Session S07: Atomic Magnetometers

Live

The optically-pumped atomic magnetometers, when operating in the spin-exchange-relaxation-free regime, provide femtotesla-level sensitivity and are therefore attractive for a host of applications including biological sensing, geosurveying and magnetic map-based navigation. Practical implementation of atomic magnetometers, especially in magnetic field imaging configurations involving multiple sensing channels, requires the sensor design to be compact, with tight integration between the atomic vapor cell and optical components. To address the need, we propose to implement metasurface-based nanophotonic components, instead of bulk birefringent optical elements, for polarization rotation and polarimetry of atomic spins in a rubidium-based magnetometer. We will present our progress on photonic component designs, material selection and fabrication, and magnetometer testbed development. Finally, we analyze the sensitivity and limits of the photonic-integrated atomic magnetometer, based on the projected and simulated performance of our optical designs.   

Live

Atom interferometers generally make use of magnetically insensitive internal states to avoid coupling environmental noise into their signal. By using magnetically sensitive states, and adding a magnetic field gradient, we produce a state dependent acceleration that we can use to increase the sensitivity of the interferometer. In order to do this, we must be able to create a linear magnetic field gradient without temporal or spatial noise, which will now be coupled into the measurement signal. By using the Raman spectra of our atoms along their ballistic path in the interferometer, we are able to map the field \emph{in situ}. We present our progress using this method to create the clean, linear field environment necessary for the primary experiment. Using small, fluxgate magnetometers next to the vacuum cell, we actively stabilize the field. Using the same Raman field mapping technique, we analyze the effects of this classical field stabilization.   

Live

We demonstrate a novel spin-exchange pumped noble gas comagnetometer which suppresses bias magnetic field noise by more than a factor of 10$^{\mathrm{3}}$. The presented device achieves $\mu $Hz-scale rotation sensitivity. Two Xe isotopes and Rb atoms are continuously polarized transverse to a pulsed bias field. Each field pulse produces 2$\pi $ Larmor precession of the Rb atoms. Both Xe isotopes' nuclear magnetic resonance conditions are simultaneously satisfied by frequency modulation of the pulse repetition rate. The Rb atoms serve as an embedded magnetometer for detection of the Xe precession. We discuss performance and the effect of magnetometer phase shifts.   

On Demand

We present a portable all-optical atomic gradiometer with two $^{87}$Rb vapor cells separated by 3 cm. A Bell-Bloom style optical pumping polarizes $^{87}$Rb into an $F=2$ edge state in a plane transverse to Earth’s field, in order to suppress Rb-Rb spin-exchange relaxation typically dominant at Earth's field. A detuned probe laser measures $^{87}$Rb free-precession frequencies, giving our sensor the large dynamic range and sub-ppb resolution necessary for a sensitive magnetometer operating in Earth’s field. The bandwidth is set by our shot-to-shot repetition rate of 300 Hz. All lasers are contained within the sensor head, and using state-of-the-art micro-fabricated vapor cells with advanced thermal insulation and custom electronics, we reduce the system power to 5 W and run the sensor from a laptop. We find unshielded sensitivity of 16 fT/cm/Hz$^{1/2}$ in Earth’s field and 10 fT/cm/Hz$^{1/2}$ at 50 $\mu$T in mu-metal shielding, close to our theoretical predictions. We use the sensor for unshielded MEG and MCG, demonstrating sensitive all-optical atomic magnetometers working unshielded in Earth-scale fields, which will prove useful for a variety of applications.   

On Demand

The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface of a sample to study iron-pnictide superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. We conduct simultaneous and spatially resolved measurements of both bulk and surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing local measurements of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope’s ability to reveal important insights into the physics of complex quantum materials.   

On Demand

We implant rubidium atoms inside cryogenic solid parahydrogen and study their electron spin coherence. We have measured the ensemble spin dephasing time T$_2^*$ to be on the order of microseconds, six orders of magnitude below the longitudinal relaxation time T$_1$. We observe that with dynamical decoupling pulse sequences, we can achieve much longer spin coherence times. A modified Hahn echo extends T$_2$ to times on the order of milliseconds, and the alternating-phase Carr-Purcell sequence gives a T$_2$ approaching 0.1~s. The physics limiting the spin coherence will be discussed. Long spin coherence times make atoms in parahydrogen promising for applications in AC magnetometry, quantum sensing, and nano-MRI.   

Not Participating

We describe quantum measurement and control of a physically rotating quantum system, where the rotation period is comparable to the T2 decoherence time of the quantum system. We use the NV center in diamond, rotating at rates up to 500,000 rpm, comparable to the nuclear spin precession frequency, to induce pseudo-fields large enough to cancel the conventional magnetic field for proximal nuclear spins while having minimal effect on the NV qubits. We have also demonstrated T2-limited sensing of static magnetic fields by upconversion of the DC field to the physical rotation frequency, allowing application of spin-echo measurement for durations 100 times longer than Ramsey experiments in a non-rotating system. Most recently, we show the first direct measurement of a quantum phase induced by physical rotation, without transduction through magnetic fields or ancillary spins. Our results highlight the profound connection between physical rotation and quantum spin through the theory of angular momentum.   

Not Participating

Understanding the relationship between brain activity and specific mental function is important for medical diagnosis of brain symptoms, such as epilepsy. Magnetoencephalography (MEG) is a promising non-invasive method for locating the brain disease. MEG uses an array of high sensitivity magnetometers, such as superconducting quantum interference devices (SQUIDs), to record magnetic fields generated from electrical currents occurring naturally in the brain. Recent years have seen rapid developments in optically pumped atomic magnetometers (OPMs), both in sensitivity and portability, which have made the replacement of SQUIDs with OPMs a general tendency within the MEG community. Currently, nearly all the MEG measurements are performed in a magnetically shielded room. Here we introduce an unshielded MEG system based on OPMs. We successfully observe the alpha-rhythm MEG signals related to open and closed eyes, and a clear auditory evoked magnetic field signal in unshielded earth field. Combined with further improvements in the miniaturization of the atomic magnetometer, our method is promising to realize a practical movable and wearable MEG system without magnetic shielding, open new applications in clinical research, and bring new insights into the medical diagnosis of brain symptoms.