Quantum: What is it, what does it mean, how is it applied?

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Theoretical physicist Richard Feynman famously said, “If you think you understand quantum mechanics, you don’t understand quantum mechanics.” For many people working in this field, that still holds true today.

Quantum sciences are still mostly nascent fields with pivotal research expanding scientists’ understanding of the world at a very small level. There are quantum properties to many quantities and processes we see in our day-to-day lives, including light and photosynthesis, and there are also far more specialized purposes for studying these fields, such as quantum computing.

You can be working in what you think are different fields … and working on different phenomena,” said K. Birgitta Whaley, a campus chemistry professor and director of the Berkeley Quantum Information and Computation Center. “You find that they’re all quite related underneath.”

The origin of quantum: What it means

As the smallest component of a physical entity, a quantum cannot be broken down any further. Quanta, the plural of quantum, are indivisible — the smallest building blocks.

Understanding the definition of this word relies on the development of scientists’ understanding of physics. According to Kasra Nowrouzi, a postdoctoral scholar at the Lawrence Berkeley National Laboratory, physics originally developed to match common sense, and the only measurements used were those at scales that could be utilized in our daily lives. As human technology advanced, however, scientists gained the ability to see the world in different ways and found that, sometimes, nature behaves differently on smaller levels.

Nowrouzi explained this through the example of scientists’ understanding of light.

“Light has both wave-like and particle-like properties,” Nowrouzi said in an email. “What brings these two together is the ‘quantum’ of light: the small (in energy), indivisible particle also known as the ‘photon.’ ”

He added that this can be used to explain colors, which are determined by wavelength. Photons, which are the smallest component, and therefore the quanta, of light, each have their own wavelength, which dictates the color humans perceive the light to be.

According to Nowrouzi, the discovery that light is made up of quanta led to physicists realizing that other physical quantities are also made up of quanta. This realization spurred the creation of the field of quantum mechanics, the branch of physics dedicated to explaining and predicting the behavior of these quantities at small scales.

The application of quantum mechanics to other fields is what lends them the prefix of “quantum.” At UC Berkeley, these subjects include quantum information science and quantum biology.

Working with computers: Quantum information science

Quantum information science relates to how information is treated using qubits when working with computers.

Classical computers — such as computers people use in day-to-day work — encode information in states called “bits.” According to Nowrouzi, these bits hold values of either zero or one, and information is processed step by step, either by changing the value of the bit or leaving it as is. This is how a computer codes.

The qubit also encodes information. Unlike a bit, it does not hold a zero or one; rather, it can hold both simultaneously. This concept is called superposition, and it refers to the qubit’s ability to hold both values at the same time before it is measured. Before experimenting, a scientist will not know for sure whether a qubit will be measured as a zero or one.

“This statistical behavior seems to be the characteristic behavior of nature at small scales, and not just a shortcoming in our knowledge,” Nowrouzi said in the email. “The exact outcome seems to be not just unknown, but unknowable.”

These qubits can also be entangled. Entanglement is the idea that scientists can learn something about one particle by measuring the behavior of another particle regardless of the distance between them.

Nowrouzi gave the example of throwing a ball at a barrier. For each throw, the person throwing the ball can predict how it will move and where it will go. When looking at very small things such as particles, however, scientists will not know if the “ball,” which represents a quantum, will make it through this barrier.

“Now, although we cannot know if our ball will make it over the barrier ahead of time, imagine if we could touch it to another ball, and in that moment, magically make it so that when the two balls are thrown at the barrier later, only one of them will make it through,” Nowrouzi said in the email. “Because they have been ‘entangled’, we can infer with certainty whether or not the other ball made it through by observing if our ball made it.”

Entangling qubits is useful, as one can store information in the correlations between qubits. Therefore, a certain number of qubits can look at a larger computational space than an equivalent amount of classical bits can.

This allows for quantum computers — computers that are built to process information this way — to look at specific types of problems that would take an extremely long time for classical computers, which have 32-bit or 64-bit processors, to execute. According to Nowrouzi, there are types of problems that would take supercomputers years to solve, but quantum computers with as few as hundreds of qubits only minutes to solve.

Currently, no such quantum computer exists. The existing quantum computers have tens of qubits, and according to Nowrouzi, it will take many years to maximize the potential of quantum computers.

Researchers from Berkeley Lab’s Advanced Quantum Testbed are attempting to construct a better quantum computer. These researchers work with collaborators from other national labs, academia and even industry to develop quantum processors and evaluate their performances.

Debayan Bandyopadhyay, a student facilitator of the “Introduction to Quantum Computing DeCal,” said he believes that while quantum computing is useful for solving specific types of problems, it is important to note the differences between the uses of a classical computer and those of a quantum computer.

“The thing to point out is that quantum computers will not replace regular computers in day-to-day tasks because it’s overkill to do regular stuff with it,” Bandyopadhyay said.

Quantum Computer

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In 1998, Whaley theorized that there are decoherence-free subspaces quantum spaces that are immune to disturbances from the environment. Even though quantum computers are kept at very low temperatures, there are always variables interacting with qubits in ways that scientists cannot control. This unwanted interference degrades the quantum system, and thus, information is lost to the environment. This is not the case, however, for super-enclosed spaces, where nothing can interact with the qubits.

Whaley discovered that by encoding information into these special states, which are immune to this interference, researchers overcome the degradation problem.

Understanding nature: Quantum biology

Quantum biology is another field that utilizes quantum mechanics. According to Whaley, who works in this field, the reactions in a biological system at the molecular level are defined by electrons, which are quanta.

“What we look at are specific phenomena at the molecular level where there are unique quantum effects,” Whaley said.

The campus Whaley Group is currently conducting research on UC Berkeley chemistry professor Graham Fleming’s discovery of the quantum mechanical effects involved in photosynthesis. Essentially, a leaf gets energy from sunlight by absorbing photons and turning them into electrons with unusually high efficiency.

The Whaley Group is studying this phenomenon by using quantum light. Quantum light is produced by emitting little packets of light that correspond to a single photon, then using another photon as a timer to see when the other is absorbed into a leaf.

“These are very sophisticated counting experiments,” Whaley said. “We’re just counting the number of photons in and out to find something about the efficiency of the tranception of energy from light to electrons, and so we’re doing this one photon at a time.”

How to get involved

Many ways exist for students to get involved in quantum sciences on campus. There are multiple physics classes and computer science classes, as well as the “Introduction to Quantum Computing DeCal.” Students can also join student-run organizations such as Quantum Computing at Berkeley, which allows students to read quantum computing research papers with a group and complete projects.

James Sud, project lead for Quantum Computing at Berkeley, said he believes there is a lot of value for all students, even those not in science, technology, engineering or math fields, to study the emerging fields of quantum sciences.

“Learning about quantum mechanics is just a fascinating thing and has philosophical implications,” Sud said. “It’s definitely a really rewarding field. … Anything that expands your mind is really important.”

Megha Krishnan is a research and ideas reporter. Contact her at [email protected] and follow her on Twitter at @_meghakrishnan_.