Andrew Dzurak of the University of New South Wales gives the workshop’s keynote address. Photo: Edoardo Charbon
It’s a cold truth about quantum computing: To operate, the leading types of processing units that could lie at the heart of quantum computers must be maintained below a chilling 15 millikelvins, or minus 459 degrees Fahrenheit, close to absolute zero. And to be useful, the electronics that process and read the data have to be nearly as cold.
Leaders in quantum science converged this summer at Fermilab for the world’s first workshop on cryogenic electronics for quantum systems. As these fields are highly competitive, the hosts worked hard to attract key global allies and leaders in the field.
Scientists and engineers from academia and industry discussed the challenges of designing electronics for processors and sensors that will work in the ultracold environment.
It’s a fundamental problem facing the field of quantum computing, which holds immense possibility across multiple disciplines. Experts say that quantum computers could someday be powerful enough to solve problems that are impossible for classical computers, potentially redefining how we see the world.
And much of it rides on designing electronics that are up to the task.
“Quantum systems won’t exist without cryogenic electronics,” said Fermilab engineer Farah Fahim, workshop co-organizer and deputy head of quantum science at Fermilab. “That’s why our community needs to collaborate, and why we’re working to establish key partnerships with academia and industry, as well as manufacturing companies that would support the fabrication of cold chips.”
Researchers across multiple sectors have called for collaboration, and pioneers in the field turned out for the meeting. They included Edoardo Charbon (also workshop co-organizer) of the Advanced Quantum Architecture Lab at the Swiss Federal Institute of Technology Lausanne, or EPFL, in Switzerland and Andrew Dzurak of the University of New South Wales and Australia, a trailblazer in the field of silicon-based qubits who gave the workshop’s keynote address. Representatives from IBM, Intel, Global Foundries, Google and Microsoft also attended.
“The Fermilab cryoelectronics workshop is a very important first step for the quantum computing community,” said Malcolm Carroll, research staff member at IBM Research. “Developing supporting electronics for future quantum computers is one of the next big hurdles. IBM looks forward to this series continuing and contributing to it as it has for this first one.”
The global cooling effort centers on accommodating the qubit — the fundamental unit of a quantum computer’s processor. Qubit information needs the extreme cold to survive — below 15 millikelvins — since any thermal energy can disturb the quantum computing operation.
“The core of any quantum-technology-based system is a very special and carefully designed electronics optimized for deep cryogenic temperatures. This is a brave new world for us electronics engineers.”
Current state-of-the-art systems use tens of qubits. But a quantum computer that surpasses the capabilities of today’s classical computer would in certain cases require millions or billions of qubits, each of which needs electronics, both to control the state of the qubit and to read out its signals.
And electronics means cables.
“As the system scales up, one bottleneck has been getting information out of the qubits and controlling the qubits themselves,” Fahim said. “It requires large numbers of wires.”
For larger systems, the qubits and the electronics need to be closely integrated. Otherwise, information can become degraded as it winds its way down lengthy wires to bulky systems. With tight integration, the electronics can deliver the fast, self-correcting feedback required to control the qubit state — on the order of ten-billionths of a second.
When you have the number of wires and cables required for a million- or billion-qubit system, close integration isn’t possible unless your electronics can operate in the cold, side-by-side with the qubit.
Fermilab engineer Farah Fahim, left, and Edoardo Charbon of the Advanced Quantum Architecture Lab at EPFL co-organized the world’s first workshop on cryogenic electronics for quantum systems. Photo: Davide Braga
“When you have lots of cables, after some point, you can’t expand in that direction anymore. You can’t integrate a million cold qubits with warm electronics,” Fahim said. “To scale up, cryogenic electronics is the only way to go. To be able to take it to the next level of integration, we need to move the room temperature control to cryogenic control. You want to be able to change the technology to meet the requirements.”
When the electronics live in the same space — the same refrigerated space — as the qubits, the system becomes practical and manageable, capable of providing accurate, real-time qubit control.
That is the challenge the workshop attendees took head-on: developing quantum-system electronics that don’t mind being left in the cold.
“Developments in cold electronics may hold the keys to scaling up quantum computing,” said Microsoft Quantum – Sydney Director David Reilly, also a professor at the University of Sydney. “As the community moves from the demonstration of single-qubit prototypes to scaled up machines that can address real problems, interest in this field is really taking off. Fermilab has deep expertise in cold electronics as well as a culture of filling the gap between academia and industry. It’s only fitting that the first workshop on this topic was at Fermilab — and I expect to see many more as government labs become pivotal players in the quantum ecosystem.”
Experts dream of a day when quantum computers can get out of the cold and sit comfortably atop your desk just like your current PC.
“We would like to reach a stage where nothing is cryocooled, but until we get there, the only way we get there is with electronics operating at very low temperatures,” Fahim said.
The workshop was a major, international step in that direction.
“Quantum technologies are the next frontier for many fields, including electronics. While quantum computers are certainly the pinnacle of such worldwide effort, many other applications are emerging, like quantum imaging, quantum sensing, quantum communications, quantum metrology, to name just a few,” Charbon said. “But the core of any quantum-technology-based system is a very special and carefully designed electronics optimized for deep cryogenic temperatures. This is a brave new world for us electronics engineers.”
To continue the dialogue on this key enabling technology, the second International Workshop on Cryogenic Electronics for Quantum Systems will be held in Neuchatel, Switzerland in 2020.
This work is supported by the DOE Office of Science.
Learn more about quantum science efforts at Fermilab.
