Nanomechanical resonators at low temperatures: from classical to quantum dissipation

当你拨动吉他弦时，它会振动，当它振动时，它会迫使周围的空气运动，从而产生你听到的声音。当然，弦最终会停止运动，因为当它使周围的空气运动时，它会失去能量，所以速度会减慢。这个项目是关于微小弦的行为，称为纳米机械谐振器，它比吉他弦小一百万倍。弦上音符的频率或音高取决于弦的长度。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，与无线电波相同，因此远远超出了我们的听觉范围。然而，我们可以通过电路来“听”纳米机械共振器的振动，并检测到音符频率的细微变化。虽然纳米机械共振器不能作为乐器，但它们可以用于许多其他用途。例如，纳米机械谐振器可以用作一组非常灵敏的称重秤。由于纳米机械谐振器本身的重量很小，因此在谐振器顶部放置一个小物体，如生物病毒，会以一种取决于物体重量的方式改变振动频率。但有一个陷阱：为了准确地称量物体，我们需要能够测量纳米机械谐振器频率的非常小的变化，并且为了能够做到这一点，谐振器必须在相同的频率下振动足够长的时间，以便进行准确的测量。就像吉他弦一样，纳米机械共振器不会永远振动。即使所有的空气都可以从纳米机械谐振器周围移除，它们仍然会将能量损失到周围环境中，甚至损失到用于测量它们的电路中。更糟糕的是，纳米机械谐振器制造得越小，其振动衰减得越快。该项目的主要目的是进行一系列实验，旨在帮助我们找出为什么纳米机械谐振器的振动衰减得如此之快。一旦我们知道了纳米机械共振器如此快地向周围环境损失能量的原因，就有可能改进它们的设计，以确保它们振动更长时间，从而可以用于测量更小的物体，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.