Thermal Noise Limit Research Articles

Resonance damping in tightly coupled d.c. SQUIDs via intra-coil resistors

Intra-coil damping is demonstrated to be an effective method for reducing resonance structure in well coupled d.c. SQUIDs with large input coils. A resistance of 1 Ω is placed across each turn of a 137 turn coil coupled to a planar washer d.c. SQUID. Non-ideal structure in the voltage–flux curve is greatly reduced, extending the range (in current bias) of satisfactory device operation. The resulting flux noise, while higher than the thermal noise limit, is less than that observed in undamped devices and can be predicted with a simple model.

Amplification of Schrödinger-cat state in a degenerate optical parametric amplifier

Amplification of the Schrödinger-cat state (even coherent-state superposition) can be accompanied by its non-exponential decoherence and strong squeezing effect in the degenerate optical parametric amplifier. In the classical pump approximation and small thermal noise limit, the Wigner function behaviour illustrates that this non-exponential effect is induced by the phase-selective character of amplification. The non-exponential decoherence is much slower, in contrast with phase-unselective amplification exhibiting faster exponential decay. Thus, if one is able to produce the Schrödinger-cat state with at least a small amplitude, then it can be partially amplified with only a weak decoherence effect.

Suspensionsthermal noise in the LIGO gravitational wave detector

We present a calculation of the maximum sensitivity achievable by theLIGO gravitational wave detector in construction, due to limitingthermal noise of its suspensions. We present a method to calculatethermal noise that allows the prediction of the suspension thermalnoise in all of its six degrees of freedom, from the energy dissipation dueto the elasticity of the suspension wires. We show how this approachencompasses and explains previous ways to approximate the thermalnoise limit in gravitational wave detectors. We show how thisapproach can be extended to more complicated suspensions to be used infuture LIGO detectors.

Conductance and Shot Noise for Particles with Exclusion Statistics

The first quantized Landauer approach to conductance and noise is generalized to particles obeying exclusion statistics. We derive an explicit formula for the crossover between the shot and thermal noise limits and argue that such a crossover can be used to determine experimentally whether charge carriers in FQHE devices obey exclusion statistics.

Cesium microwave emission without population inversion

The use of coherent population trapping (CPT) for the realization of a Cs coherent microwave emitter without population inversion is described. Preliminary experimental results are reported regarding the radio frequency spectrum of the emitted microwave radiation, the emission profile width, and the transient behavior of the output power. This new approach, based on the coherence properties of the laser radiation, allows the implementation of a microwave frequency standard where the linear light shift is absent and the thermal noise limit for the frequency instability is below 10(-12) for an integration time of 1 s.

Optimum duration of measurement for reducing thermal noise limitations in torsion pendulum experiments

Statistical estimates of thermal noise fluctuation of a linear harmonic oscillator used to detect weak forces are reexamined with special reference to gravitational experiments. Thermal noise limitation on the measurement of displacement of an oscillator is studied. It is found that the signal response of an oscillator reaches the maximum value while the thermal noise response reaches the minimum value at the same time during the transient state of the oscillator, and the precision of the experiments in the transient state could increase about two orders higher than that in the equilibrium state for typical experiments. It is also found that there is an optimum duration of measurement in which the uncertainty due to thermal noise in this period is the least.

Photothermal measurements at picowatt resolution using uncooled micro-optomechanical sensors

Deflections of bimaterial microcantilever beams were optically detected with 400 fm resolution at room temperature. This enabled photothermal radiation detection with resolutions of 40 pW for power and 10 fJ for energy. The resolution was improved by an order of magnitude by optimizing the thickness ratio of the two beam materials, as well as by modulating the incident radiation at sufficiently high frequency to be in the range of the thermal white noise limit of the cantilever vibrations. Radiative power was detected with a noise spectral density of and 250 pW/Hz and detectivity, D*, of 4.6×107 cm Hz/W.

Switch energy requirements for first- and second-order perturbations in asymmetric quantum box optical switches

We have studied the limits to the switching energy of electrically controlled optical absorption switches. Investigating a case with intraband transitions in asymmetric quantum boxes, it is shown that linear Stark shift allows switch energies of the optical switches that are comparable to the thermal noise limit at 300 K at least up to 30 GHz, provided boxes with very narrow absorption linewidth can be fabricated. This is several orders of magnitude lower energy than required by quadratic Stark shift in a similar, symmetric structure.

Displacement measurement in VIRGO super attenuators with a suspended fabry-perot interferometer

The use of a two mirrors Fabry-Pexot suspended cavity as a high sensitivity displacement detector is considered. It can be used to test the remnant noises in VIRGO Super Attenuators. The displacement sensitivity of the apparatus should be increased up to the mirrors thermal noise limit.

