arXiv subjects

Koji Usami

Publications and source records attributed to Koji Usami.

28 records · Page 2Linked to original sources

arXiv2014

Hybridizing ferromagnetic magnons and microwave photons in the quantum limit

We demonstrate large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode. Strong coupling is achieved in the quantum regime where the average number of thermally or externally excited magnons and photons is less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. A nonmonotonic temperature dependence of the Kittel-mode linewidth is observed below 1 K and is attributed to the dissipation due to the coupling with a bath of two-level systems.

quant-ph↗

arXiv2013

Graphene on silicon nitride for optoelectromechanical micromembrane resonators

Due to their exceptional mechanical and optical properties, dielectric silicon nitride (SiN) micromembrane resonators have become the centerpiece of many optomechanical experiments. Efficient capacitive coupling of the membrane to an electrical system would facilitate exciting hybrid optoelectromechanical devices. However, capacitive coupling of such dielectric membranes is rather weak. Here we add a single layer of graphene on SiN micromembranes and compare electromechanical coupling and mechanical properties to bare dielectric membranes and to membranes metallized with an aluminium layer. The electrostatic coupling of graphene coated membranes is found to be equal to a perfectly conductive membrane. Our results show that a single layer of graphene substantially enhances the electromechanical capacitive coupling without significantly adding mass, decreasing the superior mechanical quality factor or affecting the optical properties of SiN micromembrane resonators.

cond-mat.mes-hall↗

arXiv2012

Exciton-mediated photothermal cooling in GaAs membranes

Cooling of the mechanical motion of a GaAs nano-membrane using the photothermal effect mediated by excitons was recently demonstrated by some of us [K. Usami, et al., Nature Phys. 8, 168 (2012)] and provides a clear example of the use of thermal forces to cool down mechanical motion. Here, we report on a single-free-parameter theoretical model to explain the results of this experiment which matches the experimental data remarkably well.

quant-ph↗

arXiv2011

High-Q optomechanical GaAs nanomembranes

We present a simple fabrication method for the realization of suspended GaAs nanomembranes for cavity quantum optomechanics experiments. GaAs nanomembranes with an area of 1.36 mm by 1.91 mm and a thickness of 160 nm are obtained by using a two-step selective wet-etching technique. The frequency noise spectrum reveals several mechanical modes in the kilohertz regime with mechanical Q-factors up to 2,300,000 at room temperature. The measured mechanical mode profiles agree well with a taut rectangular drumhead model. Our results show that GaAs nanomembranes provide a promising path towards quantum optical control of massive nanomechanical systems.

cond-mat.mes-hall↗

arXiv2007

Quantum-noise-limited Angular Momentum Measurement for a Micron-sized Dielectric Object

An approach is described for observing quantum features of micron-sized spinning objects. Specifically, we consider a birefringent (uniaxial positive) dielectric object in the shape of an oblate (i.e., frisbee-like) symmetric top. It can be trapped in the air, its extraordinary axis can be aligned, and its angular momentum along the extraordinary axis can be stabilized, all optically. We show that the angular momentum quantum noise of the object perpendicular to the gigantic angular momentum along the extraordinary axis can be measured as a linear birefringent phase shift of a probe laser in an analogous fashion to the spin quantum nondemolition (QND) measurement in atomic physics.

quant-ph↗

arXiv2007

Observation of Topological and Parity-dependent Phase of $m=0$ Spin States

A Ramsey interrogation scheme was used to measure the phase shift of laser-cooled $^{87}$Rb clock-transition pseudospins arising as a result of a reversal of a bias magnetic field, i.e., $\textbf{B} \to -\textbf{B}$, during the interrogation. While no phase shift occurred when the reversal was sudden, the Ramsey fringes were shifted by a factor of $π$ when the reversal was adiabatic. We thus verified the prediction that the spin states $|F,m=0 >$ acquire a purely topological and parity-dependent phase factor of $(-1)^{F}$ as a result of $\textbf{B} \to -\textbf{B}$.

quant-ph↗

arXiv2006

How to Measure the Quantum State of Collective Atomic Spin Excitation

The spin state of an atomic ensemble can be viewed as two bosonic modes, i.e., a quantum signal mode and a $c$-numbered ``local oscillator'' mode when large numbers of spin-1/2 atoms are spin-polarized along a certain axis and collectively manipulated within the vicinity of the axis. We present a concrete procedure which determines the spin-excitation-number distribution, i.e., the diagonal elements of the density matrix in the Dicke basis for the collective spin state. By seeing the collective spin state as a statistical mixture of the inherently-entangled Dicke states, the physical picture of its multi-particle entanglement is made clear.

quant-ph↗

arXiv2004

Antinormally Ordered Photodetection of Continuous-mode Field

When the electromagnetic field is detected by stimulated emission, rather than by absorption, antinormally ordered photodetection can be realized. One of the distinct features of this photodetection scheme is its sensitivity to zero-point fluctuation due to the existence of the spontaneous emission. We have recently succeeded in experimentally demonstrating the antinormally ordered photodetection by exploiting nondegenerate stimulated parametric down-conversion process. To properly account for the experiment, the detection process needs to be treated with time-dependent and continuous-mode operators because of the broadband nature of the parametric down-conversion process and the wide spectrum of the pump that we used. Here, we theoretically analyze the antinormally ordered intensity correlation of the continuous-mode fields by pursuing the detection process in the Heisenberg picture. It is shown that the excess positive correlation due to zero-point fluctuation reduces because of the frequency-distinguishability of the two emitted photon pairs.

quant-ph↗

arXiv2004

Observation of Antinormally Ordered Hanbury-Brown--Twiss Correlations

We have measured antinormally ordered Hanbury-Brown--Twiss correlations for coherent states of electromagnetic field by using stimulated parametric down-conversion process. Photons were detected by stimulated emission, rather than by absorption, so that the detection responded not only to actual photons but also to zero-point fluctuations via spontaneous emission. The observed correlations were distinct from normally ordered ones as they showed excess positive correlations, i.e., photon bunching effects, which arose from the thermal nature of zero-point fluctuations.

quant-ph↗

arXiv2002

A New Strategy of Quantum-State Estimation for Achieving the Cramer-Rao Bound

We experimentally analyzed the statistical errors in quantum-state estimation and examined whether their lower bound, which is derived from the Cramer-Rao inequality, can be truly attained or not. In the experiments, polarization states of bi-photons produced via spontaneous parametric down-conversion were estimated employing tomographic measurements. Using a new estimation strategy based on Akaike's information criterion, we demonstrated that the errors actually approach the lower bound, while they fail to approach it using the conventional estimation strategy.

quant-ph↗