### Introduction: Theoretical Physicists: Laying Foundations For A New World

The realm of quantum physics has opened up a whole new world of unknowns; a frontier vaster and wilder than any we have faced before. Theoretical physicists are like wandering path builders. They roam these uncharted zones using all the tools and theories they have gathered in their travels to carve new paths. But it is only when their theories are tested in the lab that knowledge truly advances. The aim of a theoretical physicist is to lure the experimentalists onto their paths. The theorist’s playground is the imagination. The experimentalist’s is the lab.

Theorists have been free to wander the quantum realm for a century but only in the last few years have the experimentalists discovered the technology to follow. The development of the laser in the sixties was a major breakthrough giving experimentalists a precise tool to probe the quantum world. Now cold atom experiments have taken an enormous step further. By cooling atoms down to extremely low temperatures, experimentalists can control individual atoms and can induce and observe quantum phenomena with incredible precision.

### Howard Carmichael: Quantum Pioneer and Trailblazer

Howard Carmichael is a theoretical physicist and a pioneer in the quantum world. He is most famous for his Quantum Trajectory Theory, which he developed in the eighties and which has become a foundation for modern quantum technology and photonics.

Howard was one of the first two PhD students of renowned scientist Dan Walls at Waikato University. He began in 1971 just as quantum optics was taking off. Together their group launched one of the most stimulating and productive periods of science in New Zealand history.

Howard is now a professor of Physics at Auckland University where he continues to lay the theoretical foundations for the latest advances in quantum technology.

### What is Quantum Trajectory Theory

"So for example if you have a problem that is one million units in size, then in the quantum trajectory world it shrinks to one thousand so you can fit it in your computer."

The exciting thing about Quantum Trajectory Theory is that it offers a pragmatic way of working with quantum systems in the lab. The standard quantum theory developed by Shrödinger in the 1920s gives an overview of the quantum system and how it evolves. This works ok when you have millions of atoms. It’s a bit like knowing a behaviour trend for a whole country. It can give you the probability for an individual to behave in a certain way.

But it doesn’t tell you what particular individuals will do. With new technologies it is like experimentalists can zoom in and talk to individuals. For this they need a more specific and detailed theory of the quantum world and this is what Howard’s Quantum Trajectory Theory provides. It randomly generates pathways (or trajectories) for individual atoms in keeping with the general rules. In this way it bridges the world of theory with the practical world of making measurements in the lab.

### How has Quantum Trajectory Theory Changed the World?

The art of theoretical physics lies in predicting where the next experimental breakthroughs might be, getting there first and laying down some theory. In this sense Quantum Trajectory Theory hit the jackpot.

Its first wave of popularity started in the eighties when theorists began using it to run computer simulations of quantum systems, which are usually way too complex to fit in a computer. By testing individual trajectories it vastly reduces the amount of data to process.

This allows you to build artificial experiments on the computer which can be tested against real experiments.

“It joins the thinking together with the measuring and experimenting. For me this is the exciting part of science.” says Howard.

Computer toolboxes which use Quantum Trajectory Theory are now very widely used to simulate experiments.

But the really big breakthrough has come in the last few years since experimentalists have begun to work out how to “talk to” single atoms and other single quantum systems. Quantum Trajectory Theory is the perfect language for these ground-breaking experiments. It is so successful at modelling individual quantum systems that you can’t tell the difference between the results of a computer simulation and a real life experiment. This is the ultimate test of a theory.

The big holy grail and the reason governments are pouring huge sums of money into these experiments is quantum computing. If any group was able to crack this problem and create a quantum computer of as few as ten qubits it would be more powerful than the combined computing power currently in the world. Without Howard’s Quantum Trajectory Theory none of this could happen.

### Quantum Physics: The Untold Intellectual Revolution

Coming up with a description of Quantum Physics that makes sense has been a main focus of Howard’s career.

“The thing about quantum mechanics for all of us on the theory side is that it really is the untold intellectual revolution,” says Howard. “It overthrows all the sensible common sense ways of understanding the world.”

Generally in science, if you want to understand something complicated, the idea is to break it up into its parts and study them in isolation. If you can find the basic building blocks and understand how they work, then you can understand the whole. But this approach just doesn’t work with quantum physics.

“When you try to get inside everything to find the ultimate building blocks and how they work, you completely lose sight of the bits and pieces in the traditional way,” Howard explains. “You need a completely different paradigm even to talk about it.”

It is as if all the parts only make sense in relation to each other. You try to zoom in on a particle and it has turned into a wave. This is what makes quantum physics so alluring and challenging for theoretical physicists like Howard.

Quantum trajectory Theory has certainly helped to make more sense of the quantum world.