Let’s know more about Qubits

Imagine a computer suspended from a ceiling. Loops of silvery wires and delicate lines and tubes interlock on gold-colored platforms. It reminds you of a science-fiction movie, a steam-punk cousin of HAL in 2001: A Space Odyssey. The makers of the 1968 film imagined the computers to be the size of a spaceship; modern technology would have never crossed their minds — a quantum computer.

Quantum computers can solve problems that conventional computers can’t. Traditional computer chips can only process so much information at once, and we’re already reaching their physical limits. In contrast, the exploration of the unique properties of materials for quantum computing can process more information much faster.

These advances can certainly revolutionize certain areas of scientific research. For instance, identifying materials with specific characteristics, understanding photosynthesis, and discovering new medicines require enormous calculations. Quantum computing could solve these problems more efficiently. Quantum computing can also open up possibilities scientists never even considered. It’s similar to a microwave oven versus a conventional oven — different technologies for different purposes.

We’re not there yet. So far, Google claimed its quantum supremacy, i.e, quantum computer can complete a specific calculation faster than the world’s fastest conventional supercomputers. Scientists around the globe routinely using quantum computers to answer scientific questions is still a distinct memory.

To deploy quantum computers on a large scale, we need to improve the technology which constitutes its heart — qubits. Qubits are the quantum version of conventional computers’ fundamental unit of information i.e, bits. The DOE’s Office of Science supports research into developing the ingredients and recipes to build these challenging qubits.

The weird Quantum Theory

At the atomic scale or quantum scale, physics gets very weird. Electrons, atoms, photons and other quantum particles interact with each other differently than ordinary objects. We can harness these strange behaviors in certain materials, . Several of these properties — superposition and entanglement particularly- can be extremely useful in computing technology.

The principle of superposition is the concept when a qubit can be in multiple states at the same time. While traditional bits, only have two states: 1 or 0. These binary numbers carry all of the information on any computer. Qubits are more complicated.

Imagine a pot with water in it. When you have water in a bowl with a top on it, you don’t know if it’s boiling. Real water can either boil or not and looking at it doesn’t change its state. If the same pot was in the quantum realm, the water (representing a quantum particle) could boil and not boil at the same time or be in any linear superposition of the two states. If you took the lid off the water would immediately be in one state or the other. The quantum measurement forces the quantum particle (or water) into a single observable state.

Entanglement happens when qubits have a relationship with each other that prevents them to act independently. It happens when a quantum particle has a state and that is linked to another quantum particle’s state. This relationship continues even after the particles are physically apart. Even far beyond atomic distances.

These properties of superposition, entanglement, and measurement allow quantum computers to process more information than conventional bits that can only be in a single state and act independently from each other.

Harnessing Quantum Properties

But to get any of these excellent properties, you need to have fine control over a material’s electrons or other quantum particles. In some ways, this isn’t so different from conventional computers. Whether electrons move or not through a traditional transistor determines the bit’s value, making it either 1 or 0.

Rather than merely switching electron flow on or off, qubits require control over other tricky properties like an electron spin. In order to create a qubit, scientists have to find a place in a material where they can access and control these quantum properties. Once they obtain such spots, they can then use light or magnetic fields to create superposition, entanglement, and other quantum properties.

In many materials, scientists do this by manipulating the spin of individual electrons. Explaining electron spin, it is similar to the spin of a playing top with a direction, an angle, and a momentum. Each electron’s spin is either up or down. As a quantum mechanical property, spin can also exist in a combination of up and down. To influence electron spin, scientists apply microwaves (similar to the ones in your microwave oven) and magnets. The magnets and microwaves together allow scientists to control the qubit.

Since the 1990s, scientists have been able to gain better and better control over electron spin. That’s allowed them to access quantum states and manipulate quantum information more than ever before.

“To see where that’s gone today, it’s remarkable,” said David Awschalom, a quantum physicist at DOE’s Argonne National Laboratory and the University of Chicago and Director of the Chicago Quantum Exchange.

Whether they use an electron spin or another approach, all qubits face significant challenges before we can scale them. Two of the biggest ones are error correction and coherence time.

When you run a computer, you need to create and store in memory a piece of information, leave it alone, and then come back later to retrieve it. If the system that holds the information changes on its own, it’s useless for computing. Moreover qubits are too sensitive to their environment and don’t maintain their state for a very long time.

Right now, quantum systems are subject to a lot of “noise,” things that cause them to have a low coherence time (the time they can maintain their condition) or produce errors. “Making sure that you get the right answer is one of the biggest hurdles in quantum computing,” said Danna Freedman, an associate professor in chemistry at Northwestern University.

Even after reducing that noise, there will still be errors. “We will have to build technology that can do error correction before we can make a big difference with quantum computing,” said Giulia Galli, a quantum chemist, and physicist at DOE’s Argonne National Laboratory and the University of Chicago.

The more qubits you have in play, the more these problems multiply. While today’s most potent quantum computers have about 50 qubits, they will likely need hundreds or thousands to solve the problems that we want them to.

