caused by the centrifugal forces but rather a non-trivial interplay between shear induced alignment and locomotion of bacteria.

References

1. S. Zhou, A. Sokolov, O.D. Lavrentovich, and I.S. Aranson, PNAS 111, 1265 (2014)

2. A. Sokolov, S. Zhou, O.D. Lavrentovich, and I.S. Aranson, Phys. Rev. E 91, 013009 (2015)

3. A. Sokolov and I.S. Aranson, Nature Communications, 2016

Functionalization of ferroelectric materials emerged the growing interest in exploration of such nano-scale self-organized structures as polarization domains. For a long time it was believed that the regular domain patterns can appear only in ferromagnetic systems.

However, it was convincingly demonstrated over the last decade that the steady Landau-Kittel polarization domains do arise in the nano-scale strained films, superlattices nanoroads and nanodots.
Several recent results concerning their dynamic and static properties will be reviewed.
First, we consider the formation of domains in ultrathin films and superlattices of ferroelectric oxides and report a comprehensive description of their electrodynamic response. In particular we demonstrate that the field-induced domain wall motion can be in the origin of the recently observed fascinating “negative capacitance effect” effect.
Then, we show that the frequency-dependent permittivity of such system reveals the collective resonance mode in the near-terahertz frequency range that can be excited and detected by the methods of reflection absorption spectrometry. This finding can provide an important impact for the development of the ferroelectric-based terahertz optics and plasmonics.
Finally we consider the unconventional multibit polarization switching in ferroelectric nanodots, with formation of the intermediate domain states or/and chiral polarization skyrmions that can be useful for design of the memory-storage devices.

The field of quantum information science aims to leverage the properties of quantum mechanics to realize new approaches for computing and communication.  In this presentation, I will review the basic building blocks of a classical optical communication system.  From there, I will describe a communication system in which the photonic wavepackets have less than one photon - and hence entering the quantum regime.  Surprisingly,  the number of bits that can be encoded on a single quanta is nearly limitless.  I will show how this property, along with the indistinguishability of certain quantum states, can be used to realize a quantum communication system for sharing a secret key between two parties.  This system is secure against an attack by an eavesdropper due to the fundamental properties of quantum mechanics.

Plasmonics has grown in recent years into a well established area of research with a great potential. Our interest to this area roots in mechanisms involved in plasmonic resonance responses and implied pretty narrow spatial dimension range between 1nm and 25nm. We entertain an idea that the very existence of surface plasmons with sizes in that range suggest a possibility of a new fundamental scale such as the size 5nm of a free electron in our neoclassical theory. This theory features a new spatial scale - the size a_e of a free electron. This scale is special to our theory and does not appear in either classical EM theory nor in the quantum mechanics where electron is always a point-like object. Our current assessed value for this scale is a_e≈100a_B where a_B is the Bohr radius, and consequently a_e≈5 nm. In our theory any elementary charge is a distributed in space quantity. Its size is understood as the localization radius which can vary depending on the situation. For instance, if an electron is bound to a proton in the Hydrogen atom then its the size of is approximately 1 Bohr radius, that is a_B≈0.05 nm, and when the electron is free its size is

a_e≈100a_B≈5 nm.

Interestingly, the upper bound 25 nm is the skin depth and that implies that a nanosystem of size smaller than 25nm is transparent to the external field. The same transparency should hold for a nanostructured surface indicating such a surface is better for nearly ideal field electron emission. There is an experimental evidence showing that the highest current densities were obtained for nanotips with sizes ∼1nm yet another important fact supporting a possibility of a fundamental nonoscale.

1. The role of XPS and resonant XPS in understanding materials as diverse as graphene, chemotherapy drugs [1] and intermediate band photovoltaics.

2. ARPES as a tool for understanding surface and non-surface localised confined states for organic electronics and quantum computation applications [2].

3. Spin-resolved ARPES for a fundamental understanding of topological insulators and spin-orbit coupled systems [3,4].

4. Engineering novel confined systems using photoemission microscopy, and 

5. Creating and understanding novel phonon interactions through a combination of ARPES, many-body simulations and transport measurements [5,6].

Geographic tongue (GT) is a medical condition affecting approximately 2% of the population, whereby the papillae (i.e., tiny hair-like protrusions) covering the upper part of the tongue are lost due to a slowly expanding inflammation. The resultant dynamical appearance of the tongue has striking similarities with well known out-of-equilibrium phenomena observed in excitable media, such as forest fires, cardiac dynamics and chemically driven reaction-diffusion systems. Here we explore the dynamics associated with GT from a dynamical systems perspective, utilizing cellular automata simulations. Our results shed light on the evolution of the inflammation and suggest a practical way to classify the severity of the condition, based on the characteristic patterns observed in GT patients.

ההרצאה תועבר בשני חלקים בנות 45 דקות כל אחת עם הפסקה של כ- 10 דקות ביניהן.

We study relaxation in a one-dimensional two-mass mixture of hard-core particles. A special attention is payed to the region of light-to-heavy mass ratios around m/M = sqrt(5)-2 = 0.236... . At this mass ratio, each heavy-light-heavy subsystem constitutes a little known non-equal-mass generalization of the Newton Cradle, and an anomalous slow-down of relaxation is expected as a result. We further list and classify all other instances of integrability in the one-dimensional three-body hard-core systems; there, integrability is especially prominent at the quantum level, leading to the famous "scattering without diffraction" phenomenon. The principal experimental application of our results is the two-specie mixtures in optical lattices; there, the effective masses---that can be controlled at will---are assumed to replace the real ones.

