Developing Qubits:
Building a Quantum Computer's Core


A qubit, or quantum bit, is the fundamental unit of quantum information in quantum computing. It is the quantum equivalent of the classic binary bit, which is physically realized with a two-state device. A qubit is a two-state quantum mechanical system, one of the simplest quantum systems to demonstrate quantum mechanics' peculiarities.

Suspended from the ceiling is a computer. Gold-colored platforms are linked by silvery wires and tubes. A steampunk cousin to HAL from 2001: A Space Odyssey, it looks like it belongs in a science fiction film. This technology – a quantum computer – would not have occurred to the movie's creators when they imagined computers the size of a spaceship in 1968.

It's possible that quantum computers will be able to solve problems that conventional computers cannot. There is a physical limit to the amount of data that can be processed by conventional computer chips, and that limit is rapidly approaching. However, the special properties of quantum computing materials hold the promise of processing more data much faster.

They could have a significant impact on certain scientific fields. Material characterization, photosynthesis, and drug discovery all necessitate enormous amounts of computation. Quantum computing has the potential to provide a more rapid and effective solution to these issues. Quantum computing has the potential to open up new avenues of exploration. As with microwaves and conventional ovens, each has its own set of advantages and disadvantages.

However, we haven't quite arrived yet. One company has claimed that its quantum computer can outperform the world's fastest conventional supercomputers in a specific calculation. Scientists are still a long way from routinely using quantum computers to solve scientific problems.

The qubits at the heart of quantum computers must be improved in order to use them on a large scale. Information stored in qubits is a quantum computer's equivalent of bits. Researchers are working with the Department of Energy's Office of Science to find new ways to build these difficult qubits.

Quantum Oddity

The physics of the atomic scale is extremely bizarre. Quantum particles, such as electrons and atoms, behave differently from ordinary objects. We can take advantage of these bizarre properties in a variety of materials. Superposition and entanglement are two properties that can be very helpful in computing technology.

For example, one qubit can be in multiple states at once, according to the principle of superposition. Bits are binary in nature, meaning there are only two possible outcomes: 1 or 0. All of a computer's data is represented as a set of binary numbers. Qubits are more difficult to deal with than bits.

Assume that you have a pot full of water. You can't tell if the water in a pot with a lid is boiling or not if you don't open it. Looking at real water doesn't make it hotter or colder. A quantum pot, on the other hand, could have water (representing a quantum particle) boiling and not boiling at the same time, or any such linear superposition of these two states as well. The water in that quantum pot would change instantly if you removed the lid. Quantum particles (or water) are forced into a specific state by the measurement.

Entanglement occurs when two qubits share a relationship that prevents them from acting on their own. To put it simply, it occurs when two quantum particles share the same state (such as spin or electric charge). Even if the particles are separated by distances greater than atomic distances, this relationship persists.

Quantum computers are able to process more data because of these properties, which distinguish them from conventional bits, which can only exist in a single state and act independently of one another.

Exploitation of Quantum Properties

To get any of these amazing properties, you need to control electrons or other quantum particles. In some ways, it's like a regular computer. A transistor's ability to conduct electrons determines the bit's value, which is either 1 or 0.

Qubits require control over electron spin rather than just electron flow. An ideal qubit is created by accessing and controlling quantum properties. To access them, scientists can use light or magnetic fields to create superposition, entanglement, etc.

In many materials, scientists do this by modifying electron spin. Like a top, electron spin has a direction, angle, and momentum. It can be up or down. However, as a quantum mechanical property, spin can be both up and down. Scientists use magnets and microwaves to influence electron spin. Scientists can control the qubit using magnets and microwaves.

Scientists have been able to control electron spin since the 1990s. They can now access quantum states and manipulate quantum information more easily.

Argonne National Laboratory quantum physicist David Awschalom, director of the Chicago Quantum Exchange, says the change is remarkable.

Whether using electron spin or another method, scaling qubits is a major challenge. Coherence time and error correction are two major ones.

A computer needs to be able to create and store data, then retrieve it later. However, if the information system itself changes, it is useless for computing. Sadly, qubits are sensitive to their surroundings and lose their state quickly.

Currently, quantum systems are subject to a lot of “noise,” causing them to lose coherence time or produce errors. “One of the biggest challenges in quantum computing is getting the right answer every time,” said Danna Freedman, an associate professor of chemistry at Northwestern.

Even if you reduce the noise, errors will still occur. We need to build technology that can correct errors before we can use quantum computing effectively, says Giulia Galli of Argonne National Laboratory and the University of Chicago.