Building Qubits

There is still a debate on which qubit technology will be the best. “No real winner has been identified,” said Galli. “[Different ones] may have their place for different applications.” In addition to computing, different quantum materials may be useful for quantum sensing or networked quantum communications.

To help move qubits forward, DOE’s Office of Science supports research on several different technologies. “To realize quantum computing’s enormous scientific potential, we need to reimagine quantum R&D by simultaneously exploring a range of possible solutions,” said Irfan Siddiqi, a quantum physicist at the DOE Lawrence Berkeley National Laboratory and the University of California, Berkeley.

Superconducting Qubits

Superconducting qubits are the most advanced qubit technology today. Most existing quantum computers are already using superconducting qubits, including the one that “beat” the world’s fastest supercomputer. They use metal-insulator-metal sandwiches called Josephson junctions. To turn these materials into superconductors — materials that electricity can run through with no loss — scientists lower them to freezing temperatures. Also pairs of electrons coherently move through this material as if they’re single particles. This movement makes the quantum states more long-lived than in conventional materials.

Siddiqi and his colleagues are studying how to build them even better with support from the Office of Science. His team explored improvements to a Josephson junction, a thin insulating barrier between two superconductors in the qubit. By affecting how electrons flow, this barrier makes it possible to control electrons’ energy levels. Making this junction as consistent and small as possible can increase the qubit’s coherence time. In one paper on these junctions, Siddiqi’s team provides a recipe to build an eight-qubit quantum processor, complete with experimental ingredients and step-by-step instructions.

Qubits Using Defects

Defects are spaces in a material where atoms are missing or misplaced. These spaces change how electrons move in the materials. In certain quantum materials, these spaces trap electrons, allowing researchers to access and control their spins. Unlike superconductors, these qubits don’t always need to be at ultra-low temperatures. They have the potential to have long coherence times and be manufactured at scale.

While diamonds are usually valued for their lack of imperfections, their defects are quite useful for qubits. Adding a nitrogen atom to a place where there would typically be a carbon atom in diamonds creates a nitrogen-vacancy center. Researchers using the Center for Functional Nanomaterials, a DOE Office of Science user facility, found a way to create a stencil just two nanometers long to develop these defect patterns. This spacing helped increase these qubits’ coherence time and made it easier to entangle them.

But useful defects aren’t limited to diamonds. Diamonds are expensive, small, and hard to control. Aluminum nitride and silicon carbide are cheaper, easier to use, and already common in everyday electronics. Galli and her team used theory to predict how to physically strain aluminum nitride in just the right way to create electron states for qubits. As nitrogen vacancies occur naturally in aluminum nitride, scientists should be able to control electron spin in it just as they do in diamonds. Another option, silicon carbide, is already used in LED lights, high-powered electronics, and electronic displays. Awschalom’s team found that certain defects in silicon carbide have coherence times comparable to or longer than those in nitrogen-vacancy centers in diamonds. In complementary work, Galli’s group developed theoretical models explaining the longer coherence times.

“Based on theoretical work, we began to examine these materials at the atomic scale. We found that the quantum states were always there, but no one had looked for them,” said Awschalom. “Their presence and healthy behavior in these materials were unexpected. We imagined that their quantum properties would be short-lived due to interactions with nearby nuclear spins.” Since then, his team has embedded these qubits in commercial electronic wafers and found that they do surprisingly well. It can allow them to connect the qubits with electronics.

Materials by Design

While some scientists are investigating how to use existing materials, others take a different tack — designing materials from scratch. This approach builds custom materials molecule by molecule. By customizing metals, the molecules or ions bound to metals, and the surrounding environment, scientists can potentially control quantum states at the level of a single particle.

“When you’re talking about both understanding and optimizing the properties of a qubit, knowing that every atom in a quantum system is exactly where you want it is essential,” said Freedman.

With this approach, scientists can limit the amount of nuclear spin (the spin of the nucleus of an atom) in the qubit’s environment. A lot of atoms that contain nuclear spin cause magnetic noise that makes it hard to maintain and control electron spin. That reduces the qubit’s coherence time. Freedman and her team developed an environment that had a minimal nuclear spin. By testing different combinations of solvents, temperatures, and ions/molecules attached to the metal, they achieved a 1 msec coherence time in a molecule that contains the metal vanadium. That was a much longer coherence time than anyone had achieved in a molecule before. Previous molecular qubits had coherence times that were five times shorter than diamond nitrogen-vacancy centers’ times, which matched coherence times in diamonds.

“That was genuinely shocking to me because I thought molecules would necessarily be the underdogs in this game,” said Freedman. “[It] opens up a gigantic space for us to play in.”

The surprises in quantum just keep coming. Awschalom compared our present-day situation to the 1950s when scientists were exploring the potential of transistors. At the time, transistors were less than half an inch long. Now laptops have billions of them. Quantum computing stands in a similar place.

“The overall idea that we could completely transform the way that computation is done and the way nature is studied by doing quantum simulation is very exciting,” said Galli. “Our fundamental way of looking at materials, based on quantum simulations, can finally be useful to develop technologically relevant devices and materials.”

Originally published at https://www.firstqbit.com on July 22, 2020.