Nuclear-spin noise and spontaneous emission

Sleator, Tycho; Hahn, E. L.; Hilbert, Claude; Clarke, John

doi:10.1103/physrevb.36.1969

Cited by 70 publications

(93 citation statements)

References 13 publications

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“…This conclusion is consistent with the finding of the authors of Ref. [13] that the fluctuation-dissipation theorem predicts a temperature-independent injection of spin noise from a system of spins 1/2 into a resonant circuit. Since the contribution of vacuum fluctuations to d I z /dt is proportional to I z , however, the net rate of spontaneous emission is temperature dependent.…”

Section: B Equation Of Motion For I Zsupporting

confidence: 82%

“…Since the time constant for spontaneous emissions by a nuclear spin into free space is many orders of magnitude larger than the time available for NMR experiments [12], however, spontaneous emission normally makes a negligible contribution to relaxation. Enhancing the emission rate by coupling spins to an inductive resonator has enabled the detection of emitted energy in macroscopic samples in the high-temperature limit [13,14], but the enhancement provided by the resonator still yielded relaxation time constants far too long to be relevant for NMR applications.…”

Section: Introductionmentioning

confidence: 99%

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Polarization of nuclear spins by a cold nanoscale resonator

Butler

Weitekamp

2011

Phys. Rev. A

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A cold nanoscale resonator coupled to a system of nuclear spins can induce spin relaxation. In the lowtemperature limit where spin-lattice interactions are "frozen out," spontaneous emission by nuclear spins into a resonant mechanical mode can become the dominant mechanism for cooling the spins to thermal equilibrium with their environment. We provide a theoretical framework for the study of resonator-induced cooling of nuclear spins in this low-temperature regime. Relaxation equations are derived from first principles, in the limit where energy donated by the spins to the resonator is quickly dissipated into the cold bath that damps it. A physical interpretation of the processes contributing to spin polarization is given. For a system of spins that have identical couplings to the resonator, the interaction Hamiltonian conserves spin angular momentum, and the resonator cannot relax the spins to thermal equilibrium unless this symmetry is broken by the spin Hamiltonian. The mechanism by which such a spin system becomes "trapped" away from thermal equilibrium can be visualized using a semiclassical model, which shows how an indirect spin-spin interaction arises from the coupling of multiple spins to one resonator. The internal spin Hamiltonian can affect the polarization process in two ways: (1) By modifying the structure of the spin-spin correlations in the energy eigenstates, and (2) by splitting the degeneracy within a manifold of energy eigenstates, so that zero-frequency off-diagonal terms in the density matrix are converted to oscillating coherences. Shifting the frequencies of these coherences sufficiently far from zero suppresses the development of resonator-induced correlations within the manifold during polarization from a totally disordered state. Modification of the spin-spin correlations by means of either mechanism affects the strength of the fluctuating spin dipole that drives the resonator. In the case where product states can be chosen as energy eigenstates, spontaneous emission from eigenstate populations into the resonant mode can be interpreted as independent emission by individual spins, and the spins relax exponentially to thermal equilibrium if the development of resonator-induced correlations is suppressed. When the spin Hamiltonian includes a significant contribution from the homonuclear dipolar coupling, the energy eigenstates entail a correlation specific to the coupling network. Simulations of dipole-dipole coupled systems of up to five spins suggest that these systems contain weakly emitting eigenstates that can trap a fraction of the population for time periods 100/R 0 , where R 0 is the rate constant for resonator-enhanced spontaneous emission by a single spin 1/2. Much of the polarization, however, relaxes with rates comparable to R 0 . A distribution of characteristic high-field chemical shifts tends to increase the relaxation rates of weakly emitting states, enabling transitions to states that can quickly relax to thermal equilibrium. The theoretical framework presented in this pa...

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Section: B Equation Of Motion For I Zsupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Polarization of nuclear spins by a cold nanoscale resonator

Butler

Weitekamp

2011

Phys. Rev. A

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“…In yet another experiment, Sleator et al 86 observed "spin noise" in 35 Cl. An rf signal at the NQR frequency equalized the populations of the two nuclear spin levels, and then was turned off to leave a zero-spin state.…”

Section: E Magnetic Resonancementioning

confidence: 96%

SQUID Concepts and Systems

Clarke

1989

Superconducting Electronics

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“…Sleator et al (10,11) achieved the first experimental observation of nuclear spin noise by detecting a weak nuclear quadrupole resonance noise spectrum of a sample cooled to 1.5 K in a tuned circuit using a super conducting quantum interference device (SQUID) detector. These researchers related the phenomenon of spin noise to spontaneous emission being enhanced by the coupling to cavity modes (10, 11).…”

mentioning

confidence: 99%

Nuclear spin noise imaging

Müller

Jerschow

2006

Proc. Natl. Acad. Sci. U.S.A.

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NMR images were obtained from the proton spin noise signals of a water-containing phantom, which was placed in the highly tuned, low-noise resonant circuit of a cryogenically cooled NMR probe in the presence of systematically varied magnetic field gradients. The spatially resolved proton spin density was obtained from the raw signal by a modified projection-reconstruction protocol. Although spin noise imaging is inherently less sensitive than conventional magnetic resonance imaging, it affords an entirely noninvasive visualization of the interior of opaque objects or subjects. Thus, tomography becomes possible even when neither x-ray nor radio frequency radiation can be applied for technical or safety reasons.magnetic resonance imaging ͉ radiation-free imaging ͉ sensitivity M RI is a powerful noninvasive tomographic technique. In medicine MRI is used as a versatile diagnostic tool offering superior contrast of soft tissue in the interior of the human body (1, 2). In addition, MRI is an important methodology in biological and material sciences (3). In conventional MRI, the subjects are placed in a uniform static magnetic field B 0 and irradiated with a series of high-power radio frequency (rf) pulses to excite coherent superpositions of spin states. Rapidly switched, spatially dependent magnetic fields (1, 2) are used to encode the spatial coordinates in the phases of the coherent states. These states induce a detectable signal in an rf coil arranged perpendicularly to the static magnetic field. The resonance frequencies can then be mapped to locations in space. One-dimensional profiles may be acquired quasicontinuously, and experiments with various magnetic field gradients along orthogonal axes are required to obtain two-and threedimensional information. Either two-and three-dimensional Fourier transformation (3) or projection-reconstruction algorithms (4) are generally used to produce the images, in which the signal amplitude at particular frequency coordinates is proportional to the spin density at the corresponding location.The major portion of the rf power applied in MRI and NMR spectroscopy (through, e.g., excitation pulses, refocusing pulses, and decoupling) is deposited within the sample as a result of resistive losses (5). Notwithstanding the potential direct effects of rf irradiation on living cells or tissue (6, 7), the primary biological effect is heating due to the thermogenic properties of the electromagnetic field. Therefore, safety regulations have been established for medical applications of MRI (8, 9) limiting the energy deposition in patients and medical staff. The working frequency o of MRI is linearly proportional to the applied magnetic field B 0 , as given by the Larmor equation: v 0 ϭ ␥B 0 ͞2, where ␥ is the magnetogyric ratio of the observed nucleus ( 1 H for most applications). Although MRI at higher fields provides better sensitivity and better resolution, the rf power deposited in a dielectric sample increases approximately with the second power of the magnetic field and the second pow...

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Nuclear-spin noise and spontaneous emission

Cited by 70 publications

References 13 publications

Polarization of nuclear spins by a cold nanoscale resonator

Polarization of nuclear spins by a cold nanoscale resonator

SQUID Concepts and Systems

Nuclear spin noise imaging

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