Qubits are the core of quantum computing. They are also your greatest weakness.

The quantum computing revolution is upon us, with a wide array of spectacularly powerful sensors, communications systems and computers all tantalizingly close by.

Working prototypes of quantum computers, such as Google’s Sycamore machine, are already in use around the world. At the heart of these technologies are “quantum bits” or “qubits”, the fundamental units of quantum computing, just as bits are the fundamental units of traditional computing.

Now researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new method for creating tiny luminous dots called color centers; They form on defects in crystals, take a known material and flip it. These controllable color centers can in turn be used as a new method of generating qubits.

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But the big question is, if there are already quantum computers that use qubits, why do we need a new way to make these quantum bits? It turns out that the phenomena that drive qubits, and thus quantum computers, are also the biggest weaknesses of this emerging technology.

How qubits use the quantum world

The additional processing power of quantum computers lies in the fact that qubits use the surprising and often disturbing phenomena of the quantum world to function.

For example, while bits can take on two values, 0 or 1, essentially “on” or “off,” the quantum phenomenon of superposition, where multiple states of a system overlap, allows qubits to take on multiple values ​​at once. contradictory.

Therefore, a single qubit can be “on”. j “off” state, just an “on” state, or just an “off” state; These possible states increase when qubits are massively combined into a quantum network. This means that an infinite number of qubits can exist in an enormous number of states.

Leibniz data center in Garching

A quantum computer cryostat at the data center in Leibniz, Germany, on July 14, 2022. A quantum computer does not store information in the form of bits, which can have only two possible states, one or zero. Instead, a qubit in a quantum computer can be one and zero at the same time.

alliance image//fake photos

Entanglement is another important quantum phenomenon that allows qubits to exchange information in a quantum computer. This is the idea that particles can be linked in such a way that they cannot be described independently. Changing a particle in a measurement instantly changes its entangled partner no matter how far apart the two particles are, even if they are at opposite ends of the universe. This worried Albert Einstein so much that he described the jumble as “spooky action from a distance”.

Applying these quantum elements means that adding bits to a conventional computer scales its computing power linearly, but adding qubits to a quantum computer scales exponentially. Mathematically, this means that when a quantum computer north Qubits, these can exist in a superposition of 2north Disease

Entangling 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 is a catch. A big. Quantum states such as entanglement and superposition are incredibly delicate and easy to destroy. And that is a huge setback for the reliability of quantum computers.

a noisy problem

In the laboratory, entangled states and superpositions in quantum systems are destroyed by measurements. The problem is that this “measurement” is only one form of interference, and interference can come from many sources around a quantum system.

Layer collapse or loss of entanglement can also be caused by an interaction with a particle, a magnetic field, or something as simple as a change in temperature.

This means that quantum computers must be operated under extremely well-controlled conditions, such as extremely low temperatures, to shield them from ambient noise. However, due to the fragility of these states, quantum computers are not yet able to accurately create large computational chains.

For this reason, teams like those at the Berkeley Lab are working on new ways to make qubits, hoping to develop a system that is more protected against “noise”.

A colorful twist on qubits

from left to right Shaul Aloni, Cong Su, Alex Zettl and Steven Louie at Molecular Foundry

Shaul Aloni, Cong Su, Alex Zettl and Steven Louie from Molecular Foundry. The researchers synthesized a device made of twisted layers of hexagonal boron nitride with colored centers that can be turned on and off with a simple switch.

(Credit: Marilyn Sargent/Berkeley Lab)

“Qubits can be made in many different ways,” said Cong Su, a Berkeley Lab researcher involved in the new qubit work. folk mechanics. “One way is to use color centers in semiconductors, which are essentially defect emissions.”

Led by Shaul Aloni, a fellow at Berkeley Lab’s Molecular Foundry, the team used a material in solid “twisted” crystalline layers to create these color centers. His work appeared in the magazine last summer materials from nature.

“In our experiment we used hexagonal boron nitride, which has a honeycomb structure of boron and nitrogen atoms. This structure is very similar to graphene, so hexagonal boron nitride is also called white graphene,” explains Su. “In this material, we used the emissions from the defects, either intrinsically embedded or intentionally created by bombarding particles in the hexagonal boron nitride, to create color centers.”

Because they are microscopic defects in crystalline materials such as diamond that emit light of a specific color when hit by a laser or an alternative energy source such as an electron beam, color sensors can be connected to devices that control light to detect components to be connected to a quantum way. processor.

⚛️ The 4 most common types of qubits

Spin: Since quantum particles act like magnets and always point up or down, never in the middle, this property can be exploited to define spin qubits (0 = point up, 1 = point down).

Trapped atoms and ions: In their natural state, electrons try to inhabit the lowest possible energy levels. But when excited by lasers, they can reach higher levels (0 = low energy state, 1 = high energy state).

Photons: There are several ways to use individual light particles as qubits.

*Qubit Polarization: Photons have electromagnetic fields with specific orientations called polarization (0 = horizontal, 1 = vertical).

*Qubit Path: By using beam splitters to place the photons in a superposition state, they can be used to define a qubit based on the paths it follows (0=upper path, 1=lower path).

*Stop time: This is based on the arrival time of a photon, also in a quantum superposition (0 = photon arrives earlier, 1 = photon arrives later).

Superconducting circuits: Superconductors are materials that, when cooled to a low temperature, allow an electric current to flow without resistance. This type of qubit is made up of billions of atoms and still functions as one quantum system. They are defined according to the direction in which current flows in a circuit (0 = clockwise current, 1 = counterclockwise current).

Source: Institute for Quantum Computing, University of Waterloo

The color centers in hexagonal boron nitride are actually brighter than those in diamond, but until this research, scientists had difficulty using this material, a common additive in paints, due to its difficulty in creating defects in specific locations.

“Traditionally, color centers are created by ion implantation. However, due to the lack of spatial control, this creates color centers in random locations,” explains Su. “We are trying to use the hexagonal boron nitride and electron beam interface to constrain the location of these color centers.”

Another obstacle to this research was the fact that until now researchers had no reliable way to turn color centers on and off in this synthesized material. The team solved these problems by sandwiching and rotating hexagonal layers of boron nitride, rotating the top bread layer relative to the bottom. This had the effect of activating and increasing the ultraviolet (UV) emissions from the color centers.

“We were very surprised to see that simply rotating layers can improve the clarity of color centers by almost two orders of magnitude,” says Su.

The team hopes the research will be the first step toward a color-center device that engineers can use to build a quantum system or adapt for use in existing quantum systems. However, more work is needed before this is possible, with improvement in color center fidelity needed to reduce errors during quantum computing.

“It still takes a lot of effort to create a quantum device based on color center systems,” says Su. “For example, a waveguide is needed to connect different qubits so that they can entangle and communicate with each other.

“We want to consciously discover and create more color centers with better properties, and we also want to find other ways to control them.”

Portrait photo of Robert Lea

Robert Lea is a freelance science journalist specializing in space travel, astronomy and physics. Rob’s articles have been published in news week, place, put, living science, astronomy Magazines new scientist. He lives in North West England with lots of cats and comics.

Source: La Neta Neta

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