Robert Flack’s research while affiliated with University College London and other places

We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that the beam velocity can be determined with a precision below 1% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as the Van der Waals (VdW) co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The VdW co-efficient was determined to be C_{3} = 1.84 ± 0.17 a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96% in the m = +2 state.

We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that beam velocity can be determined with a precision below 1 \% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as Van der Waals co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The Van der Waals co-efficient was determined to be $C_3$=1.84$\pm$0.17\,a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96 \% in the m=+2 state.

A method for measuring the weak value of spin for atoms is proposed using a variant of the original Stern–Gerlach apparatus. A full simulation of an experiment for observing the real part of the weak value using the impulsive approximation has been carried out. Our predictions show a displacement of the beam of helium atoms in the metastable 23S1 state, Δw, that is within the resolution of conventional microchannel plate detectors indicating that this type of experiment is feasible. Our analysis also determines the experimental parameters that will give an accurate determination of the weak value of spin. Preliminary experimental results are shown for helium, neon and argon in the 23S1 and 3P2 metastable states, respectively.

There has been a recent revival of interest in the notion of a 'trajectory' of a quantum particle. In this paper, we detail the relationship between Dirac's ideas, Feynman paths and the Bohm approach. The key to the relationship is the weak value of the momentum which Feynman calls a transition probability amplitude. With this identification we are able to conclude that a Bohm 'trajectory' is the average of an ensemble of actual individual stochastic Feynman paths. This implies that they can be interpreted as the mean momentum flow of a set of individual quantum processes and not the path of an individual particle. This enables us to give a clearer account of the experimental two-slit results of Kocsis et al.

A method for measuring the real part of the weak value of spin for non-zero rest mass atoms is presented using a variant on the original Stern-Gerlach apparatus. The experiment utilises helium in the metastable 23S1 state. A full simulation for observing the real part of the weak value using the impulsive approximation has been carried out and it predicts a displacement of the beam, Δw, that is within the resolution of our detector. It also indicates how this shift might be increased. The full analysis also indicated that there is a limit, L, to the applicability of the weak value approximation and has been evaluated for our apparatus. This experiment has the possibility to be expanded to utilise other nobal gas species such as neon and argon in the 3P2 metastable state, but we shall restrict this paper to metastable helium only.

There has been a recent revival of interest in the notion of a `trajectory' of a quantum particle. In this paper we detail the relationship between Dirac's ideas, Feynman paths and the Bohm approach. The key to the relationship is the weak value of the momentum which Feynman calls a transition probability amplitude. With this identification we are able to conclude that a Bohm `trajectory' is the average of an ensemble of actual individual stochastic Feynman paths. This implies that they can be interpreted as the mean momentum flow of a set of individual quantum processes and not the path of an individual particle. This enables us to give a clearer account of the experimental two-slit results of Kocsis {\em et al.}}

A method for measuring the real part of the weak (local) value of spin is presented using a variant on the original Stern-Gerlach apparatus. The experiment utilises metastable helium in the $\rm 2^{3}S_{1}$ state. A full simulation using the impulsive approximation has been carried out and it predicts a displacement of the beam by $\rm \Delta_{w} = 17 - 33\,\mu m$. This is on the limit of our detector resolution and we will discuss ways of increasing $\rm \Delta_{w}$. The simulation also indicates how we might observe the imaginary part of the weak value.

In a recent paper Mahler {\em et al.} have argued that the experiments of Kocsis {\em et al.} provide experimental evidence for Bohmian mechanics. Unfortunately these experiments used relativistic, zero rest mass photons whereas Bohmian mechanics is based on non-relativistic Schr\"{o}dinger particles having non-zero rest mass. The experimental results can be correctly understood in terms of a different approach based on the electromagnetic field that was already outlined by Bohm in an appendix of the second of his 1952 papers. A subsequent development of this approach by Bohm, Hiley, Kaloyerou and Holland, show in detail how this theory accounts for the experimental results. We are led to the conclusion that the experiments have constructed mean momentum flow lines by measuring the real part of the weak Poynting vector. These results support and clarify the analysis of Bliokh {\em et al}. The experimental flow lines can be constructed independently of the number of photons in the beam leading to the conclusion that flow lines cannot be interpreted as `photon trajectories'. We discuss exactly how the notion of a photon arises in the field approach.

A system is being designed and constructed in order to measure the weak value of spin for atomic systems. The experiment utilises spin-1 metastable helium atoms in the 23S 1 state. This paper outlines the experiment and its features.

Protective measurements offer an intriguing method for measuring the wave function of a single quantum system. With contributions from leading physicists and philosophers of physics - including two of the original discoverers of this important method - this book explores the concept of protective measurement, investigating its broad applications and deep implications. Addressing both physical and philosophical aspects, it covers a diverse range of topics, including experimental possibility of protective measurements, connections with the PBR theorem, and the implications of protective measurements for understanding the nature of quantum reality. Including a clear and concise introduction to standard quantum mechanics, conventional measurements, and the fundamentals of protective measurements, this is a valuable resource for graduate students and researchers interested in the conceptual foundations of quantum mechanics.