Hello, and welcome to the Physics World Weekly
podcast. I'm Hamish Johnston.
This week, I'm in conversation with Tim Shea
of Canada's Perimeter Institute
for Theoretical Physics.
We explore some of the hottest topics in
quantum science and technology,
including
emergent phenomena,
quantum error correction,
and quantum simulation.
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Earlier this autumn, I had the pleasure of
visiting the Perimeter Institute
where I interviewed four physicists
about their research.
This is the third of those conversations
to appear on the podcast,
And it's with Tim Hsieh,
whose research focuses on quantum information
and quantum matter.
Here's that conversation.
I'm at the Peruner Institute in Waterloo, Ontario,
and I'm very pleased to be joined by
Tim Shea.
Hi, Tim. Welcome to the podcast.
Thanks a lot, Himesh. Thanks for, being here
visiting with us. So Tim, we're gonna talk
about quantum matter,
and I think we need to we need
a definition here.
I mean, aren't most, if not all material
properties
defined
by quantum mechanics. So what is quantum matter
as opposed to
just matter that's
defined by quantum mechanics? Yeah. That's a that's
a great question. So it it's true that,
you know, everything is dictated by the laws
of quantum mechanics, but it turns out that
in certain materials,
the really counterintuitive
laws of quantum mechanics,
play a much bigger role than in other
materials, which look more classical.
Right? Like, so for example,
you know, given our phones, our our computers,
you know, we have a deep appreciation of,
semiconductor
chips, right, like silicon,
things like that. Right?
And these,
you know, for for these these,
very useful materials,
quantum mechanics already plays some role. Right? Like,
the the the power exclusion principle, for example,
is,
really important. Right? We wouldn't be here if
it wasn't for the Pauli exclusion principle. Yeah.
Yeah. Ex exactly.
And and so, you know, in in coming
up with,
a theory of, band gaps in semiconductors,
for example,
we already need quantum mechanics. But this is
more quantum mechanics at, at a single particle
level.
Right? We're basically dealing with,
you know, how,
a single electron moves in in a whole
crystal array.
And, by analyzing that, we can already, you
know, derive a lot of
the, useful properties of of semiconductors, for example.
Okay. So that's, like, that's kinda like level
one quantum materials.
But then it turns out that there are
even more exotic,
like, higher level quantum materials where,
quantum mechanics plays a much
deeper and complex role.
So some of which we haven't even completely
understood.
Right? And and that's because, for these, you
know, more exotic quantum materials,
we're not dealing with a a single particle
moving into crystal. We're dealing with, like, a
whole collection of interacting
electrons
or or spins, magnetic moments.
And so now,
you know, we have
a a a many body problem in which
we have to apply the laws of quantum
mechanics are.
We have to apply the laws of quantum
mechanics.
And so that that can give rise to
a lot of new,
emergent
phenomena that we didn't expect at the single
particle level.
Right? And so one one example is, for
example, superconductivity,
right, or,
our what what we call quantum spin liquids.
These are where we really have to deal
with the the whole system, like, you know,
10 to the 23 or more,
electrons degrees of freedom interacting.
Right? So so these, I would say, are,
much of the focus of modern,
quantum matter research.
And I I wanted to ask you about
emergent
phenomena. I think it was it Philip Anderson
who said more is different Right. Right. In
the sense that when you when you have
lots of things interacting
Mhmm. You can have very strange well,
structures pop out of it. Right. Right. And,
you know, I suppose these are it's not
just in the quantum world. You know, if
you go to a beach, for example, you'll
see lovely ripples in the sand. Mhmm. Things
like that. So we're we're sort of used
to,
collections of small things
organizing themselves into big patterns. Right. But how,
how does this work in in the quantum
world? Yeah. Yes. You gave a few examples.
Can you maybe give a a few more?
Sure. Sure. Yeah. So in in indeed, it's
true that even in, you know, macroscopic
classical systems, you know, without any, you know,
quantum mechanics necessary, there's already immersion phenomena, like
like the type you described.
For for large or, you know, macroscopic
quantum systems, you can have even more interesting
phenomena.
One of my favorites is, something called
the topological order.
And that that's when,
you know, you can have
systems of, you know, individual,
bosonic
degrees of freedom. Right?
But whose whose interaction
give rise to some state in which you
have immersion fermion
particles,
coming out. Oh, really? That's that's really interesting.
So the the
how can bosons team up to make a
fermion? Yeah. Yeah. So I could see fermions
teaming up to make a boson, but Right.
