Optomechanical quantum sensors at room temperature (QuaSeRT)

This project is a part of the consortium QuaSeRT (Optomechanical quantum sensors at room temperature) with partners within experimental and theoretical physics from universities in Italy, France, Austria, the Netherlands, and Germany. The goal of the project is fundamental research on measurement devices or sensors that are so sensitive that they must be described by the laws of quantum physics. The precision of many sensors depends on how well one can measure the position of a mechanical element. Such a position measurement can be made very precise by measuring light reflected off of the element. Extreme examples of this are the recent observations of gravitational waves from outer space. The precision of a position measurement will nevertheless be limited by thermal noise, i.e. the fact that all objects that have a temperature will shake a little bit. This research project deals with optomechanical systems where electromagnetic radiation (f.ex. light) and the motion of small objects interact. Light can not only detect motion, but it can also influence motion by so-called radiation pressure. Recently, experimentalists have succeeded in using light to damp or cool the motion of objects such that the thermal jitter is essentially removed. The minute motion that remains is called quantum noise. This project seeked to develop better systems of this type. In particular, it is desireable to realize devices where light can be used to cool motion to this so-called quantum regime without other means of cooling, which means starting from room temperature. Kjetil Børkje at the University of South-Eastern Norway were to focus on the theoretical part of the project. This consists of developing new, alternative measurement protocols that avoid the obstacles set by thermal noise, as well as developing new ideas for preparing optomechanical systems in nonclassical states. In addition to enabling extremely sensitive measurements, the project is also of relevance to quantum information technology and to fundamental questions in quantum mechanics. In the effort to develop new sensing protocols, we have explored the dynamics of an optically damped mechanical oscillator that is also subjected to a large, oscillatory force. This setup unavoidably leads to highly nonlinear dynamics and the phenomenon of multistability, where the steady-state mechanical oscillation amplitude can take several distinct values. The sensing protocol proposed relies on the existence of a region of bistability of the steady-state coherent mechanical oscillation amplitude. If the system is driven such that it sits close to a bifurcation point, where one of the stable solutions vanishes, small signals (either mechanical or optical) can lead to switching from one stable oscillation amplitude to the other, which can then easily be read out optically. We have conducted a theoretical study of the potential for nonlinear dynamics also in the quantum regime, given the constraint that virtually all experimental realizations of optomechanical systems have so-called weak coupling on the level of individual quanta of light and motion (photons and phonons). We have found that carefully engineered optomechanical systems that involve several mechanical oscillators can display nonlinear dynamics even if the average number of photons is less than unity. This study is a step towards creating systems with fundamentally new dynamics, which can also open the door to new measurement protocols. We have also identified new methods to detect nonclassical behavior in optomechanical systems. This is possible by statistical analysis of the light the system emits. The measurements involve frequency filtering of the light and detection single photons who have been frequency converted through the optomechanical interaction. This type of measurement is possible in several different implementations of optomechanics. In this theoretical study, we have identified model-independent signatures of nonclassicality, which means that the interpretation of the measurement results does not depend on our theoretical model of the system. Finally, we have provided theoretical support to experiments conducted by other partners in the consortium. This involves characterization of the quantum, two-dimensional motion of a nanoparticle trapped in an optical tweezer and cooled to the quantum regime by strong, optomechanical interactions. The characterization is more complicated than in earlier experiments, both due to the strong interactions and due to the two-dimensional nature of the motion. The project QuaSeRT as a whole has lead to several results that has moved the frontier of quantum optomechanics. The central goal of using light to cool motion to the quantum regime from room temperature has been achieved with nanoparticles trapped in optical tweezers.

-

The main target of this project is the creation of optomechanical sensing devices achieving the quantum limit in the measurement process, and exploiting peculiar quantum properties, of both the mechanical oscillator and the interacting radiation field, to enhance the efficiency of the measurement and to integrate the extracted information in quantum communication systems. We will develop three different platforms that, according to the present state of the art, are the most suitable to achieve our goal: (i) semiconductor nano-optomechanical disks (ii) tensioned dielectric membranes (iii) levitating nanoparticles. This parallel approach allows increasing the success probability, to extend the operating frequency range and diversify the systems for a larger versatility. Moreover, in order to study specific quantum protocols, we will exploit nano-electro-mechanical systems which have been shown to be the most suitable classical test-bench for this purpose thanks to their long coherence even at room temperature and their unprecedented control. Mechanical and optical properties of the different resonators will be improved, in order to increase the coherent coupling rate and reduce the decoherence rate, eventually achieving quantum performance of the devices at room temperature, a crucial requirement for a realistic application scenario as sensors. Producing and manipulating quantum states of a sensor is an important pre-requisite for the quantum revolution, e.g., for implementing a quantum network that collects information from the environment and transfers it into quantum communication channels. We will produce prototype portable sensing systems, evaluate and compare the performance of the different platforms as acceleration sensors, study the possibilities of system integration and of functionalization for future extended sensing capability.