Optimal quantum control with poor statistics

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.

Using the Zeeman effect to manipulate radicals and study ion-radical reactions

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.

[1] J. Toscano, C. J. Rennick, T. P. Softley and B. R. Heazlewood, J. Chem. Phys. 149, 174201, (2018).
[2] J. Toscano, M. Hejduk, H. G. McGhee and B. R. Heazlewood, Rev. Sci. Instrum. 90, 033201, (2019).

Precursor of the Higgs Mode in Ultracold Few-Fermion systems

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.

Trapping long-lived Rydberg states of nitric oxide

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.

 

Towards enhanced interferometry using quantum states of light

On Wednesday 17th of October we had the pleasure to welcome Dr. Chris Wade from Oxford University to give a seminar to the AMOPP group about progress towards interferometry with exotic quantum states of light, more specifically Holland-Burnett states. This was a very interesting talk with a great mix of theory and experimental results. The abstract can be seen below.

Towards enhanced interferometry using quantum states of light

Quantum metrology is concerned with the enhanced measurement precision that may be gained by exploiting quantum mechanical correlations. In the scenario presented by optical inteferometry, several successful implementations have already been demonstrated including gravitational wave detectors [1], and lab-scale experiments [2,3]. However there are still open problems to be solved, including loss tolerance and scalability. In this seminar I will present progress implementing loss-tolerant Holland-Burnett states [4], and work searching for other practical states to implement [5].

[1] Schnabel et al. Nat. Comm. 1. 121 (2010)
[2] Slussarenko et al. Nat. Phot. 11, 700 (2017)
[3] Yonezawa et al. Science, 337, 1514 (2012)
[4] Holland and Burnett, PRL, 71, 1355 (1993)
[5] Knott et al, PRA, 93, 033859 (2016)

The Measurement Postulates of Quantum Mechanics are Redundant

On Wednesday 10th October we had Dr. Luis  Masanes from within the UCL AMOPP group give a very interesting seminar. His talk was focused on the fundamental questions posed by the measurement postulates of quantum mechanics, and how they are redundant given the other postulates that form the basis of quantum mechanics. Dr. Masanes was kind enough to provide a copy of his slides here and the abstract can be seen below.

The Measurement Postulates of Quantum Mechanics are Redundant

Understanding the core content of quantum mechanics requires us to disentangle the hidden logical relationships between the postulates of this theory. The theorem presented in this work shows that the mathematical structure of quantum measurements, the formula for assigning outcome probabilities (Born’s rule) and the post-measurement state-update rule, can be deduced from the other quantum postulates, often referred to as “unitary quantum mechanics”. This result unveils a deep connection between the dynamical and probabilistic parts of quantum mechanics, and it brings us one step closer to understand what is this theory telling us about the inner workings of Nature.

Levitated Quantum Nanophotonics

On Wednesday 1st November we had Professor Lukas Novotny from the Photonics Laboratory in ETH, Zürich give an insightful AMOPP seminar. His expertise spans many  areas, from optical antennas, near-field optics, nonlinear plasmonics and more. However, during his talk he focused on nanoparticle trapping and cooilng. The abstract for his talk can be found below, and he agreed to provide a copy of his slides which you can download here.

Abstract:

Levitated Quantum Nanophotonics
Vijay Jain [a], Martin Frimmer [a], Erik Hebestreit [a], Jan Gieseler[a], Romain Quidant [b], Christoph Dellago [c], and Lukas Novotny [a]
a) ETH Zurich, Photonics Laboratory, 8093 Zurich, Switzerland.
b) ICFO, Mediterranean Technology Park, 08860 Castelldefels, Spain.
c) University of Vienna, Faculty of Physics, 1090 Vienna, Austria.

I discuss our experiments with optically levitated nanoparticles in ultrahigh vacuum. Using active parametric feedback we cool the particle’s center-of-mass temperature to T = 100μK and reach mean quantum occupation numbers of n = 15. I show that mechanical quality factors of Q = 109 can be reached and that damping is dominated by photon recoil heating. The vacuum-trapped nanoparticle forms an ideal model system for studying non-equilibrium processes, nonlinear interactions, and ultrasmall forces.