Quantum spin dephasing is caused by inhomogeneous coupling to the environment, with resulting limits to the measurement time and precision of spin-based sensors. The effects of spin dephasing can be especially pernicious for dense ensembles of electronic spins in the solid-state, such as nitrogenvacancy (NV) color centers in diamond. We report the use of two complementary techniques, spin bath driving, and double quantum coherence magnetometry, to enhance the inhomogeneous spin dephasing time (T * 2 ) for NV ensembles by more than an order of magnitude. In combination, these quantum control techniques (i) eliminate the effects of the dominant NV spin ensemble dephasing mechanisms, including crystal strain gradients and dipolar interactions with paramagnetic bath spins, and (ii) increase the effective NV gyromagnetic ratio by a factor of two. Applied independently, spin bath driving and double quantum coherence magnetometry elucidate the sources of spin ensemble dephasing over a wide range of NV and bath spin concentrations. These results demonstrate the longest reported T * 2 in a solid-state electronic spin ensemble at room temperature, and outline a path towards NV-diamond DC magnetometers with broadband femtotesla sensitivity.
We analyze a quantum force sensor that uses coherent quantum noise cancellation (CQNC) to beat the Standard Quantum Limit (SQL). This sensor, which allows for the continuous, broad-band detection of feeble forces, is a hybrid dual-cavity system comprised of a mesoscopic mechanical resonator optically coupled to an ensemble of ultracold atoms. In contrast to the stringent constraints on dissipation typically associated with purely optical schemes of CQNC, the dissipation rate of the mechanical resonator only needs to be matched to the decoherence rate of the atomic ensemble -- a condition that is experimentally achievable even for the technologically relevant regime of low frequency mechanical resonators with large quality factors. The modular nature of the system further allows the atomic ensemble to aid in the cooling of the mechanical resonator, thereby combining atom-mediated state preparation with sensing deep in the quantum regime.Comment: 7 pages, 3 figures; updated reference
Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very high Q-factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1-1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For thermal noise limited sensitivity, we find that strain fields on the order of~h 10 Hz 23 are detectable. Measuring such strains is possible by implementing state of the art microwave transducer technology. We show that the proposed system can compete with interferometric detectors and potentially surpass the gravitational strain limits set by them for certain pulsar sources within a few months of integration time.He 8 for superfluid 4 He, which appears to be limited by a combination of 3 He impurities, sample temperature, and radiation loss. All of these dissipation mechanisms can be reduced and we assume quality factors of 10 11 are possible in future experiments with isotropically pure samples at lower temperatures of around 10 mK.The power spectrum of GWs is expected to be extremely broad and is estimated to range from 10 −16 to 10 3 Hz [7-9] for known sources. Ground-based optical interferometers (such as LIGO, Virgo, GEO, TAMA) allow OPEN ACCESS RECEIVED for broad-band search for GWs in the frequency range 10 Hz-5 kHz. These detectors are expected to be predominantly sensitive to the chirped, transient, GW impulse resulting from the last moments of coalescing binaries involving compact objects (BHs and/or NSs) [10]. Space-based interferometric detectors can be sensitive to lower frequency GWs, as they are not limited by seismic noise [11].Unlike broadband impulse sources, rapidly rotating compact objects such as pulsars are expected to generate highly coherent, continuous GW signals due to the off-axis rotating mass, with frequencies spanning from ∼1 kHz for millisecond pulsars (MSPs) in binaries, to 1 Hz for very old pulsars [7,[12][13][14][15]. Given the unknown mass distribution of the pulsar, one can only estimate the strain field here at earth. Several mechanisms give upper bounds to the strength of GWs on earth. One such limit is the 'spin down limit', which is given by the observed spin-down rate of the pulsar, and the assumption that all of the rotational kinetic energy which is lost is in the form of GWs [16]. Another limit is given by the yield strength of the material which makes up the NS, and how much strain the crust can sustain before breaking apart due to centripetal forces [17,18]. The presence of strong magnetic fields indicate a potential mechanism for producing and sustaining such strains due to deformation of the NS [19,20]. However, without knowing the strength and direction...
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