Hello, and welcome to the Physics World weekly
podcast.
This episode is part of our ongoing
celebration
of the International Year of Quantum Science and
Technology,
and it features the Nobel Laureate William Phillips
in conversation
with Physics World's Margaret Harris.
This episode is supported by Atlas Technologies.
Atlas custom aluminum and titanium
vacuum chambers and hermetically sealed,
bonded bimetal components
are used in quantum applications,
physics labs, and semiconductor
fabs.
And they are built in a fully integrated
facility with on-site design, development,
and manufacturing
capabilities.
Let Atlas help you solve your next engineering
challenge.
Learn more at atlasuhv.com.
Here's that conversation
with Bill Phillips.
He talks about quantum entanglement,
atomic clocks,
the future of quantum technologies,
and much more.
It's the International Year of Quantum Science and
Technology. I'm speaking with a real pioneer in
this field. Bill Phillips shared the 1997
Nobel Prize in Physics for his contributions to
laser cooling, which uses light from precisely tuned
laser beams to slow atoms down and reduce
their temperature to just above absolute zero. Hello,
Bill. Welcome to the podcast.
Glad to be here.
So how did you first become interested in
quantum physics? Well, I guess that,
as an undergraduate,
I was invited by,
one of the, the professors at the college
I went to, a small college.
There was only four physics majors who graduated
with me. But I was invited to participate
in research that he was doing
on what we called electron spin resonance. In
other words, what we were doing was
using
the,
flipping of of,
unpaired spins in a in this case, in
a solid,
sample as a way of investigating
the the structure
behavior
of,
of a particular,
compound.
And this is a fundamentally quantum mechanical thing.
The the direction in which the spins can
point is a quantized,
quantity.
It, unlike
a spinning top, which in principle, you could
have the axis pointing any direction you want.
Spins, electron spins, or any other,
spins, proton spins can only have certain
directions in which they can spin. A spin,
what we call spin one half. That is
something that has the the smallest amount of
angular momentum allowed,
by by quantum mechanics. And that kind of
a spin only has two different,
possible orientations.
Itself, a pretty weird thing. Why why is
this? It's it's one of the things that
I found
really fascinating,
at the beginning of my scientific career, the
this what we called space quantization, the fact
that, that you could only have two possible
directions.
So I was beginning
to to be part of the quantum adventure
as as an undergraduate. And in a certain
sense,
everything is quantum mechanical. I mean, we wouldn't
be here.
We wouldn't be able to have this conversation.
There wouldn't even be rocks around if it
wasn't for quantum mechanics.
Matter as we know it could not exist
without quantum mechanics. So to to say, you
know, how did I get started in quantum
mechanics? Well,
that's all there is. But
interestingly then,
I I did a,
a semester study at, Argonne National Laboratory,
outside of Chicago and working on the same
kind of thing,
with,
scientists who became my mentors,
two,
physicists from Argentina
who became my my mentors for that that
semester,
also working on on electron spin resonance
essentially full time. You know, this is my
first introduction
to people who are doing research,
full time. And then I was invited by
the the man who became my thesis adviser
at MIT, Dan Kleppner,
an amazing physicist.
He taught me so much, really taught me
how to think like a physicist.
And,
again, doing resonance,
in different ways in different,
systems, but using this quantum mechanical,
idea of,
of quantum mechanical spins in a different way.
And it was in his laboratory that I
first encountered
tunable lasers,
another wonderful tool
for
using the the quantum properties
of matter
to to explore what's what's going on at
the atomic level.
That was the thing that that led me
to laser cooling.
Now you mentioned that the idea that spin
is quantized is not like a clock, that
it can only have, you know, spin up,
spin down.
You mentioned that being sort of counterintuitive.
A lot of people come across quantum mechanics
and think, oh, this is this is so
weird. This is so strange. Did you did
you find it strange? Well, probably not at
the beginning.
That is the my my first introductions
to quantum mechanics I mean, certainly the the
the spin quantization is strange, but probably the
things that are most weird about quantum mechanics,
superposition and entanglement
were not the first things that I learned
about, quantum mechanics. Now the fact of the
matter is I'm the kind of person
that is just enthralled by everything.
My my colleagues,
sometimes joke that I'm I'm I'm just
so entranced by everything that happens in the
natural world. You know, drop something. Oh, wow.
It's accelerating.
Yeah.
So so I guess that that when I
first was introduced to quantum mechanics as a
boy reading,
books about about quantum mechanics. I was I
was enthralled with with the ideas, but I
don't think it was until
it wasn't until I was in graduate school
that I understood
the deeply weird nature
of,
of quantum mechanics. And I don't think even
in graduate school did I understand
how strange,
entanglement is. Only when I started to study,
what John Bell
had done. Okay. Maybe I should back up
a little bit. Nineteen thirty five, Einstein with
two of his friends, Podolsky and Rosen, write
a paper in which they claim
quantum mechanics just can't be the whole story.
Because if it were,
then things would be just so weird it
couldn't possibly you couldn't possibly believe that the
world was this weird. And what they were
talking about was
they they said, here's here's a a system
in which two
quantum objects are are entangled. What do we
mean by entangled?
Okay. When you measure something quantum mechanic, let's
say it's a spin.
It could be either up or down. But
the wonderful and amazing thing about quantum mechanics
is it can be in a superposition state
of being up and down. Now how I
describe that
is very difficult to do because there's no
classical analog. Sometimes I say it's both up
and down at the same time.
I have colleagues who say the right way
to describe it is to say it's neither
up nor down.
But but in any case, somehow or other,
it's not up or down.
