Magnetic Field Noise Research Articles

Leakage mitigation for quantum error correction using a mixed qubit scheme

Leakage errors take qubits out of the computational subspace and will accumulate if not addressed. A leaked qubit will reduce the effectiveness of quantum error correction protocols due to the cost of implementing leakage reduction circuits and the harm caused by interacting leaked states with qubit states. Ion trap qubits driven by Raman gates have a natural choice between qubits encoded in magnetically insensitive hyperfine states that can leak and qubits encoded in magnetically sensitive Zeeman states of the electron spin that cannot leak. In our previous work, we compared these two qubits in the context of the toric code with a depolarizing leakage error model and found that for magnetic field noise with a standard deviation less than 32 $\mu$G that the $^{174}$Yb$^+$ Zeeman qubit outperforms the $^{171}$Yb$^+$ hyperfine qubit. Here we examine a physically motivated leakage error model based on ions interacting via the Molmer-Sorenson gate. We find that this greatly improves the performance of hyperfine qubits but the Zeeman qubits are more effective for magnetic field noise with a standard deviation less than 10 $\mu$G. At these low magnetic fields, we find that the best choice is a mixed qubit scheme where the hyperfine qubits are the ancilla and the leakage is handled without the need of an additional leakage reduction circuit.

Positron emission tomography visualized stimulation of the vestibular organ is localized in Heschl's gyrus.

The existence of a human primary vestibular cortex is still debated. Current knowledge mainly derives from functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) acquisitions during artificial vestibular stimulation. This may be problematic as artificial vestibular stimulation entails coactivation of other sensory receptors. The use of fMRI is challenging as the strong magnetic field and loud noise during MRI may both stimulate the vestibular organ. This study aimed to characterize the cortical activity during natural stimulation of the human vestibular organ. Two fluorodeoxyglucose (FDG)‐PET scans were obtained after natural vestibular stimulation in a self‐propelled chair. Two types of stimuli were applied: (a) rotation (horizontal semicircular canal) and (b) linear sideways movement (utriculus). A comparable baseline FDG‐PET scan was obtained after sitting motion‐less in the chair. In both stimulation paradigms, significantly increased FDG uptake was measured bilaterally in the medial part of Heschl's gyrus, with some overlap into the posterior insula. This is the first neuroimaging study to visualize cortical processing of natural vestibular stimuli. FDG uptake was demonstrated in the medial‐most part of Heschl's gyrus, normally associated with the primary auditory cortex. This anatomical localization seems plausible, considering that the labyrinth contains both the vestibular organ and the cochlea.

Reduction of Environmental Magnetic Field Noise for Detection of Small Magnetic Contaminants

When performing high-sensitivity magnetic measurements via a high-sensitivity magnetic sensor such as a SQUID (Superconducting QUantum Interference Device) magnetometer, it is necessary to shield from environmental magnetic noise by employing a magnetic shield box. In general, however, the low frequency of such noise decreases the shielding performance of the magnetic shield box. Hence, the magnetic metallic contaminant signal and the low-frequency ambient magnetic field noise overlap, which makes filtering a challenging task. Therefore, we studied a system that suppresses gradient magnetic noise such as low-frequency environmental noise superimposed on a contaminant signal and commercial power supply noise localized spatially.

Fine-Tuning and Optimization of Superconducting Quantum Magnetic Sensors by Thermal Annealing.

In the present article, we present the experimental results concerning the fine-tuning and optimization of superconducting quantum interference device (SQUID) parameters by thermal annealing. This treatment allows for the modification of the parameters in order to meet a specific application or to adjust the device parameters to prevent the increase of magnetic field noise and work instability conditions due to a different critical current with respect to the design value. In particular, we report the sensor critical current, the voltage–flux (V–Φ) characteristics and the spectral density of the magnetic field of SQUID magnetometers for different annealing temperatures. The measurements demonstrate that it is possible to achieve a fine control of the most important device parameters. In particular, we show that thermal annealing allows for the reduction of SQUID noise by more than a factor of 5 and makes the device working operations very stable. These results are very useful in view of quantum technology applications related to superconducting quantum computing where the correct functioning of the quantum bit depends on the fine control of the superconducting quantum device parameters and selectable annealing is possible by using a suitable laser as a thermal source.

