# past events

## OPTICS BBQ

**Organisation:** hbar OMEGA OSA student chapter

**Place:** Barbecue area of the university's physics department (behind the tandem accelerator at Erwin Rommel Straße 1)

## IMPRS get-together

**Place:** large seminar room / MPL

**Organisation:** Daqing Wang (MPL / Sandoghdar Division)

**Talk: **Fundamentals and applications of Interferometric Scattering

**Speaker:** Prof. Leonardo Menezes (Universidade Federal de Pernambuco, Brazil)

**Abstract:**

In this talk, I'll present the principles of the interferometric scattering (iScat) technique, which allows performing microscopy and spectroscopy of various nanoscopic systems in the diffraction limited regime. Then I'll discuss applications of iScat in different situations: for detection of particles down to 5 nm, for studying single molecule absorption of light, for tracing the diffusion of a virus on a lipidic membrane and for 3D tracking of gold nanoparticles in a static electric potential.

## Distinguished Lecturer Series

Quantum Microwave Photonics

### Speaker:

Prof. Andreas Wallraff (ETH Zürich, Schweiz)

### Place:

MPL / large seminar room (*435)

### Abstract:

Using modern micro and nano-fabrication techniques combined with superconducting materials we realize quantum electronic circuits in which we create, store, and manipulate individual microwave photons. The strong interaction of photons with superconducting quantum two-level systems allows us to probe fundamental quantum effects of microwave radiation and also to develop components for applications in quantum technology. Previously we have realized on-demand single photon sources which we have characterized using correlation function measurements [1] and full quantum state tomography [2]. For this purpose we have developed efficient methods to separate the quantum signals of interest from the noise added by the linear amplifiers used for quadrature amplitude detection [3]. We now regularly employ superconducting parametric amplifiers [4] to perform nearly quantum limited detection of propagating electromagnetic fields. These enable us to probe the entanglement which we generate on demand between stationary qubits and microwave photons freely propagating down a transmission line [5]. Using two independent microwave single photon sources, we have recently performed Hong-Ou-Mandel experiments at microwave frequencies [6] and have probed the coherence of two-mode multi-photon states at the out-put of a beam-splitter. The non-local nature of such states may prove to be useful for distributing entanglement in future small-scale quantum networks.

[1] D. Bozyigit et al., Nat. Phys. 7, 154 (2011)

[2] C. Eichler et al., Phys. Rev. Lett. 106, 220503 (2011)

[3] C. Eichler et al., Phys. Rev. A 86, 032106 (2012)

[4] C. Eichler et al., Phys. Rev. Lett. 107, 113601 (2011)

[5] C. Eichler et al., Phys. Rev. Lett. 109, 240501 (2012)

[6] C. Lang et al., Nat. Phys. 9, 345ñ348 (2013)

## IMPRS get-together

**Place:** MPL (large seminar room)

**Organisation:** Eugene Kim (MPL / Vollmer Research Group)

**Talk: **Hybrid photonic-plasmonic whispering gallery mode sensors

**Speaker:** Dr. Matthew Foreman (Vollmer Research Group - Lab of Nanophotonics and Biosensing, MPL)

**Abstract:**

Optical resonator based biosensors are rapidly emerging as one of the most sensitive label free technologies, capable of fullling the requirements for next generation sensors. Whispering gallery mode (WGM) resonators, particularly, exhibit high sensitivity by virtue of their high quality (Q) resonances and large surface intensities. Commonly WGM biosensors utilize the reactive resonance shift produced by a protein or nanoparticle binding to the microcavity. This shift is proportional to the intensity jE(r0)j2 at the binding site r0 such that any mechanisms enhancing the local intensity while maintaining high Q factor can amplify the frequency shift. Exploiting plasmonic nanoantennae therefore represents a natural and powerful means by which to push the detection envelope beyond the current limits to the single molecule level. In this talk we discuss various theoretical aspects of WGM based sensors, including fundamental sensing principles, sensor optimisation and modelling of hybrid photonic-plasmonic WGM biosensors.

## IMPRS get-together

**Place:** lecture hall F (Staudtstraße 5, at the physics institute of the university of Erlangen)

**Organisation:** Roland Lauter (FAU / Institute for Theoretical Physics II)

**Talk: **Optomechanical Metamaterials: Dirac polaritons, Gauge fields, and Instabilities

**Speaker:** Vittorio Peano (FAU)

**Abstract:**

Freestanding photonic crystals can be used to trap both light and mechanical vibrations. These "optomechanical crystal" structures have already been experimentally demonstrated to yield strong coupling between a photon mode and a phonon mode, co-localized at a single defect site. Future devices may feature a regular superlattice of such defects, turning them into "optomechanical arrays". In this letter we predict that tailoring the optomechanical band structure of such arrays can be used to implement Dirac physics of photons and phonons, to create a photonic gauge field via mechanical vibrations, and to observe a novel optomechanical instability.

## 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.