For this weeks AMOPP seminar, we welcomed Dr. Florian Mintert from Imperial College London who talked about “Optimal quantum control with poor statistics”. He argued the case for Bayesian optimization method to examine experimental data. The abstract can be read below.
Optimal quantum control with poor statistics
Learning how to control a quantum system based on experimental data can help us to exceed the limitations imposed by theoretical modeling. Due to the intrinsic probabilistic nature of quantum mechanics, it is fundamentally necessary to repeat measurements on individual quantum systems many times in order to estimate the expectation value of an observable with good accuracy. Control algorithms requiring accurate data can thus imply an experimental effort that negates the benefits of avoiding theoretic modelling. We present a control algorithm based on Bayesian optimisation that finds optimal control solutions in the presence of large measurement shot noise and even in the limit of single shot measurements. With the explicit example of the preparation of a GHZ state, we demonstrate in numerical simulations that this method is capable of finding excellent control solutions with minimal experimental effort.
This weeks AMOPP seminar was given by Dr. Brianna Heazlewood from the University of Oxford. In this interesting lecture, Dr. Heazlewood spoke about “Using the Zeeman effect to manipulate radicals and study ion-radical reactions”. The abstract for the talk can be found below.
Using the Zeeman effect to manipulate radicals and study ion-radical reactions
In spite of their real-world importance, very few experimental methods can be applied to the precise study of gas-phase ion-radical reaction systems. This is primarily due to the significant difficulty associated with generating a pure beam of atomic or molecular gas-phase radicals with tuneable properties. In this seminar, I will present our work in generating a pure beam of velocity-selected radicals. Only the target radicals are transmitted into the detection region; all other components of the incoming beam (radical species travelling faster/slower than the target velocity. precursor molecules and seed gases) are removed. This control over the properties of the radical beam is achieved through the use of a magnetic guide, composed of four Halbach arrays (permanent magnets in a hexapolar configuration) and two skimming blades. Experimental measurements of Zeeman-decelerated H atoms transmitted through the guide, combined with extensive simulations, show that the magnetic guide removes 99% of H atoms travelling outside the narrow target velocity range [1,2]. We will shortly combined the Zeeman decelerator and magnetic guide with an ion trap, for the study of ion-radical reactions. I will present some recent work on the reaction of ions with polar molecules – and discuss how we intend to adapt this approach for the study of ion-radical processes.
 J. Toscano, C. J. Rennick, T. P. Softley and B. R. Heazlewood, J. Chem. Phys. 149, 174201, (2018).
 J. Toscano, M. Hejduk, H. G. McGhee and B. R. Heazlewood, Rev. Sci. Instrum. 90, 033201, (2019).
This week we had the pleasure of welcoming Dr. Philipp Preiss from the University of Heidelberg who gave a talk on “Precursor of the Higgs Mode in Ultracold Few-Fermion Systems”. The abstract can be read below.
Precursor of the Higgs Mode in Ultracold Few-Fermion Systems
The emergence of collective modes from single-particle excitations is one of the most striking features of strongly interacting systems. Understanding such excitations is an ongoing challenge in nuclear physics, strongly correlated electron systems, and high-energy physics. Ultracold atoms in optical potentials provide a unique setting to precisely study the appearance of collective excitations in a tunable laboratory setting.
Here we experimentally observe the “birth” of a collective mode in a few-body system of ultracold Fermions. Using optical tweezers, we deterministically prepare few Fermions in the ground state of a two dimensional trap. This system exhibits a shell structure of stable “magic” numbers of 2,6,12… particles. We perform many-body spectroscopy through a modulation of the interaction strength find both single-particle and two-particle excitations. The latter consists of pairwise excitations akin to Cooper pairs and can be identified as the precursor of the Higgs mode in a two-dimensional Fermi gas.
In the future, we will probe such mesoscopic Fermi systems with single-particle detection. We recently demonstrated spin-resolved fluorescence imaging of individual atoms in free space, which will allow us to detect the momenta of every particle in the system in time-of-flight. We expect to directly see the formation of Cooper pairs and the momentum space signature of the BEC-BCS crossover.
The first talk in this year’s series of AMOPP seminars was given by Dr. Adam Deller from Prof. Hogan’s group in UCL. Adam, who was one of the first members in the UCL Ps spectroscopy group, talked about miniaturised Rydberg-Stark decelerators used to trap nitric oxide molecules. Abstract below.
Trapping long-lived hydrogenic Rydberg states of nitric oxide
High Rydberg states of atoms or molecules can have extremely large static electric dipole moments, upon which an inhomogeneous electric field will exert a sizable force. Electrostatic or time-varying electric fields have been utilised to exploit this effect to guide or decelerate and trap H, He, Ps and also H2.
I will describe a compact chip-based Rydberg-Stark decelerator comprised of a linear array of 115 electrodes. And I will present the results of recent experiments in which this device was employed to decelerate and trap laser excited NO molecules. An average lifetime of approximately 300 us was measured for molecules in the cryogenic trap. These cold, trapped NO molecules are of interest for studying low-temperature inelastic scattering processes for which long-range interaction play an important role.
The Ps Spectroscopy team had the delight of attending the 15th International Workshop on Slow Positron Beam Techniques and Applications (SLOPOS-15) in Prague on 2-6 September, 2019. The workshop was held in the Faculty of Mathematics and Physics, Charles University in the centre of the Lesser Town square.
A wide range of topics, from Ps physics to defect studies, were discussed by over 100 participants in the workshop. David outlined the recent experimental progress in Ps-laser physics in his talk, while Ross talked about the versatility of a multi-ring guide for possible Ps deceleration and trapping studies. Lokesh presented the results of the new precision microwave spectroscopy experiment (stay tuned for more details). The complete list of abstracts can be found here. It was a pleasure discussing the ongoing work in the field of slow positron physics in the wonderful city of Prague.
We’d like to thank the local organising committee, led by Jakub Cizek, for organising an excellent meeting and we look forward to the next SLOPOS which will be held in Orléans, France in 2022.
The Ps spectroscopy team recently attended the XX International Workshop on Low-Energy Positron and Positronium Physics and XXI International Symposium on Electron-Molecule Collisions and Swarms i.e. POSMOL-2019. The conference was organised by the Serbian Academy of Sciences and Arts (SASA) in Belgrade on 18-20th July.
Our most recent work on the microwave spectroscopy of Ps n=2 fine structure (more on this in the next blog post) was presented by Lokesh. It was a great opportunity to discuss our research with the positron (positronium) and the electron community as well as explore ongoing and future research from other groups. The complete list of abstract is available here, as well as the photo gallery.
We’d like to thank the organisers for planning this excellent conference and we look forward to the next meeting. The UCL Ps Spectroscopy group will be attending SLOPOS-15 in Prague in September. See you then!