A superconducting bolometer with strong electrothermal feedback

We present a theoretical analysis and experimental evaluation of a transition-edge superconducting bolometer for detecting infrared and millimeter waves. The superconducting film is voltage biased and the current is read by a superconducting quantum interference device ammeter. Strong electrothermal feedback maintains the sensor temperature within the transition, gives a current responsivity that is simply the inverse of the bias voltage, and reduces the response time by several orders of magnitude below the intrinsic time constant C/G. We evaluated a voltage-biased bolometer that operates on the Tc∼95 mK transition of a tungsten film with a thermal conductance of G∼1.2×10−9 W/K. As expected, the electrical noise equivalent power of 3.3×10−17/W√Hz is close to the thermal fluctuation noise limit and is lower than that of other technologies for these values of G and temperature. The measured time constant of 10 μs is ∼100 times faster than the intrinsic time constant.

Practical implications of micromachined ultrasonic transducers.

Micromachined ultrasonic transducers capable of airborne transmission of 1‐ to 12‐MHz ultrasound have been reported. It has also been reported that these transducers should enable ultrasonic systems with 100 dB of dynamic range. A justification of the predicted dynamic range involves the careful consideration of the thermal noise limits of the system. Such analysis is developed and is based on the fluctuation dissipation theorem. In addition, the first scans produced with the new transducers are presented along with interpretations of their significant impact to nondestructive testing. Pictures of surface and bulk defects in composites and metals are included. Finally, the fabrication process is summarized and the issues involved in the development of immersion transducers are highlighted. [Work supported by the U.S. Office of Naval Research.]

Combined magnetic fields increased net calcium flux in bone cells

Low energy electromagnetic fields (EMF) exhibit a large number of biological effects. A major issue to be determined is "What is the lowest threshold of detection in which cells can respond to an EMF?" In these studies we demonstrate that a low-amplitude combined magnetic field (CMF) which induces a maximum potential gradient of 10(-5) V/m is capable of increasing net calcium flux in human osteoblast-like cells. The increase in net calcium flux was frequency dependent, with a peak in the 15.3-16.3 Hz range with an apparent bandwidth of approximately 1 Hz. A model that characterizes the thermal noise limit indicates that non-spherical cell shape, resonant type dynamics, and signal averaging may all play a role in the transduction of low-amplitude EMF effects in biological systems.

Gap junction impedance, tissue dielectrics and thermal noise limits for electromagnetic field bioeffects

The model presented in this study quantitatively examines the effect of gap junctions and gap junction impedance on electromagnetic field (EMF) dosimetry in a tissue target. A simple linear distributed-parameter electrical model evaluates the effect of tissue structure on the thermal threshold (signal-to-thermal-noise ratio) for detectable induced transmembrane voltage. Analysis of the angular frequency response of the array model, using a membrane impedance which includes ion binding and coupled surface chemical reaction kinetics, suggests that the frequency range, over which maximum detectable induced transmembrane voltage could be achieved, is orders of magnitude lower than that for a single cell. Gap junction impedance has negligible effect on both the frequency response and the increased transmembrane voltage due to a cell array unless its value becomes as high as that of an artificial bilayer lipid membrane. This results in a threshold for induced electric field bioeffects of approximately 10 μV cm −1 at the target site for a 1–10 mm cell array. Physiological variations in gap junction impedance appear to have little effect on this threshold. Thus, cells in gap junction contact in developing, repairing or resting state tissue structures would be expected to be able to detect significantly weaker EMF signals than isolated single cells. The lowered frequency response of a cell array reinforces the suggestion that the spectral density of the input signal should be adjusted to the bandpass of the detector pathway for dose-efficient and selective EMF bioeffects.

Voltage noise properties of Nb-(fine Nb wires)-NbN thin film short weak links

We have fabricated superconductive Nb-(fine Nb wires)-NbN short weak links. We find that their quasiparticle characteristics can be well explained using the theory of T. M. Klapwijk, G. E. Blonder, and M. Tinkham [Physica B and C 109 & 110B 1657 (1982)] based on the Andreev reflection phenomenon at two independent normal metal-superconductor interfaces for the case of a negligible effective barrier potential. For such devices, there is found no measurable excess noise associated with the Andreev reflection itself and device voltage noise clearly exhibits both thermal and shot noise limits. Although the device structure is fundamentally different, in the whole studied range of bias voltages, the voltage noise properties of our short weak links are found to be qualitatively comparable to those of small-area tunnel junctions.

