## Taming polar molecules for quantum experiments

On Wednesday 11th of October our weekly AMOPP seminar was given by Dr. Martin Zeppenfeld from the Rempe Group at the Max Planck institute for quantum optics in Garching, Germany. His talk focused on their experiments involving the manipulation of cold polar molecules which Dr. Zeppenfeld leads. You can visit their website to find out more about their research, and the abstract for his talk is given below.

Abstract:

Polar molecules offer fascinating opportunities for quantum experiments at cold and ultracold temperatures. For example, chemistry at low temperatures features new possibilities such as controlling chemical reactions via electric and magnetic fields or observing reactions based on tunneling through a reaction barrier. Precision measurements on molecules provide insight into fundamental physics, allowing investigation of physics beyond the standard model. Attaining sufficient control over molecules provides opportunities for quantum simulations and quantum information processing.

In my talk I will present two aspects of our work on polar molecules. First, I will present our toolbox of techniques to produce molecule ensembles at very low temperatures. This includes centrifuge deceleration of cryogenic-buffer-gas cooled molecular beams as well as optoelectrical Sisyphus cooling of formaldehyde to sub-millikelvin temperatures. Second, I will present our progress towards quantum experiments coupling polar molecules to Rydberg atoms. As a first step, we have investigated electric field controlled collisions between polar molecules.

## Optical spectroscopy for nuclear and atomic science at JYFL, Finland

On Wednesday 4th of October, we had the privilege of having Professor Iain D. Moore from the University of Jyväskylä as an invited speaker for one of our AMOPP seminars. The seminar focused mainly on the work they have been doing at the various accelerator facilities at JYFL and had a good mix of nuclear and atomic physics content.

Professor Iain D. Moore kindly agreed to provide a copy of his presentation slides which you can download here.

Abstract:

# Optical spectroscopy for nuclear and atomic science at JYFL, Finland

## Iain D. Moore, University of Jyväskylä

High-resolution optical measurements of the atomic level structure readily yield fundamental and model-independent data on nuclear ground and isomeric states, namely changes in the size and shape of the nucleus, as well as the nuclear spin and electromagnetic moments [1]. Laser spectroscopy combined with on-line isotope separators and novel ion manipulation techniques provides the only mechanism for such studies in exotic nuclear systems.

Internationally, there are a myriad of tools in use however these are traditionally variants of two main workhorses in the field – collinear laser spectroscopy and resonance ionization spectroscopy. Following a short overview of the Accelerator Laboratory at the University of Jyväskylä, I will briefly present both techniques and their use in accessing the heavy element region of the nuclear landscape which exhibits rather scarce information from optical studies. This reflects a combination of the difficulty in producing such elements (low production cross sections) and a lack of stable isotopes (thus few optical transitions available in literature). Indeed, this past year has seen a number of exciting developments including optical studies of exotic atoms produced at the level of one atom-at-a-time [2], and high-resolution spectroscopy in supersonic gas expansions [3].

Recently, we have initiated a new program on the actinide elements in collaboration with the University of Mainz. I will summarize the current status of the work which includes collinear laser spectroscopy on Pu, the heaviest element attempted with this particular technique [4]. Our focus has recently turned to the study of the lowest-lying isomeric state in the nuclear chart, 229Th.  Almost 40 years of research has been invested into efforts to observe the isomeric transition which, if found, may be directly accessed by lasers. In 2016, the community was given a tremendous boost with the unambiguous identification of the state by a group in Munich, providing a stepping stone towards a future realization of a “nuclear clock” [5].

[1] P. Campbell, I.D. Moore and M.R. Pearson, Progress in Part. and Nucl. Phys. 86 (2016) 127.

[2] M. Laataioui et al., Nature 538 (2016) 495.

[3] R. Ferrer et al., Nature Communications 8 (2017) 14520.

[4] A. Voss et al., Phys. Rev. A 95 (2017) 032506.

[5] L. von der Wense et al., Nature 533 (2016) 47.

## 12th International Workshop on Positron and Positronium Chemistry

Our research group was recently represented by Dr David Cassidy at the 12th International Workshop on Positron and Positronium Chemistry (PPC12), which took place between the 28th of August and 1st of September in in Ludblin, Poland. The main focus of this meeting was the interaction of positrons and positronium with other materials and atoms, including polymers, soft matter, surface states and more.