It has the potential
for being either when I measure it. And
when I measure it, it will be one
or the other. And there's no way that
I could predict beforehand
which one it'll be if I initially prepare
it in this state we call a superposition,
which is hard to describe because it's quantum
mechanical, and we don't have an analog
in our
ordinary
everyday classical world.
But the but the cool thing about entanglement
is that you can have two particles that
are in a state
such that when measured,
if you measure one of the particles to
be up, the other particle will necessarily be
down.
But before you do it, you have no
idea whether that first particle is gonna be
up or down. But once you've measured that
one, you know what the other one's gonna
be even though beforehand,
you'd have no way of predicting what that
other one would be and even though there's
no connection
between those two particles, they could be so
far apart that no signal traveling at the
speed of light could get from one particle
to the other to tell that second particle,
you'd better be down now,
because the other the first one was measured
to be up.
So
Einstein and Podolsky and Rosen had a not
exactly that situation,
but a similar situation
and showed,
in in their paper that this is just
too weird. How could nature be like this?
Bohr wrote a response
that everybody agrees was totally inadequate,
saying no. This is just the way things
are.
And what Bell did was he said, look.
Let's make some very simple assumptions. Let's make
the assumption that the property of an object
exists before you measure it.
Seems like a reasonable assumption.
You know, permanence of objects that we learn
as babies, you know, that, that things exist
even though we're not looking at them.
And then let's also assume that I only
need to understand
what happens in the immediate vicinity of something
in order to know everything I need to
know about that. In other words, if I'm
gonna be making some measurements in the next
microsecond,
I don't need to know what's going on
more than a microsecond away traveling at the,
the speed of light. Those two things sound
like perfectly reasonable ideas.
And what Bell showed was that if you
made those assumptions,
you would
predict results that were in contradiction to quantum
mechanics.
So now it was basically what Einstein had
already done, but in a very clear way
that could capture the attention of every physicist.
The trouble with Einstein's nineteen thirty five paper,
what we call the EPR paper for Einstein
Podolsky Rosen, was that most people thought, this
is some technical thing. It's it's a very
subtle point. We don't need to worry about
this. Well, Bell said, look,
if you believe in quantum mechanics, then that
means you can't believe in these completely reasonable
ideas.
And I think that captured people's attention in
a way that that Einstein's paper had not
because people said we don't have to worry
about this. This is a technicality. It's not
gonna affect us. But the amazing thing was
that nobody had yet done an experiment,
to show
whether quantum mechanics was right in this situation
or whether what we call local realism was
right. That what I've described you is basically
a way of of of describing what we
call local realism.
There's a perfectly reasonable idea that quantum mechanics
says isn't true.
And now we have the experiments
done by first John Clauser and then by
Alan Aspey and then all sorts of extra
things by,
Anton Zeilinger.
And I mentioned those three because the three
of them got the Nobel Prize in, what,
2022,
for their experiments basically proved that Einstein was
wrong. Nature, yes, is indeed that weird.
I didn't understand that even as a graduate
student. How how
deliciously weird nature is
because of quantum mechanics.
Would you say that entangled then was the
most challenging concept in quantum
mechanics you've come across in your work?
Most challenging? Let's say the most deliciously weird,
because it's it's easy enough to describe.
I can write down an entangled state. It's
pretty easy. But to understand
how weird that is, that's the thing that's,
that's really, I think, so impressive
about quantum mechanics is that it's so deliciously
weird. So it's not like it's that hard
to it's not that hard to understand in
a formal sense,
but it's hard to get your mind wrapped
around it because it is so weird and
so
distinct from the kinds of things that we,
experience on a day to day basis. And
the the thing that it violates, local realism,
is so reasonable.
You know, Einstein said, are you telling me
the moon doesn't exist when I when I
don't look at it? You know, it
What quantum principle would you say has had
the biggest impact on your own turning from
your your university experiences onto the work you've
done and the work you continue to do
now? For me, look, the things that I've
done with laser cooling are pretty garden variety
compared to,
entanglement and,
and and superposition.
So
laser cooling, I would say it's just, you
know, the quantization of energy levels. That's the
thing that that has driven,
the idea of laser cooling,
but it has enabled
laser cooling has enabled
the creation of,
atomic clocks of incredible precision. And these atomic
clocks
fundamentally use superposition. It's the bread and butter
of atomic clocks.
The process known as, Ramsey spectroscopy
uses entanglements. It's one of the early
practical,
applications of entanglement.
So you take, an atomic system that has
two energy levels.
And the Ramsey method, what it does is
it you put in a pulse of, say,
radio frequency or microwaves or laser light,
in more
the more modern way of doing it and
put the atom into a superposition of these
two states.
What that means is that the the atomic
system will evolve in a certain sense. That
is something is pulsating
or rotating in the atomic system if you
like,
at a frequency that is equal to the
frequency difference between those two states. And then
after a certain length of time, the longer
the better, you give it another pulse. And
depending upon the phase of of that pulse,
be it a, you know, radio frequency or
light or whatever, it's some oscillating thing. The
phase of that thing compared to the phase
of the atomic system, you'll either promote,
the atom
to the state different from the one you
started in or put it back in the
state that you started with. And the longer
you wait, the more
precisely
you'll know
how much the phase was different. So this
is the principle behind
Ramsey's
method for, making an atomic
clock, and that's the way almost all atomic
clocks work today.
And my work
and the work of my my group and
other groups around the world who have contributed
to this whole laser cooling enterprise
has made atomic clocks
of just incredible precision
possible.
