Nanomechanical resonators at low temperatures: from classical to quantum dissipation

当你敲击一根吉他弦时，它会振动，当它这样做的时候，它会迫使周围的空气运动，产生你听到的声音。当然，绳子最终会停止运动，因为当它让周围的空气运动时，它会失去能量，因此速度会变慢。这个项目是关于被称为纳米机械谐振器的微小琴弦的行为，这种琴弦大约比吉他琴弦小100万倍。一根弦发出的音符的频率或音高取决于这根弦的长度。对于吉他来说，频率正好适合人类听到，但对于纳米机械谐振器来说，频率要高得多，与无线电波的频率大致相同，因此远远超出了我们的听力范围。然而，我们有可能利用电路“监听”纳米机械谐振器的振动，并检测音符频率的微妙变化。尽管纳米机械谐振器不是很好的乐器，但它们可以用来做很多其他事情。例如，纳米机械谐振器可以用作一套极其灵敏的称重秤。因为纳米机械谐振器本身的重量很小，所以在谐振器上放置一个小物体，比如生物病毒，会改变振动的频率，这种方式取决于物体的重量。但有一个问题：为了准确称重，我们需要能够测量纳米机械谐振器频率的微小变化，并且为了能够做到这一点，谐振器必须以相同的频率振动足够长的时间，才能进行准确的测量。就像吉他弦一样，纳米机械谐振器不会永远振动。即使纳米机械谐振器周围的所有空气都可以被去除，它们仍然会损失能量给周围的环境，甚至损失到用来测量它们的电路。更糟糕的是，纳米机械谐振器越小，其振动衰减的速度就越快。这个项目的主要目标是进行一系列实验，旨在帮助我们找出为什么纳米机械谐振器的振动衰减得这么快。一旦我们知道了纳米机械谐振器对周围环境的能量损失如此之快的原因，就应该有可能改进它们的设计，以确保它们振动的时间更长，从而可以用来测量更小的物体的重量，比如单个分子。

When you twang a guitar string it vibrates and as it does so it forces the air around it into motion, giving rise to the sound you hear. Of course the string eventually stops moving because as it sets the air in motion about it, it loses energy and so slows down. This project is about the behaviour of tiny strings, called nanomechanical resonators, which are about a million times smaller than a guitar string. The frequency, or pitch, of a note that comes from a string depends on how long the string is. For a guitar the frequency is just right for humans to hear it, but for nanomechanical resonators the frequency is much higher, about the same as for radio waves, and so is well beyond our hearing. However, it is possible to `listen` to the vibrations of a nanomechanical resonator using electrical circuitry and detect very subtle changes in the frequency of the notes.Although nanomechanical resonators are no good as musical instruments, they can be useful for a lot of other things. For example, nanomechanical resonators can be used as a fantastically sensitive set of weighing scales. Because a nanomechanical resonator itself weighs very little, placing a small object, like a biological virus, on top of the resonator changes the frequency of the vibrations in a way that depends on how much the object weighs. But there is a catch: In order to weigh things accurately we need to be able to measure very small changes in the frequency of the nanomechanical resonator, and to be able to do that the resonator must vibrate long enough at the same frequency for an accurate measurement to be made. Just like guitar strings, nanomechanical resonators don't vibrate forever. Even though all the air can be removed from around nanomechanical resonators, they still lose energy to their surroundings or even to the electrical circuitry which is used to measure them. What is worse, the smaller a nanomechanical resonator is made, the faster its vibrations tend to decay away. The key aims of this project is to carry out a series of experiments designed to help us work out why the vibrations of nanomechanical resonators damp away so fast. Once we know the reasons why nanomechanical resonators lose energy so fast to their surroundings it should be possible to improve their design to ensure that they vibrate for longer and so can be used to weigh even smaller objects, like individual molecules.