Hyperpolarized xenon nuclear spins detected by optical atomic magnetometry

We report the use of an atomic magnetometer based on nonlinear magneto-optical rotation with frequency modulated light (FM NMOR) to detect nuclear magnetization of xenon gas. The magnetization of a spin-exchange-polarized xenon sample ($1.7$cm$^3$ at a pressure of $5$bar, natural isotopic abundance, polarization 1%), prepared remotely to the detection apparatus, is measured with an atomic sensor (which is insensitive to the leading field of 0.45 G applied to the sample; an independent bias field at the sensor is $140 \mu$G). An average magnetic field of $\sim 10$nG induced by the xenon sample on the 10-cm diameter atomic sensor is detected with signal-to-noise ratio $\sim 10$, limited by residual noise in the magnetic environment. The possibility of using modern atomic magnetometers as detectors of nuclear magnetic resonance and in magnetic resonance imaging is discussed. Atomic magnetometers appear to be ideally suited for emerging low-field and remote-detection magnetic resonance applications.Comment: 4 pages, 4 figure

μSQUID susceptometry of molecular qubits

El objetivo fundamental de este trabajo es la investigación de las propiedades magnéticas de distintos tipos de partículas pertenecientes a dos mundos: el mundo cuántico, representado por una nueva familia de imanes moleculares, y el mundo clásico, representado por una serie de nanopartículas magnéticas sintetizadas en ferritina. Estos estudios han sido realizados por medio de medidas de susceptibilidad magnética ac a muy bajas temperaturas, utilizando para ello susceptómetros uSQUID enormemente sensibles desarrollados también como parte de este trabajo

Broken Ergodicity and 1/f1/f Noise from Finite, Local Entropy Baths

abstract: Fluctuations with a power spectral density depending on frequency as $1/f^\alpha$ ($0<\alpha<2$) are found in a wide class of systems. The number of systems exhibiting $1/f$ noise means it has far-reaching practical implications; it also suggests a possibly universal explanation, or at least a set of shared properties. Given this diversity, there are numerous models of $1/f$ noise. In this dissertation, I summarize my research into models based on linking the characteristic times of fluctuations of a quantity to its multiplicity of states. With this condition satisfied, I show that a quantity will undergo $1/f$ fluctuations and exhibit associated properties, such as slow dynamics, divergence of time scales, and ergodicity breaking. I propose that multiplicity-dependent characteristic times come about when a system shares a constant, maximized amount of entropy with a finite bath. This may be the case when systems are imperfectly coupled to their thermal environment and the exchange of conserved quantities is mediated through their local environment. To demonstrate the effects of multiplicity-dependent characteristic times, I present numerical simulations of two models. The first consists of non-interacting spins in $0$-field coupled to an explicit finite bath. This model has the advantage of being degenerate, so that its multiplicity alone determines the dynamics. Fluctuations of the alignment of this model will be compared to voltage fluctuations across a mesoscopic metal-insulator-metal junction. The second model consists of classical, interacting Heisenberg spins with a dynamic constraint that slows fluctuations according to the multiplicity of the system's alignment. Fluctuations in one component of the alignment will be compared to the flux noise in superconducting quantum interference devices (SQUIDs). Finally, I will compare both of these models to each other and some of the most popular models of $1/f$ noise, including those based on a superposition of exponential relaxation processes and those based on power law renewal processes.Dissertation/ThesisDoctoral Dissertation Physics 201

Radio-frequency atomic magnetometry with a rubidium Bose-Einstein condensate

This thesis details progress in radio-frequency atomic magnetometry with ultracold rubidium atoms. Motivations and context are first covered, before an introduction of the main concepts required to understand the underlying physics is given. At first, a cold atom magnetometer is designed, built and characterised. Consistent 20 µK atoms are produced. Radio-frequency (RF) atomic magnetometry (AM) is performed by placing the atoms in a bias magnetic field and generating coherent precession with an external AC field. A noise floor at 330 pT/√Hz defines the sensor’s sensitivity, with a range of applications. RF-AM is then performed with a Bose-Einstein condensate (BEC). The 20 µK atoms are loaded into a magnetic trap, where RF evaporation increases their phase space density (PSD = nλ^3dB, n is the density and λdB is the thermal de Broglie wavelength of the atoms). Next, atoms are transferred into a hybrid dipole trap, collecting in a dimple created at the intersect of two high power laser beams. Production and stabilisation of these beams is described, which are focused down to a 75 µm beam waist at the trap position with a total power of 7 W. Optimisation of the evaporation process in both traps leads to consistent BEC production. A pure condensate with 4x10^4 atoms at 25 nK is reported. Radio-frequency magnetometry is performed at various probe volumes. With systematic optimisation a best AC sensitivity of 24 pT/√Hz with 3.4 × 10^8 atoms in the magnetic trap before evaporation is achieved. This is extended to the BEC with 4 × 10^4 atoms, where an AC sensitivity of 84 nT/√Hz and DC sensitivity of 14 nT/√Hz is reported, bringing previously achieved atomic magnetometry into the micrometer regime. A trade-off must be considered due to reduction in sensitivity at lower probe volumes. Volumes between 1.4×10−7 m^3 and 1.6×10−14 m^3 can be accessed, highlighting the sensors adaptability and tunability for different applications. The results are contextualised in the background of previously achieved magnetometers of various types. Finally, proof-of-concept electromagnetic induction imaging (EMI) measurements are made to confirm the sensor’s viability for high resolution imaging