A mathematical framework that uses rigged Hilbert spaces to unify the standard formalisms of classical mechanics, relativity and quantum theory will be introduced. In the framework states of a classical particle are identified with Dirac delta functions. The classical space (or space-time)  is "made" of these functions and is a submanifold in a Hilbert space of quantum states of the particle. The resulting embedding of the classical space into the space of states is highly non-trivial and accounts for numerous deep relations between classical and quantum physics and relativity. One of the most striking results is the proof that the normal probability distribution of position of a macroscopic particle (equivalently, position of the corresponding delta state within the classical space submanifold) yields the Born rule for probability of transitions between arbitrary quantum states.

We describe work a NIST to develop precision instruments based on atomic spectroscopy, advanced semiconductor lasers and micro-electro-mechanical systems (MEMS). These millimeter-scale instruments achieve useful levels of stability or sensitivity but with reduced power consumption and potentially reduced manufacturing cost compared to their larger counterparts. Physics packages for atomic frequency references with fractional frequency stabilities in the range of 10^-11 over one hour have been demonstrated. Using similar device designs and processing, magnetometers with sensitivities below 10 fT/ÖHz have been demonstrated, making them competitive with commercial SQUID-based sensors without the need for cryogenic cooling. The design, fabrication and performance of these instruments will be described, as well as a number of applications to which the devices are well-suited. 

Note: Dr. Kitching will give a more specialized seminar, later on the same day at Resnick bulg.

The search problem played an important role in the development of the field of operations research in the 1940's.  We study the search problem under a variety of circumstances where different search patterns are available and can be alternated and the search time is fixed.

The flux qubit is often considered as a major design for the future of quantum integrated circuits and its properties have triggered intense interest in the last decade [1-2]. This superconducting circuit behaves as a two-level system, each level being characterized by the direction of a macroscopic permanent current flowing in the loop of the qubit.  The permanent current, typically of the order of several hundreds of nAs, generates a large magnetic dipole, which offers interesting prospects for hybrid quantum circuits [3]. However, the flux qubit suffers from limited and irreproducible lifetimes which prevent these potential applications. Recently, a novel architecture where qubits are placed in a three dimensional cavity was introduced for transmon qubit [4]. It was shown that coherence properties can be greatly improved.

I will present the first measurements of flux qubits in a three dimensional cavity and show that they can reach long and more reproducible T1. The qubits were fabricated on a sapphire substrate and were measured by coupling them inductively to an on-chip superconducting resonator embedded in a three dimensional copper cavity. All the measured flux qubits exhibit an intrinsic T1 comprised between 5 and 13 us.

These long and reproducible depolarization times are a key element to reach the strong coupling limit with a single spin as suggested in [3]. I will describe our current experimental efforts towards this long term objective.

[1] I. Chiorescu et al., Science, 299, 5614 (2003).

[2] J. Bylander et al., Nature Physics, 7, 565 (2011).

[3] D. Marcos et al., PRL , 105, 210501 (2010).

[4] H. Paik et al., Phys. Rev. Lett., 107, 240501 (2011).

This talk will describe quantitative analyses of particle tracking data for 
systems with cytoskeletally associated motors to better understand the 
motions contributing to intracellular transport and, more generally, means 
for characterizing systems far from equilibrium. We exploit stochastic properties 
of single particle trajectories to establish "sanity" tests for experimentally 
collected data. Development of simple order parameters for 2d motion 
and studying its behavior for different models of transport in disordered media 
permits us to distinguish between random motion inside dense physical systems 
and molecular motors mediated transport. Furthermore, the stochastic properties 
of insulin-containg vesicles transport in beta cells provide a simple mechanism for how 
important beta cell functioning (biphasic insulin secretion) can arise.

The human organism is an integrated complex network of interconnected and interacting organ  systems, where the behavior of one system may affect the dynamics of all other systems.  Due to these interactions, failure of one system can trigger a breakdown of the entire network.  We introduce a systematic method to identify a network of interactions between diverse physiologic  systems, to quantify the hierarchical structure and dynamics of this network, and to track its  evolution under different physiologic states. We find a robust relation between network structure  and physiologic states: every state is characterized by specific network topology, node connectivity  and links strength -- a behavior we consistently observe across individual subjects. Further, we find  that transitions from one physiologic state to another trigger a markedly fast reorganization of physiologic
interactions on time scales of just a few minutes, indicating high network flexibility in response to  perturbations. Surprisingly, this reorganization occurs simultaneously and globally in the entire  network as well as at the level of individual network nodes, while preserving a hierarchical order in  the strength of network links. In the context of sleep-stage transitions, we demonstrate that  network connectivity and overall strength of physiologic interactions are significantly higher during  wake and light sleep, intermediate during rapid eye movement (REM) sleep and much lower during  deep sleep -- a stable stratification pattern which indicates that physiologic systems are highly and
strongly connected during light sleep, and practically disconnected during deep sleep. Such
pronounced difference in network organization during light and deep sleep is in contrast to the similarity  in the output dynamics of individual physiologic systems during these sleep stages. Our findings  highlight the need of an integrated network approach to understand physiologic function, since the framework we develop provides new information which can not be obtained by studying individual systems.