Invisible to the naked eye and highly counterintuitive, the atomic-scale quantum world is a bizarre place. Here, one object can occupy multiple states and multiple objects can occupy the same state. Particles can correlate with each other regardless of distance and collective behaviors emerge that can’t be predicted by the actions of individual bodies. How these tiny, strange phenomena underlie our rather orderly, far more intuitive human-scale universe is difficult to know, and physicists who work in this realm are constantly reminded of how fuzzy things can be.

“Even quantum mechanics is an approximation,” says David Weld, an experimental physicist with expertise in ultracold atomic physics and quantum simulation. It’s science’s best and quite successful attempt at explaining the fundamental processes of the universe, while leaving room for things that are not yet known or understood. “Physicists are always approximating,” he adds, “and we just try to be honest and smart about the approximations we use.”

Unlike our orderly, deterministic classical physics, with its well-defined locations and velocities, and discrete objects, the quantum world leans toward randomness. Uncertainty is baked into the math, and scientists speak in terms of probabilities. Beneath all this is one of the fundamental oddities of quantum mechanics, which renowned physicist Richard Feynman called “the only mystery”: Particles are also waves. This duality of matter gives rise to some signature quantum behaviors that when better understood will help us on our quest to understand nature. Harnessed, they can lead to powerful new technologies.

 

Relationships: Coherence, Superposition, Entanglement

 

The promise of a quantum computer lies in the ability to perform exceedingly complex calculations at speeds that would be out of reach for even the most sophisticated of classical computers. Enabled by their ability to encode multiple, entangled states, arrays of quantum bits (or “qubits,” the basic unit of information in a quantum computer) could hold multitudes of possibilities, solving problems that would require countless calculations. Imagine being able to predict the interactions of molecules to design the next big antibiotic, or to accurately model complex systems, like our climate.

But before the quantum computer can truly take off, scientists must overcome the problem of decoherence, which results in the loss of quantum information to the environment. Qubits are extremely sensitive to their surroundings and even the slightest perturbation will interrupt their “quantumness” — those behaviors brought on by the wavy nature of matter — including superposition and entanglement. Much effort is put toward isolating quantum bits away from the environment, and at near absolute zero temperatures, to maintain, for example, a lossless state of superconductivity.

Isolation isn’t the only way to maintain quantumness. Stephen Wilson, a materials scientist and co-principal investigator at UC Santa Barbara’s National Science Foundation-supported Quantum Foundry, is developing materials from the molecule up — materials that could generate and maintain quantum coherence over long periods of time, and at higher temperatures.

 

Stephen Wilson

“One way to think about coherence is a sort of time scale over which the quantum information that you’re reading — either the wavelength or the phase — remains well defined,” Wilson says.

Imagine a waveform — a line gently meandering up and down across a horizontal axis. Now add a second waveform along the same axis. If each part of one waveform remains fixed or constant to the corresponding part of the other wave, congratulations — you now have coherence, and a state in which a wave function from one object can split into two states that can interfere with itself is possible. The general tendency of quantum coherence is to be fleeting, but if materials can be made that can enhance or extend this ability, they will be to quantum computing what silicon and transistors have become to classical computers.

The Quantum Foundry is where quantum materials get their start. In this place, akin to an iron foundry where metals are mixed and shaped into functional forms, Wilson and his team seek to merge elements whose molecular structures could give rise to useful quantum behaviors. In one room, they grind and mix material together under vacuum conditions to minimize impurities and unwanted interactions with air, material they then cook at high temperatures to meld together. In another room, a futuristic forge heats elements to 3,000 degrees Fahrenheit — one third the heat of the sun’s surface — to create compounds that would otherwise never be seen on Earth.

One goal for the foundry is to develop new types of superconductors, materials that can conduct, at near room temperatures, with zero resistance. By using the superconducting state for information, qubits can be made that retain their coherence, but special types of superconductors must first be discovered in real materials — a hurdle for those who want to enable wide use of quantum computers.

Wilson and his crew are already scratching the surface of this holy grail of quantum materials, however. Recently, they developed a kagome (KAH-go-meh) superconductor whose electrons self-organize in such a way that a new electronic state of matter emerges that can potentially host quantum information for extended periods of time.     

“These types of superconductors are predicted to host anyons,” Wilson says, referring to the electronic states that are of interest to those pursuing topological quantum computing — a branch of quantum computing in which information can be distributed over the surface of the material in “braids,” as opposed to more conventional, trapped quantum particles. This type of logic is thought to make the system more robust to errors and perturbations and could potentially be the key to fault-tolerant quantum computing.

“Another weird thing about this superconductor is that it may actually not be a pairing of two electrons; it may be a pairing of four or six electrons, which has never been seen before,” says Wilson. “And those would also have potential uses in quantum information and perhaps quantum sensing.”

Characterizing Qubits


Physicist Ania Bleszynski Jayich’s qubit of choice: a point defect in diamond called the nitrogen-vacancy (NV) center. The NV center is created when one of two missing adjacent carbon atoms in a diamond’s carbon lattice is replaced with a nitrogen atom, resulting in a nitrogen atom next to an empty spot, a vacancy. It’s a system that has given physicists the kind of control needed to peer into the tiny aspects of the quantum world.

Importantly, this atomic-scale object makes an excellent sensor of magnetic fields. By subjecting the NV center to the tiny magnetic fields emitted by other atoms in a material, for instance, Jayich can gain information about the identity of the atoms in question, as well as their local environment. She does so by exciting the center with a laser and noting how much light it emits. It’s essentially the same method that MRIs use to create images of the body, only now, due to the individual addressability of the NV center, MRI can be done on length scales 1,000 times smaller, approaching nanometer-scale.

