Volume 9, Issue 5 - Applied Quantum Technologies / Guest Editors: Markus Krutzik, John Close, and Daniel Oi

Atom interferometry is one of the most promising technologies for high precision measurements. It has the potential to revolutionise many different sectors, such as navigation and positioning, resource exploration, geophysical studies, and fundamental physics. After decades of research in the field of cold atoms, the technology has reached a stage where commercialisation of cold atom interferometers has become possible. This article describes recent developments, challenges, and prospects for quantum sensors for inertial sensing based on cold atom interferometry techniques.

Following the first demonstration of a levitated nanosphere cooled to the quantum ground state in 2020 (U. Delić, et al. Science , vol. 367, p. 892, 2020), macroscopic quantum sensors are seemingly on the horizon. The nanosphere’s large mass as compared to other quantum systems enhances the susceptibility of the nanoparticle to gravitational and inertial forces. In this viewpoint, we describe the features of experiments with optically levitated nanoparticles (J. Millen, T. S. Monteiro, R. Pettit, and A. N. Vamivakas, “Optomechanics with levitated particles,” Rep. Prog. Phys. , vol. 83, 2020, Art no. 026401) and their proposed utility for acceleration sensing. Unique to the levitated nanoparticle platform is the ability to implement not only quantum noise limited transduction, predicted by quantum metrology to reach sensitivities on the order of 10 −15  ms −2 (S. Qvarfort, A. Serafini, P. F. Barker, and S. Bose, “Gravimetry through non-linear optomechanics,” Nat. Commun. , vol. 9, 2018, Art no. 3690) but also long-lived quantum spatial superpositions for enhanced gravimetry. This follows a global trend in developing sensors, such as cold-atom interferometers, that exploit superposition or entanglement. Thanks to significant commercial development of these existing quantum technologies, we discuss the feasibility of translating levitated nanoparticle research into applications.

We implement slow-light under electromagnetically induced transparency condition to measure the motion of cold atoms in an optical lattice undergoing Bloch oscillation. The motion of atoms is mapped out through the phase shift of light without perturbing the external and internal state of the atoms. Our results can be used to construct a continuous motional sensor of cold atoms.

The electrophysiological activities in the human body generate electric and magnetic fields that can be measured noninvasively by electrodes on the skin, or even, not requiring any contact, by magnetometers. This includes the measurement of electrical activity of brain, heart, muscles and nerves that can be measured in vivo and allows to analyze functional processes with high temporal resolution. To measure these extremely small magnetic biosignals, traditionally highly sensitive superconducting quantum-interference devices have been used, together with advanced magnetic shields. Recently, they have been complemented in usability by a new class of sensors, optically pumped magnetometers (OPMs). These quantum sensors offer a high sensitivity without requiring cryogenic temperatures, allowing the design of small and flexible sensors for clinical applications. In this letter, we describe the advantages of these upcoming OPMs in two exemplary applications that were recently carried out at Physikalisch-Technische Bundesanstalt (PTB): (1) magnetocardiography (MCG) recorded during exercise and (2) auditory-evoked fields registered by magnetoencephalography.

Owing to physical orientations and birefringence effects, practical quantum information protocols utilizing optical polarization need to handle misalignment between preparation and measurement reference frames. For any such capable system, an important question is how many resources – for example, measured single photons – are needed to reliably achieve alignment precision sufficient for the desired quantum protocol. Here, we study the performance of a polarization-frame alignment scheme used in prior laboratory and field quantum key distribution (QKD) experiments by performing Monte Carlo numerical simulations. The scheme utilizes, to the extent possible, the same single-photon-level signals and measurements as for the QKD protocol being supported. Even with detector noise and imperfect sources, our analysis shows that only a small fraction of resources from the overall signal – a few hundred photon detections, in total – are required for good performance, restoring the state to better than 99% of its original quality.

