A team of researchers led by Nick Vamivakas of the University of Rochester has successfully used laser to levitate individual nanodiamonds in vacuum.
This achievement could help create physically macroscopic Schrödinger Cat states, and instruments of high sensitivity that are capable of sensing minute torques and forces. Macroscopic Schrödinger Cat states are larger-scale quantum systems.
Previous studies had been successful in trapping optically inactive nanoparticles in vacuum. However, nanodiamonds have nitrogen-vacancy (NV) centers that are capable of emitting light. Additionally, they possess a spin quantum number of one. The research team believes that their achievement could pave the way towards a "hybrid quantum system”, which is a levitated system that combines mechanical, optical and spin degrees of freedom. In this system, the nanodiamond’s mechanical motion is combined with the vacancy’s internal spin, and its optical properties.
The research team had earlier demonstrated that a trapping laser could be utilized for levitating nanodiamonds in air. In the current study, it has been shown that the levitation is possible in a vacuum, and this is considered to be a significant advancement over earlier experiments conducted with nanodiamond optical tweezers at atmospheric pressure conditions or in liquids.
When nanodiamonds are trapped at atmospheric pressure, their collisions with the surrounding air molecules agitate them continuously. Vacuum conditions, however, do away with the effects exerted by the air molecules. "This allows us to exert mechanical control over them," said Levi Neukirch, lead author of the paper and a Ph.D. student in Vamivakas' group at Rochester. "They turn into little harmonic oscillators."
We can measure the position of the diamond in 3D and we create a feedback signal based on the position and velocity of the nanodiamond. This lets us actively damp its motion.
The trapping potential seen by the diamond can be changed to achieve this. The trapping potential can be explained by considering a diamond to be at a valley’s bottom. When it moves, it has to go uphill, but in the end, it has to comeback to its original position at the bottom. The optical potential’s shape is also changed by the feedback mechanism. When the diamond climbs the hill, the hill would be steep, and the diamond would roll back down slowly as the hill would become gradual. This would eventually lead to dampening of the diamond's motion until it reaches its ground state, and this would make the system act like a quantum mechanical oscillator.
In earlier experiments, the diamonds that were used had hundreds of vacancies. When laser excites these vacancies, all of them emit light and the diamonds shone brightly. The researchers selected diamonds that had a single or a few vacancies in the present study. When the system worked like a quantum mechanical oscillator, and the NV center had a single spin, the researchers exerted mechanical control on the nanodiamonds to achieve the tiny defect’s spin state inside the nanodiamonds.
The system had to be in vacuum for this process to take place. Neukirch stated that destruction of nanodiamonds occurred at very low pressures. They may be sublimating or melting at low pressures where the excess internal heat in the diamonds generated by the laser used for exciting the system is removed with the help of only fewer air molecules.
Eva von Haartman and Jessica M. Rosenholm at Abo Academy, Finland, provided the silica-encased nanodiamonds for this study. The bare nanodiamonds that had been used until now were replaced with the silica-encased nanodiamonds to determine if the nanodiamonds could be protected by the encasing. The researchers observed that though the nanodiamonds did not get protected, they became homogenous and spherical that could be considered as favorable properties for future studies.
The team utilized two lasers for measuring and controlling the system. One laser was used for exciting the NV center, while the other laser was used for trapping the nanodiamond. A photon gets emitted when a defect reaches a lower energy state from its excited state, and this is known as photoluminescence. The emitted photon’s energy helps understanding the system’s energy structure. Photoluminescence also helps exert control and modify the system’s energy.
At lower pressures, nanodiamonds can vanish within seconds, and researchers will have to find out ways to prevent this from taking place. Only after that, they would be able to mechanically cool the nanodiamonds into their ground state. However, Neukirch considers that these systems have great potential.
"We have demonstrated the ability to control the NV center's spin in these levitated nanodiamonds," Neukirch said. This would mean that defect's electrons had to take particular spin states. Among these two are normally "degenerate". In this case, the same energy is possessed by states having +1 or -1 as spin values.
"Without applying a magnetic field these two energy levels are the same, but we can separate them with magnetic field, and they react differently to it. If there was an electron in the spin +1 state and you then applied a magnetic field, the whole nanodiamond would feel a push, but if it was in the spin -1 state it would feel a pull," he said. "Because the electron spins are intrinsically quantum mechanical, they can exist in something called superpositions. We can create a state where a single spin is in both the +1 and -1 states simultaneously. If we can mechanically place the nanodiamond in the ground state, this would allow us to both push and pull on the spin, hopefully generating a mechanical superposition of the entire diamond. This is a curious phenomenon that physicists are interested in studying, and it is called a macroscopic Schrödinger Cat state."
Very tiny forces or torques can also be measured by levitating nanodiamonds in vacuum. Basically, the nanodiamonds act as nano-oscillators, and even minute forces can move them. Neukirch added that their "setup is capable of detecting these small motions."
Neukirch will continue to conduct research to develop levitated, optically active nanodiamonds that can survive low pressures.
This study paper has been published in Nature Photonics.
The Office of Naval Research supported this study.