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arXiv:1411.7893 · Physical Review Letters

Ultrasensitive Magnetometer using a Single Atom

I. Baumgart, Jianming Cai, Alex Retzker, Martin Bodo Plenio, Ch. Wunderlich

Published Jun 17, 2016• 106 citations
arXivHTMLPDF

Abstract

Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. The precision, and thus the sensitivity of magnetometry, scales as 1/sqrt[T_{2}] with the phase coherence time T_{2} of the sensing system playing the role of a key determinant. Adapting a dynamical decoupling scheme that allows for extending T_{2} by orders of magnitude and merging it with a magnetic sensing protocol, we achieve a measurement sensitivity even for high frequency fields close to the standard quantum limit. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of 4.6 pT/sqrt[Hz] for an alternating-current magnetic field near 14 MHz. Based on the principle demonstrated here, this unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.

DOI
10.1103/physrevlett.116.240801

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Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. The precision, and thus the sensitivity of magnetometry, scales as 1/sqrt[T_{2}] with the phase coherence time T_{2} of the sensing system playing the role of a key determinant. Adapting a dynamical decoupling scheme that allows for extending T_{2} by orders of magnitude and merging it with a magnetic sensing protocol, we achieve a measurement sensitivity even for high frequency fields close to the standard quantum limit. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of 4.6 pT/sqrt[Hz] for an alternating-current magnetic field near 14 MHz. Based on the principle demonstrated here, this unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.
Quantum magnetometry represents a fundamental component of quantum metrology, where trapped-ion systems have achieved pT/Hz\rm{pT}/\sqrt{\rm{Hz}}pT/Hz​ sensitivity in single-ion radio-frequency magnetic field measurements via dressed states based dynamical decoupling. Here we propose a scalable trapped-ion magnetometer utilizing the mixed dynamical decoupling method, combining dressed states with periodic sequences to suppress decoherence and spatial magnetic field inhomogeneity. With numerical simulations for a 10410^4104 ion system with realistic experimental parameters, we demonstrate that a sensitivity of 13 fT/Hz\rm{fT}/\sqrt{\rm{Hz}}fT/Hz​ for the radio-frequency field could be reached. Such a sensitivity could be obtained via robust resilience to magnetic field drift noise and inhomogeneity, where coherence time could be extended to the order of several minutes on average. This method enables scalable trapped-ion magnetometry, demonstrating its potential as a robust and practical solution for advancing quantum sensing applications.
A scheme of an ultra-sensitive magnetometer in the cavity quantum electromagnonics where the intracavity microwave mode coupled to a magnonic mode via magnetic dipole interaction is proposed. It is shown that by driving both magnonic and microwave modes with external classical fields and controlling the system parameters, one can reduce the added noise of magnetic field measurement below the standard quantum limit (SQL). Surprisingly, we show that beyond the rotating wave approximation (RWA), not only the added noise can be suppressed, but also the output cavity response to the input signal can be substantially amplified in order to achieve a precise magnetic-field measurement. The sensitivity of the proposed magnetic amplifier-sensor is approximately in the order of 10−18T/Hz10^{-18}T/\sqrt{Hz}10−18T/Hz​ which is competitive compared to the current state-of-the-art magnetometers like superconducting quantum interference devices (SQUIDs) and atomic magnetometers. The advantage of the proposed sensor in comparison with the other magnetometers is its high sensitivity at room temperature and sensing in a wide range of frequency up to MHz as well as its capability to signal-response amplification.
Abstract Quantum sensors are highly sensitive since they capitalise on fragile quantum properties such as coherence, while enabling ultra-high spatial resolution. For sensing, the crux is to minimise the measurement uncertainty in a chosen range within a given time. However, basic quantum sensing protocols cannot simultaneously achieve both a high sensitivity and a large range. Here, we demonstrate a non-adaptive algorithm for increasing this range, in principle without limit, for alternating-current field sensing, while being able to get arbitrarily close to the best possible sensitivity. Therefore, it outperforms the standard measurement concept in both sensitivity and range. Also, we explore this algorithm thoroughly by simulation, and discuss the T −2 scaling that this algorithm approaches in the coherent regime, as opposed to the T −1/2 of the standard measurement. The same algorithm can be applied to any modulo-limited sensor.
