# 通向生命体的量子叠加态?

[1] Oriol Romero-Isart, Mathieu L. Juan, Romain Quidant, & J. Ignacio Cirac (2009). Towards Quantum Superposition of Living Organisms, arXiv: 0909.1469v1
[2] D. E. Chang, C. A. Regal, S. B. Papp, D. J. Wilson, J. Ye, O. Painter, H. J. Kimble, & P. Zoller (2009). Cavity optomechanics using an optically levitated nanosphere, arXiv: 0909.1548v1

# New paper: cooling limits and measurement of optomechanical oscillator

New paper dance now.

arXiv:0906.1379

Title: Phase noise and laser cooling limits of opto-mechanical oscillators
Author: Zhang-qi Yin

Abstract: The noise from laser phase fluctuation sets a major technical obstacle to cool the nano-mechanical oscillators to the quantum region. We propose a cooling configuration based on the opto-mechanical coupling with two cavity modes to significantly reduce this phase noise. After optimization of the cavity parameters, we show through simple arguments that the intrinsic cooling limit of the opto-mechanical oscillator is set by $T_{\text{env}}/Q$, where $T_{\text{env}}$ is the environment temperature and $Q$ is the mechanical quality factor. We also discuss detection of the phonon number when the mechanical oscillator is cooled near the quantum region and specify the required conditions for this detection.

Update in 17th June: Today I found a similar paper published in PRL: Three-Mode Optoacoustic Parametric Amplifier: A Tool for Macroscopic Quantum Experiments, by Chunnong Zhao and et al.. The earlier version of the paper was posted in arXiv:0710.2383v3, which didn’t investigate the phase noise in detail and was already cited in my paper.

# Several interesting papers in the last months

It has been for a long time since my last post on quantum physics. Here I select some interesting papers on cavity and optomechanics.

One most interesting paper I read in the last month is from Prof. Vahala and Prof. Painter’s groups. They demonstrated that micro-mechanical oscillator can be driven and cooled by the optical gradient force, other than traditional scattering radiation pressure. This approach makes it possible that photon momentum to be transferred over a length scale approaching the wavelength of light. They use double-disk structure, which provides back-action several orders larger than the previous one. They also demonstrate the cooling factor of 13 dB under heavily damped conditions (mechanical Q=4).

Another paper is discussing the proposal for a search for the cosmic axions using an optical cavity.  Axions, which were postulated 30 years ago, remain an attractive candidate for the cold dark matter of the universe. The proposal uses some stokes-like processes to detect the axions. The axions are absorbed by an optical cavity field of frequency $\omega_o$. The sidebands $\omega_\pm = \omega_0 \pm \omega_a$ appear on the carrier. The displacement of the sidebands is the axion frequency $\omega=E_a=m$. The proposal is very sensitive.

The last one is discussing the possibility of realizing the strong coupling between a mechanical oscillator and a single atom, from Prof. Kimble’s group. The strong coupling between a cavity mode and a single atom has been accomplished for about 10 years. Once the strong coupling between a mechanical oscillator and a single atom was realized, there are countless application the the technique. Everything you have done in the Cavity-QED systems can be transferred to the optomechanical plus atomic systems. It allows us to coherent manipulation, preparation and measurement  of micromechanical objects.

# Ultrahigh quality cavities

Recently, two groups reported their progresses in manipulating ultrahigh Q cavities.

Ivan S. Grudinin et al. demonstrated a record quality factor of (6.3 pm 0.8) times 10^10 in crystalline whispering gallery mode resonator, corresponding to cavity ring-down time of tau approx 36 {mu}s. They found for a 100 {mu}m LiNbO3 cavity, a single photon would shift the cavity resonance by as much as 6 Hz. Such shift could be detected with optical techniques. This allows the quantum nondemoliion measurement for the number of photons in a cavity. I hope the shift can be enlarged in future. If the shift is large enough, this resonator may be used as a device to entangle photon qubits.

S. Kuhr et al. built a Fabry-Perot supeconducting resonator with quality factor Q = 4.2 times 10^10 and finesse 4.6time 10^9. The demping time T_c of the cavity is as long as 130 ms at resonante frequency 51 GHz and temperature 0.8 K. In previous experiments, T_c was limited to 1 ms. The field damping time of their cavity is 100 times longer than previous ones. I think this microwave F-P resonator is very powerful for realizing quantum information processes. The coupling strength g between atom and cavity is about 310 kHz, which is 4 orders larger than cavity decay rate. As I discussed before, realizing a entangling gate in this system only requires time of 1.7/g, and realizing a nearly perfect controlled-Z gate only needs 3.4/g.. Therefore, the effects of cavity damping can be neglected. Besides, for Rydberg atoms with quantum number 51 and 50, the spontaneous emission rate is of the order 10^2 which is 3 orders lower than time needed in quantum processes. Therefore, a almost perfect quantum logic gate (with fidelity larger than 0.99) may be realized in this cavity.