# past events

## Distinguished Lecturer Series

The Quantum Way of Doing Computations

### Speaker:

Prof. Rainer Blatt (Institute for Experimental Physics, University of Innsbruck and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Innsbruck, Austria)

### Place:

MPL / large seminar room (*435)

### Abstract:

Since the mid nineties of the 20th century it became apparent that one of the centuries’ most important technological inventions, computers in general and many of their applications could possibly be further enormously enhanced by using operations based on quantum physics. This is timely since the classical roadmaps for the development of computational devices, commonly known as Moore’s law, will cease to be applicable within the next decade due to the ever smaller sizes of the electronic components that soon will enter the quantum physics realm. Computations, whether they happen in our heads or with any computational device, always rely on real physical processes, which are data input, data representation in a memory, data manipulation using algorithms and finally, the data output. Building a quantum computer then requires the implementation of quantum bits (qubits) as storage sites for quantum information, quantum registers and quantum gates for data handling and processing and the development of quantum algorithms.

In this talk, the basic functional principle of a quantum computer will be reviewed. It will be shown how strings of trapped ions can be used to build a quantum information processor and how basic computations can be performed using quantum techniques. In particular, the quantum way of doing computations will be illustrated by analog and digital quantum simulations and the basic scheme for quantum error correction will be introduced and discussed. Scaling-up the ion-trap quantum computer can be achieved with interfaces for ion-photon entanglement based on high-finesse optical cavities and cavity-QED protocols, which will be exemplified by recent experimental results.

## Imperial-MPL µ-Symposium

**Organisation:** MPL / Russell Division

**Place:** Large seminar room, MPL

**Programme:**

10:30 – 10:45

**Opening remarks**

*Philip Russell*

10:45 – 11:25

**Prospects for measuring few to sub-fs dynamics in matter**

*Jon Marangos, Imperial College London*

In this seminar I will look at various approaches that are using new types of light source or new methodologies to extract few to sub-femtosecond dynamics in molecules triggered by sudden photoionization. I will begin by reviewing some of the motivations for pursuing these questions and why it is important to find techniques that offer, not only extreme time resolution, but that are also suitable for application to problems in molecules larger than those hitherto investigated by attosecond methods. I will then review the viability and status of methods such as HHG spectroscopy and attosecond pump-probe spectroscopy. The advantages of various transient absorption related techniques using HHG and XFEL sources will be discussed. I will conclude by summarising the highest priority light source requirements for this research.

11:25 – 11:40

**Fabrication of photonic crystal fibres**

*Michael Frosz*

The "stack-and-draw" procedure is used to fabricate a wide variety of photonic crystal fibres, ranging from simple capillary fibres and solid-core fibres to the more advanced hollow-core fibres and exotic nanostructures. A short overview is given of possibilities and limitations.

11:40 – 11:55

**Temporal compression and UV generation in gas-filled hollow-core kagome PCF**

*Ka Fai Mak*

By operating in the anomalous dispersion regime solitonic propagation dynamics can be realized. This allows the temporal self-compression of uJ-level, 25 fs pulses down to few-optical-cycles, corresponding to an octave-spanning spectrum. By utilizing a related phenomenon known as dispersive-wave emission, a significant amount of energy can be converted into the deep- and vacuum-UV in a tunable manner.

11:55 – 12:10

**Spatio-temporal characterisation of single cycle laser pulses**

*Tobias Witting*,* Imperial College London*

We discuss advanced methods for the spatio-temporal characterisation of laser pulses in the single cycle regime. The SEA-F-SPIDER technique is given special emphasis.

12:10 – 12:25 Break

12:25 – 13:05

**New light sources for attosecond science**

*John Tisch*, *Imperial College London*

This talk will describe recent efforts at Imperial College London to develop new sources for ultrafast and attosecond science, including attosecond pulses in the challenging VUV range and spectrally isolated attosecond pulses from resonantly enhanced high harmonic generation (HHG) in plasma plumes. Underpinning these sources is the process of few-cycle pulse generation by hollow fibre pulse compression. Recent results on the energy scaling and carrier-envelope phase stability of hollow fibre pulse compression will also be discussed.

13:05 – 13:20

**Ultrafast Stimulated Raman Scattering in hollow core photonic crystal fiber**

*Federico Belli*

We investigate the propagation of ultra-short pulses in kagomé HC-PCF filled with hydrogen. By means of the soliton self-compression mechanism we impulsively excite large rotational and vibrational Raman coherences in the system, with time scales of 57 and 8 fs respectively.

13:20 – 13:35

**Generation and characterization of few-cycle phase-controlled 1.7 ****?****m pulses**

*Dane Austin, Imperial College London*

We generate carrier-envelope phase stabilised, 1.6 cycle pulses with a wavelength of 1.7 micron and 650 microjoule energy using spectral broadening in a hollow fibre. Characterisation with SEA-F-SPIDER demonstrates, for the first time, the spatio-temporal quality of the source, whilst simulations elucidate the role of self-steepening, ionization and higher-order Kerr nonlinearities.

