hello and thank you all for coming here
my name is r D'Amico chintz key and i'm
a researcher at the Santa Fe Institute
before proceeding I would like to thank
our sponsors first and foremost we want
to thank Thornburg investment management
for generously underwriting SF ice
community lecture series without their
funding these lectures would not be
possible we also want to thank the
lensing Performing Arts Center for
allowing us to host these lectures in
this beautiful space lastly we want to
thank Enterprise Holdings foundation for
their recent renewed support in today's
lecture we will hear from dr. chris
monroe dr. Monroe is a leading atomic
physicist and quantum information
scientist she received his undergraduate
degree in physics from MIT his PhD in
physics from the University of Colorado
Boulder he then did postdoctoral work
and worked as a resident researcher at
the National Institute of Standards and
Technology and while working there in
the mid 90s he demonstrated the first
quantum logic gate an essential
component of quantum computers chris is
currently distinguished University
professor at the University of Maryland
in College Park he is a member of the
joint quantum Institute in the center
for quantum information and computer
science among various other awards chris
is an elected member of the National
Academy of Sciences beyond that today
learned that chris is also percussionist
in the Columbia Symphony Orchestra in
Columbia Maryland and he has even
manufactured some instruments for this
Orchestra such as a machine that makes
wind noises and is now in his garage
this talk is part of the SFI community
lecture series this series allows
researchers both from the Institute and
from afar to share their ideas and
cutting-edge research with a broad
public today's talk connects with
several recent community lectures which
concern fundament
questions in physics and computation
this includes past events such as an
interdisciplinary panel on the nature of
time with Sean Carroll James hurdle and
David Krakauer a lecture by dr. Sabine
Hassan Felder which considered the
relationship between aesthetics and the
search for fundamental physical laws and
not least the most recent lectures bio
suffice own Chris Moore on the limits of
computers in science and society
which was a wonderful introduction to
the ideas behind the theory of
computation and the growing impact of
computing and machine learning on
society and before proceeding I'd like
to tell you that the next lecture in
this series which will be the last one
of the season will take place on
November 13th it will be Michelle Gervin
who will talk about predicting chaos
with machine learning which is another
fascinating topic on the frontier of
computer science so today's lecture will
be about quantum computers which are
computers which operate according to the
principles of quantum physics quantum
computation is one of the most exciting
and active and may be hyped research
fields in physics and in computer
science and at the same time the
development of large-scale quantum
computers may hold the potential to
revolutionize technology science and
society apart from the applications in
my opinion it is one of the most
intellectually exciting areas in
contemporary science because
understanding the relationship between
physics and computation involves far
more than the simple application of
physical knowledge to engineering
problems rather we increasingly see
information and computation essential
principles that underlie much of the
structure of statistical physics and
quantum physics for this reason
understanding the relationship between
computation and physics is as much about
design designing the next generation of
computers as it is about understanding
the fundamental nature of reality so
with that please join me in welcoming
dr. Chris Monroe
okay thank thank you you guys thank you
so much it's lovely to be here one of my
favorite places in the world and thank
you for the kind introduction it gets us
started right off being Santa Fe I
thought I would start I have I have lots
of pretty pictures not too many
equations the topic is indeed quantum
computers so we're gonna have to delve
into what we're gonna have to delve into
quantum physics which has a sort of
mystical following and I'll tell you
right off that you're not gonna leave
today understanding any more about
quantum physics than you had before on
the other hand I hope you're gonna leave
with with the knowledge that you don't
need to and that's maybe the point here
another way to say it is that is that a
line Stein didn't believe in it and so
we're in we're in good company if we
don't believe in it either it's not
about believing it's it's a very
interesting theory but it does call into
question what what is science and you
know and what is what is religion in a
sense for quantum physics calls us to
basically define what reality is which
is very strange
and so I'm a working experimental
physicist likes to work on cars and
build things and to come up against such
very high-level philosophical thoughts
is interesting to say the least but I'm
of the opinion that I used the theory
because it works very well and maybe
you'll indulge me in this story about
how this strange theory might play a
role in our ability to compute this is a
painting one of my former students gave
to me about 15 years ago
Boris Bleen office and he's at
Washington Seattle now and at the time
so I'm an atomic physicist I work with
individual atoms and we that they're
wonderful test beds for for quantum
physics experiments including quantum
computing and at the time Boris worked
in our laboratory we were we were
playing around with individual cadmium
atoms why cadmium because they had
certain features that were amenable to
what we wanted to do and if you're in
oil Pina you know that that
the bright yellow pigment in oil paints
is cadmium it's called cadmium yellow so
he he he painted this for me and I he
didn't want to interpret it but he is as
being too cadmium atoms and they're
entangled and I'll talk a little later
about what entanglement means but that's
what this stuff is it's sort of the
fabric of space it's and I don't mean to
sound mystical but entanglement is
rather mystical you can lose your mind
thinking about it too much so that's
that's a painting of two cadmium atoms
in and at the end of the lecture I'll
talk a little bit about how individual
atoms might play a role in the next next