And so I feel a a real kinship
to the idea of of superposition
because the major application of laser cooling
in which I've been so so privileged to
to to be a part of is, is
using this,
superposition principle. So let me tell you a
story. Mhmm. Sure. When I first
came to NIST, where I am now, the
the National Institute of Standards and Technology, at
that time, it was called the National Bureau
of Standards.
And, when I first came to to to
our metrology institute in, 1978,
The very best clock in the world was
in our laboratories in Boulder, Colorado. Now Boulder,
Colorado is about a kilometer and a half
above sea level, and here I am in
Washington close to sea level.
Because of Einstein's
theory of general relativity,
he showed that clocks would run slower
if they're deeper in a gravitational potential.
The effect is not very big.
At a kilometer and a half, it would
be about
one and a half parts in 10 to
the 13.
The very best clock operating in Boulder at
an at a an altitude of one and
a half kilometers
was good to a part in 10 to
the 13.
So what that means is if you had
two such clocks that were at the very
best level, which we didn't have two such
clocks, only had one. But if you had
two such clocks, one at sea level and
one at,
the height of our Boulder labs,
you would just barely not be able to
resolve the difference.
Today, we can resolve a difference of less
than a millimeter
with the clocks that exist
today
due in part to laser cooling and in
part due to any number of other developments
that have happened along the way. I just
find that so amazing. Turning now to the
present, actually. What are you working on now?
Are you still involved in atomic clocks or
in laser cooling or Well, not so much
involved in atomic clocks. I I think of
of our laboratory
as having
been a generator
of ideas and techniques that could be used
by people who make atomic clocks
and that we're continuing
to do that that that kind of thing
to doing some of the fundamental work that
enables other people to do some amazing things
that that, that they're doing. So one of
those people is,
Junyi at in our laboratories out in Boulder
making these clocks that are good to better
than a part in 10 to the 18.
Okay? But the same kind of work is
going on at the National Physical Laboratory. So
there's atomic clock groups there where they're doing
microwave clocks and optical clocks
and,
just amazing, amazing things. Again, using laser cooling
techniques.
But but we're doing all kinds of,
adventures using ultra cold atoms. So ultra cold
atoms is the main theme of what our
research group is doing,
which came from laser cooling, but other tricks
as well.
So after you laser cool it, the coldest
temperatures we get using laser cooling is a
mere
one millionth of a degree above absolute zero.
So I know that sounds pretty cold, but
by today's standards, it's not that cold. It's
where the atomic clocks,
that define what we mean by a second
today are operating at that temperature.
But
those clocks, which are based on a microwave
transition in cesium,
They're good to about a part in 10
of the 16. I mean, you might say,
well, part in 10 of the 16.
Who needs to do better than that? And
the answer is we are never satisfied. We
always wanna do better. And so people are
making clocks at a part in 10 of
the 18, but not with cesium,
with,
atoms like strontium or ytterbium, depending on what,
lab you're in, aluminum ions.
So people all over the world are making
atomic clocks
using laser cooling techniques, using other techniques that
have developed in our laboratories
and in other laboratories
that are that are good to on the
order of a part in 10 to the
18. So in other words, two orders of
magnitude better
than the the cesium clocks, but cesium
is the definition of the second. That is
the formal internationally agreed upon definition of the
second is a certain number of oscillations
of a cesium atom.
That's the definition of the second. And that
definition
is two orders of magnitude
worse than what we can do with a
ton of clocks. So what that means is
we're gonna redefine what we mean by a
second.
And I am serving on a committee along
with people from from all over the world
to
decide
or to recommend
to the international bodies that make the final
decisions
what should be the new definition of the
second.
And
there's no question. The new definition of the
second is going to be
the oscillations
of some atom,
maybe some set of atoms at optical frequencies,
not at microwave frequencies, not at nine
gigahertz,
but at
something on the order of 10 to the
15 hertz.
Now why? I mean, when it's going so
much more rapidly,
it means that a lot of errors are
much smaller as a fraction. So that's one
of the reasons why we wanna do,
optical quarks as as we say. And, and
so instead of microwave
shining onto atoms, we're gonna be shining lasers
onto atoms. Lasers
that have a stability
that is
so amazing compared to what was possible when
I first got into the laser business.
These lasers are are stable to
a tiny fraction
of a cycle per second when the light
is oscillating
at 10 to the 15 cycles per second.
It's just it's just amazing. And
so
we have
contributed
to the laser cooling part of that. We've
contributed to some of the ways in which
the atoms are trapped because you gotta hold
on to these atoms. If,
if you let go of the atoms, they'll
just fall in the gravitational field. Now we
actually use that in the cesium clock. We
toss the atoms up and they fall back
down after about a second.
But we want something
longer than that,
so we hold them in traps. And these
traps have to be very carefully arranged so
that they don't
themselves distort
the the ticking frequency of the atoms.
But we're doing other things like, quantum information.
So this is one of the big deals
in current day,
investigations having to do with quantum mechanics,
is using
single atoms or other single quantum
entities
as what we call qubits.
Quantum bits,
ordinary digital information
is stored and processed using what we call
bits. And those bits
represent a mathematical
zero or one that in binary,
mathematics is the thing that
allows you to store and and process information.
And now we're gonna make those zeros or
ones
be represented by, say, for example, a spin
that points up or down. The two spin
states that we talked about earlier
fit very nicely into the idea of binary
logic. But the beauty is or the the
amazingly
difficult thing is that they can be zero
or one, but they could be in this
superposition state, which is neither zero or one
or both zero or one depending on how
you wanna describe it, which we don't really
know how to do in classical terms.
