We investigate theoretically the use of an electron in a Penning trap as a detector of single microwave photons. At the University of Sussex we are developing a chip Penning trap technology, designed to be integrated within quantum circuits. Microwave photons are guided into the trap and interact with the electron’s quantum cyclotron motion. This is an electric dipole transition, where the near field of the microwave radiation induces quantum jumps of the cyclotron harmonic oscillator. The quantum jumps can be monitored using the continuous Stern-Gerlach effect, providing the quantum non demolition signal of the microwave quanta. We calculate the quantum efficiency of photon detection and discuss the main features and technical challenges for the trapped electron as a quantum microwave sensor.

We describe a novel use for superconducting flux pumping, namely a method for magnetizing a planar magnetic field source. The method is useful for many applications, but we illustrate it with the planar Penning trap with applications in quantum technology. A flux pumping mechanism and associated mathematical model are described. The method allows for complete control of the induced persistent current in a double-loop superconducting structure, whereby the final current is a known function of the applied flux and the loop geometry. We have tested our flux pumping technique and found excellent agreement with the theory.

The measurement of the g-factor of the electron bound in a hydrogen-like ion is a high-accuracy test of the theory of quantum electrodynamics (QED) in strong fields. Here we report on the measurement of the g-factor of the bound electron in hydrogen-like oxygen (¹⁶O⁷⁺). In our experiment a single highly charged ion is stored in a Penning trap. The electronic spin state of the ion is monitored via the continuous Stern–Gerlach effect in a quantum non-demolition measurement. Quantum jumps between the two spin states (spin up and spin down) are induced by a microwave field at the spin precession frequency of the bound electron. The g-factor of the bound electron is obtained by varying the microwave frequency and counting the number of spin flips. The comparison of our experimental values for the g-factor of the bound electron with the theoretical values shows excellent agreement and confirms the recent non-perturbative QED calculations.

The measurement of the magnetic moment (or g‐factor) of the antiproton and of the proton is a sensitive test of CPT invariance. In our experiment we will store and detect a single (anti)proton in a cryogenic Penning trap. The g‐factor will be measured by detection of quantum jumps via the continuous Stern‐Gerlach effect. Most of the experimental techniques to be used have been already successfully employed by our group for the measurement of the g‐factor of the bound electron in hydrogen‐like ions. However, the magnetic moment of the proton is smaller than that of the electron by a factor of 658. Our hybrid trap design combines cylindrical electrodes with a toroidal ferromagnetic ring electrode. With this novel trap, spin‐flip transitions of the (anti)proton can be detected by observation of tiny differences in the axial frequency by a phase‐sensitive method. With our apparatus, we envisage to determine the g‐factor of the (anti)proton with an accuracy of 10⁻⁹ or better.