A Quantum Jump Sensor for Dark Matter Detection

值得注意的是，在宇宙尺度上，我们不知道是什么构成了宇宙中84%的物质。这种暗物质对恒星的运动、星系的形成以及大爆炸余辉中的模式有着深远的影响，但我们只能假设它的真实本质是什么。我们将建造一种新型的量子传感器，使用一个孤立的电子，它将足够灵敏，可以判断暗物质是否由某些类型的新粒子组成。似乎大部分失踪的物质都是某种新型物质，它们几乎不与普通物质发生电磁相互作用。强核力和弱核力的不同对称性为这一现象提供了一个线索，这使得粒子物理学家提出了一种新的粒子——轴子。预测轴子的理论并没有预测到它的质量，但是在早期的宇宙中，大约比电子质量小10^9-10^12倍的轻轴子会被创造出来，并且今天仍然以暗物质的形式存在。除了粒子物理学的提示外，宇宙学也有关于暗物质特性的暗示。对早期宇宙中氢的微波跃迁频率的观测表明，在宇宙黎明时期，它比预期的要冷。这也是暗物质与普通物质碰撞并降低其温度的时期。带有微小电荷的奇异粒子——被称为微带电粒子——可以解释这一现象。人们进行了许多实验来探测轴子和带电粒子，但都没有发现。探测轴子最灵敏的实验是利用强磁场促使轴子衰变为微波光子，其频率与轴子质量直接相关。然后它们探测到这些微波。不幸的是，对于一个重要的轴子质量范围，最先进的微波探测器有一个基本的和不可避免的噪声源，使轴子信号相形见绌。这个最小的噪声，被称为标准量子极限，可以通过计算构成电磁场的光子的数量来克服。在30-60 GHz范围内没有合适的单光子计数器，所以我们将发明一个。我们选择的技术是一个电子，被困在电场和磁场的组合中。当电子吸收微波光子时，其量子轨道发生可探测的变化。被捕获的电子对与任何带电暗物质的碰撞也很敏感，碰撞后也会改变其轨道。这种轨道上的变化可以用量子跃迁光谱来测量，这种光谱以前被用来测量电子的磁矩。如果我们能揭开暗物质的本质，我们就能最终了解宇宙中最丰富的物质，而描述它的精确特性将对天体物理学的许多方面产生影响。轴子或带电粒子的发现只会是粒子物理学新时代的开始，因为这两种粒子都被认为会伴随着其他粒子。在轴子的情况下，这些可能是更重的希格斯粒子，这为在欧洲核子研究中心建造未来的圆形对撞机来发现它们提供了额外的有力论据。最后，该装置是一种可探测到的最弱微波信号传感器，可应用于改进的微波天文学、分子光谱的化学物质识别和传感。

It is remarkable that on a cosmic scale, we do not know what makes up 84% of the matter in the universe. This dark matter has a profound impact on the movement of stars, the formation of galaxies and the patterns in the afterglow of the Big Bang, but we can only hypothesise what its true nature might be. We will build a new type of quantum sensor using a single isolated electron that will be sensitive enough to tell if dark matter is made from certain types of new particles.It seems likely that most of the missing matter is some new type of substance which barely interacts with ordinary matter electromagnetically. One hint for what this might be comes from the differing symmetries of the strong and weak nuclear forces, which led particle physicists to propose a new particle, the axion. The theory which predicted the axion does not predict its mass, but light axions around 10^9-10^12 times less than the mass of an electron would have been created in the early universe and still be present today as dark matter.As well as hints from particle physics, there are also indications from cosmology as to the properties of dark matter. Observations of the microwave transition frequencies of hydrogen in the period of the early universe known as the cosmic dawn suggest that it was colder than expected. This was also the period where dark matter could collide with ordinary mater and reduce its temperature. Exotic particles with tiny charges - known as millicharged particles - would account for this observation. Many experiments have been carried out to detect axions and millicharged particles, but none have been discovered. The most sensitive experiments to detect axions use a strong magnetic field to encourage the axions to decay into microwave photons with a frequency directly related to the axion mass. They then detect those microwaves. Unfortunately, for an important axion mass range, state-of-the art microwave detectors have a fundamental and unavoidable noise source which dwarfs the axion signal. This minimum noise, referred to as the Standard Quantum Limit can be overcome by counting the number of photons which make up the electromagnetic field. No suitable single photon counter exists in the range 30-60 GHz, so we will invent one. The technology we have chosen is a single electron, trapped in a combination of electric and magnetic fields. As the electron absorbs a microwave photon, its quantum orbit changes detectably. A trapped electron is also sensitive to collisions with any millicharged dark matter, also changing its orbit after a collision. This change in orbit can be measured using quantum jump spectroscopy, which was previously used to measure the electron's magnetic moment.If we could uncover the nature of dark matter, we would finally have understood the most abundant substance in the universe and characterising its precise properties would have implications for many aspects of astrophysics. A discovery of the axion or millicharged particle would only be the start of a new era of particle physics since both particles would be expected to be accompanied by others. In the case of the axion, these could be much heavier Higgs particles, giving an additional strong argument for the construction of a Future Circular Collider at CERN to discover them. Finally, this device is a sensor for the weakest detectable microwave signals, which could be applied to improved microwave astronomy, molecular spectroscopy for the identification of chemical substances and sensing.