Quantum computing with molecular magnets

En esta tesis se ha pretendido realizar y testar los componentes básicos necesarios de un futuro ordenador cuántico usando para ello moléculas magnéticas sencillas. Con este fin se ha comprobado que estas moléculas cumplen con los tres ingredientes principales que necesita reunir un buen candidato: qubits bien definidos y diferenciables, coherencia cuántica y la posibilidad de acoplarse a dispositivos. También se ha estudiado, por tanto, si estas moléculas conservan sus propiedades magnéticas, tales como el espín, la anisotropía magnética, la interacción entre distintos iones, etc., cuando se integran en un sensor. El trabajo que aquí se presenta se divide fundamentalmente en tres partes. Desarrollo, mejora y testado de dispositivos superconductores de interferencia cuántica (SQUID) En esta parte de la tesis se realiza un análisis detallado de varios modelos de dispositivos pertenecientes a nueva generación de susceptómetros SQUID, especialmente diseñados para medir las propiedades magnéticas de muestras de tamaño micrométrico, hasta 30 um de diámetro (capítulo 3). En particular, presentamos un estudio del ruido de estos sensores, teniendo en cuenta las distintas contribuciones extrínsecas al SQUID y provenientes de la electrónica. También se estudia la posibilidad de incluir una etapa amplificadora de bajo ruido, que permita reducir esta contribución. Por otro lado, se ha hecho un cuidadoso análisis sobre el origen de la respuesta magnética de los susceptómetros vacíos, con el fin de llegar a un protocolo común de tratamiento de los datos de susceptibilidad ac obtenidos para distintas muestras. Por último, y con el objetivo de mejorar la sensibilidad de los sensores en la detección de muestras de tamaño nanométrico, en este trabajo se presenta la realización mediante Focused Ion Beam (FIB) de una nanoespira de unos 500 nm de diámetro, en serie con la bobina detectora del SQUID. Esta nanoespira permite mejorar la sensibilidad del dispositivo en esa zona. Para comprobarlo, se incluyen varias medidas con muestras cuyo comportamiento magnético es conocido. Caracterización de cristales de moléculas magnéticas con los que se pueden llevar a cabo puertas lógicas computacionales La segunda parte incluye un estudio de imanes moleculares que contienen uno, dos o tres espines magnéticos débilmente acoplados, con el fin de comprobar que pueden utilizarse para llevar a cabo las operaciones básicas de computación cuántica. Por un lado, se estudian dímeros constituidos por dos iones Ln(III) donde Ln es un lantánido (capítulo 4). Cada uno de los espines que forman el dímero tiene una estructura de niveles tal que los dos niveles de menor energía están fuertemente separados de los niveles superiores, formando, por tanto, sistemas de dos niveles efectivos o qubits. Por otro lado, estos espines están débilmente acoplados entre sí, lo que constituye un sistema de cuatro niveles entrelazados con los que es posible realizar puertas cuánticas de dos qubits, en particular las puertas CNOT y SWAP. En esta tesis se presentan los primeros resultados de realización de estas puertas, en los dímeros de Tb2, Dy2 y ErCe. En cuanto a los trímeros, o imanes moleculares con tres espines magnéticos, en esta tesis se presenta un estudio para el trímero compuesto por dos iones Cu(II) y un ion Er(III) (capítulo 5). En primer lugar se analiza el comportamiento magnético de los dos iones de cobre en el trímero CuLaCu, donde el ion La(III) es diamagnético, y se comprueba que existe un acoplo entre sus espines, permitiendo la realización de puertas de dos qubits, también para este material. Incluyendo la información obtenida para este compuesto, se presenta por último un análisis del comportamiento magnético del trímero CuErCu constatando la existencia de un acoplo muy débil entre los iones cobre (II) y el ion erbio y, por tanto, la posibilidad de realizar puertas cuánticas de tres qubits, como las puertas Fredkin y Toffoli. Estudio de procesos de relajación magnética y deposición de imanes moleculares sobre superficies Esta última parte pretende aunar las dos partes anteriores, depositando las moléculas estudiadas en la parte anterior sobre los dispositivos superconductores del principio mediante la técnica de Dip-Pen Nanolitography (DPN). La intención última es comprobar si estas moléculas mantienen sus propiedades magnéticas al ser sometidas a los efectos de un sustrato sólido ajeno a su estructura cristalina, y comprobar si se trata de sistemas lo suficientemente coherentes como para realizar operaciones cuánticas sin que los estados se degraden demasiado rápido. En primer lugar, en el capítulo 6 se presenta un estudio de los procesos de relajación de estas moléculas en su estado original, atendiendo al origen y las causas de estos procesos. Para ello se ofrecen los resultados en una serie de muestras en las que hay cada vez más niveles accesibles, partiendo de un monómero ErLa con un sólo espín magnético y sin apenas interacciones hiperfinas, hasta llegar al caso de un dímero con dos espines magnéticos y una considerable contribución nuclear, Dy2. Una vez entendidos los procesos de relajación en las muestras cristalinas, se han depositado unas pocas monocapas de estas muestras sobre los SQUID utilizando la técnica DPN, con el fin de comprobar su integridad física (capítulo 7). En particular se describen experimentos para dos muestras diferentes, un imán molecular cuyo comportamiento es bastante conocido, el Mn12, y un dímero de Dy2. En este capítulo se muestran los resultados de las medidas de susceptibilidad ac en comparación con las medidas previas, realizadas sobre microcristales