Fermilab is a member of the IBM Q Hub at the Oak Ridge National Laboratory and part of joint research with the IBM Q Network in the context of the Chicago Quantum Exchange.
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Charged particles, like protons and electrons, can be characterized by the trails of atoms these particles ionize. In contrast, neutrinos and their antiparticle partners almost never ionize atoms, so their interactions have to be pieced together by how they break nuclei apart.
But when the breakup produces a neutron, it can silently carry away a critical piece of information: some of the antineutrino’s energy.
Fermilab’s MINERvA collaboration recently published a paper to quantify the neutrons produced by antineutrinos interacting on a plastic target.
The way antineutrinos change between their various types could help explain why the modern universe is dominated by matter. The most promising model of how this behavior relates particles and antiparticles depends on antineutrino energy. However, neutrons can leave holes in the puzzle of an antineutrino’s identity because they carry away energy and are produced in different quantities by neutrinos and antineutrinos. This MINERvA result is aimed at improving predictions of how neutrons could affect current and future neutrino experiments, including the international Deep Underground Neutrino Experiment, hosted by Fermilab.
The MINERvA detector at Fermilab helps scientists analyze neutrino interactions with atomic nuclei. Photo: Reidar Hahn
In this study, MINERvA looked for antineutrino interactions that produce neutrons. The antineutrino interactions that MINERvA studies look like one or more trails of ionized atoms all pointing back to a single nucleus. Unlike charged particles, neutrons can travel many tens of centimeters from an antineutrino interaction before being detected. So, the MINERvA collaboration characterized neutron activity as pockets of ionized atoms spatially isolated from both charged particle tracks and the interaction point.
An antineutrino interaction can produce other types of neutral particles, which can fake a neutron interaction, and charged particles, which can confuse a neutron counting measurement by themselves ejecting neutrons from nuclei. In addition, when these charged particles have low momentum, they can end up in a mass of ionization too close to the interaction point to be counted separately that also masks evidence for neutral particles. So, neutrons can be counted more accurately in antineutrino interactions that produce few additional particles. MINERvA scientists used conservation of momentum calculations to avoid interactions that produced many charged particles.
This graphic illustrates a neutrino interaction in the MINERvA detector. The rectangular box highlights the spot where a neutrino interacted inside the detector. The square box just above it highlights the appearance of a neutron resulting from the neutrino interaction. Image: MINERvA
Other experiments’ measurements of neutrons from antineutrinos have waited for each neutron to lose most of its energy before it can be counted. However, neutrons from MINERvA’s antineutrino sample have enough energy to knock other neutrons out of nuclei they collide with. This chain reaction changes both the original neutrons’ energies and the number of neutrons detected. This result focuses on signs of neutrons within tens of nanoseconds of an antineutrino interaction.
By understanding neutron production in concert with MINERvA’s characterization of antineutrino interactions on many nuclei, future oscillation studies can quantify how undetected neutrons could affect their conclusions about the differences between neutrinos and antineutrinos.
Andrew Olivier is a physicist at the University of Rochester and member of the MINERvA collaboration.
Why is our universe accelerating in its expansion? If Einstein’s theory of general relativity is correct, then the dark energy that drives this expansion accounts for nearly 70% of the total energy in the universe. However, precise measurements of the history of this expansion may reveal that new dynamic forces are in play. The Dark Energy Survey has combined its four primary cosmological probes for the first time in order to constrain the properties of dark energy. These first combined constraints are competitive with previous experiments and will improve as more data is analyzed.
The Dark Energy Survey is the first experiment to demonstrate the immense power and promise of this combined-probes approach to survey design. The combined-probes approach is the basis for all major next-generation dark energy experiments in the 2020s including the Large Synoptic Survey Telescope. It enables scientists to make the most precise measurement of dark energy possible while protecting against measurement bias.
Researchers used the Blanco telescope in conducting the Dark Energy Survey. The Milky Way is on the left of the sky, with the Magellanic clouds in the center. Photo: Reidar Hahn
Dark energy is the mysterious phenomenon that is accelerating the universe’s expansion. To get a firmer grasp on dark energy’s nature, scientists take various measurements of celestial objects, analyzing the data to determine how dark energy affects the growth of our universe.
Researchers model dark energy with an equation of state. This is related to the rate at which the universe grows over time. That this equation of state is constant in time (with a value of -1) is the prediction of a cosmological constant in Einstein’s field equations in general relativity.
For the first time, the Dark Energy Survey has combined four approaches to inform the dark energy equation of state. The four approaches measure the distances to the explosions of dying stars called supernovae, the regular variations in the density of galaxies called baryon acoustic oscillations, the way galaxies cluster together, and the way light from distant galaxies is distorted (lensed) by structure in the universe. This combination is one of the most powerful measurements ever made by a dark energy experiment. This combined result agrees with the result obtained by combining many previous cosmological data sets: that the dark energy equation of state appears consistent with a cosmological constant. The Dark Energy Survey also demonstrates for the first time that researchers can use similar surveys to independently constrain the amount of ordinary matter in the universe, an important check against measurements from the primordial universe nearly 14 billion years ago.
Arguably, the most important aspect of this measurement is that it is the first time scientists have confirmed this result to such precision in an analysis that was protected against observer bias. This is important because the Dark Energy Survey is now making the most precise measurements of dark energy ever, and when the standard cosmological model and all previous evidence suggests a cosmological constant explanation for dark energy, researchers must do everything they can to limit the possibility of unconscious biases in their analyses.
Michael Troxel is a Duke University physicist on the Dark Energy Survey.