Sounds like a disaster
because one of the great strengths
of binary
information is
that,
the things that store those those bits are
things like a transistor
that is turned on or off like a
switch,
a spot on a, a DVD that's burned
or not burned,
a patch on a magnetic
disc
that is magnetized in one direction
or another. These are the things that store
classical information, and they're so good
that you can be really, really confident
that this thing is one thing or the
other. No
uncertainty. The the the uncertainties
the the the errors where you might get
something that was, you know, only half burned
or or some so you wouldn't be able
to tell what it was. That's so tiny.
It's just it's just ridiculous.
And that's a wonderful strength
of binary
information.
And now I'm telling you, we wanna make
it so so nobody could know
what the state of this thing is before
you measure. That sounds like a disaster,
but it's not.
If you do the right kinds of things
for certain kinds of problems,
the ability
to put the bits, now quantum bits, into
superpositions
means that you can do the problem
in a lot fewer operations
than would be
needed to do that same problem if you
used a classical computer with ordinary bits.
So one of the first examples of that
was,
done by a guy named Peter Shor,
nineteen ninety five. He came up with an
algorithm
that
showed it was possible to factor
numbers
using a quantum mechanical
approach
in a time that would be much shorter
or at least the number of operations
that would be much smaller than the number
of operations needed
by an ordinary classical computer.
Factoring is what is technically known as a
hard problem.
What that means is that the number of
operations required to do that problem, to solve
that problem grows exponentially
with the size of the number. So if
I want to
factor a hundred digit number,
that the time it takes to do that
grows exponentially with how long how many digits
that number has.
But if you do it quantum mechanically, it
doesn't grow exponentially, so it becomes an easy
problem. This is absolutely amazing
that changing the hardware on which you do
the calculation
has changed what we call the complexity class
of a problem. Now I have to be
careful because nobody's ever proved
that
a classical
calculation of,
factoring
is exponentially hard.
It might be that some clever mathematician will
find an algorithm
that makes it easy.
Nobody thinks that's gonna happen. Everybody thinks that
this problem is gonna remain
exponentially hard, and that's one of the reasons
why
certain kinds of encryption
are
used with a lot of confidence. So let's
back up. Why was it so important that
Shor came up with this algorithm?
And the reason is that something called public
key encryption,
which you may not know what that is,
but you use it every day.
Public key encryption
depends upon
the fact that it's hard to factor numbers
but easy to multiply them. So this process,
multiplication, is the inverse of factoring.
And the fact that it's asymmetric
leads to the possibility of what we call
public key encryption.
So let's say you buy something online. Now
your credit card has to go to the
company that's selling it to you.
So your computer actually, that company sends your
computer
a number,
a a big integer,
and they know the factors
and nobody else does because they don't know
how to factor big numbers because it would
take too long. It would take years or
centuries to factor this big number, so nobody
knows the factor. But they know the factors
because they took two smaller numbers and multiplied
them together, and that was easy.
That number is then used to encrypt your
credit card number.
And the encryption is easy for the person
who knows what the factors are and essentially
impossible for the person who doesn't. So that
means somebody trying to intercept
the,
transmission between your computer and the the computer
at the ecommerce company
can't get any useful information.
Now what if that evil doer
had a quantum computer?
Then that evil doer could factor
the the number and figure out what your
credit card number is and then steal it
and use it to buy, you know, TVs
or whatever the, you know, evil doers buy.
Now the fact of the matter is that
probably
you don't have enough money in your bank
account to make it worthwhile
for some criminal who made a quantum computer.
We don't have quantum computers that can do
this yet. Okay? It's not easy to make
a quantum computer,
even one that can do very simple problems,
let alone one that can factor big numbers.
But if somebody did that,
then they could decrypt
secret messages
that really do matter.
So, for example,
the diplomatic
communications
between,
say, diplomatic offices of some country
often contain
secrets
that people don't want revealed for decades, that
it would be it it would it would
matter
to the security
of
of of a country if if these diplomatic
secrets were revealed even decades from now. Military
secrets,
intelligence secrets, you know, who
is the spy and who have they
contacted?
These these could be devastatingly
dangerous things if they were were revealed
even decades from the time that they were
first happened. And so people worry
that sometime in the future, you're gonna have
a quantum computer
that could
reveal secrets because people could wiretap
communications, but it wouldn't do them any good
because it's encrypted. Okay? But if you had
a quantum computer, you could decrypt things.
So
what are you gonna do? Well, quantum mechanics
comes to the rescue.
You can have quantum communications
that cannot be eavesdropped upon because of something
called
the no cloning theorem.
An eavesdropper
could not intercept the message,
make a duplicate of the message, send it
on so that the person
receiving the message doesn't realize that the message
has been,
intercepted and then use that intercepted message. That's
not allowed
by the laws of physics. So we're very
confident
that these quantum,
communication methods
would work, and people are actually using them.
There are places where, I'm not sure they
need to, but they can say, look, our
bank communicates with other banks using quantum,
cryptography. So we know that our our transactions
are, are secure.
So that's one way. Another way is a
classical approach where you come up with new
encryption algorithms
called post quantum
encryption that are not dependent upon
this asymmetry of factoring and,
and and multiplying.
And then these will not be able to
be attacked by a quantum computer
as far as we know.
No one has proved
that these new algorithms
can't be
attacked by some quantum computer. It's just that
nobody has figured out how.
And very clever people have been working on
it to try to figure out ways of
doing this and have failed. And so we
think these things are pretty secure.
But with so many like, with so many
things, we're not sure.
