通向生命体的量子叠加态?

量子叠加态最大能够在多大的系统中存在？目前已经在光子，原子，以及cooper对中看到了薛定谔猫态。下一步是什么，当然是微型的机械系统了。通过光 （纳米）机械振子技术，我们可能很快就可以看到振子系统的薛定谔猫态了。可是，我们能够真的在生物体中看到薛定谔猫态么？似乎不可能，但最近的理论工作告诉我们这是可行的^[1]。通过光镊技术，可以把几十个纳米大小的振子束缚在光势阱中。这个振子几乎是完全与环境脱耦的，有可能通过光驱动冷却到基态，从而制备出 薛定谔猫态出来。要知道，很多病毒是能够在真空中生存的。因此如果我们把病毒束缚在振子中，我们就可能制备出具有生物活性的系统的量子叠加态。这种量子叠 加态与薛定谔当年提出的薛定谔猫态就几乎一模一样了。同样的技术也可能用于其他基于光学机械振子系统的各种量子信息处理过程^[2]。
[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

科学美国人网站上也报道了这个结果,里面的解说更加详细和具体.

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 . The sidebands appear on the carrier. The displacement of the sidebands is the axion frequency . 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.

用量子光学研究凝聚态物理

这个学期我选了一门《凝聚态物理导论》，前些天学了一些光子晶体的知识。在与主讲老师讨论时，他建议我思考一下用量子光学的视角来研究凝聚态物理。这实在是一个大的思考问题的角度，我现在还没有什么头绪。不过用量子光学的手段来模拟凝聚态系统，是现在的一个研究的热门领域。比如在冷原子光学晶格中模拟Bose-Hubbard模型等许多重要的凝聚态物理模型，从而观测到超流态和Mott绝域态。我找来了原始的论文，可是无法完全理解，因为我缺乏凝聚态物理的背景。最近我看到了另外三篇篇论文(quant-ph/0606097，quant-ph/0606159，cond-mat/0609050)，讨论了如何在光子晶体，或者通过光纤联接的多个光学腔系统中实现Bose-Hubbard模型。相比前者而言，这是更加纯粹的量子光学系统，我对这个系统更加熟悉，因此从这里着手学习一下用量子光学的手段模拟凝聚态物理系统是很好的。

写到这里，我发现自己似乎对很多东西都感兴趣。比如我对黑洞量子信息有些兴趣，于是准备学点广义相对论。不久又开始学习凝聚态物理，用量子光学实现各种凝聚态物理模型。我也有点怀疑是否能够把这些都学到手。不过我还是会注意的，量力而行，从已有的基础出发来学习新的东西。我知道这些与量子光学，量子信息有关的东西我不可能都学好，但是通过学习它们能够进一步的加深我对自己的专业量子光学与量子信息的理解，让我知道自己所学的东西有这么广泛的应用。而且也开拓了我的视野，不再局限在物理学的一隅。有了这些收获，花费时间是很值得了。