13:35 – 13:50

**Pulse propagation instabilities in gas-filled kagome PCFs**

*Francesco Tani*

With the emergence of Kagome PCFs in nonlinear optics, investigation of both conventional nonlinear processes, within a range of energies before inaccessible, and novel nonlinear dynamics became accessible. In these conditions instabilities may play a strong role in the pulse evolution dynamics. Investigation of these is here discussed both experimentally and numerically.

## IMPRS get-together

**Place:** ECAP (Erwin-Rommel-Straße 1) in seminar room 307

**Organisation:** Mykhaylo Filipenko (ECAP / Radiation Detection Group)

**Talk:** ** **Discovering dark matter at the LHC

**Speaker:** Dr. Björn Penning (FermiLab/CERN)

**Abstract:**

Dark Matter (DM) is a long standing puzzle in high energy physics and goal of a diverse research program. Recent tantalizing excesses in underground DM detectors increase emphasis on low masses. At the LHC we have great sensitivity in this region and the ability to test parts of the parameter space that can not be accessed otherwise. I am presenting first searches using events containing jets, photons and W-bosons to search for DM pair production. By utilizing an effective field approach we achieve complementary and comparable sensitivities to direct and indirect dark matter searches in regions where recent excesses have been reported. Furthermore new signatures involving heavy quarks (b-quarks or top-quarks), theoretical developments and comparisons to underground and indirect searches will be discussed.

## IMPRS get-together

**Place:** MPL (large seminar room *429/435, 4th floor)

**Organisation:** Georg Epple (MPL / Russell Division)

**Talk:** ** **A single Rydberg electron in a Bose-Einstein Condensate

**Speaker:** Prof. Tilman Pfau (5. Physikalisches Institut, Universität Stuttgart)

**Abstract:**

Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1], which was first introduced to explain their findings.

At ultracold temperatures and Rydberg states with medium size principle quantum numbers *n*, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2]. These molecules can show a linear Stark effect corresponding to a permanent dipole moment [3], which if seen from a standpoint of traditional molecular physics is surprising.

At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers *n* in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, leading again to a density dependent energy shift of the Rydberg state. This allows, together with the strong van-der-Waals blockade, to excite only one single Rydberg atom in a condensate. We excite a Rydberg electron with *n *upto 202 in the BEC, the size of which becomes comparable to the size of the BEC. We study their life time in the BEC and the coupling between the electron and phonons in the BEC [3]. So the single electron that we prepare in a quantum gas allows nicely to study the transition from two- to few- to many-body interaction.

As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number and the imaging of the Rydberg electron's wavefunction by its impact onto the surrounding ultracold cloud seem to be within reach.

[1] E. Amaldi and E. Segre, Nature **133**, 141 (1934)

[2] C. H. Greene, et al. PRL. **85,** 2458 (2000); V. Bendkowsky et al., Nature **458**, 1005 (2009)

[3] W. Li, et al., Science **334**, 1110 (2011)

[4] J . B. Balewski, A. T. Krupp, A. Gaj, D. Peter, H. P. Büchler, R. Löw, S. Hofferberth, T. Pfau, Nature **502**, 664 (2013)

## Distinguished Lecturer Series

Juggling with photons and raising Schrödinger cats of light in a cavity

### Speaker:

Prof Serge Haroche (Ecole Normale Supérieure and Collège de France, Paris)

### Place:

MPL / large seminar room (*435)

### Abstract:

The founders of quantum theory assumed in “thought experiments” that they were manipulating isolated quantum systems obeying the counterintuitive laws which they had just discovered. Technological advances have recently turned these virtual experiments into real ones by making possible the actual control of isolated quantum particles. Many laboratories are realizing such experiments, in a research field at the frontier between physics and information science. Fundamentally, these studies explore the transition between the microscopic world ruled by quantum laws and our macroscopic environment which appears “classical”. Practically, physicists hope that these experiments will result in new technologies exploiting the strange quantum logic to compute, communicate or measure physical quantities better than what was previously conceivable. In Paris, we perform such experiments by juggling with photons trapped between superconducting mirrors. We count these photons in a non-destructive way and we prepare states of the quantum field reminiscent of the famous cat which Schrödinger imagined to be suspended between life and death. I will give a simple description of these studies, compare them to similar ones performed on other systems and guess about possible applications.

## MPL Distinguished Lecturer Series

Spin Hall Effect for Electrons and Photons

### Speaker:

Prof Henry van Driel (Department of Physics, University of Toronto, Canada)

### Place:

MPL / large seminar room (*435)

### Abstract:

Hall effects have provided important insights into the electronic properties of solids since 1879. In the spin Hall effect, a pure electrical current produces a pure transverse spin current while the inverse spin Hall effect describes the converse situation. I describe the generation and detection of both these spin Hall effects in GaAs using coherence control optical techniques. An analogous spin Hall effect for light is also demonstrated wherein a linearly polarized light beam incident on, e.g., GaAs is split transversely into its spin, that is, right and left circularly polarized, components. Both electron and photon spin Hall effects involve nanometer displacements of charges or photons but can nonetheless be resolved optically.