Single-spin sensing of domain-wall structure and dynamics in a thin-film skyrmion host

Skyrmions are nanoscale magnetic structures with features promising for future low-power memory or logic devices. In this work, we demonstrate novel scanning techniques based on nitrogen vacancy center magnetometry that simultaneously probe both the magnetic dynamics and structure of room temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the handedness of the Dzyaloshinskii-Moriya interaction in this material and extract the helicity angle of the skyrmion bubbles. Our measurements also show that the skyrmion bubbles in this material change size in discrete steps, dependent on the local pinning environment, with their average size determined dynamically as their domain walls hop between pinning sites. In addition, an increase in magnetic field noise is observed near all skyrmion bubble domain walls. These measurements highlight the importance of interactions between internal degrees of freedom of skyrmion bubble domain walls and pinning sites in thin film systems. Our observations have relevance for future devices based on skyrmion bubbles where pinning interactions will determine important aspects of current-driven motion.

A High-Balance Planar SQUID Gradiometer

We have developed and fabricated SQUID gradiometers in a Nb/Al-AlOx/Nb technology using an i-line stepper combined with a mask aligner lithography. The stepper and the aligner are used to fabricate fine-scale patterns such as the Josephson junctions and large-scale patterns such as pickup coil, respectively, on the same wafer with a 4-inch diameter. The SQUID is designed as a first-order gradiometer with a dual-washer configuration. Two sets of multiturn spiral input coils with opposite directions are integrated on the washer and connected to the pickup coil to form a flux transformer. In order to improve the balance of the gradiometer, a fully symmetric configuration is introduced with a mirrored SQUID. The pickup coil is a first-order gradiometric type with a base line of 3.5 cm. The size of each loop is 12.65 mm × 11.65 mm, which results in a gradient magnetic field sensitivity of 0.25 nT/(cm·Φ0). The measured gradient magnetic field noise is 75 fT/(m·√Hz) in the white noise region. Moreover, a sapphire holder is used to fix and package the chip in order to maintain the high intrinsic balance (typical 10−4). It makes the SQUID gradiometer more attractive for an unshielded operation in noisy environments.

A cryogenic radio-frequency ion trap for quantum logic spectroscopy of highly charged ions.

A cryogenic radio-frequency ion trap system designed for quantum logic spectroscopy of highly charged ions (HCI) is presented. It includes a segmented linear Paul trap, an in-vacuum imaging lens, and a helical resonator. We demonstrate ground state cooling of all three modes of motion of a single 9Be+ ion and determine their heating rates as well as excess axial micromotion. The trap shows one of the lowest levels of electric field noise published to date. We investigate the magnetic-field noise suppression in cryogenic shields made from segmented copper, the resulting magnetic field stability at the ion position and the resulting coherence time. Using this trap in conjunction with an electron beam ion trap and a deceleration beamline, we have been able to trap single highly charged Ar13+ (Ar XIV) ions concurrently with single Be+ ions, a key prerequisite for the first quantum logic spectroscopy of a HCI. This major stepping stone allows us to push highly-charged-ion spectroscopic precision from the gigahertz to the hertz level and below.

Publisher's Note: Competition between electric field and magnetic field noise in the decoherence of a single spin in diamond [Phys. Rev. B 93 , 024305 (2016)

Received 13 June 2019DOI:https://doi.org/10.1103/PhysRevB.99.249903©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFluctuations & noiseOpen quantum systems & decoherenceQuantum information with solid state qubitsPhysical SystemsNitrogen vacancy centers in diamondCondensed Matter, Materials & Applied Physics

Effect of Niobium Cylinder on Output of the Three-Dimensional SQUID Gradiometer

Because of the large ambient noise in biomagnetic measurements, it is usually necessary to use a gradiometer to reduce the environmental noise [A. Hayashi, Y. Hirata, and S. Kuriki, “Coil cancellation scheme with an electronic gradiometer for the reduction of magnetic field noise,” J. Magn. Soc. Jpn., vol. 29, no. 4, pp. 477-481, 2005.]. In order to prevent noise coupling into the SQUID from the on-chip input coil, it is also necessary to cover the chip with a niobium cylinder [S. L. Zhang et al., “A novel superconducting quantum interference device for biomagnetic measurements,” Sci. Bull., vol. 58, no. 24, pp. 2917-2919, 2013.], [J. R. Claycomb and J. H. Miller, Jr., “Superconducting magnetic shields for SQUID applications,” Rev. Sci. Instrum., vol. 70, no. 12, pp. 4562-4568, 1999.]. This paper focuses on the possible impact of introducing a niobium cylinder on the output of a SQUID gradiometer and how to reduce this effect. In this paper, the effect of the niobium cylinder on the output is equivalent to imbalance in simulation and measurement. Changing the relative position of the niobium cylinder makes the equivalent imbalance of the SQUID gradiometer coil less than 1%, which is caused by manual winding.