So this is places where quantum mechanics puts
us in danger and quantum mechanics comes to
the rescue. And so in my lab, we're
not making quantum computers, but we're doing some
of the things
that will
enable
new kinds of quantum computers to be,
to be to be made. Mhmm.
So let's talk about quantum computers. You need
you mentioned that you can make a quantum
computer or a quantum processor with cold atoms.
You can make it there's other types of
qubits out there, trapped ions, super connecting circuits,
that sort of thing.
Do you have a view about which of
these platforms is likely to succeed or which
Yeah. Well, this this this has been a
an ongoing question. What's the what's the winning
platform?
And my attitude
from the beginning and still today is
that it's too early
to settle on one particular platform.
And it may very well be that the
final quantum computer
that does things like factor numbers, it's not
even clear that that's the most important
problem that you want to do,
is likely to be a hybrid
where
computations
are done on one platform
and storage
is done on another platform. So superconducting
quantum computers are really nice and fast,
but they can't store information for very long,
whereas atoms and ions can store information for
a really long time. They're very
robust, very
isolated from the environment,
but they're kinda slow
in the in the computation process.
So it might very well be that you
wanna use the best features
of different platforms in different in in in
different parts of your of your quantum computer.
But, you know, what do I know? It's,
I think we're a long way,
and this is not a majority view or
at least is not a universal view. We're
a long way from having quantum computers
that can do interesting problems
that,
a lot faster than what,
classical computers
can do. You've probably heard
that somebody used a quantum computer
to do
a a problem that would take a classical
computer,
you know, a septillion
years to do. Maybe so,
but
it's a little misleading.
They chose
a problem.
It was easy for a quantum computer to
do and hard for a classical computer to
do, and it's a problem nobody cares about.
Nobody's gonna make any money solving that problem.
People definitely gonna make money factoring numbers. People
probably are gonna make money
by,
doing quantum chemistry,
learning how some
molecule that either you haven't made yet or
you haven't tested yet, how it would behave.
That's the sort of thing that could very
well lead to
to things that people care about that that,
are gonna make a difference in our lives.
That hasn't happened yet,
but we may not be that far from
it.
Just to give you an idea of how
far away
we are from this,
I
proposed,
a bet with a colleague of mine.
His name is Carl Williams. He's well known
in the quantum
business.
And the bet is,
at a certain future date, will we or
will we not have a quantum computer that
can factor numbers
that a
classical computer of that future time
cannot
factor?
Okay?
And
he proposed the the date of 2045,
which is
only about twenty years from now. Right? Why
2045? It's about fifty years after
Shor and after the first demonstration
of a quantum gate by Dave Wineland and
his group
at our NIST laboratories in Boulder.
And and it's a well defined problem and
not an easy one.
And,
I'm taking the point of view that, no,
we will not have a computer that can
do that, And he's taking the point of
view that, yes, we will. And we'll put
up some money and it'll go into some
fun for a scholarship or a prize depending
upon who wins. And I expect to be
dead by then. But, what I hope
is that this bet will encourage people
to work hard on solving the problems that
need to be solved in order to,
to make this work like error correction. We
haven't talked about error correction,
but that's the big thing because
these quantum processes are prone to errors
much more so than the classical computers.
So you've got to be doing something to
fix that, and that's called quantum error correction.
And people are starting to do that, and
we need more of that. And so that's
one of the reasons for having a bet
like this. When I talk to people,
other scientists working in the field, and ask
them which side of the bet they would
like to be on, it's about fifty fifty.
So I think this is a great bet
that that
we could expect that in about twenty years,
we're gonna have
or, you know, it'll be of that order
of time
where we'll have
really competent quantum computers that can do that
kind of thing. Now as I said, I
don't think factoring is the most important thing.
What I want is a quantum computer that
can do quantum mechanics.
This,
this exponential growth that I described for factoring
is also true of many problems in quantum
mechanics.
Let's say that you've got
a model
that is trying to describe magnetism. Magnetism is
one of our favorite subjects
in, in quantum mechanics. So let's imagine it's
a chain, a one dimensional chain of spin.
So on each link of the chain, there's
a spin, a quantum mechanical spin. And magnetism
is all about how do these spins interact
to produce
certain kinds of states, like all the spins
pointing in the same direction. That's what we
call ferromagnetism.
Every other spin pointing in opposite directions. That's
what we call antiferromagnetism.
And
quantum magnetism
is, at least in some circumstances,
a hard problem
because
the number of possible states of all these
spins pointing up and down
is
let's say that the spin can only point
up or down and you've got n spins.
There's two to the power n number of
ways you could do this on a chain.
And if you add one more spin to
the chain, you've doubled
the number. That's what we mean by exponential
growth.
So that means that any reasonable size chain
is impossible
for a classical computer to do.
Once you've got a few tens
of spins,
a classical computer cannot do a brute force
calculation, but a quantum computer could. And so
having
a quantum computer that can,
reliably calculate
tens of
of of qubits
that represent the spins, this will be a
big deal.
Now you've probably heard that these quantum computers
already have that many qubits.
Yeah. But
they're not what we call logical qubits.
You see, the thing is that because of
these errors, you're not gonna get a reliable
answer from the kinds of of quantum computers
we have today. You have to do error
correction. The way you do error correction is
you
assemble a number of physical qubits into what
we call a logical qubit. And then by
making measurements on the logical qubit
that are
not determining what the state of the logical
qubit is, but determining
what the relationship is between some of the
qubits
without learning what the state because you can't
learn what the state is because that would
destroy the superposition state. But you can determine
whether an error has happened and and fix
that error. That's what quantum what quantum error
correction is about, and people are just starting
to do this. And this is what one
of the things that's just so exciting right
now is that people are making
computers
that have logical qubits and doing error correction.
