arXiv subjects

Akio Kawasaki

Publications and source records attributed to Akio Kawasaki.

36 records · Page 2Linked to original sources

arXiv2020

Trapping $^{171}$Yb Atoms into a One-Dimensional Optical Lattice with a Small Waist

In most experiments with atoms trapped in optical lattices, the transverse size of the optical lattice beams is on the order of tens of micrometers, and loading many atoms into smaller optical lattices has not been carefully investigated. We report trapping 1500 $^{171}$Yb atoms in a one-dimensional optical lattice generated by a narrow cavity mode at a distance of 0.14 mm from a mirror surface. The simplest approach of loading atoms from a mirror magneto-optical trap overlapped with the cavity mode allows the adjustment of the loading position by tuning a uniform bias magnetic field. The number of atoms trapped in the optical lattice exhibits two local maxima for different lattice depths, with a global maximum in the deeper lattice. These results open a way to quantum mechanical manipulation of atoms based on strong interaction with a tightly focused light field.

physics.atom-ph↗

arXiv2020

Entanglement-Enhanced Optical Atomic Clock

State-of-the-art atomic clocks are based on the precise detection of the energy difference between two atomic levels, measured as a quantum phase accumulated in a given time interval. Optical-lattice clocks (OLCs) now operate at or near the standard quantum limit (SQL) that arises from the quantum noise associated with discrete measurement outcomes. While performance beyond the SQL has been achieved in microwave clocks and other atomic sensors by engineering quantum correlations (entanglement) between the atoms, the generation of entanglement on an optical-clock transition and operation of such a clock beyond the SQL represent major goals in quantum metrology that have never been demonstrated. Here we report creation of a many-atom entangled state on an optical transition, and demonstrate an OLC with an Allan deviation below the SQL. We report a metrological gain of $4.4^{+0.6}_{-0.4}$ dB over the SQL using an ensemble consisting of a few hundred 171Yb atoms, allowing us to reach a given stability $2.8{\pm}0.3$ times faster than the same clock operated at the SQL. Our results should be readily applicable to other systems, thus enabling further advances in timekeeping precision and accuracy. Entanglement-enhanced OLCs will have many scientific and technological applications, including precision tests of the fundamental laws of physics, geodesy, or gravitational wave detection.

quant-ph↗

arXiv2020

Measurement of the Newtonian Constant of Gravitation $G$ by Precision Displacement Sensors

The Newtonian constant of gravitation $G$ historically has the largest relative uncertainty over all other fundamental constants with some discrepancies in values between different measurements. We propose a new scheme to measure $G$ by detecting the position of a test mass in a precision displacement sensor induced by a force modulation from periodically rotating source masses. To seek different kinds of experimental setups, laser interferometers for the gravitational wave detection and optically-levitated microspheres are analyzed. The high sensitivity of the gravitational wave detectors to the displacement is advantageous to have a high signal-to-noise ratio of $10^{-6}$ with a few hours of the measurement time, whereas the tunability of parameters in optically-levitated microspheres can enable competitive measurements with a smaller scale setup dedicated to the $G$ measurement. To achieve an accuracy of $G$ better than currently available measurements, developments in force calibration is essential. These measurements can provide an alternative method to measure $G$ precisely, potentially leading to the improvement in the accuracy of $G$, as well as a better search for non-Newtonian gravity at a length scale of $\sim1$ m.

physics.ins-det↗

arXiv2020

Absolute pressure and gas species identification with an optically levitated rotor