Right. Right. The opposite seems a bit odd.
It's it's it's pretty amazing. Like,
one way to think of this is that,
you know, you can you can imagine the
individual
constituents of the system
as being composed of even smaller degrees of
freedom.
Right? So so in in in, you know,
fundamental particle physics, there's there's this old idea
of, partons. Right? Like, our our,
you know, our protons, our neutrons are made
out of smaller particles called quarks.
Right?
So it turns out to be
somewhat valuable in thinking in terms of that
perspective even for, like, you know, these these
tabletop systems or materials. So you you can
imagine your individual constituents consist of smaller
degrees of freedom. Like, you can imagine a
boson that's composed of two fermions bound together.
Right?
And for,
you know, what I call trivial
phases of matter, you can imagine that these
these these, you know, imaginary fermions are, like,
bound together tightly, and each boson is doing
its own thing. So you never really see
the fermions by themselves. Right?
But you could imagine the possibility that these
bosons,
each of which have, like, a pair of
bound fermions,
are interacting so much that these bound fermions
become deconfined.
Right? It's just like how in again,
using a particle physics analogy,
if you have, like, protons and neutrons, like,
high enough temperature or pressure,
in principle, you could have quarks that are
deconfied.
And you you could see the individual constituents
in in a very, you know, extreme setting.
Right? Here, it's like the the strong interactions
between,
in this case, like bosons,
that lead to the deconfident
of this imaginary
constituent of this fermion. And this is how
this fermion becomes, like, unbound due to strong
interactions
of many collective degrees of freedom.
And and some of these emergent phenomena, they're
not I mean, it's not just a theoretical
curiosity, is it? Right.
Right. Right. Yeah. You you know, you mentioned
topological
Yeah. That's definitely not. So useful for computing,
couldn't you? Yeah. So this this has, you
know, gone all the way back until the,
the the nineteen eighties. So,
there there's a system called the the fractional
quantum hall effect, right, which basically is like
a two d electron gas in a very
high magnetic field at low temperature. And
and here, there there's already this phenomenon of,
this fractionalization
of individual particles into some smaller constituents.
So one of the simplest example is, like,
you have these this this two d electron
system. You know, it's built out of electrons,
but, again, due to strong interactions,
you have fractionalization
into,
pieces of electron. For example, a third of
the electron charge with with a third of
its,
particle statistics in some sense. And so, yeah,
these these fractional quantum ball systems have been,
seen experimentally,
since since the eighties. So these systems, you
know, remarkably exist. And,
yeah, that that that's what makes this phenomenon,
of course, even more, interesting and relevant. And
and and it can be useful as well
because I think, you know, for example, topological
properties,
perhaps resistant to noise, and that could be
handy when you're trying to build a quantum
Exactly. Exactly. So exactly. One one approach toward
encoding this, you know, very fragile quantum information
is to encode the information into
the fractionalized
pieces of the individual degrees of freedom. Right?
So so if you if you didn't have
fractionalization, if you just have information encoded directly
at the physical level,
then, you know, the that physical
that physical degree of freedom can be corrupted
easily by some some noise. But if you
encode it non locally in in terms of,
you know, fractions of the original degrees of
freedom that are separated in space,
then it's much less likely that some noise
event
will collectively corrupt all three and lead to
a logical error. Right? So this this nonlocal
encoding of of information is more,
robust, and this is why,
you know, we have this pretty promising approach
of topological computation using these fractional oxide fractionalized
excitations,
to do computation.
And, Tim, I wanted to move on to,
quantum computers.
Mhmm. I mean, I suppose you could think
of a quantum computer as essentially
a piece of quantum matter that physicists can
control very precisely.
Mhmm. If we were able to create
large scale quantum computers, let's say using those
topological
qubits Mhmm.
What sort of emergent
phenomena do you think we could see? Or
or maybe we don't know and we'll have
to wait.
Yeah. Exactly. I think I think this is,
at least for me, the most
exciting
thing would be to, you know, see some
phenomenon that we don't know yet how to
understand. Right? I think that that's always the
most exciting mode
of of physics being done. Right?
This for example, this is what happened for
the the factional quantum hall effect. Right? Like,
like, there was no prediction
of this very interesting phenomenon beforehand.
And so that, I think, is also true
for this,
you know,
for for the quantum computers being developed right
now. You you can think of it as
kind of probing a new
probing a new extreme,
of, complexity
access. Right? So,
as you probably know, like, a lot of
the the big breakthroughs,
you know, in seeing new phenomenon from experiments
leading to new theories has come from probing,
in a extreme,
point in some parameter.
