The physicists of Stanford have developed a sensitive "quantum microphone" that can measure individual sound particles, called phonons. The journal Nature could eventually become lead smaller, more efficient quantum computers, which work by manipulation of sound instead of light.
"We expect this device to enable new types of quantum quantum sensors, transducers and storage devices for future quantum machines," said study leader Amir Safavi-Naeini, assistant professor of applied physics at the Stanford School of Humanities and Sciences.
Quantum of motion
Phonons, first proposed in 1
Like photons, which are quantum carriers of light, phonons are quantized, which means that their vibrational energies are limited to discrete values - similar to how a staircase is composed of different stages.
"Sound has this graininess that we normally do not experience," said Safavi-Naeini. "Sound at quantum level crackles."
The energy of a mechanical system can be represented as different "jib" states – 0, 1, 2, etc. – based on the number of phonons it generates. For example, a "1-jib state" consists of one phonon of a given energy, a "2 jib state" of two phonons of the same energy, and so on. Higher phoneme states correspond to louder sounds.
Previously, scientists could not directly measure phonon states in constructed structures, because the energy differences between states – in the staircase analogy, the distance between stages – are negligible. "A phonon equals an energy ten billion trillion times smaller than the energy required to keep a light bulb on for a second," co-lead author of the study, Patricio Arrangoiz-Arriola, said.
The Stanford team developed the world's most sensitive microphone – one that uses quantum principles to eavesdrop on the whisper of atoms.
In a normal microphone, incoming sound waves wobble with an inner membrane, and this physical displacement is transformed into a measurable voltage. This approach does not work for the detection of single phonons because the position of a quantum object can not be determined exactly without changing the Heisenberg uncertainty principle.
"If you tried to measure the number of phonons with a regular microphone, the measurement will inject energy into the system that will mask exactly the energy you want to measure," said Safavi-Naeini.
Instead, physicists developed a method to measure the jib states and therefore the number of states phonons – directly in sound waves. "Quantum mechanics tells us that position and momentum are not exactly known – but it does not say anything about energy," Safavi-Naeini said. "Energy can be detected with infinite precision."
The quantum microphone developed by the group consists of a series of supercooled nanomechanical resonators that are so small that they can only be seen through an electron microscope. The resonators are coupled to a superconducting circuit that contains electron pairs that move without resistance. The circuit forms a quantum bit or qubit which can exist in two states simultaneously and has a natural frequency that can be read electronically. When the mechanical resonators vibrate like an eardrum, they produce phonons in different states.
"The resonators consist of periodic structures that act like sound mirrors, and by introducing a defect into these artificial lattices, we can place the phonons in the middle of the structures," said Arrangoiz-Arriola.
Like defiant inmates, the captured phonons clatter on the walls of their prisons, and these mechanical movements are transmitted to the qubit by ultra-thin wires. "The sensitivity of the qubit to shifts is particularly high when the frequencies of the qubit and the resonators are almost equal," said joint lead author Alex Wollack, also a graduate student in Stanford The qubit and resonators vibrate at very different frequencies, weakening the researchers This mechanical connection triggers a kind of quantum interaction, known as the dispersive interaction, which connects the qubit directly to the phonons.
This binding causes the frequency of the qubit to shift in proportion to the number of phonons in the resonators. By measuring the mood changes of the qubit, the researchers were able to determine the quantized energy levels of the vibrating resonators – effectively dissolving the phonons themselves.
"Different phonon energy levels appear as distinct peaks in the qubit spectrum," said Safavi-Naeini. "These peaks correspond to Fock states 0, 1, 2, and so on. These multiple peaks have never been seen before."
Mechanical Quantum Mechanics
Mastering the ability to precisely create and capture phonons could help pave the way for novel quantum devices that are capable of storing and retrieving information encoded as sound particles seamlessly convert between optical and optical elements mechanical signals.
Such devices could potentially be made more compact and efficient than quantum photon machines because phonons are easier to manipulate and have wavelengths that are a thousand times smaller than light particles.
"Right now, people use photons to encode these states, we want to use phonons, which has many advantages," said Safavi-Naeini. "Our device is an important step in making a 'mechanical quantum mechanical' computer."
Other Stanford co-authors include graduate students such as Zhaoyou Wang, Wentao Jiang, Timothy McKenna and Jeremy Witmer, and post-docs Marek Pechal and Raphël Van Laer.
The research was funded by the David and Lucile Packard Fellowship, the Stanford University Terman Fellowship, and the US Office of Naval Research.