Amir Hossein Ghadimi
Mechanical oscillators are among the most important scientific tools in the modern physics. From the pioneering experiments in 18th by founding fathers of modern physics such as Newton, Hooke and Cavendish to the ground braking experiments in the 21th century where the merge of two massive black holes 1.3 billion light-year away detected on earth by a gravitational wave detectors, the high Q mechanical oscillators were at the core of many monumental experiments in physics. Their ability to couple to many different physical quantities such as mass, charge, acceleration, electro-magnetic forces and optical fields makes them an ideal candidate for sensing applications. In addition, their intrinsically low dissipation rates (¿m) results in reduced coupling to the thermal bath. Since the invention of micro/nano-technology in the second half of the 20th century and ability to control the dimensions at micro and nano-scales, new horizon was opened up for mirco/nanomechanical oscillators. Miniaturization of the mechanical oscillators made them small and stiff enough to be used in our handheld electronics where dozens of mechanical sensors such as accelerometers and gyroscopes are used in our laptops and smartphones everyday.
Besides these technological advancements, since the beginning of 21th century, a new opportunity for mechanical oscillators emerged: the idea of ¿putting mechanics into quantum mechanics¿ and observing the quantum effects of these massive classical oscillators. Aside from the numerous technical challenges for achieving this goal, two fundamental obstacles has to solved: I) Even the smallest nano-mechanical oscillators still consist of billions of atoms and molecules and are orders of magnitude more massive that the traditional ¿quantum objects¿ such as atoms and molecules. Larger mass results in smaller zero point motion¿the length scale where quantum effects are visible¿which means in order to ¿see¿ these quantum effects, we have to detect smaller displacement than ever before. II) The second challenge is the low frequency of the mechanical oscillators which makes their thermal Brownian energy, orders of magnitude larger than the quantum ground state of the oscillator¿the energy scale where the quantum effects are visible¿as n_th = kBT/h¿>>1 even for a ¿/2¿ ~ 1GHz oscillator at room temperature.
Both of these obstacles, can be seen as the competition between few fundamental rates: thermal decoherence and measurement rate/mechanical frequency. Thermal decoherence is the rate at which the mechanical oscillator exchange phonons ¿ quanta of mechanical energy (h¿)¿ with its thermal environment and is given by ¿decoherence = ¯ n_th*¿m. The first obstacle translates to having the measurement rate being faster than the decoherence rate of the mechanical oscillator, ¿measurement>¿decoherence. This means in order to see the quantum coherent motion of the mechanical oscillator, we have to ¿look¿ at it before it has time to exchange random thermal energy with its environment. In other words, the life time of the quantum states of macroscopic objects are limited by their thermal decoherence rate and we have to interact with the oscillator in this short lifetime. The second obstacle on the other hand, reduces to having the mechanical frequency larger than its thermal decoherence rate, ¿m >¿decoherence. Over the past 15 years, the field of cavity opto-mechanics was very successful in improving the measurement schemes and designing ...
EPFL2018