It's not good enough yet, but it's getting
there.
So would you say that's one development that
our listeners, this podcast, should look out for
in this field? You know? Who are the
people to watch in this area? What are
the things we should be seeing happen in
the between now and 2045, let's say Yeah.
I think when the bet's gonna happen? Right.
So I think the thing you should be
looking for is things like how many logical
qubits
you've got
that can be entangled with each other. And
this is one of the things that some
recent news,
has shown that this number is starting to
become larger than it was in the past.
And how long
or for how many operations
can these logical qubits
maintain
their
coherence? That is their
their quantum ness,
without,
having some sort of an error with and
that those errors occur because they're connected weekly,
we hope, to the environment. The environment comes
in and and and messes things up, and
that's what we have to correct by quantum
error correction. First, we have to prevent it.
That's one of the things that using cold
atoms allows you to do is to prevent
the connection to the environment, but it's never
perfect,
and
correct it when when the environment does mess
you up. And it's that correction.
How long
is the coherence of this,
of this state lasting?
What I often say the thing we need
is an immortal qubit,
something that is not killed by the environment
that lasts
throughout the whole
calculation
and and for a calculation that's long enough
to do interesting things. So those are the
the things you wanna be,
looking for is how many operations
can you do and still maintain
the the quantumness of of that qubit, and
how many,
logical qubits can you have. Those are are
some of the key things
that are gonna determine whether you really have
a,
a competent quantum computer.
And what about in the other areas we
talked about in quantum information and, optical clocks?
Yeah. So so optical clocks
are already wonderful
and getting better all the time. One of
the things that recently came up was a
nuclear clock.
So now, I mean, it's optical if you
call,
sort of far
ultraviolet
optics. Okay?
I mean, it's not so
far. Okay. So let me back up and
say why nuclear clocks are so wonderful.
There's one nucleus, thorium,
that I forget which isotope it is, that
has
an isomer
state,
an excited state of the nucleus
that is only a little bit above the
ground state.
It's low enough in energy. I think it's
about eight electron volts, whereas, like, optical things
like two electron volts. So it's not not
not not so far above, which means that
we can use well known techniques to produce
what is effectively laser light to shine on
the nucleus and have it make a transition.
Most nuclear states
are kilovolts or more above the ground state,
and we just don't have any way of
making
laser
like light at those energies. But, you know,
eight electron volts, yeah, we can do it
and have done it. That's the thing.
So in the just in the past year,
starting
at PTB, they they used a broadband laser
to excite,
the state for the first time. Now starting
to really pin down
what the energy was because people didn't even
know what the energy was. This is amazing
thing. So a guy at PTB,
I don't know, two decades ago said this
looks like a really great idea.
They didn't even know what the energy was
to within a factor of two.
And now they know what it is to
to some really fine,
level. I forget what, but, you know, many,
many digits.
So so this has been a huge
improvement over the years. And just this year,
laser excitation at PTB and then a couple
of other groups,
one of them being,
our NIST laboratories in Boulder, have done it
with with really narrow band lasers.
So this is the beginning of the possibility
of a nuclear clock. It's still years,
in the future, but the beauty is really
high frequency. So that means, again,
various perturbations are gonna be a smaller
fraction,
but a nucleus. The nucleus is much less
susceptible
to outside
influences
than the atom
where its outer electrons are the ones that
are doing the work. And those outer electrons
are basically exposed
to all kinds of bad things that might
happen. Whereas the nucleus is protected. It's sitting
sitting in deep inside the atom with lots
of electrons around it sort of protecting it.
And,
and the hope is that you'd be able
to make a clock that is much less
susceptible
to the kinds of things that that mess
up our our atomic clock, but probably not
for some decades.
Is it gonna be good enough that it's
really gonna compete with the part in 10
of the 18 atoms that are going on?
So one of the things you're gonna look
for is is the international community gonna
come to a decision
about what is gonna be the new,
the new definition of the second? What atom
is it gonna be? We haven't even decided
whether it's gonna be a single atom or
whether it's gonna be a bunch of atoms.
I'm very much in favor of the single
atom approach
because Simpler. Simpler. Everybody knows what you mean
when you say this is what a second
is. So,
but we'll see. You know, there's there's,
arguments to be made,
on, on both sides of that. And,
so that's another thing to watch, and those
things are getting better all the time. So
here's one of the the really wonderful things
that's being done with these
incredibly accurate clocks.
One of the the really fundamental questions we
might ask ourselves
about the way
nature works is, are the fundamental constants of
nature,
in fact,
constant?
Now the things for which this is important
are not things like, say, the charge of
the electron or Planck's constant,
because
the value of these constants depends upon
what we choose for our,
unit system of units.
In fact, right now,
the charge in the electron
and Planck's constant
cannot change because it would be illegal.
Those
constants have been set by international agreement.
The things that matter are things like the
fine structure constant. What is that?
It's a combination of constants like the charge
electron Planck's constant
and the, what's called the, electric,
permittivity
of of the vacuum,
and the speed of light all combined together
to create a constant that is dimensionless.
It is a dimensionless
way of describing
how strong
electric interactions or electromagnetic interactions are. And we
have similar
dimensionless constants that describe how strong nuclear,
interactions are, strong force, weak force. Well, anyway,
the the the fine structure constant
describes
how,
how strong electrical interactions are, and that means
it's basically
the thing that sets the scale
for almost everything that's part of our daily
lives. Chemistry, which is,
you know, basically what makes our bodies work,
is defined with the kinds of things that
are possible defined by the fine structure constant.
