Precessing Ferromagnetic Needle Magnetometer

Abstract

A ferromagnetic needle is predicted to precess about the magnetic field axis at a Larmor frequency Ω under conditions where its intrinsic spin dominates over its rotational angular momentum, N ≫ IΩ (I is the moment of inertia of the needle about the precession axis and N is the number of polarized spins in the needle). In this regime the needle behaves as a gyroscope with spin N maintained along the easy axis of the needle by the crystalline and shape anisotropy. A precessing ferromagnetic needle is a correlated system of N spins which can be used to measure magnetic fields for long times. In principle, by taking advantage of rapid averaging of quantum uncertainty, the sensitivity of a precessing needle magnetometer can far surpass that of magnetometers based on spin precession of atoms in the gas phase. Under conditions where noise from coupling to the environment is subdominant, the scaling with measurement time t of the quantum-and detectionlimited magnetometric sensitivity is t −3/2 . The phenomenon of ferromagnetic needle precession may be of particular interest for precision measurements testing fundamental physics.For an ensemble of N independent particles prepared in a coherent superposition of quantum states, the standard quantum limit (SQL) on the precision of a measurement of the phase φ is given byafter time t ≫ 1/Γ rel , where Γ rel is the relaxation rate of the coherence. Equation (1) represents a random walk in phase with step size 1/ √ N consisting of Γ rel t steps. In cases where the goal is to measure a frequency Ω = φ/t, there is an analogous SQL on the precision of a frequency measurement,For a measurement subject to the SQL, the minimum possible measurement uncertainty is obtained when Γ rel is made as small as possible. In the limit where Γ rel → 0, the precision becomes constrained by the duration of the measurement, so in Eqs. (1) and (2), Γ rel is replaced by 1/t. However, if the particles' time evolution is correlated, the SQL can be circumvented for times shorter than the coherence time (1/Γ rel ) [2][3][4]. Extensive experimental efforts involving quantum entanglement, squeezed states, and quantum nondemolition (QND) measurement strategies have been made to take advantage of this potential improvement in measurement sensitivity [5][6][7][8]. In this Letter we draw attention to a system which can, in principle, surpass the SQL on measurement of spin precession in a different way: by rapid averaging of quantum uncertainty.In particular, we consider the measurement of magnetic fields. The most precise magnetic field measurements are based on the techniques of optical atomic magnetometry [9, 10]: N atomic spins are optically polarized and their precession in a magnetic field B is measured using optical rotation of probe light [11]. Depending on its magnitude, the value of B is either extracted from measurement of the Larmor frequency Ω = gµ B B/ or the accrued spin precession angle φ = Ωt if φ ≪ 1 during the measurement time t (g is the Landé g-factor and µ B is the Bohr magneton). Optica...

Colloquium : Quantum limits to the energy resolution of magnetic field sensors

“…A similar "freezing-out" of non-spin degrees of freedom is predicted for needle-shaped solid-state ferromagnets in the single-domain size regime (Jackson Kimball et al, 2016). As with the BEC case described above, the ferromagnetic interactions impose full polarization, and at low temperatures no intrinsic fluctuations cause diffusion of the polarization angle.…”

“…Examples of point-like sensors are single NVDs (Ariyaratne et al, 2018; and single trapped ions (Ruster et al, 2017). Linear sensors include ferromagnetic needles (Jackson Kimball et al, 2016;Band et al, 2018) and some cold atomic ensembles . Planar sensors include superconducting sensors of various types Kher et al, 2013;Kher et al, 2016;Kher, 2017), Hall-effect sensors and several others (Grosz et al, 2016;.…”

Hidden photons in Aharonov-Bohm-type experiments

“…Nonetheless, according to Fig. (2) of [34], the benchmark sensitivity considered here can easily be obtained even if the magnetometer does not work beyond the SQL limit, in a measuring time of about 100 s. In that sense, our assumption is a conservative one.…”

“…In the following we assume that the detector has a sensitivity to magnetic fields of order b det ¼ 1 × 10 −18 T. These sensitivities are within reach of the most precise magnetometers for integration times of order of one week, see for instance [32,33]. In a recent work [34] it has been proposed to use a ferromagnetic needle as a micron-scale magnetometer. In this setup the ferromagnetic needle consists of a correlated system of N spins, which will precess in the presence of a background magnetic if the intrinsic angular momentum of spin dominates over the rotational angular momentum.…”

Exceptional magnetic sensitivity of PT -symmetric cavity magnon polaritons

“…On the other hand, highly sensitive magnetometers are indispensable tools which assisted humankind through a wide range of practical applications in geology, navigation, archaeology, magnetic storage, and medicine [23][24][25]. The technologies used for magnetic field sensing encompass many aspects of physics, such as search coil, fluxgate, Hall effect, magnetoelectric coupling, spin mechanics, and magnetoresistance [26][27][28][29][30], to name a few. The state-of-the-art magnetic sensor can reach an ultrahigh sensitivity of subfetotesla, like a superconducting quantum interference device [31,32] and atomic magnetometer [33,34], however with limitations such as extreme temperature [31,32] or low working frequency [33,34].…”