Quantum information meets condensed matter: inside the mind of Xie Chen

August 13, 2014 by Cynthia Eller, California Institute of Technology
Quantum information meets condensed matter: inside the mind of Xie Chen

Perhaps you have heard of Schrödinger's cat, a fictional feline created by physicist Erwin Schrödinger as a thought experiment in 1935. According to her inventor, Schrödinger's cat can be simultaneously alive and dead inside a sealed box, although upon an observer's opening the box, the cat will always be found either alive or dead.

If you are among the quantum confused—those who think Schrödinger was engaged in some serious crazy talk when he invented his dead-and-alive superposed cat—take heart. You are in good company. Xie Chen, who joined the Caltech faculty as an assistant professor of theoretical physics on July 1, admits that she too finds of matter, like superposition and entanglement, counterintuitive . . . though impressively congruent with .

Chen comes to Caltech after a two-year postdoctoral fellowship at UC Berkeley. Originally from China, Chen received her BS from Tsinghua University in Beijing and her PhD from MIT. She recently discussed with us her research interests and ambitions for her Caltech career.

What will you be working on at Caltech?

I'm a theoretical physicist, and I work in both and . Work at this intersection is being pursued here at Caltech in the physics, math, and astronomy division, and also through IQIM [the Institute for Quantum Information and Matter]. It's a new trend to bring these two areas of theoretical study together.

What is quantum information?

At the heart of quantum information is the idea that computers or cell phones, and the computation and communication protocols through which they operate, can run more efficiently and securely if we make use of the quantum properties of matter. For example, if a cell phone's hardware relies on the quantum behavior of its components, it will be possible to develop encryption protocols that, at least theoretically, are totally secure.

When did you first become interested in quantum information?

My interest started in college, and it was the focus of the first part of my graduate study. My goal was to devise ways to make more resistant to noise, such as heating or the effect of stray electromagnetic fields from the environment. The quantum properties of things can be so fragile. If you raise the temperature a little bit or have a little disturbance in your lab, that could cause a quantum computer to break down. This has been bothering people for the last 20 years. The principle of quantum computation is well established, but the conditions to make it work in the lab are just not there. This continues to be a big part of my work: making quantum computation more reliable and more resistant to noise.

How do quantum information and condensed matter relate to one another?

Condensed matter is a much older and broader topic in physics. Analyzing the properties of solid and liquid materials—particularly things like semiconductivity, superconductivity, and magnetism—has been around for hundreds of years. I shifted into it in the second half of my graduate study, as people began to realize that ideas about quantum information could be very helpful in the study of condensed matter.

That need came out of the laboratory: peculiar quantum properties of condensed matter were appearing in the lab and people wanted to understand them. Previously, classical physics or simple quantum theories were able to account for everything that was seen in condensed matter. However, with modern technologies it's now possible to access and observe the quantum aspects of materials at a deeper level. For example, strange conducting materials were found which do not allow current to flow through the interior but only on the surface of the material with precisely quantized conductance. Conventional methods cannot explain such a topological property of the material. We need new methods to explain that observation. This is where quantum information ideas are very useful.

Quantum properties are so counterintuitive for the average person. Does that change when you're immersed in quantum physics on a daily basis, as you are?

No. Quantum mechanics has never fully become part of my intuition. I think that people still haven't truly understood the quantum level yet. The quantum community generally takes the attitude that we can set those questions aside for a while, because quantum theory predicts experimental results so well. But if you think deeply enough about quantum reality, and sometimes I do, you get confused. There are so many fundamental questions that we cannot yet fully explain. The is more a mathematical formulation than an intuition we are born with, because we mostly deal with the classical world.

But if technologies are developed that rely on quantum properties, do you think that will change—that we will start to internalize a quantum view of things?

Eventually, maybe. When we deal with the quantum world frequently enough and have reached a better consensus on how to interpret it, it may be possible to develop a quantum intuition of the world. That would be an amazing state of mind to have, because with it we could not only push science forward but also have a totally new perspective on the universe and the meaning of life!

What are you looking forward to at Caltech?

Putting "quantum" and "materials" together isn't easy and it's an amazing subject that's developing quickly as people are trying to figure out what's going on. With IQIM and all the specialized faculty here, Caltech is a leader in this. People on the quantum side and the materials side talk to each other and work on projects together at Caltech. I'm very excited to be a part of that.

