Andrew Horsley

Widefield microwave imaging in alkali vapor cells with sub-100 mum resolution

We report on widefield microwave vector field imaging with sub um resolution using a microfabricated alkali vapor cell. The setup can additionally image dc magnetic fields, and can be configured to image microwave electric fields. Our camera-based widefield imaging system records 2D images with a 6x6 mm2 field of view at a rate of 10 Hz. It provides up to 50 um spatial resolution, and allows imaging of fields as close as 150 um above structures, through the use of thin external cell walls. This is crucial in allowing us to take practical advantage of the high spatial resolution, as feature sizes in near-fields are on the order of the distance from their source, and represents an order of magnitude improvement in surface-feature resolution compared to previous vapor cell experiments. We present microwave and dc magnetic field images above a selection of devices, demonstrating a microwave sensitivity of 1.4 uT/sqrt-Hz per 50x50x140 um3 voxel, at present limited by the speed of our camera system. Since we image 120x120 voxels in parallel, a single scanned sensor would require a sensitivity of at least 12 nT/sqrt-Hz to produce images with the same sensitivity. Our technique could prove transformative in the design, characterisation, and debugging of microwave devices, as there are currently no satisfactory established microwave imaging techniques. Moreover, it could find applications in medical imaging.

Imaging Microwave and DC Magnetic Fields in a Vapor-Cell Rb Atomic Clock

We report on the experimental measurement of the dc and microwave magnetic field distributions inside a recently developed compact magnetron-type microwave cavity mounted inside the physics package of a high-performance vapor-cell atomic frequency standard. Images of the microwave field distribution with sub-100-μm lateral spatial resolution are obtained by pulsed optical-microwave Rabi measurements, using the Rb atoms inside the cell as field probes and detecting with a CCD camera. Asymmetries observed in the microwave field images can be attributed to the precise practical realization of the cavity and the Rb vapor cell. Similar spatially resolved images of the dc magnetic field distribution are obtained by Ramsey-type measurements. The T2 relaxation time in the Rb vapor cell is found to be position dependent and correlates with the gradient of the dc magnetic field. The presented method is highly useful for experimental in situ characterization of dc magnetic fields and resonant microwave structures, for atomic clocks or other atom-based sensors and instrumentation.

Frequency-tunable microwave field detection in an atomic vapor cell

We use an atomic vapor cell as a frequency tunable microwave field detector operating at frequencies from GHz to tens of GHz. We detect microwave magnetic fields from 2.3 GHz to 26.4 GHz, and measure the amplitude of the sigma+ component of an 18 GHz microwave field. Our proof-of-principle demonstration represents a four orders of magnitude extension of the frequency tunable range of atomic magnetometers from their previous dc to several MHz range. When integrated with a high resolution microwave imaging system, this will allow for the complete reconstruction of the vector components of a microwave magnetic field and the relative phase between them. Potential applications include near-field characterisation of microwave circuitry and devices, and medical microwave sensing and imaging.

Experimental and numerical study of the microwave field distribution in a compact magnetron-type microwave cavity

The homogeneity of the microwave magnetic field distribution inside a microwave cavity resonator is of crucial importance for the performance of high-stability vapor-cell atomic clocks, in particular when operating in the pulsed regime. Here we report on measurements and numerical simulations of the microwave field distribution inside a highly compact magnetron-type microwave cavity. A microwave field imaging technique is used to measure the microwave field distribution over the cavity volume occupied by the Rb vapor cell. Over the measured volume, we find a variation in the microwave fields z-component (relevant for the clock performance) of 20%, with good agreement between experiments and simulations. The presented results are relevant for assessing clock performance limitations arising from the cavity studied, and give design guidelines for future improved cavities of similar type.

An atomic memory suitable for semiconductor quantum dot single photons

Quantum networks consist of many quantum memory nodes that are interconnected via photonic links, transporting single photons carrying quantum information. In the future, such quantum networks may enable: high-speed quantum cryptography for unconditionally secure communication; large-scale quantum computers; and quantum simulators that will allow for exponential speed-up in solving specific complex problems. A promising route towards functional quantum network nodes is the heterogeneous approach [1], where different and separately optimized physical systems are used for single photon generation and storage. For example semiconductor quantum dots may be used as efficient, fast and deterministic single photon sources, while atomic ensembles allow for efficient storage of these photons.

Widefield microwave imaging using NV centres

Microwave devices form the backbone of many scientific and technological applications, from quantum devices (atom chips, ion traps, atomic clocks, qubits…) to telecommunications (wifi, mobile phones…). There is great interest in techniques to image the microwave near-fields close to such devices, which promise to transform device development, characterisation, and debugging. We have developed techniques for imaging magnetic fields at microwave frequencies (GHz to tens of GHz) [Fig. 1] using both atomic vapor cells [1-3] and scanning probe nitrogen-vacancy (NV) centres in diamond [4-6]. We detect the fields through coherent Rabi oscillations driven on transitions within the atoms or NV centres. Compared to traditional antenna-based microwave imaging, our techniques represent a fundamentally new approach to microwave sensing, providing intrinsically calibrated measurements with high spatial and temporal resolution.

Imaging the static magnetic field distribution in a vapor cell atomic clock

We use a Ramsey-type interaction scheme to measure spatially-resolved images of the static magnetic field (C-field) amplitude Bdc applied across the Rb cell in the physics package of a high-performance vapor-cell atomic clock. Low field variations of <; 0.5% are found across the recorded images, and Fourier analysis of the data indicates low variations of Bdc also along the direction of laser propagation. Images of the T2 relaxation time are obtained in a similar way, and show a distribution that correlates with the Bdc distribution. This indicates inhomogeneous dephasing due to C-field gradients, which also results in spatial variation of the T2 time for the clock transition.