The authors describe a novel variety of spinning-rotor vacuum gauge in which the rotor is a ${\sim}4.7{\text -}μ$m-diameter silica microsphere, optically levitated. A rotating electrostatic field is used to apply torque to the permanent electric dipole moment of the silica microsphere and control its rotational degrees of freedom. When released from a driving field, the microsphere's angular velocity decays exponentially with a damping time inversely proportional to the residual gas pressure, and dependent on gas composition. The gauge is calibrated by measuring the rotor mass with electrostatic co-levitation, and assuming a spherical shape, confirmed separately, and uniform density. The gauge is cross-checked against a capacitance manometer by observing the torsional drag due to a number of different gas species. The techniques presented can be used to perform absolute vacuum measurements localized in space, owing to the small dimensions of the microsphere and the ability to translate the optical trap in three dimensions, as well as measurements in magnetic field environments. In addition, the dynamics of the microsphere, paired with a calibrated vacuum gauge, can be used to measure the effective molecular mass of a gas mixture without the need for ionization and at pressures up to approximately 1 mbar.

physics.ins-det↗

arXiv2019

Precision Mass and Density Measurement of Individual Optically Levitated Microspheres

We report an $\textit{in situ}$ mass measurement of approximately-$4.7{\text -}μ$m-diameter, optically levitated microspheres with an electrostatic co-levitation technique. The mass of a trapped, charged microsphere is measured by holding its axial (vertical) position fixed with an optical feedback force, under the influence of a known electrostatic force. A mass measurement with $1.8\%$ systematic uncertainty is obtained by extrapolating to the electrostatic force required to support the microsphere against gravity in the absence of optical power. In three cases, the microspheres are recovered from the trap on a polymer-coated silicon beam and imaged with an electron microscope to measure their radii. The simultaneous precision characterization of the mass and radius of individual microspheres implies a density of $1.55\pm0.08~$g/cm$^3$. The ability to recover individual microspheres from an optical trap opens the door to further diagnostics.

physics.ins-det↗

arXiv2019

Robust kHz-linewidth distributed Bragg reflector laser with optoelectronic feedback

We demonstrate a combination of optical and electronic feedback that significantly narrows the linewidth of distributed Bragg reflector lasers (DBRs). We use optical feedback from a long external fiber path to reduce the high-frequency noise of the laser. An electro-optic modulator placed inside the optical feedback path allows us to apply electronic feedback to the laser frequency with very large bandwidth, enabling robust and stable locking to a reference cavity that suppresses low-frequency components of laser noise. The combination of optical and electronic feedback allows us to significantly lower the frequency noise power spectral density of the laser across all frequencies and narrow its linewidth from a free-running value of 1.1 MHz to a stabilized value of 1.9 kHz, limited by the detection system resolution. This approach enables the construction of robust lasers with sub-kHz linewidth based on DBRs across a broad range of wavelengths.

physics.optics↗

arXiv2019

Electrically Driven, Optically Levitated Microscopic Rotors

We report on the electrically driven rotation of $2.4~μ$m-radius, optically levitated dielectric microspheres. Electric fields are used to apply torques to a microsphere's permanent electric dipole moment, while angular displacement is measured by detecting the change in polarization state of light transmitted through the microsphere (MS). This technique enables greater control than previously achieved with purely optical means because the direction and magnitude of the electric torque can be set arbitrarily. We measure the spin-down of a microsphere released from a rotating electric field, the harmonic motion of the dipole relative to the instantaneous direction of the field, and the phase lag between the driving electric field and the dipole moment of the MS due to drag from residual gas. We also observe the gyroscopic precession of the MS when the axis of rotation of the driving field and the angular momentum of the microsphere are orthogonal. These observations are in quantitative agreement with the equation of motion. The control offered by the electrical drive enables precise measurements of microsphere properties and torque as well as a method for addressing the direction of angular momentum for an optically levitated particle.

physics.optics↗

arXiv2019

Near-Unitary Spin Squeezing in $^{171}$Yb

Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in $^{171}$Yb, an optical lattice clock atom. The collective nuclear spin of $\sim 10^3$ atoms is squeezed by cavity feedback, using light detuned from the system's resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical clock transition of $^{171}$Yb.

