Stanford physicists have developed a "quantum microphone" so sensitive that it can measure individual particles of sound, called phonons.
The device, which is detailed in July 24 in the journal Nature could eventually lead to smaller, more efficient quantum computers that operate by manipulating sound rather than light
"We expect this device to allow new types of quantum sensors, transducers and storage devices for future quantum machines, "said Amir Safavi-Naeini, an assistant professor of applied physics at Stanford's School of Humanities and Sciences
Quantum of motion
First proposed by Albert Einstein in 1907, phonons are packets of vibrational energy emitted by jittery atoms.
Like photons, which are the quantum carriers of light, phonons are quantized, meaning their vibrational energies are restricted to discrete values-similar to
"Sound has this granularity that we do not normally experience," said Safavi-Naeini. "Sound, at the quantum level, crackles."
The energy of a mechanical system can be represented as different "Fock" states-0, 1, 2, and so on-based on the number of phonons it generates. For example, "1 Fock State" consists of one phonon of a particular energy, and "2 Fock State" consists of two phonons with the same energy, and so on. Higher phonon states correspond to louder sounds
Until now, scientists have been unable to measure phonon states in engineered structures directly because the energy differences between states-in the staircase analogy, the spacing between steps-is vanishingly small. "One phonon corresponds to an energy of three trillion trillion times smaller than the energy required to keep a lightbulb on for one second," said Patricio Arrangoiz-Arriola, a co-author of the study
To address this issue , the Stanford team engineered the world's most sensitive microphone-one that exploits the quantum principles to eavesdrop on the whispers of atoms
In a normal microphone, the incoming sound waves jiggle an internal membrane, and this physical displacement is converted into a measurable voltage . This approach does not work for detecting individual phonons because, according to the Heisenberg uncertainty principle, and the quantum object's position can not be precisely known without changing it
"If you tried to measure the number of phonons with a regular microphone , the act of measurement injects energy into the system that masks the very energy you are trying to measure, "Safavi-Naeini said.
Instead, the physicists devised a way to measure Fock states-and thus, the number of phonons-in sound waves directly. "Quantum mechanics tells us that the position and momentum can not be known precisely-but it says no such thing about energy," said Safavi-Naeini. Singing qubits
The quantum microphone of the group developed consists of a series of supercooled nanomechanical resonators, so small that they are only visible through an electron microscope
. The resonators are coupled to a superconducting circuit that contains electron pairs that move around without resistance. The circuit forms a quantum bit, or qubit, that can exist in two states at once and has a natural frequency that can be read electronically. When the mechanical resonators vibrate like a drumhead, they generate phonons in different states
"The resonators are formed from periodic structures that act like mirrors for sound.
Like unruly inmates, trapped phonons rattle the walls of their prisons, and these mechanical motions are conveyed to the qubit by ultra-thin wires. "The qubit's sensitivity to displacement is especially strong when the frequencies of the qubit and the resonators are almost the same," said Alex Wollack, a joint-first author, also a graduate student at Stanford.
However, by detuning the system so that the qubit and the resonators vibrate at very different frequencies, researchers have weakened this mechanical connection and triggered a type of quantum interaction, known as a dispersive interaction, which directly links the qubit to the phonons
This bond causes the frequency of qubit to shift in proportion to the number of phonons in the resonators. "Different phonon energy levels appear as distinct peaks in the qubit spectrum," Safavi-Naeini said. The researchers have been able to determine the quantized energy levels of the vibrating resonators – effectively resolving the phonons themselves
. "These peaks correspond to Fock states of 0, 1, 2 and so on."
Mechanical quantum mechanical
Mastering the ability to precisely generate and detect phonons could help pave the way for new types of quantum devices that are capable of storing and retrieving information encoded as particles of sound or that can convert seamlessly between optical and mechanical signals.
Such devices could be made more compact and efficient than quantum machines that use photons, since phonons are easier to manipulate and have wavelengths that are thousands of times smaller than light particles
"Right now, people are using photons to encode these states. it's a lot of advantages, "said Safavi-Naeini. "Our device is an important step towards making a 'mechanical quantum mechanical' computer."
Coupling qubits to sound in a multimode cavity
Patricio Arrangoiz-Arriola et al. Resolving the energy levels of a nanomechanical oscillator, Nature (2019). DOI: 10.1038 / s41586-019-1386-x
Physicists count sound particles with quantum microphone (2019, July 27)
retrieved 27 July 2019
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