EMN-Q - Strategic Research Agenda and Quantum Technologies Roadmaps

Together with the EMN-Q web platform, EMN-Q events and workshops this Strategic Research Agendanrepresents, in the intention of the EMN-Q members, one of the main communication channels to QT stakeholders from academia, industry, and standardisation bodies. The Strategic Research Agenda once developed will be periodically updated by the EMN-Q, with the support of the QT community, to ensure that QT metrology needs, even if evolving, are taken into account and national metrology activities are coordinated, prioritised and focussed. The roadmaps will ensure that NMIs can align their research programmes in anticipation of future needs so that European QTs will be supported far into the future. In turn, QT stakeholders will acquire confidence that their metrological needs will be addressed by a long-term and coherent approach; improving on the present situation where short-term projects are based on ad-hoc needs. NMIs' awareness and involvement early on in device development will provide assurances to the user community and help the QT market to grow

Application of quantum magnetometers to security and defence screening

Over recent years the sensitivity of alkali-metal vapour magnetometers has been demonstrated to surpass that of even Superconducting Quantum Interference Devices (SQUIDs), the current commercial gold standard in laboratory weak- field magnetometry sensing. Here we present a proof-of-principle approach to building an RF atomic magnetometer which is robust, portable, tunable, non-invasive and operable at room temperature in an unshielded environment. In view of these characteristics, we discuss the potential application of alkali-metal magnetometry in imaging concealed objects, non-destructive evaluation of the structural integrity of metallic objects (e.g. pipelines and aircraft), and detection of rotating motors. We present a cost-effective approach to operating an atomic magnetometer in a Magnetic Induction Tomography (MIT) modality, to non-invasively map the conductivity of conductive objects concealed by conductive materials remotely and in real time. This is achieved by measuring the secondary eld in the subject due to eddy currents circulating as a result of application of a tunable radio-frequency oscillating eld, which overcomes the bandwidth and sensitivity limitations of using coils for sensing as in conventional MIT. In addition, we demonstrate the use of the atomic magnetometer for the remote detection of DC and AC electric motors with an improved response compared with a commercial fluxgate magnetometer in the sub 50 Hz regime (particularly detection down to 15 Hz). Its capability for non-invasive measurement through concrete walls is established, with potential for use in industrial monitoring and detection of illicit activity. Finally, the possibility of detection of submerged targets or for the atomic magnetometer to be mounted on submarine vehicles was explored. Promising results were obtained, but further investigation is required in this environment to establish this as a viable marine detector

NanoSQUIDs for Millikelvin Magnetometry

Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate