## Gerard Milburn on Quantum Computers

#### From the Special Topic of Quantum Computing, Published March 2010

In our Special Topics analysis on quantum computers, the work of Professor Gerard Milburn ranks at #10 by total cites, based on 48 papers cited a total of 2,603 times. One of these papers is the most-cited paper in the topic for the decade.

His record in
Essential Science Indicators^{SM} from Clarivate Analytics
includes 99 papers, the majority of which are classified in the
field of Physics, cited a total of 3,651 times from January 1, 2000
to February 28, 2010.

At present, Milburn is an Australian Research Council Federation Fellow at the University of Queensland's Centre for Quantum Computer Technology.

**Would you tell us a bit about your
educational background and research experiences?**

I obtained a Ph.D. in theoretical physics from the University of Waikato in 1982 for work on squeezed states of light and quantum nondemolition measurements. I was appointed to a postdoctoral research assistantship in the Department of Mathematics, Imperial College London in 1983. In 1984 I was awarded a Royal Society Fellowship to work in the Quantum Optics group of Professor P. Knight, at Imperial College.

I am currently an Australian Research Council Federation Fellow at the University of Queensland. I am the Chair of the Scientific Advisory Committee of the Perimeter Institute for Theoretical Physics in Canada and a Fellow of the Australian Academy of Science and the American Physical Society.

I have worked in the fields of quantum optics, quantum measurement and stochastic processes, atom optics, quantum chaos, mesoscopic electronics, quantum information and quantum computation, and most recently in quantum nanomechanics and superconducting circuit QED.

"I am working on different ways to encode information on a single photon."

I have published over 200 papers in international journals, and published
four books: *Quantum Optics*, with Dan Walls, (Springer 1994;
updated with a new edition in 2008); two non-technical books on quantum
technology and quantum computing (*Schroedinger's Machines*, Allen
and Unwin, 1996; *The Feynman Processor*, Allen and Unwin 1998), and
I have just completed a book on quantum measurement and control with Howard
Wiseman (Cambridge University Press, December, 2009).

**What first drew your interest to the field
of quantum computing?**

In 1986, I came across a past issue of the *International Journal of
Theoretical Physics* which contained an article by Feynman called
"Simulating physics with computers." After reading this paper, and related
papers by Fredkin and Toffoli in the same journal, I sat down to see if one
could build a reversible quantum gate using quantum optics.

I quickly came up with a way to do it using nonlinear optics, but was
unable to complete the paper as I moved to the University of Queensland at
about the same time. However soon after I got to Brisbane I sent the paper
to *Physical Review Letters* and it was published in 1989. I learned
later that a similar idea had been developed quite independently by Dr.
Yamamoto in Tokyo and presented at a conference in 1988.

Subsequently, together with my long-time collaborator, Carl Caves, now at the University of New Mexico, I began to think about how one could read out a quantum computer and what effect measurement noise would have. Unfortunately, in 1990 my grant application to work on optical quantum computing was rejected so I turned to the new field of atom optics and ion trapping.

**What is your main focus in the
field?**

In quantum information I am interested in how we can use ideas from linear optical quantum computing to build physical simulators of other physical systems, such as nonlinear fields.

My research interests are somewhat distant from quantum computing now. I am much more interested in a new class of engineered systems (such as nanomechanics and superconducting quantum circuits) in which quantum coherence can be controlled for applications other than quantum computing. This is likely to have an impact on technology much sooner than quantum computing.

That said, however, these new fields partly owe their existence to the massive investment in quantum computing research over the last 10 years.

**You were a coauthor on the #1 paper in our
analysis, the 2001 Nature article, "A scheme for efficient
quantum computation with linear optics." Would you tell us about this
paper and why it is cited so much?**

"I am interested in how we can use ideas from linear optical quantum computing to build physical simulators of other physical systems, such as nonlinear fields."

The idea behind this paper was really due to my coauthors, Ray Laflamme and Manny Knill. I was very doubtful that a conditional gate would work using only linear optics. My original idea for a quantum gate from 1988 required nonlinear optics.

While I had a great deal of experience in describing the effect of conditional measurements in quantum mechanics from my previous work with Carl Caves, I had not fully appreciated how such measurements could be used to conditionally prepare very non-classical states.

As it happened, Ray, Manny, and I found ourselves together at a program in the Aspen Center for Physics in 2000, where we had the time to think about it carefully and discuss it daily. It soon became clear that not only was the idea sound, but that a simple CNOT gate could be demonstrated with current technology.

**How has the Nature paper shaped or
influenced your subsequent work?**

The paper had a big impact on experimental research in our group. It led to a new quantum optics lab being established with significant funding from Australia and the US. The lab was led by Andrew White. They were the first to demonstrate a quantum gate using our scheme, and have gone onto produce a number of important experiments in the field.

**According to your website, one of the aspects
of your work is "frequency multiplexing of quantum information on
optical pulses." Would you tell us about this, and some of the key
papers you have published in this area (on our list or not)?**

I am working on different ways to encode information on a single photon. A single photon pulse is a coherent superposition of a single excitation across many frequency modes. By coherently controlling this superposition we control the temporal structure of the pulse. This gives us a way to encode quantum information in a single photon.

That is very different from the more conventional on-off coding. By using quantum interference to do the decoding we hope to get a much greater bandwidth. Much of this work is not yet published, although I have presented some ideas at conferences over the last two years.

**How has the field of quantum computing
changed in the past decade? Where do you hope to see it go in the
next?**

No matter what technology is ultimately used for quantum computing, optics will necessarily be used for short- and long-distance communication both within and between quantum computers (as is currently the case for conventional computing).

I was thus somewhat puzzled to learn that the US government had recently stopped funding research on optical quantum information processing. We certainly will continue to develop optical quantum information processing. At some point the solid-state QC community will need to return to quantum optical interconnects.

**Professor Gerard J. Milburn**

**Centre for Quantum Computer Technology**

**The University of Queensland**

**St Lucia****, Queensland, Australia**

GERARD MILBURN'S CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:

Knill E, Laflamme R, Milburn GJ, "A scheme for efficient quantum
computation with linear optics," *Nature* 409(6816): 46-52, 4
January 2001, with 1,186 cites. Source:
*Essential Science Indicators* from
*Thomson
Reuters* .

KEYWORDS: QUANTUM COMPUTATION, QUANTUM OPTICS, QUANTUM MEASUREMENT, STOCHASTIC PROCESSES, ATOM OPTICS, QUANTUM CHAOS, MESOSCOPIC ELECTRONICS, QUANTUM INFORMATION, QUANTUM NANOMECHANICS, SUPERCONDUCTING CIRCUIT QED, REVERSIBLE QUANTUM GATE, NONLINEAR OPTICS, LINEAR OPTICS, SINGLE PHOTON PULSE.