Magnetic Gradiometer for the Detection of Zero- to Ultralow-Field Nuclear Magnetic Resonance

Magnetic sensors are important for detecting nuclear magnetization signals in nuclear magnetic resonance (NMR). As a complementary analysis tool to conventional high-field NMR, zero- and ultralow-field (ZULF) NMR detects nuclear magnetization signals in the sub-microtesla regime. Current ZULF NMR systems are always equipped with high-quality magnetic shieldings to ensure that ambient magnetic field noise does not dwarf the magnetization signal. An alternative approach is to separate the magnetization signal from the noise based on their differing spatial profiles, as can be achieved using a magnetic gradiometer. Here, we present a gradiometric ZULF NMR spectrometer with a magnetic gradient noise of 17 fT_{rms}{cm}^{-1}{Hz}^{-1/2} in the frequency range of 100-400 Hz, based on a single vapor cell (0.7x0.7x1.0{cm}^3). With applied white magnetic-field noise, we show that the gradiometric spectrometer achieves 13-fold enhancement in the signal-to-noise ratio (SNR) compared to the single-channel configuration. By reducing the influence of common-mode magnetic noise, this work enables the use of compact and low-cost magnetic shields. Gradiometric detection may also prove to be beneficial for eliminating systematic errors in ZULF-NMR experiments searching for exotic spin-dependent interactions and molecular parity violation.

Noise performance of SQUID magnetometers as a function of critical current value: a statistical-like approach

The critical current value is a key parameter of a Superconducting Quantum Interference Device. In this paper we investigate the effects of different critical current values on the noise performance of SQUID magnetometers. By using a statistical-like approach, we measure the spectral density of magnetic field noise of 200 magnetometers relating the values to the critical current change. The magnetometer consists of a dc-SQUID in a washer shape magnetically coupled to a square superconducting pickup coil. The fully integrated design includes a feedback coil to work in Flux Locked Loop mode and an Additional Positive Feedback circuit to increase its voltage responsivity. The main application field of these sensors is the magnetoencephalography that exploits multichannel systems based on SQUID magnetometers, in which the uniformity of noise performance of the sensors is needed. We found that a best value of critical current is 20 μΑ and also that a value spread ranging from 8 to 40 μm, does not affect in an evident way the sensitivity of such magnetometers. The corresponding noise spectral density remains within the range 2 – 6 fT/⎷Hz.

PKBPNN-Based Tracking Range Extending Approach for TMR Magnetic Tracking System

The magnetic tracking system, which is based on a permanent magnet and a magnetometer array, has numerous potential applications in the biomedical and industrial area. However, its tracking accuracy drops off sharply with the increase of tracking distance because of both the interference of environmental magnetic field and sensor measurement noise. To extend the magnetic tracking range, two novel methods were proposed in this study. First, the state-of-the-art tri-axial tunnel magnetoresistance (TMR) sensors, which possess the most significant field sensitivity and signal-to-noise ratio (SNR) over other types of magnetoresistive sensors, were adopted to construct the sensor array. Second, a fusion approach was proposed for tracking range extending. The particle swarm optimization-Levenberg Marquardt (PSO-LM) method based on the magnetic dipole model was applied in the near field (i.e., near-source zone), and the prior knowledge based back propagation neural network (PKBPNN) was adopted to the far field (i.e., far-source zone). In the transition zone, these two algorithms were fused by using an adaptive sigmoid function. The trained artificial neural network (ANN) model embodied the physical model errors, the sensor installation errors, and the inherent characteristics of adopted magnetometers. Therefore, it has greater tracking performance than singly using the PSO-LM in the far-source zone. The experimental results show that the tracking errors decrease from (18.24 ± 9.37mm, 12.45 ± 3.37°) to (8.95 ± 1.74mm, 7.97 ± 2.08°) in the tracking range between 216 and 296 mm. Besides, the tracking distance is extended to 396 mm, with the position error of less than 25 mm. It can be concluded that this approach has significantly extended the tracking range of the magnetic tracking system.