Some people pointed out that if fine structure
constant were different by 10%,
we couldn't exist.
Our body chemistry wouldn't work in the way
that it does. So, you know, it's kind
of a either a happy accident or, you
know, some divine
intervention that makes our,
our body chemistry work. Or we won the
cosmic lottery and there's 10 of the 500
other universes where the flying structure comes in
something different and there's nobody interesting there.
But
one of the questions we could ask ourselves
is,
does it change with time?
Well, now
because of these atomic clocks, we could have
two atomic clocks that operate on different atoms
or maybe on different transitions even within the
same atom
that depend upon the fine structure constant in
different ways. So depending on what the transition
is, it could depend very strongly on the
fine structure constant or very weakly on the
fine structure constant. So if you've got two
such clocks
and you
just let them, you know, compare their frequencies
year after year,
you can determine whether from one year to
the next the fine structure constant is changing.
And this has allowed us to say
that we know the fine structure constant is
not changing by about a part in 10
to the eighteenth per year.
And as the clocks get better, we'll be
able to put a finer,
determination on that, or you might find that
indeed it does change. That would change everything.
We would have to change our our view
of of the way the world works. Another
thing which is really exciting is
Einstein's
theory of general relativity tells us that if
we have two clocks
and
that are of different sorts
and we move them together in a gravitational
field. And these are clocks that don't depend
on gravity, so a pendulum clock wouldn't be
the right the right thing, but an atomic
clock would be. If we move them into
different,
places in a gravitational potential,
those two clocks better stay in the same
ratio.
If they don't, it means that the most
fundamental principle of general relativity is wrong, what
we call the equivalence principle.
So putting two such clocks,
maybe one in a satellite, one on the
earth, maybe two of them together in a
satellite with an eccentric orbit, this would really
be be exciting. I mean, we've done this
to some extent, but but with these new
clocks,
we could do this in in a way
that has been unparalleled in the past. And
so we could
test
this feature of general relativity,
at a level that's never been possible before.
This is really important because we don't know
how to have a unified theory of quantum
mechanics in general relativity.
Well, if we could find out
that
the equivalence principle fails
in this particular way, it might give us
some insight
into how we can we can solve this
great unsolved problem, this great challenge of our
current time of how do we reconcile quantum
mechanics with, with gravity.
There's a lot of big things there.
Taking it back to your own career just
for a moment, reflecting on your career so
far,
what are you most proud of?
Well,
back in around 1988,
we accidentally
discovered
that
the
temperature
to which you could laser cool atoms
was lower
than what
everybody said was possible based on the theory
of laser cooling
at that time.
And it was an accidental discovery. I mean,
we were not looking for this. We were
just fooling around in the lab
trying to to see whether laser cooling was
working the way it was supposed to, and
the first indications was it looks every like,
everything's great. And then we started to see
some problems,
like, everything wasn't great. And we started to
pound on that and eventually learned that the
temperature
was was too low. And that was the
thing that made people pay attention. There's other
little things, things weren't working out. You know,
there's some detail. We'll figure it out. But
when the temperature started to be lower, you
see I mean, the whole idea of laser
cooling is to make the temperature as low
as you can. And we had apparently made
it lower than you can.
So that got people excited. And then people
came up with explanations. Our our our friends
in Paris at the Ecole Normale came up
with, with explanations for what was what was
going on. Steve Chu, who was at that
point at Stanford,
was also working on on
understanding the the the theory behind it, and
that really changed things in an important way.
It made possible
laser cooled atomic clocks. So all the laser
cooled atomic clocks that use cesium today
use,
that feature that the temperature is lower than
what the original theory of laser cooling said
it was.
You know, even these optical clocks can be,
made to work pretty well with that, but
there's other techniques. And it made those other
techniques like, I haven't even talked about Bose
Einstein condensation.
An amazing process that happens
because of a purely quantum mechanical feature
that makes atoms of the same kind
fundamentally indistinguishable. And this is a feature that
that quantum mechanics gave us, this idea of
indistinguishability,
which was not something
that that was understood before
before quantum mechanics. In fact, in 2024,
a hundred years ago, Satyendra Bose
came up with the idea
that photons were indistinguishable
and therefore the statistical
mechanics of photons
would be different
from the usual statistical mechanics that Boltzmann or
Maxwell
and and and such people came up with
because of the fact that the particles are
indistinguishable.
And the way you count the number of
possible states is different
because,
if you interchange the particles, that's not a
new state
because,
because of the indistinguishability.
And
that indistinguishability
has led to this new
what some people call a new state of
matter called a Bose Einstein condensate where all
of the atoms almost all the atoms are
in the same quantum state. Well, that was
facilitated
by this discovery
that the, that the temperature could be so
much lower because it meant in order to
get this state of Bose Einstein condensation, you've
got to
cool the atoms to a very low temperature.
You've got to have a high density of
atoms, and
you start off in a better place
if if the atoms are colder. And and
our accidental
discovery
allowed that that to happen.
So that's probably the thing that I'm most
proud of. But we also accidentally discovered what
are called optical losses.
Now I shouldn't say that we accidentally discovered
it because people already had the idea.
In fact, one of the earliest
ideas of laser trapped atoms was was in
fact 1968.
So this is
laser cooling. The idea wasn't even came out
until '75, and it wasn't even demonstrated until
'78, '10 years later. But in 1968,
a Russian,
physicist named,
Vladland Litokov came up with the idea of
trapping atoms in a standing wave of light.
And in that way,
eliminating the Doppler shift because the atoms would
be trapped in over such a small distance
that effectively the thing called, there's a thing
called Dicke narrowing that,
that that gets rid of the Doppler shift.
