Stanford University researchers have developed a key experimental device for
future quantum physics-based technologies that borrows a page from current,
everyday mechanical devices.
Reliable, compact, durable, and efficient, acoustic devices harness
mechanical motion to perform useful tasks. A prime example of such a device
is the mechanical oscillator. When displaced by a force—like sound, for
instance—components of the device begin moving back-and-forth about their
original position. Creating this periodic motion is a handy way to keep
time, filter signals, and sense motion in ubiquitous electronics, including
phones, computers, and watches.
Researchers have sought to bring the benefits of mechanical systems down
into the extremely small scales of the mysterious quantum realm, where atoms
delicately interact and behave in counterintuitive ways. Toward this end,
Stanford researchers led by Amir Safavi-Naeini have demonstrated new
capabilities by coupling tiny nanomechanical oscillators with a type of
circuit that can store and process energy in the form of a qubit, or quantum
"bit" of information. Using the device's qubit, the researchers can
manipulate the quantum state of mechanical oscillators, generating the kinds
of quantum mechanical effects that could someday empower advanced computing
and ultraprecise sensing systems.
"With this device, we've shown an important next step in trying to build
quantum computers and other useful quantum devices based on mechanical
systems," said Safavi-Naeini, an associate professor in the Department of
Applied Physics at Stanford's School of Humanities and Sciences.
Safavi-Naeini is senior author of a new study published April 20 in the
journal Nature describing the findings. "We're in essence looking to build
'mechanical quantum mechanical' systems," he said.
Mustering quantum effects on computer chips
The joint first authors of the study, Alex Wollack and Agnetta Cleland, both
Ph.D. candidates at Stanford, spearheaded the effort to develop this new
mechanics-based quantum hardware. Using the Stanford Nano Shared Facilities
on campus, the researchers worked in cleanrooms while outfitted in the
body-covering white "bunny suits" worn at semiconductor manufacturing plants
in order to prevent impurities from contaminating the sensitive materials in
play.
With specialized equipment, Wollack and Cleland fabricated hardware
components at nanometer-scale resolutions onto two silicon computer chips.
The researchers then adhered the two chips together so the components on the
bottom chip faced those on the top half, sandwich-style.
On the bottom chip, Wollack and Cleland fashioned an aluminum
superconducting circuit that forms the device's qubit. Sending microwave
pulses into this circuit generates photons (particles of light), which
encode a qubit of information in the device. Unlike conventional electrical
devices, which store bits as voltages representing either a 0 or a 1, qubits
in quantum mechanical devices can also represent weighted combinations of 0
and 1 simultaneously. This is because of the quantum mechanical phenomenon
known as superposition, where a quantum system exists in multiple quantum
states at once until the system is measured.
"The way reality works at the quantum mechanical level is very different
from our macroscopic experience of the world," said Safavi-Naeini.
The top chip contains two nanomechanical resonators formed by suspended,
bridge-like crystal structures just a few tens of nanometers—or billionths
of a meter—long. The crystals are made of lithium niobate, a piezoelectric
material. Materials with this property can convert an electrical force into
motion, which in the case of this device means the electric field conveyed
by the qubit photon is converted into a quantum (or a single unit) of
vibrational energy called a phonon.
"Just like light waves, which are quantized into photons, sound waves are
quantized into 'particles' called phonons," said Cleland, "and by combining
energy of these different forms in our device, we create a hybrid quantum
technology that harnesses the advantages of both."
The generation of these phonons allowed each nanomechanical oscillator to
act like a register, which is the smallest possible data-holding element in
a computer, and with the qubit supplying the data. Like the qubit, the
oscillators accordingly can also be in a superposition state—they can be
both excited (representing 1) and not excited (representing 0) at the same
time. The superconducting circuit enabled the researchers to prepare, read
out, and modify the data stored in the registers, conceptually similar to
how conventional (non-quantum) computers work.
"The dream is to make a device that works in the same way as silicon
computer chips, for example, in your phone or on a thumb drive, where
registers store bits," said Safavi-Naeini. "And while we can't store quantum
bits on a thumb drive just yet, we're showing the same sort of thing with
mechanical resonators."
Leveraging entanglement
Beyond superposition, the connection between the photons and resonators in
the device further leveraged another important quantum mechanical phenomenon
called entanglement. What makes entangled states so counterintuitive, and
also notoriously difficult to create in the lab, is that the information
about the state of the system is distributed across a number of components.
In these systems, it is possible to know everything about two particles
together, but nothing about one of the particles observed individually.
Imagine two coins that are flipped in two different places, and that are
observed to land as heads or tails randomly with equal probability, but when
measurements at the different places are compared, they are always
correlated; that is, if one coin lands as tails, the other coin is
guaranteed to land as heads.
The manipulation of multiple qubits, all in superposition and entangled, is
the one-two punch powering computation and sensing in sought-after
quantum-based technologies. "Without superposition and lots of entanglement,
you can't build a quantum computer," said Safavi-Naeini.
To demonstrate these quantum effects in the experiment, the Stanford
researchers generated a single qubit, stored as a photon in the circuit on
the bottom chip. The circuit was then allowed to exchange energy with one of
the mechanical oscillators on the top chip before transferring the remaining
information to the second mechanical device. By exchanging energy in this
way—first with one mechanical oscillator, and then with the second
oscillator—the researchers used the circuit as a tool to quantum
mechanically entangle the two mechanical resonators with each other.
"The bizarreness of quantum mechanics is on full display here," said
Wollack. "Not only does sound come in discrete units, but a single particle
of sound can be shared between the two entangled macroscopic objects, each
with trillions of atoms moving—or not moving—in concert."
For eventually performing practical calculations, the period of sustained
entanglement, or coherence, would need to be significantly longer—on the
order of seconds instead of the fractions of seconds achieved so far.
Superposition and entanglement are both highly delicate conditions,
vulnerable to even slight disturbances in the form of heat or other energy,
and accordingly endow proposed quantum sensing devices with exquisite
sensitivity. But Safavi-Naeini and his co-authors believe longer coherence
times can be readily achievable by honing the fabrication processes and
optimizing the materials involved.
"We've improved the performance of our system over the last four years by
nearly 10 times every year," said Safavi-Naeini. "Moving forward, we will
continue to make concrete steps toward devising quantum mechanical devices,
like computers and sensors, and bring the benefits of mechanical systems
into the quantum domain."
Reference:
E. Alex Wollack et al, Quantum state preparation and tomography of entangled
mechanical resonators, Nature (2022).
DOI: 10.1038/s41586-022-04500-y
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Physics