Counter-ion polarization and low frequency, low electric field intensity biological effects

By analysis of published experimental data for the dielectric properties of tissue at extremely low frequencies it is shown that the “thermal noise problem” is much less severe for field effects depending on counter-ion polarization than for effects that require a change of transmembrane potential. Application of dielectric mixture theory and the theory of counter-ion polarization is used to show that the fraction of charges near the cell surface involved in counter-ion polarization is of the order of 10−6 at the thermal noise limit when the frequency of the applied field is 15 Hz. It is also shown that the exponential decay distance for polarization-related charge motion can be more than 50% of the intercell spacing. Thus extremely low frequency electric fields that cause counter-ion polarization can affect the ionic environment of those membrane-embedded proteins which extend far into the intercellular medium and can possibly also influence cell-to-cell communication.

Counter-ion polarization and low frequency, low electric field intensity biological effects

By analysis of published experimental data for the dielectric properties of tissue at extremely low frequencies it is shown that the “thermal noise problem” is much less severe for field effects depending on counter-ion polarization than for effects that require a change of transmembrane potential. Application of dielectric mixture theory and the theory of counter-ion polarization is used to show that the fraction of charges near the cell surface involved in counter-ion polarization is of the order of 10 −6 at the thermal noise limit when the frequency of the applied field is 15 Hz, It is also shown that the exponential decay distance for polarization-related charge motion can be more than 50% of the intercell spacing. Thus extremely low frequency electric fields that cause counter-ion polarization can affect the ionic environment of those membrane-embedded proteins which extend far into the intercellular medium and can possibly also influence cell-to-cell communication.

High dynamic range measurements of an all metal isolator using a sapphire transducer (for gravitational wave detectors)

The authors report the performance and mechanical properties of an all metal mass-spring isolator with corner frequency about 44 Hz. Noise floor measurements using a prototype sapphire transducer and results of active damping of the low-frequency normal modes are given. The measurements described cover a total dynamic range of 160 dB, and show that no excess noise is generated above the thermal noise limit, even in the presence of high-amplitude excitation.

Probability-density function of noise at the output of a two-beam interferometer

The probability-density function (PDF) of the fluctuating output intensity of a two-beam interferometer, illuminated by a semiconductor laser, is described theoretically and experimentally. The PDF, while dominated by the laser phase noise, may be substantially modified both by the laser intensity noise and by the thermal noise and bandwidth limitations imposed by the detection system. Two regimes of operation are explored. In the coherent regime, for which the interferometer ’ s differential delay τ is much smaller than the laser coherence time τc, the form of the PDF is highly sensitive to the mean phase difference between the interfering beams: The PDF is strongly asymmetrical for mean phase differences of 0 and π (experimentally reported here for the first time to our knowledge) but is symmetrical when the interferometer is in quadrature. In the incoherent regime, for which τ ≫ τc, the PDF is insensitive to such phase differences. The experimental results are in excellent agreement with a numerical simulation for which it is assumed that the laser phase noise is a Wiener process and for which the finite bandwidth of the detection system, as well as the effects of laser intensity noise and system thermal noise, is taken into account.

Indoor Radon: A Little Less to Worry About

In the report "The response of living cells to very weak electric fields: The thermal noise limit" by J. C. Weaver and R. D. Astumian (26 Jan., p. 459), figures 2 and 3 on page 461 were inadvertently interchanged. The captions were correct.

THE EFFECT OF AN ELECTRICAL FIELD ON GERMINATION AND GROWTH OF SEEDLINGS

A weak electrical field has a stimulatory effect on germination of seeds and the early growth of young plants. Waldo bean seeds were placed in a chamber with moistened filter paper. Oppositely charged aluminum plates were used to stimulate growth. The top plate was,suspended above the filter paper to eliminate the possibility of a direct current flow through the paper. Each set of samples was examined after 5 days. The average root length of the test groups was significantly greater than that of the control groups. Additional experiments suggested that the rate of growth of the plants in soil was enhanced by applying a weak electric field to the soil. Using electricity to stimulate better germination has many practical applications. A current agricultural practice is to germinate seeds in dissolving tubes for planting as individual plants. Results of these experiments suggest that applying an electric current to the germinating seeds would stimulate them to grow more vigorously for more successful growth in the field, The use of electricity may have important applications as art adjunct to organic farming practices. Experiments are under way to apply the use of electricity to hydroponic systems. Foster, K.R. and H.P. Schwann. 1986. In: C. Polk and E. Postrow (eds.). CRC handbook of biological effects of electromagnetic fields. CRC Press, Inc., Boca Raton, Fla. Weaver, J.C. and R.D. Astumian. 1990. The response of living cells to very weak electric fields: The thermal noise limit. Science 247:459-462.