physics.atom-ph↗

arXiv2019

Three-dimensional force-field microscopy with optically levitated microspheres

We report on the use of 4.7-$μ$m-diameter, optically levitated, charged microspheres to image the three-dimensional force field produced by charge distributions on an Au-coated, microfabricated Si beam in vacuum. An upward-propagating, single-beam optical trap, combined with an interferometric imaging technique, provides optimal access to the microspheres for microscopy. In this demonstration, the Au-coated surface of the Si beam can be brought as close as ${\sim}10~μ$m from the center of the microsphere while forces are simultaneously measured along all three orthogonal axes, fully mapping the vector force field over a total volume of ${\sim}10^6~μ$m$^3$. We report a force sensitivity of $(2.5 \pm 1.0) \times 10^{-17}~{\rm N / \sqrt{Hz}}$, in each of the three degrees of freedom, with a linear response to up to ${\sim}10^{-13}~{\rm N}$. While we discuss the case of mapping static electric fields using charged microspheres, it is expected that the technique can be extended to other force fields, using microspheres with different properties.

physics.ins-det↗

arXiv2019

Geometrically asymmetric optical cavity for strong atom-photon coupling

Optical cavities are widely used to enhance the interaction between atoms and light. Typical designs using a geometrically symmetric structure in the near-concentric regime face a tradeoff between mechanical stability and high single-atom cooperativity. To overcome this limitation, we design and implement a geometrically asymmetric standing-wave cavity. This structure, with mirrors of very different radii of curvature, allows strong atom-light coupling while exhibiting good stability against misalignment. We observe effective cooperativities ranging from $η_{\rm eff}=10$ to $η_{\rm eff}=0.2$ by shifting the location of the atoms in the cavity mode. By loading $^{171}$Yb atoms directly from a mirror magneto-optical trap into a one-dimensional optical lattice along the cavity mode, we produce atomic ensembles with collective cooperativities up to $Nη=2\times 10^4$. This system opens a way to preparing spin squeezing for an optical lattice clock and to accessing a range of nonclassical collective states.

physics.atom-ph↗

arXiv2019

Search for Kilogram-scale Dark Matter with Precision Displacement Sensors

The search for dark matter has been performed mainly for weakly interacting massive particles and massive compact halo objects, and the intermediate mass region has not been investigated experimentally. A method to search dark matter with precision displacement sensors is suggested for this mass range. The search is performed by detecting a characteristic motion of a test mass when it is attracted by a dark matter particle through gravity. Two different types of displacement sensors are examined: optically levitated microspheres and laser interferometers for gravitational wave detection. The state-of-the-art detectors' sensitivity is several orders of magnitude lower to put constraints on dark matter particles. Among the two types of detectors, gravitational wave detectors have higher sensitivities, and a sensitivity 10 times more than the next generation detector can potentially address the existence of dark matter particles of a few kilograms.

physics.ins-det↗

arXiv2018

Impact of Non-Unitary Spin Squeezing on Atomic Clock Performance

Spin squeezing is a form of entanglement that can improve the stability of quantum sensors operating with multiple particles, by inducing inter-particle correlations that redistribute the quantum projection noise. Previous analyses of potential metrological gain when using spin squeezing were performed on theoretically ideal states, without incorporating experimental imperfections or inherent limitations which result in non-unitary quantum state evolution. Here, we show that potential gains in clock stability are substantially reduced when the spin squeezing is non-unitary, and derive analytic formulas for the clock performance as a function of squeezing, excess spin noise, and interferometer contrast. Our results highlight the importance of creating and employing nearly pure entangled states for improving atomic clocks.

quant-ph↗

arXiv2015

Magic Wavelength for Hydrogen 1S-2S Transition

The magic wavelength for an optical lattice for hydrogen atoms that cancels the lowest order AC Stark shift of the 1S-2S transition is calculated to be 513 nm. The magnitude of AC Stark shift $ΔE=-1.19$ kHz/(10kW/cm$^2$) and the slope $dΔE/dν= -27.7$ Hz/(GHz $\cdot$ 10 kW/cm$^2$) at the magic wavelength suggests that a stable and narrow linewidth trapping laser is necessary to achieve a deep enough optical lattice to confine hydrogen atoms in a way that gives a small enough light shift for the precision spectroscopy of the 1S-2S transition.