David presented our recent advances in producing a beam of Rydberg positronium atoms (PRL 117, 073202 & PRA 95, 053409) and the prospects of using such techniques to form the yet-unobserved positron-atom bound states (PRA 93, 052712).

You can have a look at the abstracts in the conference website. We are grateful to the organizers for this opportunity and their hard work.

## Rydberg Positronium-electron scattering

In our last experiment, we showed how a curved quadrupole guide can be used to transport Rydberg Positronium (Ps*) around a 45 degree bend (blue path in picture below) into a region away from the positron beam line (yellow path below), which can then be implemented into a scattering region (PRA 95 053409). In addition, this new off-axis setup eliminated detection difficulties that were encountered in our previous straight guide experiments.

The scattering chamber setup is shown in the right hand side picture below. Guided Ps* atoms emerging from the end of the quadrupole are introduced into a scattering region where they collide with electrons that are thermoinically ejected (orange path) from a Molybdenum filament . This region is surrounded by gamma-ray detectors coupled to photomultiplier tubes (PMT) which record annihilation events inside the chamber. However, these PMTs in themselves are not sufficient to conclude what kind of interaction has taken place, we can only conclude that a Ps atom annihilated if an event is detected, but not if there were intermediate steps (such as charge exchange between the free electrons and the valence electron) before annihilation. To overcome this, we used a micro-channel plate (MCP) which has the benefit of being able to detect only certain charged particles (i.e. differentiating between electrons and positrons) depending on the electric fields applied in the detection region.

When the Ps*collides with free electrons a few different things can happen; the electron could break up the Ps* into electron and positron (ionisation: $e^{-} + Ps^{*} \rightarrow e^{-} + e^{+} + e^{-}$ ) or, the positron from the Ps* will be stolen away by the incoming free electron, resulting in a new short lived Ps which annihilates before hitting the MCP, thus giving a diminished signal (charge exchange: $e^{-} + Ps^{*} \rightarrow Ps + e^{-}$). Additionally, Positronium and an electron can form a negative Ps ion (Ps¯ ), which is the bound state of a Ps atom and an orbiting electron, this ions have been shown to be quite unstable, having a lifetime of <1ns.

Above you can see some experimental Time of Flight (TOF) data showing the possibility of having detected a charge exchange process. The blue curve on the left panel is a TOF signal of guided Positronium atoms after hitting the MCP detector. The Ps* atoms have a mean flight time of around 10$\mu$s. When we let the electron beam into the scattering region  we observe a suppression of the TOF spectra (green curve), this is due to the electric fields in front of the MCP detector, which should only allow positrons to be detected. The PMT signal for both cases (shown in the right panel) are the same indicating that this drop in MCP signal is in fact electron related, not as a result of less Positronium being guided. All that remains to do now is to determine the nature of this signal but nonetheless, it’s intriguing how readily Ps* interacts with electrons (maybe other particles too), especially when considering that a direct method of producing anti-Hydrogen is via charge exchange collisions between Ps* and antiproton ( $Ps^{*} + \overline{p} \rightarrow \overline{H}+ e^{-}$ ).

## Optimizing Positronium Rydberg manipulation

In recent experiments we showed how we successfully manipulated the trajectories of Rydberg Positronium atoms using electric field inside a curved quadrupole guide, which allows us to perform velocity selection on the Positronium atoms that enter the quadrupole field.

These curved guide experiments were performed by exciting Positronium to n = 14, which is a state that has a relatively low mean lifetime of ~8 $\mu$s, which is comparable to the flight time of our experiment, meaning that many of the n = 14 atoms undergo radiative decay before they can be detected. However, when judging what the most efficient n for guiding is, we need to have into consideration a few other factors. The lifetime of different n states goes up as $n^3$, but the field-ionisation rate goes up as ~$n^{-4}$, and the dipole moment as $n^2$. Combining all of these factors and implementing them into trajectory-simulations based on the electric fields generated by the curved guide, we were able to determine that the most efficient n for curved guiding is n = 14, as it can be seen from the graph below.

These simulations agree with our data and were instrumental in determining the optimum parameters of the experiment.

