We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized 3 He or 129 Xe samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron as well as in a new type of 3 He/ 129 Xe clock comparison experiment which should be sensitive to a sidereal variation of the relative spin precession frequency. Characteristic spin precession times after one day. Even in that sensitivity range, the magnetometer performance is statistically limited, and noise sources inherent to the magnetometer are not limiting. The reason is that free precessing 3 He ( 129 Xe) nuclear spins are almost completely decoupled from the environment. That makes this type of magnetometer in particular attractive for precision field measurements where a long-term stability is required.

couple to the spins of standard model particles like the electron, proton, and nucleon (mostly the bound neutron) [23]. These terms have set the most stringent limits on CPT and Lorentz violations. To determine the leading-order effects of a Lorentz violating potential V, it suffices to use a non-relativistic description for the particles involved given by [23](with J = X, Y, Z ; w = e, p, n) .(1)

The detection of the free precession of co-located 3 He/ 129 Xe nuclear spins (clock comparison) is used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out in the weighted frequency difference, i.e., ω = ω He - Features of frequency standards and clocksSince Galileo Galilei and Christiaan Huygens invented the pendulum clock, time and frequency have been the quantities that we can measure with the highest precision. Since 1967 the Cs atomic clock defines our unit of time, the second, as the period during which a cesium-133 atom oscillates 9,192,631,770 number of cycles on the hyperfine clock transition |F = 4, m F = 0 → |F = 3, m F = 0 in the 6 2 S 1/2 atomic ground state. Cesium atomic clocks have been gradually improved to the point where modern cesium-fountain clocks realize the definition of the second with a relative uncertainty of about 4 × 10 −16 [1]. In the near future, the cesium clock defining the fundamental timing reference will be replaced with an optical clock, since suppression of systematic effects shifting the frequency of a standard is greatly facilitated by the use of higher frequencies.Thanks to the incredible high relative accuracy of frequency determination, atomic clocks may touch the μHz range on an absolute scale, but will essentially not go far below.To address fundamental questions in physics often associated with the experimental search for violation of fundamental symmetries in nature, much smaller frequencies or frequency shifts as a result of tiny changes in the clock transition must be tracked. From that point of view it is more appropriate to develop a "clock" that oscillates at low frequencies (∼10 Hz), but shows the same relative accuracy as a Cs atomic clock. Thus, frequency shifts in the pHz range caused by hypothetical interaction potentials might be accessible."Spin clocks" which are based on nuclear spin precession are the most promising approach to reach such sensitivity limits. The ,,spin clock" described here is based on the detection of free spin-precession of gaseous, nuclear spin-polarized 3 He or 129 Xe samples [2]. Like in a free induction decay (FID) measurement, the decay of the transverse magnetization is monitored and the Larmor frequency ω of the precessing sample magnetization is related to the magnetic field B 0 through ω = γ · B 0 , where γ is the gyromagnetic ratio of the corresponding nucleus. Since this type of clock will preferably operate at low magnetic fields and thus at low frequencies, using a SQUID as magnetic field detector is appropriate due to its high sensitivity in that spectral range. The 3 He/ 129 Xe nuclear spins are polarized by means of optical pumping. Thus, the nuclear polarization obtained exceeds the Boltzmann polarization as used in typical NMR experiments by four to five orders of magnitude.

He/ 129 Xe Clock-Comparison ExperimentsThe detection of the free precession of co-located 3 He/ 129 Xe sample spins can be used as ultra-sensitive probe for non-magnetic spin interactions  Search for a Lorentz Invariance violating sidereal modulation of the Larmor frequency: Search for spin-dependent short-range interactions:Detection of magnetic field produced by oriented nuclei Cohen-Tannoudji et al., PRL 22 (1969),758 Results:3 He spin precession: T 2 * = 2h 20min  Sensitivity of Rb-magnetometer: 100fT@ BW 0.3 Hz  P He  5% @ 4 mbar 10 -11 tesla 10 -13 tesla  B  BMSR-2, PTB Berlin LT c -SQUID

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To test Lorentz symmetry we used a 3 He/ 129 Xe co-magnetometer. We will give a short summary of our experimental setup and the results of our latest measurements. We obtained preliminary results for the equatorial component of the background field interacting with the spin of the bound neutron:b n ⊥ < 3.72 × 10 −32 GeV (95 % CL).

Repetitive cranial magnetic resonance imaging (MRI) showed lesions typical of multiple sclerosis (MS) with one Gd-enhancing focus in the left parietal cerebrum which correlated to the clinical symptoms of a secondary progressive female MS patient. Since this Gd-enhancing lesion lasted over more than two years even after numerous intravenous high-dose methylprednisolone administrations and intrathecal triamcinolone injections the infl ammatory etiology of this lesion was questioned and T2* echo-gradient images and MR angiography showed a capillary telangiectasia mimicking an active infl ammatory MS-lesion.