星上剩磁对惯性传感器的影响

The requirement of space gravitational wave detection on residual acceleration is extremely high (10(-15) ms(-2) Hz(-1/2)), and the environmental magnetic field will cause magnetic force and Lorentz force. To ensure the accurately detection of gravitational wave, the environmental magnetic field and its gradient must be controlled within a low range. In this paper, we mainly research the effect of the on-board residual magnetism on internal sensors. Then, the relationship between residual magnetism and acceleration is explored from the aspects of interstellar magnetic field, residual magnetism of satellite components and time-varying magnetic field detection. Moreover, the simulation and detection of magnetic field are discussed. The results show that the remanence magnetic field can be reduced by optimizing the location and the orientation of the magnetic source. It is necessary to control magnetic field noise by real-time monitoring of interstellar magnetic field and time-varying magnetic field by adopting weak magnetic detection device for obtaining high-precision gravitational wave detection data. It can be concluded that it's necessary to analyze the influence of on-board residual magnetism on the inertial sensors, and the magnetic field evaluation schemes and weak magnetic detection methods for satellite platform should be developed.

Zero Magnetic Field Calibration for Single-Beam Atomic Magnetometers Using Second Harmonics

In this letter, we propose a simple and rapid method for zero field calibration of single elliptically polarized beam atomic magnetometers. This method uses a wobble adjustment that applies a small amplitude and low-frequency (70 Hz) ac magnetic field and sweeps the dc magnetic field. The second harmonic signal reaches its maximum when the dc magnetic field noise is zero. Our single-beam atomic magnetometer operates with elliptically polarized light, which has both circularly and linearly polarized light components. The wavelength of a single beam for a rubidium atom was determined to be 780.206 nm with a power of 2.72 mW.

Electric Noise Spectra of a Near-Surface Nitrogen-Vacancy Center in Diamond with a Protective Layer

Surface noise is a detrimental issue for sensing devices based on shallow nitrogen vacancy (NV) color center diamonds. A recent experiment indicates that electric field noise is significant compared to magnetic field noise. They also found that the electric field noise can be reduced with a protective surface layer, though the mechanism of noise reduction is not well understood. We examine the effect of a protective surface layer on the noise spectrum, which is caused by surface charge fluctuations. We use the fluctuation-dissipation theorem to calculate and analyze the noise spectrum for six different surface layer materials typically used for NV center diamond devices. We find that four parameters largely affect the noise spectrum: effective relaxation time, effective loss tangent, power law exponent of the noise spectrum, and layer thickness. Our results suggest that a surface covering layer is indeed useful for decreasing surface noise, but which material is most suitable depends on the device operational frequency range.

Realization of two-stage crossed beam cooling and the comparison with Delta-kick cooling in experiment.

We report the first experimental realization of the two-stage crossed beam cooling (TSCBC) method that we proposed in 2013 [L. Wang et al., J. Phys. B: At., Mol. Opt. Phys. 46, 195302 (2013)]. With the 87Rb Bose-Einstein condensation apparatus and electromagnet coils providing the magnetic levitation to counteract the gravitation, we simulated the micro-gravity environment and realized the TSCBC with 4 × 104 87Rb atoms. We estimated that the lowest temperature of atoms can be at 3.56 nK with a new method and verified that the cooling process is adiabatic enough with time-of-flight images. According to analysis, we believed that the noise of magnetic field was the main obstacle that hinders the further cooling of the atomic ensemble. Under the same experimental conditions, we carried out the Delta-kick cooling method and got a lowest temperature of 23.3 nK also with 4 × 104 87Rb atoms. According to the results of comparing experiments, we can see that the TSCBC method is more effective.

