## Hans Briegel on the Advances and Applications of Quantum Computers

#### Interview for the Special Topic of Quantum Computers, August 2010

Photo: C. Lackner

*Quantum computation refers to the direct use of quantum mechanics
to perform operations on data. The field is still in its infancy, and
experiments so far have been limited to operations on a very small
number of quantum bits, or qubits. Research continues at a lively pace
because large-scale quantum computers would far exceed the performance
of classical computers, and they would have important applications in
cryptanalysis because of their potential to factorize very large
numbers.*

*According to our Special Topics analysis of quantum computers
research over the past decade, the work of Professor Hans Briegel ranks
at #9 by total cites, based on 41 papers cited a total of 2,672 times.
Two of these papers are among the 20 most-cited over the past
decade—one of them is the second most-cited of the past
decade.*

*In* Essential
Science Indicators^{SM} *from*
*Thomson
Reuters**, his record includes 56 papers cited a total of
2,899 times between January 1, 2000 and April 30, 2010 in the field of
Physics. His record in the*
Web of Science^{®}includes 73 papers cited a total
of 3,240 times between January 1, 2000 and July 10, 2010.

*He is head of the quantum information group at the Institute for
Theoretical Physics, University of Innsbruck, and a Scientific Director
at the Institute for Quantum Optics and Quantum Information of the
Austrian Academy of Sciences.*

**In order to set the scene, can I ask you for
a layperson's definition of what quantum computation is, and what it is
trying to achieve?**

Quantum computation uses the fundamental properties of quantum systems, such as atoms, or photons, for new ways of information processing. Technological achievements, such as high-precision laser technology, allow experimenters today to control and manipulate matter on the level of individual atoms.

One can, for example, use the internal states of atoms as switches to store
and process information. By the amazing properties of quantum mechanics,
these atomic states can exist in arbitrary super-positions, representing
something like *on* and *off* at the same time.

When you run a quantum computer, wave-like super-positions of different atomic states can interfere, much like in an interferometer, and these are used to enhance certain outcomes of the computation.

**What is the advantage of quantum
computation?**

With a quantum computer one could do a number of tasks much faster than it seems possible with any existing classical computer. One example is the problem of factoring larger integers, the difficulty of which plays an important role in modern public-key cryptographic systems, or the problem of simulating the behavior of complex quantum systems, which seems to be a key challenge in several fields of science.

**How would you characterize in general terms
the main intellectual challenges being worked on by you and your
colleagues?**

We are trying to understand the implications of quantum mechanics for novel ways of information processing, both in man-made devices and in natural systems. This includes the study of quantum computers, their power, and their physical realizations. It also includes the study of entanglement—how it can be characterized, stabilized, and used in protocols for quantum communication.

"Where does the power of a quantum computer come from? How is it related to the entanglement of the resource state, and what is special about the cluster states?"

Furthermore, we are interested in the fundamental problem of simulation and how it relates to notions of complexity and entanglement. I think, ultimately, we would like to understand to what extent nature can be simulated by machines, be they quantum or classical. Part of it is just fun, driven by a fresh look on quantum mechanics.

**Turning to your papers published in the last
10 years, I would like to begin with the top three, all published in
Physical Review Letters. Let's discuss this trio in order of
publication, beginning with "Entanglement of atoms via cold controlled
collisions" (82: 1975-8, 1999).**

The first author of this paper is Dieter Jaksch, who these days has his own research group at Oxford. This paper, now 10 years old, was the first proposal to show how neutral atoms, trapped in standing laser fields—the so-called optical lattice—can be entangled by controlled collisions.

The paper should be seen in context with earlier work by (co-authors) Peter Zoller, Ignacio Cirac, and colleagues, who had already shown how atoms in such a lattice can be made to arrange in an ordered way, like in a box of eggs (through a quantum phase transition).

My main contribution to this work was to show how the parallelism of this
system could be fruitfully exploited to realize quantum error correction
and elements quantum algorithms, by entangling entire blocks of atoms by
simple lattice manipulations, and this is further elaborated in the 2000
*Journal of Modern Optics* paper "Quantum computing with neutral
atoms" (47: 415-51).

I think this 1999 *Physical Review Letters* paper is highly cited
because it introduced a large community of people, working on cooling and
trapping of atoms, into the field, and also because part of it has in the
meantime been realized in experiments.

**Your two most-cited papers are written
jointly with Robert Raussendorf. What is the content of these key
papers, and why have they made such an impact?**

These papers had a lot of impact because, first, they introduced a completely new scheme of a quantum computer, based on measurement rather than unitary quantum gates, and second they gave a new (or at least much extended) meaning to entanglement as a resource in quantum information processing.

In "Persistent entanglement in arrays of interacting particles," (*Phys.
Rev. Lett*. 86: 910-3, 2001) with Robert Raussendorf, who is now a
professor at University of British Columbia, Vancouver, we introduced the
cluster states as a new family of entangled states, together with some of
their rather unusual properties.

We showed that cluster states can be created efficiently, for example in an optical lattice, where one can entangle large arrays of many particles with a few simple laser manipulations (this was later realized in experiments by Immanuel Bloch and his group).

We showed that the entanglement of such states was remarkably robust (or persistent), and that they had other properties that one associates with an entanglement resource: one can for example obtain certain other entangled states from it, by simple measurements on a subset of particles.

In the second paper, we introduced the one-way quantum computer, which used
the cluster state as its essential resource. We called it *one-way*
because the computation is driven by one-qubit measurements, which
successively destroy the entanglement of the cluster ("A one-way quantum
computer," *Phys. Rev. Lett*. 86: 5188-91, 2001).

This paper had the strongest impact; it broke with the paradigm that a quantum computation must necessarily be a coherent process, like a sequence of quantum gates. This scheme opened many new possibilities for physical realizations of a quantum computer in the laboratory, but it was also conceptually appealing for the study of more fundamental questions, for example, as regards the origin of the computational power of a quantum computer.