David Awschalom knows how to control the quantum state

Researchers have figured out how to use diamonds and laser light to trap electrons and control the quantum state.
Written by Boonsri Dickinson, Contributing Editor

Unlike traditional computers that store information in two states -- ones and zeros -- quantum computers store information at the atomic level, under the rules of quantum mechanics.

That's important because a fully functional quantum processor could change the way we communicate, increasing the performance of computers.

But David Awschalom thinks building a quantum computer remains a dream out of reach.

That doesn't mean he isn't trying. The director of the Center for Spintronics and Quantum Computation, Awschalom tasked his team at University of California Santa Barbara to develop a way to control the quantum state -- a step in the right direction toward that goal. The researchers showed they can measure quantum activity without destroying it.

One favorable aspect of quantum information is security -- the data can’t be replicated, promising more secure bank transactions and even improvements in drug design.

I spoke with Awschalom about how his team controlled the quantum state -- and why quantum computing still has a long way to go before it's ready to change our everyday lives.

SmartPlanet: So can you build a quantum computer?

The idea of quantum computing is still a vision. If such a computer could be built, there are certain kinds of computations that it could do much, much faster than regular digital computers. Examples are searching databases, factoring large number and a few others. Some of these problems would take a lifetime on a conventional computer.

SmartPlanet: What will quantum computers be made of?

These defects within diamond could someday form the individual bits of a quantum computer – solid state qubits. They may also form only a specialized part, like memory bits or a way to transform quantum states into light: a type of quantum transducer. We are a very long way from actually building a full-scale computer; our experiment works with only one bit and one photon as a proof of concept experiment.

SmartPlanet: When you combined laser light with trapped electrons to detect and control the electrons' fragile quantum state without erasing it. How exactly did you do this?

The atom-sized diamond defect that we study can absorb light of precisely the right wavelength. If the wavelength is tuned away slightly, it turns out that the photons that pass by the defect can actually interact with the electrons without being absorbed.

This is quite unusual. That novel interaction produces a measurable change in both the light passing by, and the quantum state of the electrons within the defect.

SmartPlanet: What did you do?

Our experiment was aimed to do precisely that: we focused laser light onto the defect and, after the light had passed through, separately measured both the light and the electrons. We found that we could measure quantum properties of the electrons without erasing their state by measuring the light. We also found that the light could be used to control certain quantum properties.

What was the point?

This experiment was hatched from an effort to understand the quantum properties of these diamond-based defects.

SmartPlanet: Why is preserving quantum states important?

The quantum states encode the information within the bits. Unfortunately, however, they are very fragile.

One of the major obstacles is finding ways of preserving them while the computer is running. This is one of the intriguing aspect of quantum physics: the act of observing a state changes the state; to exploit the unique properties of quantum mechanics we need to find new schemes to measure physical properties of matter.

SmartPlanet: Why is it that information can never be copied in quantum computers?

This is a unique property of quantum states. They can be measured, but not ‘cloned’ or copied. The act of copying them intrinsically changes the original. This might be useful for secure communication, since the person receiving the message would know if it was intercepted and read.

SmartPlanet: How do you see your tool being applied in 5 to 10 years? How might something like this change computing or current communication systems?

Our experiment provides a method of using light to control and measure the quantum state of diamond defects. It may be also extended to do long-distance communication of quantum states, providing the possibility of building a quantum repeater.

Just as digital signals need a ‘repeater’ to amplify and re-broadcast over a long distances, quantum signals based on light also have a limited range unless they can be re-broadcast. This is a vital component of future quantum networks.

This post was originally published on Smartplanet.com

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