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1 / 5 (3) Aug 13, 2014
I am an amateur scientific researcher, and I know a small amount about quantum mechanics. I like the new direction of the sound droplet experiments that indicate De Broglie's pilot wave that support the modified Newtonian dynamic(M.O.N.D.) using the water droplet experiment with sound vibration in water. I am wanting to construct the experiment at home in my workshop, but in a set of vibrating water droplets i wanting to position the Intel Corp.'s quarks (atomic) computer to have the water droplet itself have other vibrations that have smaller and smaller sub water droplets to put smaller and smaller computers in to see if there is a physical limit to this experiment in the smaller and smaller micro world or the size of the quantum realm can yield any significant data to be useful to uncover quantum pertinent conclusion. I have 3 different Technicians degrees. One in USAF as an aircraft tech. I just like doing research in my small and insignificant way. I stay info wise cutting edge
not rated yet Aug 13, 2014
Schrödinger's cat, a macroscopic set of quantum mechanical entities, is a fine analogue of the QM Universe of microscopic entities; that does not collapse into reality until observed in the instant now.
not rated yet Aug 13, 2014
I use a "guitar" analogy to give people some hints and ways to think about superposition:

"A guitar contains all songs; the guitarist reveals them by sequentially measuring (by plucking) the strings."
2.3 / 5 (3) Aug 13, 2014
strange conducting materials were found which do not allow current to flow through the interior but only on the surface of the material with precisely quantized conductance
Such a materials expel the electrons toward their surface like the Teflon sponge soaked with mercury. Their conductivity is not "quantized" and the principle of this mechanism is essentially classical in the same way, like at the case of the various room temperature superconductors. Because the physicists don't understand the quantum mechanics, they're pushing it everywhere and ignore the fact, the pure quantum states are very fragile at room temperature. But we already know, that many physical system can provide quite faithful macroscopic analogies of quantum mechanics and this is the key understanding of the actual role of quantum mechanics for consciousness.
1 / 5 (3) Aug 13, 2014
IMO What the human brain does it simulates the perceptions with sending of neural spike solitons across neural network, where they collide in time and space dimensions. The spikes are doing neuron synapses more conductive, so that the high coincidence of spikes at certain place of network makes this place more conductive for another spikes and it creates a permanent path for them. In this way, the random sending of spikes across neural network leads to fast finding of the optimal paths for spikes.

For being able to work in this way, the spikes must be formed with selfinforcing nondispersive solitons (similar to waves, which we can send along rope). The transverse and longitudinal wave character of spikes must be well balanced with material of neural membrane in similar way, like this one of vacuum, which is able to send the particle solitons at huge distance without scattering. This is where the similarity with QM begins.
2 / 5 (4) Aug 13, 2014
The vacuum behaves quantum mechanically due the presence of tiny density fluctuations, which are known as a Higgs field in vacuum and as a Brownian noise at the water surface. This is the reason, why these two environments are so similar each other. The undulation of vacuum or water surface exposes more density fluctuations at its place, which behaves like more dense area for another waves. Analogously the higher frequency of spikes at certain place of human brain makes it more susceptible for another spikes. Such a spikes are focused by itself after then and their density represents a probability function for another spikes. As a whole the neural network behaves like quantum environment after then. But this is the point, where the similarity with quantum mechanics ends. The quantum wave character of neural network applies at much larger scale and energy density, so its able to work even at room temperature. Whereas the true quantum wave phenomena don't survive few Kelvin temperatures.
2 / 5 (4) Aug 13, 2014
I know about theories of quantum consciousness of David Hamerhoff and Roger Penrose, which is trying to apply the quantum mechanics at much smaller scope, but frankly I don't see any logics in it. These guys are IMO at the right track, but they already missed their point, particularly because they're ignoring the experiments of recent era and they're adhering on sixty years old textbook examples of quantum mechanics. The water surface, our brain or superconductors don't need to be very microscopic for still being able to emulate the quantum mechanics at room temperature quite faithfully. Also, the recent experiments demonstrate, that the holographic character of consciousness isn't very delocalized. The zebrafish brain handles the paramecium perception in rather localized way, thus effectively disproving the Pribram's holographic theory of consciousness.
1 / 5 (3) Aug 13, 2014
Errata: David Hamerhoff = Stuart Hameroff
Aug 14, 2014
This comment has been removed by a moderator.
Bob Wilplast
not rated yet Aug 16, 2014
It is "an amazing state of mind to have", trust me.

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