From the Special Topic of
In our Special Topic on Quantum Computers, the work of
Prof. Dr. Ignacio Cirac ranks at #1 by cites and #2 by
papers, based on 100 papers cited a total of 5,555 times.
Prof. Dr. Cirac's record in
Essential Science IndicatorsSM from
Reuters includes 194 papers, the majority of which are
classified in Physics, cited a total of 10,551 times
between January 1, 1999 and December 31, 2009. He is also a
Highly Cited Researcher in the field of Physics.
Prof. Dr. Cirac is Director of the Theory Division of the Max-Planck
Institut für Quantenoptik in Garching, Germany. He also holds the
title of Honorarprofessor in the Department of Physics at the Technical
University of Munich, sits on the editorial board of several journals, and
is one of the 2010 recipients of the Benjamin Franklin Medal.
Below, he talks with ScienceWatch.com correspondent
Gary Taubes about his highly cited work in the area of quantum
What were you hoping to accomplish with your
2000 Physical Review A article, "Three qubits can be entangled
in two inequivalent ways" (Dur W, Vidal G, Cirac, JI, 62: art. no.
062314, December 2000)? What was the context of that research?
At that time, we were interested in understanding how to crack
entanglement. If you think about quantum computation, not only quantum
computing but applications that quantum mechanics might have in information
technology, you see there's this very strange property of quantum mechanics
that is key, and it's this property of entanglement. So it was very natural
to try to characterize this property and quantify it.
This is what led us to that paper. People had thought before about how to
characterize that property when you had two objects. We realized that if
you consider three objects, then you find many new and interesting
features. And that's what that paper is about.
Can you explain what entanglement is for those of us without a
Well, for somebody who's not a physicist, you can think of entanglement as
a very special kind of correlation. When we have two or more particles,
they will behave in a correlated way even though they don't communicate
with each other. You measure something in one, and you measure something on
the second one, and what you obtain is very much related to that first
measurement. If you have three objects, then your third measurement is also
related to the first and the second.
"I think we'll be able to do simulations that simply
can't be done with normal computers..."
These correlations are called entanglement, and they don't exist in
classical mechanics. And these particles, these qubits, can be atoms in
different laboratories, for example. They have to be microscopic systems,
so they really behave according to quantum mechanics. They have to be
qubits so they have to have two possible values. Atoms have that property.
They could be photons also, or electrons, or molecules—whatever
microscopic objects that can have two values in this quantum mechanical
Why are two qubits insufficient, and what do you gain by moving to
Some of the applications that exist in information technology based on
quantum systems require several objects, not only two. Of course the
mathematics gets more complicated when you're dealing with more objects.
People had studied two.
We decided to see what happens with more and more objects. So the first
step was to take three. This is where quantum effects are very important in
What was the biggest challenge or obstacle you encountered in this
I'd say understanding how to properly formulate the problem, and then
realizing that we could actually classify all possible properties of three
objects according to one specific way, which is the one that gives you all
the information you want.
That was the first challenge: to find the right question, in effect. And
the second challenge, of course, was to solve it in practice.
What is it about this paper that makes it so influential? Why has
it garnered so many citations?
Because later on people found applications for this three-qubit state, and
then of course they used the properties derived from the paper for these
What exactly are these applications, and did you anticipate them
when you wrote the paper?
Well, methods for distributing secret information, things related to
security in communication or protocols for doing digital signatures or
whatever protocols that involve three partners. This is not quantum
computation now, but quantum communication or secure communications based
on quantum mechanics.
And, no, we didn't anticipate this connection. It was unexpected. It came
later on. We were just studying the entangled properties of qubits, of more
than two qubits, and then we later realized that the theory we were
developing may have applications in other fields.
How did you decide where to submit or publish your paper? Why
Physical Review A in this case, for instance, rather than
Physical Review Letters?
There are some papers that have a significant amount of content, more than
can be easily summarized in four pages. Even though the impact factor of
Physical Review A is not as high as Physical Review
Letters, sometimes it's better to write a long paper where you can
explain everything in detail and publish it in a journal that will accept
it than it is to rush and publish in Physical Review Letters.
That's what we did here.
On the other hand, I think all of us are seduced by the fact that
Physical Review Letters has greater impact and so we tend to
submit there more often.
How have your research interests evolved in the decade since that
paper came out?
"We were just studying the entangled properties of
qubits, of more than two qubits, and then we later realized
that the theory we were developing may have applications in
I have been basically working on two different topics related to quantum
information. One is related to this paper, further developing the theory of
quantum information. The other is thinking about physical systems and how
to build quantum computers from them.
During the last year, that first path has now evolved into many-body
systems. The theoretical tools we've developed now can be applied in other
fields of physics, specifically in condensed matter physics.
As for that second path, I've continued working with experimentalists,
trying to build more robust quantum devices based on atoms, ions,
electrons, and photons—on these microscopic systems.
What do you think is achievable in quantum computing in the next
Maybe reaching something on the order of 50 or 100 qubits, and also making
significant progress in the related field of quantum simulation. I think
we'll be able to do simulations that simply can't be done with normal
computers—simulations of condensed matter systems, in particular,
that will allow us to study properties of magnets or magnetism,
conductivity or superconductivity.
These problems are very hard to study with normal computers. I think with
quantum simulators we'll be able to study these systems, simulate them, and
understand the physics behind them.
Is there one goal in particular that you'd like to achieve in your
One of my major goals would be to develop theoretical tools that are useful
in this context of quantum information, in building a quantum computer or
quantum communication device, and in studying these many-body quantum
Is there any fundamental or technological limit to the number of
qubits that can be used in quantum computing?
There's no fundamental reason why we couldn't work with 100,000 qubits.
It's a huge technological challenge, but I don't see any fundamental reason
why it can't be done. Of course, it will take a very long time to get
there. Technology doesn't advance as fast as we want.
What would you rate as your most difficult or trying professional
Probably when I had to choose to move from Spain. I got an offer from
Innsbruck and I had to decide whether to stay in Spain, where I'm from, or
move to Austria. That was a very difficult decision. And it was difficult
in the beginning, but later it worked out well.
Which of your professional achievements brings you the most
I'd say working with my collaborators, and particularly with Peter Zoller,
who has been one of my closest collaborators for many years
Prof. Dr. Ignacio Cirac
Max-Planck Institut für Quantenoptik
Ignacio Cirac's current most-cited paper in Essential Science
Indicators, with 715 cites:
Dur W, Vidal G, Cirac JI, "Three qubits can be entangled in two
inequivalent ways," Phys. Rev. A 62(6): art. no. 062314, December
Essential Science Indicators from
Ignacio Cirac is featured in
KEYWORDS: QUANTUM COMPUTATION, ENTANGLEMENT, QUBITS, QUANTUM MECHANICS,
THREE OBJECTS, APPLICATIONS, QUANTUM INFORMATION, MANY-BODY SYSTEMS,
CONDENSED MATTER PHYSICS, QUANTUM SIMULATION.