Nuclear orientation below 1 mK: Development and applications

The Oxford top-loading PrNi₅ nuclear demagnetisation stage /1/ has been further developed to reach temperatures below 1 mK. The hyperfine field for⁷¹As in nickel has been measured by low temperature nuclear orientation as 14.61(36)T. Measurements on transitions in⁷¹Ge yield spin values 3/2 (1299 kev and 1139 kev) and 5/2 (1027 kev) and 5 E2/M1 mixing ratios are given.     

Search for superconductivity in pure Au below 1 mK

An experiment is reported in which a polycrystalline sample of 99.9999% Au was cooled to 0.22 mK in a residual magnetic field of 0.32 mG. No superconductivity was seen.     

Optical dilution and feedback cooling of a gram-scale oscillator to 6.9 mK

We report on the use of a radiation pressure induced restoring force, the optical spring effect, to optically dilute the mechanical damping of a 1 g suspended mirror, which is then cooled by active feedback (cold damping). Optical dilution relaxes the limit on cooling imposed by mechanical losses, allowing the oscillator mode to reach a minimum temperature of 6.9 mK, a factor of approximately 40 000 below the environmental temperature. A further advantage of the optical spring effect is that it can increase the number of oscillations before decoherence by several orders of magnitude. In the present experiment we infer an increase in the dynamical lifetime of the state by a factor of approximately 200.     

Feedback cooling of a one-electron oscillator

A one-electron oscillator is cooled from 5.2 K to 850 mK using electronic feedback. Novel quantum jump thermometry reveals a Boltzmann distribution of oscillator energies and directly measures the corresponding temperature. The ratio of electron temperature and damping rate (also directly measured) is observed to be a fluctuation-dissipation invariant, independent of feedback gain, as predicted for noiseless feedback. The sharply reduced linewidth that results from feedback cooling illustrates the likely importance for improved fundamental measurements and symmetry tests.     

Disappearance of metal-like behavior in GaAs two-dimensional holes below 30 mK

The zero-field temperature dependence of the resistivity of two-dimensional holes is observed to exhibit two qualitatively different characteristics for a fixed carrier density for which only the metallic behavior of the so-called metal-insulator transition is anticipated. As T is lowered from 150 to 0.5 mK, the sign of the derivative of the resistivity with respect to T changes from being positive to negative when the temperature is lowered below approximately 30 mK and the resistivity continuously rises with cooling down to 0.5 mK, suggesting a crossover from being metal-like to insulatorlike.     

Thermal contact between powdered CMN and solid He below 35 mK

The thermal relaxation time measurements between powdered CMN and solid ³He show that the thermal boundary resistance is independent of temperature in the range between 4 mK and 35 mK.     

Feedback cooling of a single trapped ion

Based on a real-time measurement of the motion of a single ion in a Paul trap, we demonstrate its electromechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical master equation.     

Feedback cooling of a single neutral atom

We demonstrate feedback cooling of the motion of a single rubidium atom trapped in a high-finesse optical resonator to a temperature of about 160 μK. Time-dependent transmission and intensity-correlation measurements prove the reduction of the atomic position uncertainty. The feedback increases the 1/e storage time into the 1 s regime, 30 times longer than without feedback. Feedback cooling therefore rivals state-of-the-art laser cooling, but with the advantages that it requires less optical access and exhibits less optical pumping.     

Observation of the magnetic phase transition in PrCu2 below 58 mK by neutron diffraction

A neutron diffraction experiment on the coupled electron-nuclear compound PrCu₂ has been carried out at mili-Kelvin temperatures. Below 58 mK, several magnetic diffraction peaks have been observed at positions in the reciprocal space represented by G±τ, where G is the reciprocal lattice vector and $\text{τ}\text{= 0.24a}{}^1\text{±0.68c}{}^1$. The peak intensity of one of these diffraction peaks has also been measured as a function of temperature down to 10.5 mK.     

Observation of classical rotational inertia and nonclassical supersolid signals in solid 4He below 250 mK

We have confirmed the existence, as first reported by Kim and Chan, of a supersolid state in solid 4He at temperatures below 250 mK. We have employed a torsional oscillator cell with a square cross section to insure a locking of the solid to the oscillating cell. We find that the nonclassical rotational inertia signal is not a universal property of solid 4He but can be eliminated through an annealing of the solid helium sample. This result has important implications for our understanding of the supersolid state.     

Gaussian approximation of the fundamental mode in single mode elliptic core fibers

In this paper, a scalar variational analysis based on gaussian approximation of the fundamental mode of single mode elliptical core fibers is presented. Curves depicting the variation of the normalised modal spot sizes (along the major and minor axes) of step and parabolic index fibers with the normalized frequency are given. The variation of spot size noncircularity with core noncircularity is shown to match well with some recent experimental result. The analysis is used to calculate various splice losses at a joint between two elliptical core fibers and compared with the available experimental results.     

Vibration tailoring of inhomogeneous rod that possesses a trigonometric fundamental mode shape

In this study, a special class of closed-form solutions for inhomogeneous rod is investigated. Namely, the following problem is considered: determine the distribution axial rigidity when the material density is given of an inhomogeneous rod so that the postulated fundamental trigonometric mode shape serves as an exact vibration mode. In this study, the associated semi-inverse problem is solved that results in the distributions of axial rigidity that together with a specified law of material density satisfy the governing eigenvalue problem. For comparison, the obtained closed-form solutions are contrasted with approximate solutions based on an appropriate polynomial shapes, serving as trial functions in an energy method. The obtained results are utilized for vibration tailoring, i.e. construction of the rod with a given natural frequency.     

Photonic crystal fiber with a flattened fundamental mode for the fiber lasers

The 1-hole-missing and 7-hole-missing photonic crystal fibers (PCFs) with flattened fundamental modes (FMs) are proposed by introducing a layer of up-doped silica into the core of the PCFs. The transverse mode competitions are compared between the 7-hole-missing PCF lasers with and without flattened-FMs. The numerical results show that the flattened-FM PCF lasers can support the single transverse mode operation, even for a large value of the ratio of air hole diameter to the spacing between holes (up to 0.53).