How to levitate a diamond Wednesday, 14 October 2015

American researchers have demonstrated how to levitate nanodiamonds in a vacuum using lasers, opening the possibility of creating instruments for sensing tiny forces and torques.

Levi Neukirch from the Institute of Optics at the University of Rochester says they had previously levitated nanodiamonds in air using a trapping laser but they are continuously agitated by collisions with the air molecules around them. Trapping the diamonds in vacuum removes the effect of all these air molecules.

“This allows us to exert mechanical control over them. They turn into little harmonic oscillators,” said Neukirch.

“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.”

While other researchers have trapped other types of nanoparticles in vacuum, those were not optically active. The nanodiamonds, on the other hand, can contain nitrogen-vacancy (NV) centers that emit light and also have a spin quantum number of one.

Neukirch said in their previous experiments the diamond shone brightly because it contained hundreds of vacancies, all which emit light after being excited with a laser. In their recent work they chose diamonds that had few vacancies and were even able to select diamonds with a single vacancy. With a single spin in the NV center, and the system functioning as a quantum mechanical oscillator, the researchers would be able to affect the spin state of the tiny defect inside the nanodiamond by exerting mechanical control on the entire nanodiamond.

He explained that the defect’s electrons had to take on specific spin states, two of which are normally “degenerate,” meaning in this case that states with spin values of +1 or -1 have the same energy.

“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.”

There is still much to be done as the nanodiamonds are being destroyed in seconds at very low pressures. Neukirch believes the nanodiamonds are either melting or sublimating, because at lower pressures there are fewer air molecules to remove the excess internal heat from the diamonds, which is injected by a laser that is used to excite the system as part of the experiment. But he says the  potential for these systems is very exciting.

 

The green dot is the levitating nanodiamond. Photo: J. Adam Fenster, Univeresity of Rochester.