# Blog

## 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.

## Charles W. Clark, Twisting the neutron wavefunction

Last Wednesday (February 8th 2016) we were lucky enough to have Charles W. Clark from the Joint Quantum Institute at NIST as an invited speaker to give a talk about “Twisting the neutron wavefunction”. The talk focused on the importance of fundamental wave theory and how the wave-particle duality of neutrons studied with interferometers (particularly on a Mach-Zehnder configuration) can provide great insight about basic optical principles being applied to matter waves, such as the addition of angular momentum to neutron wavefunctions.

The abstract for his talk can be found below, and he kindly agreed to provide a copy of his presentation slides which you can download here.

We will be updating our blog with subsequent talks from visiting speakers that may visit UCL, so stay tuned!

Abstract:

Twisting the neutron wavefunction

Charles W. Clark, Joint Quantum Institute, University of Maryland , USA

Wave motions in nature were known to the ancients, and their mathematical expression in physics today is essentially the same as that first provided by d’Alembert and Euler in the mid-18th century. Yet it was only in the early 1990s that physicists managed to control a basic property of light waves: their capability of swirling around their own axis of propagation. During the past decade such techniques of control have also been developed for quantum particles: atoms, electrons and neutrons. I will present a simple description of these phenomena, emphasizing the most basic aspects of wave and quantum particle motion. Neutron interferometry offers a poignant perspective on wave- particle duality: at the time one neutron is detected, the next neutron has not yet even been born. Here, indeed, each neutron “then only interferes with itself.” Yet, using macroscopically-machined objects, we are able to twist neutron deBroglie waves with sub-nanometer wavelengths.

## Rydberg Ps electrostatically guided in curved quadrupole

The latest efforts of our research at UCL have been focused on manipulating Positronium (Ps) atoms in highly-excited principal quantum number n states (Rydberg states) [PRL. 114, 173001]. In one of our latest works we showed how we can exploit the large electric dipole moment of low-field-seeking Rydberg states (those states which have positive Stark shift) to confine them in a quadrupole “guide” [PRL 117, 073202].

As a direct follow-up to that study, we devised a modified version of a quadrupole guide with a 45° bend that would allow us to perform velocity selection on the atoms being guided by tuning the efficiency with which the Rydberg Ps atoms are transmitted through the bend, in addition, in our previous set-up we experienced technical difficulties since the detection scheme was in-line with our positron beam, so having a curved guide would also be beneficial for that reason.

The schematic figure above depicts our current experimental setup, which we have used to guide Rydber Ps atoms around a 45° bend into a region off-axis with our positron beam. We have not yet implemented velocity selection, but we have clear evidence that we can efficiently guide Ps atoms in this configuration.

The left panel in the figure above shows the time of flight (TOF) distribution of n = 14 atoms excited to high-field seeking states (as measured by the detectors at the end of the curved guide, i.e. “LYSO C” and “NaI”), and a background wavelength with is off-resonant with any transition, essentially acting like a “laser on” and “laser off” measurement. The right panel shows the background-subtracted trigger rate for this measurement (“laser on” – “laser off”), which shows clear evidence of atoms with a TOF arrival time of ~8 $\mu \mathrm{s}$.

In addition to this being a stepping stone to demonstrate velocity selection due to the acceptance of the curved section of the guide, we may also improve this set-up into eventually developing a ring-like stark decelerator, and other Ps atom optics.

## Production and time-of-flight measurements of high Rydberg states of Positronium

One of our recent studies focused on measuring the lifetimes of Rydberg states of Positronium (Ps) [PRA. 93, 062513]. However, some of the limitations that prevented us from measuring lifetimes of states with higher principal quantum number (n), is the fact that such states can be easily ionised by the electric fields generated by the electrodes in our laser-excitation region (these electrodes are normally required to achieve an excitation electric field of nominally ~ 0 V/cm).

We have recently implemented a simple scheme to overcome this complication, whereby we make use of a high-voltage switch to turn discharge the electrodes in the interaction region after the laser excitation has taken place.

The figure shown above show the Background-subtracted spectra (the SSPALS detector trace is recorded with a background and resonant wavelength, they are then normalised and subtracted from each other) for n = 18 and n = 19. It is clear from the “Switch Off” that when the high voltage switched is not utilised (and the voltages to all electrodes are always on), that most of the annihilations happen at early times, especially around ~100ns, this is the time it takes for the atoms to travel out of the low-field region, and become field-ionised by the DC voltage on the electrodes.

On the other hand, the “Switch On” curves show that both n = 18 and 19 have many more delayed events (after ~ 400 ns) due to Rydberg Ps being able to travel for much longer distances before annihilating when the switch is used to discharge the electrode biases.

The figure above shows  data taken by a detector set up for single-gamma-ray detection, approximately 12 cm away from the Ps production target, on the same experiment as described for the previous figure. It is clear from this data that the time-of-flight (TOF) to this detector is ~2 $\mu \mathrm{s}$ However, in this case it is clear that only the n = 19 state benefited from having the “switch on”, indicating that is the smallest-n state that this scheme is necessary for our current electric-field configuration.

Comparing the SSPALS and TOF figures it can be seen that even though the n = 18 SSPALS signal was changed drastically, the n = 18 TOF distribution remained the same, this is a clear example of how changes in the SSPALS spectrum discussed in the first figure are indicative of changes in atom distributions close to the Ps production region, but are not necessarily correlated to TOF distributions measured at different positions across the Ps flight paths

These methods will eventually lead to more accurate measurement of the lifetimes of higher n-states of Ps, and the possibility of using those states with higher electric dipole moments for future atom-optics experiments, such as Ps electrostatic lenses and Stark decelerators.