Raphael Thys
quantum computer, electronic circuitry
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The quantum computing revolution is almost upon us, with a vast array of sensors, communication systems, and spectacularly powerful computers all tantalizingly close to actualization.
Functional prototypes of quantum computers, like Google’s Sycamore machine, are already in operation across the globe. At the heart of such technologies are “quantum bits” or “qubits,” the fundamental units of quantum computation, similar to how bits are the foundational units of traditional computing.
Now, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new method of creating tiny light-emitting points called color centers; they arise in defects within crystals by taking a known material and giving it a twist. In turn, these controllable color centers could be used as a new way to generate qubits.
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But the big question is: if quantum computers already exist and use qubits, why do we need a new method of making these quantum bits? Turns out, the phenomena that power qubits—and thus quantum computers—are also this emergent technology’s greatest weaknesses.
How Qubits Harness the Quantum World
The added computational power of quantum computers lies in the fact that qubits use the astounding, and often downright troubling, phenomena of the quantum world to operate.
For example, while bits can take two values—0 or 1, basically “on” or “off”—the quantum phenomenon of superposition, in which multiple states of a system overlap, allows qubits to simultaneously take on multiple contradictory values.
So a single qubit could be in an “on” and “off” state, just an “on” state, or just an “off” state; these possible states increase as qubits are gathered en masse to create a quantum network. That means a multitude of qubits can exist in a tremendous number of states.
Leibniz Computing Center in Garching
A cryostat from a quantum computer at the Leibniz Computing Center in Germany, July 14, 2022. A quantum computer does not store information in the form of bits, which can only assume two possible states, one or zero. Instead, a qubit of a quantum computer can be both at the same time, i.e. one and zero.
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Entanglement is the other major quantum phenomenon enabling qubits to pass information to one another in a quantum computer. This is the idea that particles can be linked in such a way that they can’t be described independently. Changing one particle by performing a measurement instantly changes its entangled partner, no matter how far apart the two particles are, even if they’re on opposite ends of the universe. This troubled Albert Einstein so much that he described entanglement as “spooky action at a distance.”
The application of these quantum elements means that while adding bits to a traditional computer scales its computing power linearly, adding qubits to a quantum computer scales it exponentially. Mathematically, that means if a quantum computer has n qubits, these can exist in a superposition of 2n states.
The entanglement of qubits, and storing information in a superposition, makes quantum computers more powerful than classical computers, and results in a system that can solve problems exponentially faster. But, there’s a catch. A big one. Quantum states like entanglement and superposition are incredibly delicate, and easily destroyed. And that’s a major setback for the reliability of quantum computers.
A Noisy Problem
In the lab, entangled states and superposition in quantum systems are destroyed upon measurement. The problem is this “measurement” is just a form of interference, and interference can come from a number of sources around a quantum system.
The collapse of a superposition or the loss of entanglement could also be caused by an interaction with a particle, a magnetic field, or something as simple as a fluctuation in temperature.
This means quantum computers have to be operated in extremely well-controlled conditions, such as very extremely low temperatures, to protect them from any environmental “noise.” Even then, the fragility of these states means quantum computers aren’t yet capable of producing large chains of calculations accurately.
That’s why teams like the one based at Berkeley Lab are working on new ways to create qubits, hoping that they may develop a system that is better protected against “noise.”
A Colorful Twist on Qubits
from left to right shaul aloni, cong su, alex zettl, and steven louie at the molecular foundry
Shaul Aloni, Cong Su, Alex Zettl, and Steven Louie at the Molecular Foundry. The researchers synthesized a device made from twisted layers of hexagonal boron nitride with color centers that can be switched on and off with a simple switch.
(Credit: Marilyn Sargent/Berkeley Lab)
“Qubits can be realized in many different ways,” Cong Su, a researcher at Berkeley Lab involved in the new work on qubits, tells Popular Mechanics. “One way is to utilize color centers in semiconductors, which are essentially emissions coming from defects.”
Led by Shaul Aloni, staff scientist at Berkeley Lab’s Molecular Foundry, the team used a solid-state “twisted” crystalline layered material to give rise to these color centers. Their work was published last summer in the journal Nature Materials.
“We used hexagonal boron nitride in our experiment, which has a honeycomb lattice consisting of boron and nitrogen atoms. This structure is very similar to graphene, so hexagonal boron nitride is also called white graphene,” Su explains. “In this material, we used the emissions coming from the defects—that are either intrinsically embedded or intentionally created by particle bombardment inside hexagonal boron nitride—to create color centers.”
Because they are microscopic defects in crystalline materials such as diamond, that emit light of specific color when struck with a laser or an alternative energy source like a beam of electrons, color sensors can be united with devices that control light to connect components in a quantum processor.
⚛️ The 4 Most Common Types of Qubits Spin: Because quantum particles act like magnets—always pointing up or down, never in between—this property can be exploited to define spin qubits (0 = pointing up, 1 = pointing down). Trapped atoms and ions: In their natural state, electrons seek to inhabit the lowest-possible energy levels. But when excited with lasers, they can reach a higher level (0 = low energy state, 1 = high energy state). Photons: There are a handful of ways to use individual particles of light as qubits.
- Polarization qubit: Photons have electromagnetic fields with particular orientations, which is called polarization (0 = horizontal, 1 = vertical). *Path qubit: Using beam splitters to put photons into a state of superposition, they can be exploited to define a qubit based on the paths they take (0 = top path, 1 = bottom path). *Time qubit: This is based on a photon’s time of arrival, also in a quantum superposition (0 = photon arrives early, 1 = photon arrives later). Superconducting circuits: Superconductors are materials that, when cooled to a low temperature, let an electrical current flow without resistance. Made up of billions of atoms, these kinds of qubits still act like a single quantum system; they’re defined based on the direction a current flows around a circuit (0 = clockwise current, 1 = counterclockwise current). Source: Institute for Quantum Computing, University of Waterloo
Color centers in hexagonal boron nitride are actually brighter than those in diamond, but until this research, scientists have struggled to use this material—a common additive in paints—as it’s hard to produce the defects at determined locations.
“Traditionally, color centers are created using ion implantations. However, this creates color centers at random locations due to a lack of spatial control,” Su explains. “We are trying to use the interface of hexagonal boron nitride and electron beams to confine the location of these color centers.”
Another roadblock to this research has been the fact that, until now, researchers lacked a reliable way to switch color centers in this synthesized material on and off. The team solved these problems by stacking and rotating hexagonal boron nitride layers like a sandwich, with the top layer of bread rotated in relation to the bottom. This had the effect of activating and enhancing ultraviolet (UV) emissions from color centers.
“We were very surprised to see that a simple twist of layers can enhance the brightness of color centers by nearly two orders of magnitude,” Su says.
The team hopes that the research is the first step toward a color center device that engineers could use to build a quantum system, or that could be adapted to use in existing quantum systems. More work will be needed before that is possible, however, with an improvement in the fidelity of color centers needed in order to reduce the errors during quantum computation.
“There is still much effort needed to make a quantum device based on color center systems,” Su says. “For example, a waveguide is needed for connecting different qubits together to make them entangle and communicate with each other.
“We want to discover and intentionally create more color centers with better properties, and we also want to find other ways to control them.”
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