New hardware embeds mechanical devices in quantum technology
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, hearing aids harness mechanical motion to perform useful tasks. An excellent example of such a device is the mechanical oscillator. When moved by a force – like sound, for example – the components of the device begin to move back and forth around their original position. Creating this periodic motion is a convenient way to keep time, filter signals, and detect motion in ubiquitous electronics, including phones, computers, and watches.
The researchers sought to bring the benefits of mechanical systems back to the extremely small scales of the mysterious quantum realm, where atoms delicately interact and behave in counterintuitive ways. To that end, Stanford researchers led by Amir Safavi-Naeini have demonstrated new capabilities by coupling tiny nanomechanical oscillators to a type of circuit capable of storing and processing energy in the form of a qubit or a quantum “bit” of information. Using the device’s qubit, researchers can manipulate the quantum state of mechanical oscillators, generating the kinds of quantum mechanical effects that could one day power advanced computing and ultra-precise sensing systems.
“With this device, we have shown an important next step in trying to build quantum computers and other useful quantum devices based on mechanical systems,” said Safavi-Naeini, associate professor in the Department of Applied Physics at Stanford’s School. of Humanities and Sciences. Safavi-Naeini is the lead author of a new study published April 20 in the journal Nature describing the findings. “We’re basically looking to build ‘mechanical quantum mechanics’ systems,” he said.
Gathering quantum effects on computer chips
The study’s co-first authors, Alex Wollack and Agnetta Cleland, both PhD students at Stanford, led the effort to develop this new quantum mechanics-based hardware. Using Stanford Nano’s shared facilities on campus, researchers worked in clean rooms while outfitted in body-covering white “bunny suits” worn in semiconductor fabs to keep impurities out. contaminate the sensitive materials involved.
Using specialized equipment, Wollack and Cleland fabricated hardware components at nanoscale resolutions on two silicon computer chips. The researchers then glued the two chips together so that the components on the bottom chip faced those on the top half, like a sandwich.
On the bottom chip, Wollack and Cleland fashioned a superconducting aluminum circuit that forms the device’s qubit. Sending microwave pulses through 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 a 0 or a 1, qubits in quantum mechanical devices can also simultaneously represent weighted combinations of 0s and 1s. This is due to 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 level of quantum mechanics is very different from our macroscopic experience of the world,” Safavi-Naeini said.
The top chip contains two nanomechanical resonators formed by suspended bridge-like crystalline structures 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 electric force into motion, which in the case of this device means that the electric field carried by the photon qubit is converted into a quantum (or single unit) of vibrational energy called phonon.
“Just like light waves, which are quantized into photons, sound waves are quantized into ‘particles’ called phonons,” Cleland said, “and by combining the energy of these different forms in our device, we create quantum technology hybrid that exploits the advantages of both.”
Generating these phonons allowed each nanomechanical oscillator to act as a register, which is the smallest possible data storage element in a computer, and the qubit providing the data. Like the qubit, oscillators can therefore also be in a state of superposition – they can be both excited (representing 1) and unexcited (representing 0) at the same time. The superconducting circuit allowed researchers to prepare, read and modify the data stored in the registers, conceptually similar to the operation of conventional (non-quantum) computers.
“The dream is to create a device that works similar to silicon computer chips, for example, in your phone or on a USB drive, where registers store bits,” Safavi-Naeini said. “And although we can’t store quantum bits on a USB stick yet, we’re showing the same sort of thing with mechanical resonators.”
Take advantage of the entanglement
Beyond superposition, the connection between photons and resonators in the device further exploited another important quantum mechanical phenomenon called entanglement. What makes entangled states so counter-intuitive, and also notoriously difficult to create in the lab, is that information about the state of the system is spread across a number of components. In these systems, it is possible to know everything about two particles together, but nothing about any of the particles observed individually. Imagine two coins that are tossed to two different places and land as a random heads or tails with equal probability, but when the measurements at the different places are compared, they are still correlated; that is, if one piece lands opposite, the other piece is guaranteed to land opposite.
The manipulation of multiple qubits, all superimposed and entangled, is the punch that powers computation and sensing in sought-after quantum technologies. “Without superposition and lots of tangles, you can’t build a quantum computer,” Safavi-Naeini said.
To demonstrate these quantum effects in the experiment, the Stanford researchers generated a single qubit, stored as a photon in the bottom chip circuit. The circuit was then allowed to exchange power 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, 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 weirdness of quantum mechanics is on full display here,” Wollack said. “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 billions of atoms moving – or not moving – in concert.”
To possibly perform practical calculations, the period of sustained entanglement, or coherence, should be considerably longer – on the order of seconds instead of the fractions of seconds obtained so far. Both superimposition and entanglement are very delicate conditions, vulnerable to even slight disturbances in the form of heat or other energy, and therefore endow the proposed quantum sensing devices with exquisite sensitivity. But Safavi-Naeini and her co-authors believe that longer consistency times can be easily achieved by perfecting manufacturing processes and optimizing the materials involved.
“We have improved the performance of our system over the past four years by nearly 10 times per year,” Safavi-Naeini said. “Going forward, we will continue to take concrete steps towards designing quantum mechanical devices, like computers and sensors, and bringing the benefits of mechanical systems into the quantum realm.”
Other co-authors on the paper include Rachel G. Gruenke, Zhaoyou Wang, and Patricio Arrangoiz-Arriola from the Department of Applied Physics at the Stanford School of Humanities.
The research was funded by the David and Lucile Packard, Stanford Graduate and Sloan Fellowships. This work was funded by Amazon Inc., US Office of Naval Research, US Department of Energy, National Science Foundation, Army Research Office, and NTT Research.
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