Cryogenic optomechanics and the resurgence of the resonant-mass gravitational wave detector
Cryogenic optomechanics and the resurgence of the resonant-mass gravitational wave detector
复制标题
低温光力学和共振质量引力波探测器的复兴
DOI：
10.1088/1367-2630/aa8194
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发表时间：
2017
影响因子：
3.3
通讯作者：
M. Tobar
中科院分区：
文献类型：
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作者：
M. Tobar
2016年标志着历史性的突破，宣布了LIGO探测器首次直接探测引力波（GW）[1]。这一事件是一个很长的时间来，因为GW领域有着悠久的历史，这已经显着提高了精度和量子测量领域超过六十年。第一个可操作的探测器是共振棒探测器（或更一般的“共振质量探测器”），重量超过一吨，由JosephWeber在20世纪60年代首次实现[2]。30年后，这些探测器的20世纪90年代低温版本上线，在大约1 KHz的检测频率和1 Hz的带宽下具有10 Hz量级的显著灵敏度[3]。意大利有3个，美国和澳大利亚各有1个，它们都具有很高的占空比和稳定的工作，大多数探测器都有超导SQUID读出器来监测探测器的振动共振，但其中一个NIOBE [4]有微波参量传感器，实际上是一个超灵敏的光机系统。Singh等人的新论文[5]提出了一种类似于NIOBE的新型探测器，基于千克质量级的超流光学机械系统，而不是1.5吨的铌圆柱体。新的探测器是独一无二的，因为超流体的声学特性是高度可调的压力允许更多的通用系统比过去。这些现代光学机械系统通过稀释制冷机冷却至K，成为具有量子有限精度的非常敏感的设备[6]。就像1990年的版本一样，这种基于宏观质量的系统将先验地对GW敏感。与20世纪90年代的棒相比，这些系统的较小质量降低了灵敏度，但这被降低热奈奎斯特噪声的较低mK温度所抵消，而且光机械设备的最新进展已经在转换和放大机制方面有了很大的改进，通过实施反作用规避系统，可以在标准量子极限甚至超过该极限监测运动[6]。这种光学机械系统也可以在频带中敏感，这不适合自由质量激光干涉仪检测器[7-9]。更小的质量也允许一个比原来的酒吧更“桌面”样的实验，在灵敏度的偏移不仅由其较低的温度，但也非常高的mechanicalQ-因素的声学超流体系统的可能性。由于氦中的声速随气压变化50%，因此该装置易于调谐，适合于搜索已知频率的连续毫秒脉冲。据Singh等人[5]报道，该实验可以在1 kHz处接近10 Hz量级的奈奎斯特噪声限制应变灵敏度，这比20世纪90年代的原始共振质量探测器好两个数量级[3]。在理论上众所周知，奈奎斯特噪声限制声振荡器是潜在的宽带GW检测器，在振荡的谐振频率附近，因为噪声和信号经历相同的传递函数。由于其有限的噪声温度，增加传感器来读出振荡器的运动限制了带宽。理论上，一个完美的换能器将允许无限的带宽。在实践中，我们应该开发尽可能低的噪声传导。这可以通过将声振荡器配置为光学机械系统来实现，通过用微波参数上变频换能器阅读运动。为了使灵敏度最大化并允许反作用规避，微波Q因子应至少足够高以处于已分辨的边带区域[10]。Singh等人[5]以一种新颖的方式实现了这一点，用超流体填充高QNb谐振器，它既是声学振荡器又是微波参量换能器。声模的影响调制了铌腔中的介电常数，铌腔也是一个极高Q值的腔。测量的10阶Q值因子给出了100 Hz的线宽，因此当监测开放访问时，
The year 2016marked the historic occasionwith the announcement of the first direct detection of gravitational waves (GWs)with the LIGOdetectors [1]. This event was a long time coming as theGW field has a long rich history, which has significantly enhanced thefield of precision and quantummeasurement over six decades. The first operational detectors were the resonant-bar detectors (ormore generally ‘resonant-mass detectors’), of more than a ton inweight, asfirst realised in the 1960s by JosephWeber [2]. Three decades later, the 1990s cryogenic versions of these detectors came on-linewith a significant sensitivity of order 10 Hz at about a 1 KHz detection frequencywith a 1 Hz bandwidth [3]. Therewere 3 in Italy, 1 in theUSA and 1 inAustralia, all of whichwere very reliable with high duty cycle and stable operation.Most of these detectors had superconducting SQUID readouts tomonitor the vibrational resonance of the detector, however one of them,NIOBE [4], had a microwave parametric transducer, andwas in fact an ultra-sensitive optomechanical system. The newpaper by Singh et al [5] presents a novel detector analogous toNIOBE, based on a superfluid-based optomechanical systemof kgmass scale, rather than a 1.5 tonne niobium cylinder. The new detector is unique because the acoustic properties of superfluid is highly tunablewith pressure allowing amuchmore versatile system than in the past. These daysmodern optomechanical systems are cooled tomKvia a dilution refrigerator, becoming very sensitive devices with quantum limited precision [6]. Just like the 1990 versions, such systems based on macroscopicmasses will be a priori sensitive toGWs. The smallermass of these systems compared to the bars of the 1990s reduces the sensitivity, but this is offset by the lowermK temperature reducing the thermalNyquist noise, also recent advances in optomechanical devices have seen a large improvement in the transduction and amplificationmechanisms, enablingmonitoring of themotion at the standard quantum limit and even beyond this limit by implementing back action evading systems [6]. Such optomechanical systems can also be sensitive in frequency bands, which are not as suitable for the free-mass laser interferometer detectors [7–9]. The smaller mass also allows amore ‘table-top’ like experiment than the original bars, with the offset in sensitivity not only made up by its lower temperature, but also the possibility of very highmechanicalQ-factors in an acoustic superfluid system. Because the speed of sound in heliummay change by 50%depending on the pressure the devicemay be easily tuned,making it suitable for searching for continuousmillisecond pulsars of known frequencies. It is reported by Singh et al [5], that this experiment can approach theNyquist noise limited strain sensitivities of order 10 Hz at 1 kHz, which is two orders ofmagnitude better than the original resonantmass detectors of the 1990s [3]. It is well known in theory that aNyquist noise limited acoustic oscillator is potentially a broad-bandGW detector around the resonance frequency of oscillation, as the noise and signal see the same transfer function. It is the addition of the transducer to read-out themotion of the oscillator that limits the bandwidth, due to its finite noise temperature. In theory a perfect transducer would allow infinite bandwidth. In practice one should develop the lowest noise transduction possible. Thismay be achieved by configuring the acoustic oscillator as an optomechanical systemby reading out themotionwith amicrowave parametric up converting transducer. To maximise the sensitivity and allow back action evasion themicrowaveQ-factor should be at least high enough to be in the resolved sideband region [10]. This is achieved in a novel way in Singh et al [5], a high-QNb resonator is filledwith superfluid, which becomes both the acoustic oscillator and themicrowave parametric transducer. Themotion of the acousticmodemodulates the permittivity within theNb cavity, which is also an extremely high-Q cavity.MeasuredQ factors of order 10 give linewidths of order 100 Hz, and thuswhenmonitoring a OPEN ACCESS