These issues multiply as the number of qubits increases. While today's most powerful quantum computers have around 50 qubits, they will likely require hundreds or thousands to solve our problems.

Examining Alternatives

The best qubit technology is yet to be determined. “No real winner has emerged,” Galli said. “[Different ones] may have their uses.” Quantum materials may also be used for quantum sensing or networked quantum communications.

The Office of Science at the Department of Energy is funding research into a variety of technologies in order to help move qubits forward. The DOE's Lawrence Berkeley National Laboratory and the University of California, Berkeley's Irfan Siddiqi said that to fully exploit quantum computing's scientific potential, researchers must reimagine quantum research.

Qubits made of superconducting materials

Qubits based on superconducting technology are the most cutting-edge at the moment. The vast majority of existing quantum computers, including the one that "beat" the world's fastest supercomputer, make use of superconducting qubits. Josephson junctions are metal-insulator-metal sandwiches. Scientists use extreme cold to turn these materials into superconductors, which conduct electricity without loss. Also, pairs of electrons move through the material as if single particles. This movement extends the lifetime of quantum states.

With help from the Office of Science, Siddiqi and his colleagues are researching better ways to build superconducting qubits. This thin insulating barrier between two superconductors in the qubit has been studied by his team. This barrier controls electron energy levels by affecting electron flow. Increasing the qubit's coherence time by making the junction consistent and small. An eight-qubit quantum processor is described in detail in one of Siddiqi's papers on these junctions.

Defect-Based Qubits

The absence or misplacement of atoms in a material causes defects. These gaps alter electron movement in materials. These voids trap electrons, allowing researchers to access and control their spins. Unlike superconductors, these qubits don't need to be ultracold. They may have long coherence times and be mass produced.

In spite of the fact that diamonds are valued for their flawless appearance, the imperfections present in diamonds are actually quite useful for quantum computing devices. A nitrogen atom substituted for a carbon atom in a diamond creates a nitrogen-vacancy center. The Center for Functional Nanomaterials, a DOE Office of Science user facility, developed a two-nanometer long stencil to create these defect patterns. This spacing increased qubit coherence time and made entanglement easier.

Aside from diamonds, there are many other materials with useful defects. It is difficult to control diamonds. Aluminum nitride and silicon carbide are already widely used in electronics. Galli and her team predicted how to physically strain aluminum nitride to create qubit electron states using theory. Because aluminum nitride has nitrogen vacancies, scientists should be able to control electron spin like they can in diamonds. Silicium carbide is already used in LEDs, high-powered electronics, and displays. Some defects in silicon carbide have coherence times comparable to or longer than nitrogen-vacancy centers in diamonds. Galli's group also came up with theories to explain why the coherence times were longer than they were in the past.

Theoretically, we started looking at these materials at the atomic level. They were always there, but no one looked for them,” Awschalom said. “Their presence in these materials was unexpected. We assumed their quantum properties would be transient due to nearby nuclear spin interactions.” His team has since embedded these qubits in commercial electronic wafers with surprising results. This can connect qubits to electronics.

Material Design

During this time, some scientists are trying to figure out how to use materials that already exist. Other scientists are making new materials from scratch. This method creates materials molecule by molecule. Scientists can control quantum states at the level of a single particle by customizing metals, molecules or ions bound to metals, and the environment.

"To fully comprehend and optimize a qubit's properties, Freedman says it's critical to know where each atom is in the quantum system."

This method limits the amount of nuclear spin (the spin of an atom's nucleus) in the qubit's environment. Many atoms with nuclear spin produce magnetic noise, making it difficult to control electron spin. Reduces qubit coherence time. Freedman and her team created a non-nuclear environment. They achieved a 1 millisecond coherence time in a vanadium molecule by testing different solvents, temperatures, and ions/molecules attached to the metal. That was the longest coherence time ever achieved in a molecule. Previously, molecular qubit coherence times were five times faster than diamond nitrogen-vacancy centers.

“I was shocked because I assumed molecules would be the underdogs,” said Freedman. “It gives us a huge playing field.”

Quantum keeps surprising us. Awschalom compared our current situation to the 1950s, when scientists were exploring transistors' potential. Transistors were half an inch long back then. Laptops now number billions. Quantum computing is in the same boat.

“The idea that quantum simulation could completely transform the way we compute and study nature is very exciting,” said Galli. This new understanding of materials, based on quantum simulations, can finally be put to use in the development of useful devices and materials."