## IMPRS get-together

**Place:** MPL (large seminar room *429/435, 4th floor)

**Organisation:** Michael Schmidberger (MPL / Russell Division)

**Talk:** Temporal cavity solitons: ultraweak acoustic interactions, phase-modulation trapping, dispersive waves, and Kerr frequency combs

**Speaker:** Prof. Stéphane Coen (University of Auckland, New Zealand)

**Abstract:**

In this talk, I will present recent experimental results obtained with temporal cavity solitons in a passive fibre ring resonator. I will first introduce the concept and properties of temporal cavity solitons, which are persistent pulses of coherently-driven passive nonlinear resonators. I will then show how these pulses interact very weakly over extremely long ranges through transverse acoustic waves. In the most extreme case, we observe pairs of temporal cavity solitons shifting their relative position by only half an attosecond per round-trip of the 100 m-long resonator and it takes an effective propagation distance of the order of an astronomical unit for the interaction to fully develop. In the second part, I will show how a synchronous phase-modulation of the coherent-driving beam can overcome the acoustic interaction by trapping the temporal cavity solitons in potential wells. We will also demonstrate that manipulation of the trapping potential gives us the ability to manipulate individual cavity solitons in a train, leading to selective temporal shifting of solitons, forced merging of neighboring solitons, and selective erasure. Finally, I will briefly present measurements in which temporal cavity solitons in low dispersion cavities are shown to radiate dispersive waves and how this connects to the generation of frequency combs in microresonators.

## IMPRS get-together

**Place:** MPL (large seminar room *429/435, 4th floor)

**Organisation:** Federico Belli (MPL / Russell Division)

**Talk:** Optical shock waves in disordered thermal lensing

**Speaker:** Dr. Silvia Gentilini (Sapienza University, Rome, Italy)

**Abstract:**

Shock waves (SWs) are an ubiquitous phenomenon observable in different areas, ranging from water waves to gas dynamics and from plasma to supernova explosions. They also appear in non-linear optics, where can occur in systems described by universal models, such as the non-linear Schrödinger equation, when the hydrodynamical approximation holds true. In dispersion dominated systems, as are the optical systems, the shock phenomenon is regularized by the emergence of fast oscillations, the so called *undular bores*. Given the coherent nature of the *undular bores*, disorder is expected to strongly affect the occurrence of dispersive shock waves (DSWs). We present our experimental investigations on the occurrence of DSWs in disordered, non-linear media. The experimental set-up allowed the visualization of a propagating Gaussian laser beam in thermal defocusing media in presence of controllable amount of disorder. We show that, by increasing the strength of non-linearity, the shock formation is enhanced, while, on the other hand, random light scattering hampers and eventually inhibits the wave breaking phenomenon. We characterized the thermal non-linearity of the system and we quantified the effect of disorder by measuring the first shock-no shock phase diagram in terms of non-linearity strength and disorder degree.

## MPL Distinguished Lecturer Series

From Extreme Nonlinear Optics to Ultrafast Atomic Physics

### Speaker:

Prof Anne L'Huillier (Department of Physics, Lund University, Sweden)

### Place:

MPL / large seminar room (*435)

### Abstract:

The interaction of atoms with intense laser radiation leads to the generation of high-order harmonics of the laser field. In the time domain, this corresponds to a train of pulses in the extreme ultraviolet range and with attosecond duration. This presentation will introduce the physics of high-order harmonnic generation and attosecond pulses and describe recent developments including multicolor schemes or noncollinear geometries.

After the first decade where attosecond pulses were characterized, analyzed and used in –mostly– demonstration experiments, we begin to perform experiments where these pulses allow us to explore new physics. We will describe some of these applications, and in particular recent results concerning single and double photoioniza¬tion dynamics.

## MPL Distinguished Lecturer Series

### Speaker:

Prof Jérôme Faist (Quantum Optoelectronics Group, ETH Zurich, Switzerland)

### Place:

MPL / large seminar room (*435)

### Abstract:

The quantum cascade laser has demonstrated operation over an extremely wide wavelength range extending from the mid-infrared at 2.9?m to the Terahertz at 360?m. One very important feature of this device is its ability to provide gain over a very broad wavelength range. Recently, we have shown that such broadband devices, when operated in continuous wave, emit as a coherent optical comb1 in which the phase relation between the comb modes corresponds approximately to a FM modulated laser. These new comb lasers enables the fabrication of a dual comb spectrometer based on a quantum cascade laser that offers a broadband, all solid-state spectrometer with no moving parts and a ultrafast acquisition time. We discuss also the extension of these ideas to the THz.

1. A. Hugi, G. Villares, S. Blaser, H. C. Liu and J. Faist, Nature 492 (7428), 229-233 (2012).