Archived Articles
- Uncertainty and nonlocality: a description of my paper with Stephanie Wehner, Science 2010.
- Quantum information can be negative: an accessible description of my paper with Horodecki and Winter in Nature.
- Sending quantum information down channels which cannot convey quantum information: A perspective in Science. A copy is available here.
- Quantum computing as free falling: A Science perspective on quantum computation as geometry. A copy is available here.
Research Interests
I'm currently very engaged in using insights from quantum informatin theory to probe fundamental physics. This effort spans several fields, including quantum information theory, reconciling quantum mechanics and gravity, black hole thermodynamics, and the foundations of quantum theory. Although these fields are often distinct, there are many conceptual overlaps, as can be seen below. You can check out my google scholar page here.
Reconciling quantum mechanics with General Relativity (Einstein's theory
of gravity), is one of the grand challenges of modern physics.
Rather than
attempting to quantise gravity,
my latest research takes a different approach -- instead of modifying
General Relativity and leaving quantum theory untouched, we modify quantum
theory and find that consistency with General Relativity requires an
intrinsic breakdown in predictability that is
mediated by spacetime itself. The result is a consistent theory of
quantum
field theory coupled to classical spacetime.
I've set out why I believe it's reasonable to question whether we should quantise the spacetime metric
here, and
the proposal can be found in
a "postquantum
theory of classical gravity" based on a master equation approach.
With
Zach Weller-Davies, we now have a path integral formulation of it.
The theory is formally
renormalisable, which is exciting, since it allows us to think of
gravity as a theory of spacetime down to the shortest of distances. This contrasts with quantum gravity approaches like string theory, where spacetime curvature is meant to emerge from
strings propagating in a higher dimensional flat spacetime. In this sesnse, postquantum gravity takes seriously the idea that gravity is a theory of spacetime and opts to modify its
relationship with quantum theory in order to preserve this.
These theories do not need the measurement postulate of quantum
mechanics
as the Born rule follows from the dynamics. These results follow from the
derivation of the most
general form of evolution laws for classical systems coupled to quantum ones in 1811.03116 and 2203.01332. We have proposed an experiment, to test whether spacetime is classical via something we
call, the decoherence
vs diffusion trade-off. Consistent coupling of quantum fields to
classical spacetime ncessarily requires the metric to have stochastic
fluctuations. If these fluctuations are strong enough, they could provide
an alternative explaination to the phenomena we currently regard as dark
matter (see here and here). You can find more information and some links to popular articles here
Much of my work is in understanding the basic blocks of quantum information theory --
what does it mean for one quantum system to have information about another system.
This is often best understood through communication theory. If an individual has a lot
of information about my system, then I don't need to send them many qubits for them
to possess the rest of my state. It was this idea which led to state
merging and negative information.
I am also interested in the role of time in quantum mechanics,
which was the subject of my Ph.D thesis.
There, it was proposed that there is a new fundamental limitation on the accuracy
of measurements of the time of an event. It is also impossible to tell the
past from the future
Selected publications
A postquantum theory with classical spacetime
"The hardest thing of all is to find a black cat in a dark room,
especiall if there is no cat."
Quantum information theory
"You don't understand quantum mechanics, you just get used to it."
-- attributed to Feynman, borrowed from von Neumann.
Quantum information theory is currently a very exciting field,
and we are constantly learning new and surprising things about
quantum mechanics.
I am interested in quantum communication theory, quantum
cryptography, and entanglement manipulation.
Quantum Thermodynamics
"In this house, young lady, we obey the laws of thermodynamics!"
--Homer Simpson
The laws of thermodynamics govern much of the
world around us. They tell us that a hot cup of tea in a cold room will cool down rather than heat up;
they tell us that unless we are vigilant, our houses will become untidy rather than spontaneously tidy;
they tell us how efficient the best heat engines can be. But the laws of thermodynamics only apply to
large objects, when many particles are involved. Can the laws of thermodynamics be applied to small
systems, such as the kind of microscopic motors currently been fabricated in labs? Or perhaps even
quantum systems? Surprisingly, the answer is yes. I'm interested in formulating laws of thermodynamics
for quantum systems. We find for example, that at the small scale, there are many second laws,
not just one, and that we can formulate a stronger version of the zeroth law which helps us to naturally define the notion
of temperature. This builds on some of my previous work deriving thermo-majorisation criteria, which was proven to be
necessary and sufficient for thermodynamical state transformations.
Foundations of quantum mechanics
"It is very difficult to be more interesting than quantum mechanics."
--Gaspar, to the frustrated wife of a physicist (who shall remain anonymous)
As a general rule, I am interested in understanding how one might be able to generalize
our current laws of evolution and the state space these laws act on. This is not only
to understand what other laws might be possible, but also to understand what is so
special about current physical laws and states.
Other areas of interest, include for example, probabilistic interpretations of unitaries.
In quantum mechanics, outcomes of measurements
on a state have a probabilistic interpretation while the
evolution of the state is treated deterministically. However, it turns out
that one can
also treat the evolution as being probabilistic
in nature
and one can measure `which evolution' happened.