Full Circle Physics

With the help of interviews conducted by IQIM communications coordinator Crystal Dilworth (PhD ’14) and filmmaker Iram Parveen Bilal (BS ’04), E&S has delved into the thinking of several IQIM scientists about the frontiers of quantum science, the role IQIM plays in exploring that frontier, and the question oft thought but rarely spoken: Why should we care? Here—in a “conversation” assembled from separate interviews—are some of their insights into what makes the world of the tiny such a big deal.

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A Passion for Entanglement (+)

Our Fall 2015 article “Full Circle Physics” highlighted Caltech’s Institute for Quantum Information and Matter (IQIM) and its researchers who are exploring the frontiers of quantum science. We revisit the topic here from the viewpoint of SURF participant Patrick Rall, a senior who spent the summer there doing cutting-edge science.

For senior Patrick Rall, a native of Munich, Germany, the summer offers one of the year’s few chances to visit home. But for the last two summers, Rall, a Caltech physics major, has been spending his summers on campus, drawn by another opportunity—the chance to conduct cutting-edge research while being mentored by John Preskill, the Richard P. Feynman Professor of Theoretical Physics, as part of the Institute’s Summer Undergraduate Research Fellowships (SURF) program. Last year, Rall worked in the laser lab of Assistant Professor of Physics David Hsieh on a condensed matter physics experiment. This summer, he switched his attention to quantum information science, a new field that seeks to exploit quantum mechanical effects to create next-generation computers that will be faster and more secure than those currently available.

A key idea in quantum mechanics is superposition of states. Subatomic particles like electrons can be described as having multiple positions, or more than one speed or energy level. This is illustrated by the thought experiment developed in 1935 by Austrian physicist Edwin Schrödinger. In it, a cat is placed into an imaginary box containing a bottle of poison, radioactive material, and a radiation detector. If a radioactive particle decays and radiation is detected inside the box, the poison is released and the cat is killed. But according to quantum mechanics, the cat could be simultaneously alive and dead. Yet if one were to open the lid of the box, the cat would become alive or dead. By opening the box, we have destroyed the quantum nature of the state; that is to say, the observation itself affects the outcome, and yet that outcome is randomly determined.

“Where this gets really interesting is when more than one cat gets involved,” Rall says. “Then we can have states where looking at one cat determines the outcome of looking at the other, even if they are on different continents or even different planets. For example, I cannot know if I will see a live or a dead cat upon opening either box, but I can know that the cats are either both alive or both dead.”

This “spooky action at a distance”—as Einstein phrased it—is called entanglement, and an entangled state, physicists say, can store information. “When looking at systems with many cats, the amount of entanglement information is much larger than what I can obtain by looking at the cats individually,” Rall says. “To harness the sheer quantity of information stored in these so-called many-body systems, we must better understand the structure of these spooky correlations. This is what I worked on this summer.”

Quantum many-body systems are difficult to simulate on a computer, but by looking at small-enough systems and using mathematical tools, researchers can study complex entangled quantum states. Physicists have been studying many-body entanglement for a long time because of its importance in understanding certain semiconductors.

“This summer, I had the privilege to work under Professor Preskill, and that was an incredible experience,” Rall says. A central interest of Preskill’s lab is to design schemes for quantum computation. Modern computers use classical bits—ones and zeroes—to store data. A quantum computer would use quantum bits—or qubits—and use their superposition and entanglement to perform computation. Quantum computers, while still in the experimental stage (with heavy investment from companies like IBM, Microsoft, and Google), have been touted for their potential to generate unbreakable codes and to efficiently simulate many complex systems, with implications for computational chemistry and biology.

“The most interesting thing about the quantum computer is that we have no idea what it could be capable of,” says Rall. “We know some quantum algorithms that are faster than the best-known classical algorithms. But what are the limits? Nobody knows.”

Written by Rod Pyle


Photo: Caltech undergraduate Patrick Rall worked on quantum information science during last summer’s SURF program (Credit: Seth Hansen)

Superhero Physics

One day last year, Spiros Michalakis, a staff researcher at Caltech’s Institute for Quantum Information and Matter, found himself having a serious conversation with actor Paul Rudd about conservation of mass and energy. Rudd stars in this summer’s Marvel flick, Ant-Man, playing a character who steals a suit that shrinks the wearer down to the size of an ant while allowing him to retain his usual strength.

Rudd was part of a team, including writers, producers, and special effects experts working on the film, who met with Michalakis, a quantum physicist, to get his scientific input on various plot points in the script. “The Ant-Man suit has to do a lot of work to keep the hero alive,” says Michalakis. After all, a lot could go wrong if a 160-pound man suddenly became the size of an insect. Everything from his metabolism, to his breathing, to his sight would change (he would see in the ultraviolet).

Then there was the issue of mass. In order for the character to be able to ride on a flying ant (as the original Marvel character did), he would have to lose almost his entire mass. How would one account for that? “I gave them a visually stunning way to represent the loss of mass through energy dissipation,” says Michalakis. “That would lead to nuclear explosions that would destroy the earth, but who is counting?”

The filmmakers identified Michalakis as the expert they needed through the Science and Entertainment Exchange, a program of the National Academy of Sciences aimed at elevating the level of scientific accuracy in Hollywood productions. Michalakis says he doesn’t think the goal should necessarily be to get the science in the movie exactly right but to make some fundamental aspects accurate.

“It is in the conversations after the movie that the fans get into the actual science,” he says. “That’s when the experts should be front and center to answer the questions and to create wonder.”

–Written by Kimm Fesenmaier


Header photo courtesy of Marvel Entertainment/Film Frame