physics.atom-ph↗

arXiv2015

Fast Compact Laser Shutter Using a Direct Current Motor and 3D Printing

We present a mechanical laser shutter design that utilizes a DC electric motor to rotate a blade which blocks and unblocks a light beam. The blade and the main body of the shutter are modeled with computer aided design (CAD) and are produced by 3D printing. Rubber flaps are used to limit the blade's range of motion, reducing vibrations and preventing undesirable blade oscillations. At its nominal operating voltage, the shutter achieves a switching speed of (1.22 $\pm$ 0.02) m/s with 1 ms activation delay and 10 $μ$s jitter in its timing performance. The shutter design is simple, easy to replicate, and highly reliable, showing no failure or degradation in performance over more than $10^8$ cycles.

physics.atom-ph↗

arXiv2015

Observation of Rapid Change of Crystalline Structure during the Phase Transition of the Palladium-Hydrogen System

We performed an X-ray diffraction experiment while palladium bulk absorbed and desorbed hydrogen to investigate the behavior of the crystalline lattice during the phase transition between the $α$ phase and $β$ phase. Fast growth of $β$ phase was observed around x=0.1 and x=0.45 of PdH$_x$. In addition, slight compression of the lattice at high hydrogen concentration and increase in the lattice constant and the line width of the $α$ phase after a cycle of absorption and desorption of hydrogen was observed. These behavior correlated with the change in the sample length, which may infer that the change in shape was related to the phase transition.

cond-mat.mtrl-sci↗

arXiv2015

Two-Color Magneto-Optical Trap with Small Magnetic Field for Ytterbium

We report a two-color magneto-optical trap (MOT) for ytterbium atoms operating at a low magnetic field gradient down to 2 G/cm where a conventional MOT using the singlet transition (6s^2 1S0 -> 6s6p 1P1) is unable to trap atoms. By simultaneously applying laser light on both the broad-linewidth singlet transition and the narrow-linewidth triplet transition (6s^2 1S0 -> 6s6p 3P1), we load and trap 4.0 x 10^5 atoms directly from an atomic beam at 700 K. In the two-color MOT, the slowing and trapping functions are separately performed by the singlet transition light and the triplet transition light, respectively. The two-color MOT is highly robust against laser power imbalance even at very low magnetic field gradients.

physics.atom-ph↗

arXiv2014

Passive intrinsic-linewidth narrowing of ultraviolet extended-cavity diode laser by weak optical feedback

We present a simple method for narrowing the intrinsic Lorentzian linewidth of a commercial ultraviolet grating extended-cavity diode laser (TOPTICA DL Pro) using weak optical feedback from a long external cavity. We achieve a suppression in frequency noise spectral density of 20 dB measured at frequencies around 1 MHz, corresponding to the narrowing of the intrinsic Lorentzian linewidth from 200 kHz to 2 kHz. The system is suitable for experiments requiring a tunable ultraviolet laser with narrow linewidth and low high-frequency noise, such as precision spectroscopy, optical clocks, and quantum information science experiments.

physics.optics↗

arXiv2012

Deformation of Palladium Plates by a Small Stress during Hydrogen Absorption and Desorption

We observed an unexpected bending of a horizontally held palladium plate at 150$^{\circ}$C under a small external shearing stress. The deformation was fastest at the nominal composition of PdH$_{\rm 0.1-0.2}$, which is in the $α+β$ phase. We also observed that a plate warped back and forth during a cycle of absorption and desoprtion of hydrogen when it was hung vertically, which does not have a reasonable explanation.

cond-mat.mtrl-sci↗