“The NV center as a defect can do that same type of imaging, and with elemental specificity,” she says. “It can image different atoms and molecules, and with really high spatial resolution.”

One quantum property that this highly sensitive qubit exploits is superposition, in which the state of the NV center is a result of multiple possible states — imagine the ripples from two stones dropped into a pond when they meet. The quantumness of a qubit arises from this property: While a classical bit is binary — either zero or one — a quantum bit can be zero and one, with a mathematical description called a wave function that encodes the many possibilities of this NV center qubit, and in the case of sensing, what it is interacting with.

 

Ania Jayich

“So I put my little qubit in a superposition of zero and one, and I expose it to the field I want to sense,” explains Jayich. “Then as time evolves, there will be some probability amplitude for the qubit to be in the zero state and some probability amplitude to be in the one state, and those two probabilities respond differently in a magnetic field.” To find out how “zero” or how “one” the qubit is, Jayich fires a green laser at it and counts the number of fluorescing photons, which will tell her what field the qubit felt. From there she can derive the characteristics of the material.

Part of Jayich’s job as a co-principal investigator of the Quantum Foundry, a collaboration of 25 members of the UCSB faculty, is to characterize the never-before-seen compounds being forged there using the uncertainty of superposition to interact with the mysterious substances. But she’s also working to improve the properties of the quantum sensor itself, to enable its interaction with ever more complex targets, like proteins, or new states of matter and emergent properties, such as superconductivity.

One way to achieve even higher sensitivities, according to Jayich, would be to entangle qubits, another purely quantum phenomenon in which two quantum particles are so intimately connected that their properties automatically correlate with each other; they share the same wave function. Astonishingly, this can happen over any distance, prompting Albert Einstein’s description of entanglement as “spooky action at a distance.”

Mastery over quantum entanglement is the reason for this year’s Nobel Prize in Physics, and the research has powerful implications for quantum computing and quantum cryptography. Imagine being able to send information from one point to another instantaneously, while any attempt to intercept the information destroys it.

For Jayich’s purposes, entangled qubits would mean a supercharged solid-state sensor. Theoretically (only because this behavior has not yet been demonstrated in solid-state sensor systems), entangled sensors would have a performance vastly improved over separate quantum sensors working together.

“Let’s say I have a hundred sensors and I’m trying to detect a field,” she says. “One hundred sensors will usually do about 10 times better than a single sensor, if they’re not entangled. But if they’re entangled, I can actually have 100 sensors do 100 times better than one sensor. This is the kind of interesting scaling that can happen.”

Of course, Jayich notes, the more entangled and therefore more powerful these sensors can be, the more difficult it will become to control their quantum mechanics. But it is definitely a goal for the Quantum Foundry to reach for in the collaboration’s quest to generate new, useful quantum materials.

New Phenomena

 

David Weld’s laboratory is a place of ultracold atoms, photons, lenses, mirrors and lasers — some so strong that people have to shield their eyes when the lasers are on. In this lab, they use lithium and strontium atoms chilled to almost absolute zero, each suspended in standing waves of light to create synthetic solids — ghosts of solids organized in their regular, repeating lattices — that can be poked and hit and shaken with lasers to see how these atoms respond. At this low energy scale, the typically zippy atoms slow down, allowing the researchers a better look at their slow-motion quantum mechanical behaviors.

This level of control allows for investigating predictions and trying to answer questions that were mere thought experiments until recently. In the past few years, Weld and team have witnessed position-space Bloch oscillations — back-and-forth sloshings of quantum particles under a constant force predicted by Felix Bloch in 1929. They also replicated the “quantum boomerang” effect, a strange phenomenon where particles in a disordered system are “kicked” off their locations, only to return right back to where they came from. While disordered quantum insulators have been understood in broad strokes since the late 1950s, the quantum boomerang effect was first predicted just a few years ago.

But there is a problem. It’s sort of a Mount Everest of quantum physics problem, the kind that is both tantalizing and nearly unsolvable. It’s called a quantum many-body problem, and it’s actually a class of problems that seek to identify the properties of systems of many interacting quantum particles. When you have particles with multiple, overlapping wave functions and random odd behaviors, what happens when you put them together to bump into and jostle each other?

“It’s sort of an evergreen problem in physics,” Weld says. “And it’s something that is both interesting and important because assemblies of many interacting particles are what a lot of technologies are based on.”

 

David Weld

The difficulty lies in the sheer multitude of possibilities that grow exponentially with the number of particles in the system. Beyond a few particles, the calculations become unwieldy and they overwhelm even our best computers.

This is where quantum simulation shines. Using the lithium atoms and the fine control his system provides, Weld works with his team to turn the problem of computation into one of control, telling their once neatly separated lithium atoms to jump into the mosh pit. In so doing, they addressed the question: What emerges when particles in a disordered, chaotic system interact? Apparently, what emerges is a “weird quantum state” that looks suspiciously like order but, when investigated, has the hallmark of quantum chaos — a riddle within a riddle, but an answer in a sea of questions and a basis for better understanding the weird quantum world.

“We have good approximate theories for some of these many-body systems, like metals and superconductors,” says Weld, “and less good approximate theories for some others, like high-temperature superconductors.” Any advances in quantum chaos and the many-body problem could in the near term, he adds, be helpful in designing new materials. In the bigger picture, they could lead to the coveted link between the quantum and classical worlds.

Photos from Top, L to R:

Ganesh Pokharel, Wilson Group / Daipeng Yang, Jayich Lab
Eber Nolasco-Martinez, Weld Group
Andrea Capa Salinas, Wilson Group /  Simon Meynell, Jayich Lab

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