We are currently working on implementing this off-axis curved guide into a scattering experiment using helium ions, below you can see at the bottom of the image the new scattering chamber that has been coupled to the Positronium Rydberg beam.

## Eberhard Widman, In-beam hyperfine spectroscopy of (anti)hydrogen

On Wednesday 22nd of February, we had the pleasure of to host Professor Eberhard Widman as one of our weekly invited speakers for AMOPP seminars.  His research in the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration is heavily related to the kind of antimatter experiments that we do at UCL, except they deal with anti-hydrogen atoms which they produce using positrons and anti-protons from the Antiproton Decelerator (AD) at CERN. The main focus of his talk was the prospect of making measurements of the hyperfine structure of anti-hydrogen.

He kindly agreed to provide a copy of his slides, which you can download  here.

Abstract:

In-beam hyperfine spectroscopy of (anti)hydrogen

Prof. Dr. Eberhard Widmann, Director, Stefan Meyer Institute for Subatomic Physics, Austrian Academy of Sciences Boltzmanngasse 3, A-1090 Wien

The ground-state hyperfine structure (GS-HFS) of hydrogen is known from the hydrogen maser to relative precision of 10–12. It is of great interest to measure the same quantity for its antimatter counterpart, antihydrogen, to test the fundamental CPT symmetry, which states that all particles and antiparticles have exactly equal or exactly opposite properties. Since CPT is strictly conserved in the Standard Model of particle physics, a violation, if found, would point directly to theories behind this framework. The application of the maser technique requires the confinement of the atoms in a matter box for 1000 seconds and is currently not applicable to antihydrogen. Therefore, the ASACUSA collaboration at the Antiproton Decelerator of CERN has built a Rabi-type beam spectroscopy setup for a measurement of GS-HFS.

With the initial aim of characterizing the setup devised to measure the GS-HFS and to evaluate its potential, a beam of cold, polarized, monoatomic hydrogen was built and used together with the microwave cavity and sextupole magnet designed for the antihydrogen experiment. The (F,M)=(1,0) to (0,0) transition was measured to a precision of several ppb [1], more than a factor 10 better than in the previous measurement using a hydrogen beam. This result shows that the apparatus developed is capable of making a precise measurement of the GS-HFS of antihydrogen provided a beam of similar characteristics (velocity, polarization, quantum state) becomes available.

In a recent publication on the non-minimal Standard Model Extension (SME), describing possible violations of Lorentz and CPT invariance, Kostelecky and Vargas [2] conclude that the in-beam hyperfine measurements of hydrogen alone can be used to constrain certain coefficients of their model, which have never been measured before. The status and prospects of in-beam measurements of hydrogen and antihydrogen will be presented.

[1] M. Diermaier et al., arxiv : 1610.06392

[2] V.A. Kostelecky and A.J. Vargas, Physical Review D 92, 056002 (2015).

## Sougato Bose, Probing macroscopic quantum superpositions and the quantum nature of gravity through levitated objects

We often have internal speakers giving talks at the Atomic, Molecular Optical and Positron Physics (AMOPP) group about their cutting-edge research. Last wednesday (15th February 2017) Professor Sugato Bose presented some of his latest results and calculations based on experiments being performed by  Professor Peter Barker’s group on macroscopic quantum behaviour.

The talk had the broad interest of the department as you can see from the fully-occupied lecture hall above,  and Professor Sugato agreed to provide a copy of his slides, which you can download here, so you may too get an insight into this topic.

Abstract:

Probing macroscopic quantum superpositions and the quantum nature of gravity through levitated objects

Prof. Sougato Bose, Dept of Physics & Astronomy University College London

We will discuss theoretical proposals of how quantum superpositions of distinct centre of mass states of a nano-crystal may be created and probed purely by measuring a spin embedded in the object. The idea is to use a levitated diamond with an NV centre spin. Next we will also describe how to reach conditions whereby two such masses interacting purely through gravitational interaction can become entangled. Witnessing such an entanglement experimentally is equivalent to establishing the quantum nature of the gravitational field. Time permitting, we will discuss how the violation of macro-realism can be verified for a levitated nano-object in a loop-hole free manner simply by coarse grained position measurements.