A scheme of an ultrasensitive magnetometer in the cavity quantum electromagnonics is proposed, where the intracavity microwave mode is coupled to a magnonic mode via magnetic dipole interaction. It is shown that by driving both magnonic and microwave modes with external classical fields and controlling the system parameters, one can reduce the added noise of magnetic field measurement below the standard quantum limit (SQL). Surprisingly, we show that beyond the rotating wave approximation (RWA), not only can the added noise be suppressed but also the output cavity response to the input signal can be substantially amplified in order to achieve a precise magnetic-field measurement. The estimated theoretical sensitivity of the proposed magnetic amplifier-sensor is approximately on the order of ${10}^{\ensuremath{-}18}\phantom{\rule{0.28em}{0ex}}\mathrm{T}/\sqrt{\mathrm{Hz}}$, which is competitive compared to the current state-of-the-art magnetometers like superconducting quantum interference devices (SQUIDs) and atomic magnetometers. The advantages of the proposed sensor in comparison with the other magnetometers is its high sensitivity at room temperature, sensing in a wide range of frequencies up to MHz, and its capability for signal-response amplification.
This thesis describes an ion trap magnetometer, in which experimental hardware and theory for trapped ion magnetometry has been developed and demonstrated. Quantum magnetometry has been a fast expanding field in recent years due to the plethora of military, medical and security applications. The current quantum magnetometer literature has shown great improvements on the standard magnetometer success metrics such as sensitivity, spacial resolution, frequency tuneability, noise shielding and vector field resolution. Ion trap magnetometry could potentially boast several advantages over some of the better established methods such as, nano-meter spatial resolution, broadband tuneability over radio-frequency (RF) and microwave frequencies, and magnetic field noise shielding and sensitivity improvements through dynamic decoupling methods; all in room temperature environments. The experimental setup discussed uses a micro-fabricated Paul trap for ion confinement due to the scalability of this technology. The method for sensing that was demonstrated utilises long-wavelength radiation dressed states to extend the coherence time of a single 171Yb+ ion beyond the bare states and to allow for a high level of frequency tuneability. The coherence times measured were T2 = 0:645 s for RF field sensing at 15.616 MHz and T2 = 1.153 s for microwave field sensing at 12.658466 GHz. Sensitivities of SB = 125.857 ± 26.155 pT/ vHz for RF magnetic fields and SB = 102.054 ± 11.046 pT/ vHz for microwave magnetic fields were achieved experimentally. A novel micro-fabricated Paul trap for trapping multiple ion clusters was developed to improve sensitivity values by increasing the total number of trapped ions. The design confines ions over a two-dimensional surface spanning 0.65 x 1.00 mm which would also allow for magnetic field gradient measurements. Efforts have been made to miniaturise certain experimental hardware to further the development of portable trapped ion experiments. For this, a miniaturised RF resonator that could be used for radial confinement of ions within Paul traps has been developed. At the current stage of development it is capable of applying a peak RF voltage of 114.9 V with a Q value of 70 at a resonant frequency of O/2p = 12.65 MHz.
This thesis presents advancements in radio-frequency atomic magnetometry utilising cold atoms as sensors. While cold atom magnetometers demonstrate lower absolute sensitivity compared to conventional vapour cell systems due to reduced atom numbers, they offer exceptional spatial resolution at the micrometre scale. The experimental apparatus features a cold atom production system capable of reaching quantum degeneracy through a refined cooling sequence that involves optical molasses, magnetic trapping and evaporation, and optical evaporative cooling in the dipole trap, culminating in a 87Rb Bose-Einstein condensate. This work details the development and characterisation of magnetometer set-ups with progressively enhanced performance. The first magnetometer utilised atoms in the optical molasses phase as a sensor, achieving an AC sensitivity of 330 pT/√Hz with approximately 108 atoms at 20 μK. Comprehensive magnetometry optimisation methods were developed to enhance sensitivity. Following laboratory interruptions due to the COVID-19 pandemic, system rebuilding incorporated significant upgrades, including a new probe laser and gradient compensation systems. The improved magnetometer prototype achieved a substantially enhanced sensitivity of 0.72±0.33 pT/√Hz, approaching the fundamental sensitivity limit of the system. Using atoms in the magnetic trap as a sensor, a sensitivity of 1.85 pT/√Hz was demonstrated. The system was verified to be photon-shot noise limited. Investigations into a double-pass probe configuration showed that, while the measured sensitivity was similar to that of the single-pass (4.3 pT/√Hz compared to 3.1 pT/√Hz at a probe power of 6.4 μW measured at the detector), the rotation angle increased by a factor of 2.2, with significantly reduced noise levels. These findings suggest that multi-pass strategies hold promise for enhanced performance in future implementations. In addition, the first signal detection from a BEC magnetometer, achieving 4.5 nT/√Hz with only 95,000 atoms at 35 nK was demonstrated.