Right? Like, superconductivity,
we're cooling down to very low temperature.
Right? For pressure quantum ball, it's like reducing
dimensionality
to, you know, like, a two d plane
applying a really high magnetic field. Right? And
so this, you know, this this,
development of quantum computing is, like, probing,
this
new access of, like, quantum coherence.
Right? Of, like, basically, how how big of
a system can we have,
you know, superposition of states.
And right? So it's this it's this new
regime of, like, quantum complexity
that's being now, you know, newly available.
Right? And,
and, yeah, again, to be I think it'd
be the most exciting if we see, you
know, some new phenomena that might suggest
even, like, a, like, a breakdown in the
current laws upon mechanics. Right? Yeah. Now that'll
be obviously very, you know like, we we
we don't expect it right now, but,
that would be, I think, the most exciting
thing. Yeah. I mean, that is a that
is an interesting thing, isn't it? An idea
that's emerged,
you know, sorry about the pun, over the
last little while is that you, you know,
you could see that crack in in the
standard model.
Right. That's right. In a in a quantum
computer rather than smashing
particles together. You know, you could get your
first glimpse of,
of physics beyond what we know. That's right.
That's right. Yeah. Yeah. It's yeah. So for
for, I guess, beyond the center model, there
were kind of,
we're kind of looking at this, like, reductionist
paradigm, right, where, you know, we have, like,
you know, basic degrees of freedom that and
and their laws, and we're trying to see
what the complete description of those basic degrees
of freedom. But but in this in this
other axis of, like, you know, building a
a big controllable
quantum computer,
right, it's like more of like a I
don't know. The opposite of reduction is, like,
constructionist
philosophy. Again, this philosophy of of emergence,
you know, coming from basic degrees of freedom,
which we already understand,
you know, what laws are describing them, but
together, they lead to these emergent laws that
that we don't,
really know about. Right?
So so we've chatted a bit about quantum
computers, and I know that that's one
interest that you have in terms of your
research.
Where are we at the moment with quantum
computers? What what are the challenges facing people
who are trying to develop them at the
moment? Is it this this coherence problem dealing
with the noise that Yeah. Destroys your quantum
state? That's right. So that that that is
the biggest challenge because, you know, quantum information
is even more fragile than, classical information.
Right? Because quantum information, you have to
worry about noise,
pretty much in along different axes. Right? So
in in in the classical world, you only
have one basis.
Right? Like, one zero,
up or down. And if you worry about
noise, like flipping bits in that one basis.
But,
as you probably know, you know, a a
qubit is,
in some sense, almost like a like a
continuous space. Right? You can have arbitrary superpositions
of zero and one, and so you could
have noise acting along all these different directions,
along the block sphere. Right?
And so, you have to work a lot
harder to protect
any quantum information that you've you've encoded.
But but that said, there's been remarkable
experimental progress in in, a whole variety of
approaches for quantum computing, like trapped ions, superconducting
qubits,
red brick arrays, for example.
And and so I think it's it's really
exciting time where, you know, people are scaling
up, their quantum simulators in computers. The controllability
is improving. Their gate fidelities are improving.
And so now this is really,
like a new playground for for theorists
to, at this point, you know, predict,
new types of phases, for example, that can
emerge in these systems,
things like that.
And, Tim, am I right that you you
work on some quantum error correction? Is that
right? Yeah. So, yeah. I I've been Now
can you talk a bit about that? Because
that, I mean, you know, as well as
improving the fidelity or quality of a of
a quantum Right. Right. Right. The I suppose
for the time being,
error correction is is a really important Right.
Issue, isn't it? Yes. So what will what
is quantum error correction? And Yeah. So so
quantum error correction is is how you, you
know, protect
this logical infer this quantum
information you're encoding against noise. Right? And so,
typically, you have to, you know, measure
certain
syndromes
in in your in your system, repeatedly.
And based on those the syndrome measurement outcomes,
decide,
you know, what is the most likely error
that occurred. Right? And then you'll apply some
feedback to,
reverse those errors.
So that that's that's the basic idea. But,
remarkably, that like, quantum error correction, I think,
is extremely deep and has a lot of
relevance to,
beyond its, original practical intention of preserving logical
information.
For example, it, you know, it it provides,
like, a new angle for,
for interpreting,
the the holographic
correspondence, right, between,
like, a, a system in one lower dimension
and one in higher dimension with gravity.