For quantum communications, the use of Earth-orbiting satellites to extend distances has gained significant attention in recent years, exemplified in particular by the launch of the Micius satellite in 2016. The performance of applied protocols such as quantum key distribution (QKD) depends significantly on the transmission efficiency through the turbulent atmosphere, which is especially challenging for ground-to-satellite uplink scenarios. Adaptive optics (AO) techniques have been used in astronomical, communication, and other applications to reduce the detrimental effects of turbulence for many years, but their applicability to quantum protocols, and their requirements specifically in the uplink scenario, is not well established. Here, we model the effect of the atmosphere on link efficiency between an Earth station and a satellite using an optical uplink and how AO can help recover from loss due to turbulence. Examining both low Earth orbit and geostationary uplink scenarios, we find that a modest link transmissivity improvement of about 3 dB can be obtained in the case of a coaligned downward beacon, while the link can be dramatically improved, up to 7 dB, using an offset beacon, such as a laser guide star. AO coupled with a laser guide star would thus deliver a significant increase in the secret key generation rate of the QKD ground-to-space uplink system, especially as reductions of channel loss have a favourably nonlinear key-rate response within this high-loss regime.

We present a fast and robust optically pumped magnetometer that is based on a feedback-controlled spin ensemble of cesium atoms in spin-polarized vapor. The table-top system is intended for operation in unshielded environment, and its design allows conversion into a handheld sensor head. Under strongly disturbed environmental conditions in the laboratory, the sensor exhibits a speed of more than 56 kHz, while having a slew rate of 39 mT/s and a full dynamic range of 10 – 120 μT. Under these conditions a sensitivity of 33 pT/Hz$\text{pT}/\sqrt{\text{Hz}}$ is reached. By reducing the speed to 3.6 kHz the sensitivity can be improved to 6 pT/Hz$\text{pT}/\sqrt{\text{Hz}}$, which is close to the shot noise limit. We describe the sensor design and its optimization and demonstrate the performance of the sensor under conditions appropriate for magnetic susceptometry measurements.

The “quantum threat” to our current, convenient cryptographic algorithms is getting closer, with demonstrable progress by commercial quantum computing efforts. It is now more important than ever that we combine all of our tools into a new quantum-safe toolbox to develop the next generation of quantum-safe networking solutions. Here we combine an integrated quantum entropy source with quantum-resistant algorithms in the GnuGPG open-source software; leading to a fully quantum-safe version of GnuGPG. The quantum entropy source itself is capable of a raw rate of randomness in excess of 10 Gbps. After post-processing, quantum random numbers are used by the quantum-resistant algorithms to allow GnuGPG to perform its usual public-key cryptographic tasks, such as digitally signing documents, but now in a secure quantum-safe way.

A compact platform for cold atoms opens a range of exciting possibilities for portable, robust and accessible quantum sensors. In this work, we report on the development of a cold-atom microwave clock in a small package. Our work utilises the grating magneto-optical trap and high-contrast coherent population trapping in the lin⊥$\perp $lin polarisation scheme. We optically probe the atomic ground-state splitting of cold 87 Rb atoms using a Ramsey-like sequence whilst the atoms are in free-fall. We have measured a short-term fractional frequency stability of 5×10−11/τ$5{\times}{10}^{-11}/\sqrt{\tau }$ with a projected quantum projection noise limit at the 10−13/τ${10}^{-13}/\sqrt{\tau }$ level.

We present electric field measurements and imaging of a Yagi–Uda antenna near-field using a Rydberg atom–based radio frequency electric field measurement instrument. The instrument uses electromagnetically induced transparency with Rydberg states of cesium atoms in a room-temperature vapor and off-resonant RF-field–induced Rydberg-level shifts for optical SI-traceable measurements of RF electric fields over a wide amplitude and frequency range. The electric field along the antenna boresight is measured using the atomic probe at a spatial resolution of λRF/2${\lambda }_{RF}/2$ with electric field measurement uncertainties below 5.5%, an improvement to RF measurement uncertainties provided by existing antenna standards.

Recently, several reports with a strong focus on compact, nonstationary optical atomic clocks have been published, including accounts of in-field deployment of these devices for demonstrations of chronometric levelling in different types of environments. We review recent progress in this research area, comprising compact and transportable neutral atom and single-ion optical atomic clocks. The identified transportable optical clocks strive for low volume, weight and power consumption while exceeding standard microwave atomic clocks in fractional frequency instability and systematic uncertainty. Some transportable clock projects additionally address requirements for metrology or serve the joint technology development between industrial and academic stakeholders. Based on the reviewed reports on nonstationary optical atomic clocks, we suggest definitions for transportable, portable and mobile optical atomic clocks. We conclude our article with an overview of possible future directions for developments of optical clock technology.