Towards mobile quantum sensors for gravity surveys

Design and first characterisations of the mobile sensor head in the Gravity Gradient Technologies and Opportunities Programme (GGtop) are presented. The aim of the project is the development of a mobile gravity gradiometer and faces the challenge of condensing a normally lab filling experiment in a portable and robust package. A fibre network replaced free space optics light distribution and fibre based components free space equivalents. Although stable against misalignment, the systems performance is limited by polarization changes of the guided light which arise from birefringence fluctuations along the length of the network due to external temperature fluctuations and mechanical disturbances. These instabilities limit the achievable temperature of the trapped rubidium 87 cloud to approx. 18μK. In preparation for gravity measurements, Rabi oscillations and Ramsey fringes with a λ/2 time of 7.4μs were successfully demonstrated in a co-propagating Raman beam configuration. The atom cloud was launched as a first step towards gravity gradiometry. As the system was designed to be portable, the complete system fits into a 1.5m x 2m x 0.5m package, plus a 14u rack of support electronics

Application of optical single-sideband laser in Raman atom interferometry

A frequency doubled I/Q modulator based optical single-sideband (OSSB) laser system is demonstrated for atomic physics research, specifically for atom interferometry where the presence of additional sidebands causes parasitic transitions. The performance of the OSSB technique and the spectrum after second harmonic generation are measured and analyzed. The additional sidebands are removed with better than 20 dB suppression, and the influence of parasitic transitions upon stimulated Raman transitions at varying spatial positions is shown to be removed beneath experimental noise. This technique will facilitate the development of compact atom interferometry based sensors with improved accuracy and reduced complexity

Experimental Comparison of Efficient Tomography Schemes for a Six-Qubit State

Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a six-qubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers.Comment: 7 pages, 7 figure

Permutationally invariant state reconstruction

Feasible tomography schemes for large particle numbers must possess, besides an appropriate data acquisition protocol, also an efficient way to reconstruct the density operator from the observed finite data set. Since state reconstruction typically requires the solution of a non-linear large-scale optimization problem, this is a major challenge in the design of scalable tomography schemes. Here we present an efficient state reconstruction scheme for permutationally invariant quantum state tomography. It works for all common state-of-the-art reconstruction principles, including, in particular, maximum likelihood and least squares methods, which are the preferred choices in today's experiments. This high efficiency is achieved by greatly reducing the dimensionality of the problem employing a particular representation of permutationally invariant states known from spin coupling combined with convex optimization, which has clear advantages regarding speed, control and accuracy in comparison to commonly employed numerical routines. First prototype implementations easily allow reconstruction of a state of 20 qubits in a few minutes on a standard computer.Comment: 25 pages, 4 figues, 2 table

Quantum sensing for gravity cartography

The sensing of gravity has emerged as a tool in geophysics applications such as engineering and climate research(1–3), including the monitoring of temporal variations in aquifers(4) and geodesy(5). However, it is impractical to use gravity cartography to resolve metre-scale underground features because of the long measurement times needed for the removal of vibrational noise(6). Here we overcome this limitation by realizing a practical quantum gravity gradient sensor. Our design suppresses the effects of micro-seismic and laser noise, thermal and magnetic field variations, and instrument tilt. The instrument achieves a statistical uncertainty of 20 E (1 E = 10(−9) s(−2)) and is used to perform a 0.5-metre-spatial-resolution survey across an 8.5-metre-long line, detecting a 2-metre tunnel with a signal-to-noise ratio of 8. Using a Bayesian inference method, we determine the centre to ±0.19 metres horizontally and the centre depth as (1.89 −0.59/+2.3) metres. The removal of vibrational noise enables improvements in instrument performance to directly translate into reduced measurement time in mapping. The sensor parameters are compatible with applications in mapping aquifers and evaluating impacts on the water table(7), archaeology(8–11), determination of soil properties(12) and water content(13), and reducing the risk of unforeseen ground conditions in the construction of critical energy, transport and utilities infrastructure(14), providing a new window into the underground