And and also for for me, there are
a lot of deep connections between quantum error
correction
and quantum phases of matter.
Right? So so my my interest is has
been in kinda understanding,
you know, the the error correcting regime versus
the non error correcting regime as two different
types of phase of matter.
Right? Again, we'd like, going back to what
you said, we can think of this quantum
computer as some macroscopic
quantum system, right, to be analyzed
on the same footing as some quantum material,
which we would conventionally,
label as some phase of matter.
Right. And and so yeah. So does that
go back to, you know, you you were
talking about top a topological state where the
quantum
well, the as as well as the quantum
state is distributed
between,
several different
entities. Is that I mean, is that an
example
maybe of a an error corrective Exactly. Yeah.
Yeah. That that's a that's a very nice
example of a of a topological phase of
matter
serving as a quantum error correcting code.
Right? But then you can ask, you know,
if you try to corrupt this, you know,
with environmental noise, like, what's actually happening on
the experimental quantum computer, at some point, it
will lose the quantum information. Right? At some
point, you will destroy this order. Right? So
here, you have a case in which the
the error correction threshold, right, that the point
at which
you're, you know, you're you've lost the ability
to store quantum information gets,
destroyed, and that coincides with a a phase
transition. Right? So
so, what I've been very interested in is,
you know, applying this
perspective of phases of matter and phase transitions,
to,
error correction, like, viewing that as a phase
of matter and, you know, importing a lot
of the techniques we have and thinking about
phases of matter,
into this, air correction setting. Yeah. Well, that's
really interesting because I've I've always maintained that
if there's one thing a physicist loves, it's
a phase transition. Yeah. That's true.
That's true. Yeah.
There's, you know, a huge amount of insight
into a system that you can get, isn't
there, by watching? And there's a lot of,
you know, like, notions of,
universality. Right? The whole the whole idea of
thinking about a phase of matter is that
you don't have to deal with every individual
system, right, and their, you know, different microscopic
properties. You you care about the universal long
distance physics, what's common to the whole,
set of these systems.
Right? And that that's a powerful way of
thinking that I think, again, gives you some
insights into these error correcting or not regimes.
And and finally, Tim, I I wanted to,
to to, you know, ask you about,
your wish list Mhmm. For for quantum. You
know, let's say,
sometime in the future, not too far in
the future, Mhmm. You know, people are able
to build
reasonably large quantum computers.
Uh-huh. What what I mean, what sort of
well, I don't know if if is program
the right word? What would you like to
run on a quantum computer? What sort of
system would you like to simulate
or create on that computer?
What what would be your first? Mhmm. I
see. Yeah. That's that's a
that's a good question.
So
I guess, you know, given my condensed matter
background of, you know, dealing with very
hard models,
you know, that we think describe real quantum
materials that we have yet to solve, I
think I would still like to, you know,
probably use a quantum
computer to gain insights into those effective models
of quantum materials.
For example, there are, you know, these these
thing called Hubbard models, which are effective descriptions
of, like, high temperature, superconductors,
things like that. And, you know, they're they're
very
they're somewhat intractable for our classical computers to
handle. And, also, we don't have that many
great analytical techniques
often to do.
And so, you know, using some analog quantum
simulator or maybe even a digital quantum computer,
if we can get insights into, you know,
like, the finite temperature properties
of these,
you know,
strongly directing Hamiltonians,
right, or, you know, like, the dynamics,
I think that would be very, very useful
and insightful.
And I suppose beyond, you know, the
the the pure physics,
if you could get a handle on those
materials. Mhmm. That I mean, that would be
a revolution in material science, wouldn't it? Right.
Right. Yeah. You could design
some amazing
material. That's right. That's right. Yeah. Yeah. So
maybe maybe learning more about these effective models,
their, you know, their final temperature properties would
would give us insight into
maybe, you know, how to design materials with,
like, higher TC, for example.
Yeah.
Well, thanks, Tim. Thanks for for speaking to
me, and, I hope that your dream will
come true. Yeah. Yeah. You'll get to, to
do those those calculations or experiments or whatever
you want to call it Yeah. Yeah. On
the quantum computer sometime in the future. Thanks
for coming on the podcast. Thanks, Alish. Thanks.
That was Tim Shea of the Perimeter Institute
for Theoretical Physics.
Thanks to Tim for a fascinating discussion
about all things quantum.
And thank you for listening to this podcast,
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