Physicists at UCSB have proven that it’s not just the size of an object that matters when it comes to displaying quantum effects.
UCSB graduate student Aaron O’Connell, along with physics professors Andrew Cleland and John Martinis, published a paper in the March 17 advance online edition of the journal Nature about their experiment, which showed that a mechanical resonator made of trillions of atoms could still follow laws of quantum mechanics.
“We don’t see quantum effects in the everyday world,” O’Connell said. “Physicists can see quantum effects if we design the system right.”
Quantum physics tends to apply to small objects at low temperatures, whereas larger objects obey the laws of classical mechanics. According to the journal article, quantum theory is usually used to describe atomic, electric and optical systems, but mechanical systems are difficult for quantum demonstrations.
Because the mechanical resonator is visible to the naked eye and about the width of a human hair, Cleland said the team’s finding is a significant demonstration of the scope of quantum effects.
“This is an important validation of quantum theory, as well as a significant step forward for nanomechanics research,” Cleland said in a press release.
The resonator, when given an electric charge, expands or contracts. An oscillating charge causes the mechanism to vibrate at a certain frequency. The resonator is connected to a quantum bit, or qubit, which is used to excite the resonator to vibrate by transferring a quanta of energy to it and measuring the results.
“The thing that is interesting about qubits is that while a light switch can only be off or on, a qubit operates by quantum mechanics and can be in a superposition of on and off at the same time,” O’Connell said.
The system was cooled to about 0.02 degrees Kelvin. The resonator vibrates as a harmonic oscillator at a frequency of six gigahertz, allowing it to operate at a quantum level at higher temperatures than some other systems.
The resonator was excited by the qubit, which was charged with a quanta of energy. The qubit and the resonator obeyed the predictions from the laws of quantum mechanics and became “entangled.” Quantum entanglement causes measurement of the qubit to affect the state of the resonator.
“The energy leaks from the qubit to the resonator over time,” O’Connell said. “Halfway between [the transfer], the qubit and the resonator become in an entangled state, where if you measure one of them, it affects the other.”
The study also showed that the resonator could experience one of the strange effects of quantum theory: superposition.
“We took the quantum bit and put it in an excited state and ground state at the same time.” Cleland said. “We can transfer that state to our mechanical system, which would then be excited and not excited at the same time.”
According to Cleland, future experimentation with the resonators will seek to improve their ability to vibrate so that more precise measurements can be made on them. Slight imperfections within the resonator cause it to lose energy more quickly, decreasing the number of oscillations it can complete before stopping.
“The mechanical resonator has between 200-300 oscillations. If we can improve that by a factor of 10 we can have experiments that are more complete and thorough measurements.”