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Development and demonstration of a high bandwidth, ultra sensitive trapped ion magnetometer

Ethan Robert Potter

Published Sep 24, 2019• 0 citations
OpenAlex

Abstract

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.

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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.
Magnetic quantum sensors based on trapped ions utilize properties of quantum mechanics which have optimized precision and beat current limits in sensor technology. Trapped ions are highly sensitive in a large span of signal ranging from DC or static B-field to the radiofrequency range in 100s of MHz and can attain the sensitivity in the range of pT to sub pT . They are tuneable to frequencies of interest and can be used as a lock-in frequency detector. This modelling and simulation based study presents an innovative design of Surface Paul Traps, enabling the use of trapped ions as ultra-sensitive sensors for magnetic field detection and precise measurement of magnetic field gradients at a sub-millimeter spatial resolution. The novel design features multiple trapping regions, allowing for the mapping of magnetic fields across various ion-trapping zones. The study demonstrates groundbreaking advancements in ion manipulation and confinement through innovative chip architecture.
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.
Quantum sensors are a new upcoming technology that offers to push the limits of how we can detect incredibly small signals in the DC and low RF regimes using magnetometers such as super-conducting quantum interference device (SQUID); N-V centres in Diamond, atomic vapour cells and atomic traps. All these technologies are currently being heavily invested in to push the boundaries of magnetometry for commercial applications. The quantum sensor I have been working to develop is set apart in several different aspects: unlike other quantum sensors it is very sensitive at the high RF and even 12.6 GHz microwave radiation; it has the ability to be rapidly tuned to different frequencies; due to the microwave decoupling technique we employ, the sensor is not sensitive to fluctuations in the DC field and therefore does not require bulky shielding or a laboratory environment to function. To this end I have built a demonstrator system capable of sensing RF and microwave fields and have measured the sensitivity of this device in both of these regimes. There has also been work on developing a portable version of the demonstrator and this work is ongoing within the group.
Scalability is a challenging yet key aspect required for large scale quantum computing and simulation using ions trapped in radio-frequency (rf) Paul traps. In this thesis 171Yb+ ions are used to demonstrate a magnetic field insensitive qubit which has a measured coherence time of 1.5 s, making it an ideal candidate to use for storing quantum information. A magnetic field sensitive qubit is also characterised which can be used for the implementation of multi-qubit gates using a potentially very scalable scheme based on microwaves in conjunction with a static magnetic field gradient instead of using lasers. However, the measured coherence time is limited by magnetic field fluctuations and will prohibit high fidelity gate operations from being performed. To address this issue, the preparation of a dressed-state qubit using a microwave based stimulated rapid adiabatic passage (STIRAP) pulse sequence will be presented. This qubit is protected against the noisy environment making it less sensitive to magnetic field fluctuations. The lifetime of this qubit is measured to demonstrate its suitability for storing quantum information. A powerful method for manipulating the dressed-state qubit will be presented and is used to measure a coherence time of the qubit of 500 ms which is two orders of magnitude longer compared to the magnetic field sensitive qubit. It will also be shown that our method allows for the implementation of arbitrary rotations of the dressed-state qubit on the Bloch sphere using only a single rf field. This substantially simplifies the experimental setup for single and multi-qubit gates. Furthermore, this thesis will present a experimental setup capable of successfully operating microfabricated surface ion traps. This setup is then used to operate and characterise the first two-dimensional (2D) lattice of ion traps on a microchip. A unique feature of the microfabrication technique used for this device is the extremely large voltage that can be applied which allows long ion lifetimes along with large secular frequencies to be measured, demonstrating the robustness of this device. Rudimentary shuttling between neighbouring lattice sites will be shown which could be used as part of a efficient scheme to load a large lattice of ions. One of the many applications of a 2D lattice of ions lies in the field of quantum simulations where many-body systems such as quantum magnetism, high temperature superconductivity, the fractional quantum hall effect and synthetic gauge fields can be simulated. It will be shown how making only minor modifications to the microchip the ion-ion separation can be reduced sufficiently to offer an exciting platform for the successful implementation of 2D quantum simulations. A theoretical investigation on the optimal 2D ion trap lattice geometry will also be presented with the aim to maximise the ratio of ion-ion coupling strength to decoherence from motional heating of the ions and to laser induced off-resonant coupling.
In the current landscape of quantum technologies, great efforts are being made to overcome the technical challenges of realising robust and scalable devices for use in a wide variety of potentially impactful applications. This thesis therefore describes the design considerations and assembly of a purpose-built vacuum system for the realisation of high-fidelity quantum logic operations on a microfabricated surface ion trap device. The device consists of an X-junction structure with embedded current-carrying wires, which will allow for the demonstration of scalable, laser-free, magnetic field gradient-based quantum logic. Initial characterisation experiments and observations pertaining to this novel microchip design are presented, as well as demonstrations of coherent quantum operations using the 171Yb+ qubit both with and without the current-carrying wire generated magnetic field gradient. In addition, this work also shows how this static magnetic field gradient design for scalable quantum computing may be leveraged for use in metrology. By utilising the inherent gradient-induced coupling of the trapped ion qubit’s transition frequency to it’s position in this architecture, the ion becomes extremely sensitive to electric field perturbations, enabling the implementation of many well-established quantum electric and magnetic field sensing techniques. Furthermore, the nature of this novel sensing scheme permits high sensitivities within the unique DC − kHz bandwidth, which many current quantum electrometers are unable to reach. Measurements of DC and AC electric fields yield unprecedented sensitivities for the sensing of phase coherent electric fields in a linear Paul trap. In addition, an implementation of a common magnetometry technique to measure electric field noise is also exhibited. Finally, it is shown that a purpose built system for the implementation of this scheme has the potential to improve sensitivities by up to six orders of magnitude, enabling the possibility of utilising this scheme for a wide variety of applications'
This thesis describes the experimental work towards the development of a scalable quantum computer based on microfabricated ion trap using long-wavelength radiation and magnetic field gradient. There are three key elements in implementing such a quantum computer: the junction trap to shuttle ions, the structure to generate high gradient magnetic field and the structure to induce strong microwave coupling to the ions. A new dynamic simulation tool was developed addressing the problems faced by static solvers. This tool was used to aid the design and optimisation of an X junction geometry allowing the ions to be shuttled with minimised motional heating gain. The design and fabrication techniques were reported on the structure to generate a high gradient magnetic field. A discussion was given on the design of producing microwave and maximise the coupling to the cold ions. A review was given on the far-field methods and near-field methods. A novel design was reported where the single-qubit gate is predicted to be 45 times faster than a conventional setup. Two essential topics on the microfabrication of a reliable scalable quantum computer unit were discussed: breakdown and RF loss. Investigation using numerical simulations showed that dielectric can breakdown due to high voltage and local heating which are results of impedance mismatch. Microfabrication processes were improved, high-quality films were reported to have twice as much breakdown voltages. The mechanisms of RF loss were reviewed. Novel structures and smooth electroplating technique were developed to minimise the loss. A low loss ion trap was produced and tested. An experimentally observed anomalous glow discharge phenomenon was reported and investigated.