And so this would be great.
Ten years before there was even laser cooling
that would allow you to put the atoms
in such a thing, that just wasn't possible,
but he had the idea. Well, that's exactly
what Junyi does in his lab. He puts
the atoms in an optical lattice and holds
them in place and then uses that as
an atomic clock. Well, okay. So everybody knew
this was a was was a thing, but
we
saw that process
of the atoms being trapped in the optical
lattice
in our laser cooled samples when we weren't
looking for it.
We were trying to measure the temperature
of the atoms
in the,
the laser cooling configuration
Instead of turning lasers off and just letting
the atoms expand
and measuring the temperature from the distribution of
velocities, instead, we wanted to measure what the
temperature was while they were being laser cooled.
And the idea we came up with was,
let's look at the Doppler shift
induced by the scattered light. So light comes
in, and if it bounces off an atom
that's moving, there'll be a Doppler shift, and
we can measure that Doppler shift and see
what the distribution of velocities was. So we
did that, and the distribution of velocities just
floored us. It was so odd.
Instead of being a nice smooth broad distribution
showing the distribution
of of velocities,
it was that with a big sharp peak
right in the middle.
So what is this peak?
And,
and and and we fought for a very
short period of time. Did we accidentally make
a Bose condensate?
Because this is before peep anybody made a
Bose condensate, but people were talking about it.
Then we realized, no. No. That's not what
we did at all. What we're doing is
we're trapping the atoms so the Doppler shift
goes away. So this thing that we were
trying to learn about, the Doppler shift, was
partly being canceled because the atoms were being
held in what we now call an optical
lattice.
And then
people started to do it everywhere, and,
and now it's led to these wonderful clocks
that have the atoms in the optical lattice.
So that was another case.
It wasn't nearly as
astounding
as the sub Doppler laser cooling because it
was expected. It's just that we weren't looking
for it at the time that we saw
it, but that
theme of learning about things accidentally
has seems like it's been a recurring theme
in our laboratory.
And I think it's an important thing for
people
to understand about the way that science is
done. Often,
science is done not because people are looking
for a particular goal
and working towards that, but it happens because
they're fooling around
and see something
that wasn't expected.
And we have to
have
a lot of
that kind of activity.
If all of our science activity is directed
toward
specific goals, we're gonna miss
a lot of really important stuff that allows
us to get to those goals.
And this idea of what is sometimes called
curiosity driven research
just fooling around,
is so important to the development of science.
And and without it, we're just not gonna
get to where we need to go.
So final question, maybe the most nebulous one
of all. What does quantum mean to you?
Yeah.
So interestingly,
I was on a panel a couple of
years ago
that was run out of Oxford,
in which they had gathered
a handful of Nobel laureates together
in physics
to answer the question, what was the most
important discovery
of twentieth century physics?
And
most of us said quantum mechanics.
And
so when it was my turn, I said
quantum mechanics.
You know, what is quantum mechanics?
Just the I said, it's it's
the fact that particles behave like waves,
which we haven't even talked about here,
and waves behave like particles. This wave particle
duality is at the heart of quantum mechanics.
And that understanding,
which a lot of people would say was
sort of the ordinary part of quantum mechanics,
that understanding
led to a technological revolution
that completely
changed our daily lives.
We all walk around with
mobile phones
that wouldn't
exist were it not for quantum mechanics. Semiconductor
electronics depends upon the quantum mechanical nature of
electrons. And I'm just looking around at the
kind of,
electronic devices that you're using for for for
this, and and
it's it's all quantum mechanics. So for me,
quantum mechanics is this idea
that waves are particles and particles are waves
and that this has led to a technology
that has changed our daily lives in a
fundamental way.
Steve Weinberg
was also on this panel.
And when it came time for him to
say
to answer this question, what is quantum mechanics?
He says, well, quantum mechanics is the departure
from
classical mechanics where we thought of particles
as having a position and a momentum, and
we described
the state of something
by saying what the position and momentum was
of all the particles in the system. In
quantum mechanics,
it's a vector in Hilbert space.
And I thought this is the difference between
an experimentalist and a theorist.
And, of course, Hilbert space behaves differently from
from
regular space, and so everything behaves differently. But
but the perspective you see,
both of which are, of course, are true
and both of which I think embody
the
the the wonder of of quantum mechanics.
But but we had very different points of
view about what quantum mechanics was.
Bill Phillips, thank you so much. It's been
really wonderful to talk to you. It's been
a pleasure.
That was the Nobel laureate William Phillips in
conversation with Physics World's Margaret Harris.
I'm afraid that's all the time we have
for this week's podcast.
Thanks to Bill and Margaret for a fascinating
discussion,
and also to our producer,
Fred Isles.
And a very special thank you to Atlas
Technologies
for their generous support of this episode.
We'll be back again next week, but in
the meantime, do check out the latest episode
of the Physics World Stories podcast.
Host Andrew Glester is joined by three expert
guests
to explore the impact of artificial intelligence
on discovery,
research, and the future of physics.
That episode is called AI and the future
of physics,
and you can find it on the Physics
World website
or at your favorite podcast provider.
Atlas Technologies is happy to support this episode
and the exciting work being done in quantum
science.
Atlas helps solve engineering challenges everywhere.
Particle colliders,
space missions, quantum,
cryogenics,
and more.
Custom vacuum chambers and bimetal flanges and fittings
are built in Atlas' fully integrated facility
with on-site design,
development,
and manufacturing capabilities.
Learn more at atlasuhv.com.