Improvement of Performance of SQUID Magnetometer System for Highly Sensitive Geomagnetic Field Measurements

We have been developing a LTS superconducting quantum interference device (SQUID) magnetometer system aiming at highly sensitive geomagnetic field measurements with an intended magnetic field noise floor of approximately 10 fT/√Hz in the frequency range below 100 Hz. However, our previous system using SQUID magnetometers with the noise of ∼30 fT/√Hz had the system noise of ∼200 fT/√Hz, and the measurable frequency bandwidth was limited up to ∼30 Hz. This unsatisfactory performance was due to the thermal noise and cutoff frequency of an electromagnetic (EM) shield made of copper foil, which covered the SQUID magnetometers cylindrically, because its design was not optimum. To improve the instrument performance, we fabricated SQUID magnetometers having a noise floor of ∼10 fT/√Hz. We also evaluated the thermal noise and cutoff frequency of pure copper foils with different values of thickness and purity. Adopting a 0.08-mm-thick copper foil as an EM shield for the new SQUID magnetometers, the system noise and frequency bandwidth were improved to ∼25 fT/√Hz and ∼1 kHz, respectively. Using this improved system, we measured geomagnetic fields and confirmed that the background signal in the Z component was smaller compared with the measurement result obtained using the previous system.

Soft ferromagnetic amorphous microwires for GMI sensing cores

Abstract The recent work is focused on the development of suitable amorphous ferromagnetic microwires (CoFe-based) for use in giant magneto-impedance (GMI) magnetic sensing cores. Rapid solidification route is used for the preparation of microwires via in-rotating water quenching technique. The potential of the CoFeSiBCr soft magnetic microwires for GMI based sensing core is investigated by using the driving frequency of MHz range. The GMI characteristics of the developed materials are carried out via impedance-mode. The performance of the magnetic sensor is studied based on its maximum GMI ratio, magnetic field sensitivity and noise spectrum. The experimental results reveal that the maximum GMI ratio, GMImax of about 650% and field sensitivity of around 802 V/T is achieved at ac driving frequency of 5 MHz with 3 mA driving current when annealed at 750 K for 5 min. The noise level of the optimized microwire can be obtained under 10 nT/√Hz@1 Hz while driven at 5 MHz. By means of its high operating frequency regime, high sensitivity, low noise level and simple methodology for sensing, the GMI microwires working in impedance-mode can be a potential candidate for commercial sensing applications such as microfluxgate sensors.

Enhanced quantum coherence in exchange coupled spins via singlet-triplet transitions.

Manipulation of spin states at the single-atom scale underlies spin-based quantum information processing and spintronic devices. These applications require protection of the spin states against quantum decoherence due to interactions with the environment. While a single spin is easily disrupted, a coupled-spin system can resist decoherence by using a subspace of states that is immune to magnetic field fluctuations. Here, we engineered the magnetic interactions between the electron spins of two spin-1/2 atoms to create a "clock transition" and thus enhance their spin coherence. To construct and electrically access the desired spin structures, we use atom manipulation combined with electron spin resonance (ESR) in a scanning tunneling microscope. We show that a two-level system composed of a singlet state and a triplet state is insensitive to local and global magnetic field noise, resulting in much longer spin coherence times compared with individual atoms. Moreover, the spin decoherence resulting from the interaction with tunneling electrons is markedly reduced by a homodyne readout of ESR. These results demonstrate that atomically precise spin structures can be designed and assembled to yield enhanced quantum coherence.

Effect of critical current spread on the noise performance of SQUID magnetometers: An experimental study

We investigate the noise performance dependence on the critical current spread of a fully integrated magnetometer based on superconducting quantum interference device (SQUID). In particular, we analyze the behavior of spectral density of magnetic field noise carrying out an experimental study on 200 magnetometers. The magnetic sensors consist of a superconducting flux transformer connected to a SQUID in a washer shape and include both a feedback coil for flux locked loop operation and an additional positive feedback circuit to increase the intrinsic voltage responsivity. A damping resistor in parallel to the SQUID inductance has been used in order to prevent both the degradation due to the non-optimal βL value and the washer resonances. Such sensors are often employed in magnetoencephalography or more in general in ultra-high sensitivity applications requiring the magnetic noise spectral density as low as 2 fT/√Hz in the white region.We will show that a SQUID critical current varying from 8 to 40 μA does not cause an evident degradation of the magnetometer sensibility. In addition, the magnetic field noise vs critical current indicates an optimal value of SQUID critical current of 20 μA. It corresponds to an inductance parameter βL of about 2 highlighting the effectiveness of the damping resistor in parallel to the SQUID inductance.