generation computers to do certain tasks
so following our exhaustive introduction
into the topic of computing and
information as a physicist I've found
that this topic is is is great fun
because we don't usually think about
physics and information is necessarily
mixing but in every case when you think
of information what it is how you store
information it is physical it could be a
switch that's in the on or off position
we tend to think of bits information
unit of information has two states 0 and
1 you can store you can use other bases
if you would like but 0 and 1 are
sufficient so we tend to use that it
could be an electrical current going one
way or the other way on/off anything
that can be in two states can act as a
bit and store information magnetic
storage it we're the the direction of a
small magnet whether it's pointing up or
down
well information is physical and all of
the computers that we've had since the
beginning of computation which is a long
time ago have been based on physical
principles this is a picture many of you
who've probably seen the first digital
computer in this country is called ENIAC
was developed in the mid twentieth
century it was used for the defense
industry to calculate missile
trajectories and so forth but this
machine is pretty on
wieldy it it was composed of vacuum
tubes these were the switching elements
that would that would store zeros and
ones now what's interesting is a
computer when you use computers you
don't really think of what's inside of
them and that's the beauty of
information theory it doesn't matter
what's inside as long as they obey
certain certain types of rules whether
they store electrical currents or
magnetic little magnetic domains in a
hard drive for instance or even optical
discs they store little pits that can
reflect light in two different two
different ways so there is a beautiful
picture in the in the advertisement for
this talk that of this this picture here
this is the first solid-state transistor
and it's interesting 1947 it was right
after ENIAC was unveiled it's a
radically different type of a medium and
in fact it doesn't look very doesn't
look very stable this is this dark piece
of glass it looks like glass it's
actually called germanium it's a
semiconductor it doesn't really conduct
very well unless you do something to it
and what what they're doing to this
there's a piece of glass with a little
gold film and it's pressed against the
germanium and when you when you apply a
signal on this gold the germanium
conducts current very well when you turn
that signal off it doesn't conduct so
it's a it's an electronically controlled
electronic switch it doesn't look like
the basis of all computation and in over
the last six years but it is the
solid-state transistor because it's
solid-state and not a bulky vacuum tube
it could be it could be shrunk down in
size down to really tiny dimensions and
that I wish we could talk more about the
the beautiful work that has taken this
big unwieldy mess into little tiny
transistor elements that are part of
modern computers these days so you've
probably heard of Moore's Law Gordon
Moore was the founder of Intel and and
back in the day in the 50s and 60s he
and and and others aimed
to to engineer this system so it would
work well well it worked very well they
got they were able to make them really
small and over the course of time this
Moore's law represents the growth in the
number of transistors the number of
these things on a single chip over the
course of the last few decades this is a
this is an exponential growth every
every decade we get about a factor of 10
more transistors in nowadays this is
even little outdated nowadays were at 10
billion transistors in a chip if a few
you know inch or two on a side
well it's interesting I'm talking about
Moore's law
this isn't there's really nothing to do
with quantum quantum physics you might
argue there is some quantum physics
happening right at this little interface
but it's not really needed to understand
the basic principles here but this is
going to Moore's law because it's ending
is going to indeed motivate the use of
quantum physics for a new mode of
computing well why is it ending actually
it's pretty simple the transistor is
getting so small that if I if we follow
this line for another couple of decades
each transistor will be the size of an
atom and at that point there's no more
making it smaller unless you want to
split the atom inside your computer and
I don't think that's gonna that's gonna
happen that takes a lot of energy so
Moore's law in fact this laptop I bought
actually I just bought last year but the
laptop out but before this three years
ago was about as powerful the batteries
are better the display is better but the
computing prowess of processors is
already showing signs of saturating you
probably recognize that yourself so what
are we going to do I mean Moore's law
you could argue this exponential growth
was the engine behind the information
air behind the economy in the last 50 60
70 years what are we gonna do well let's
talk about quantum mechanics because in
fact why Moore's law is ending and is in
a sense because matter is granular and
we have to confront dealing with
individual atoms as circuit elements
so I'm going to quote Richard Fineman
who has a proud history in this area of
the country of course during World War
two he's also one of the father figures
of quantum physics and he had a speech a
long time ago called there's plenty of
room at the bottom and he it's a it's a
three or four page lecture it's just
wonderful and there's gems everywhere
but my favorite paragraph is right here
and he's I think he's stimulated by
these solid-state devices that can be
made really small and he says well when
you make things really small we get down
to the size of individual atoms there's
a completely new opportunity for design
why well when you get down to single
atoms when when you get down to the very
simple granular part of matter that
system obeys a new law of physics it's
not really a new law but it's its laws
emerge that are different than then then
what we use for baseballs and and things
that we see in everyday life those are
the laws of quantum mechanics
now Fineman didn't know what these
opportunities would be but he he was a
visionary and this is so long ago it
took many decades for us to find what
those opportunities are and I think the
the verdict is in that that opportunity
is quantum computing so I want to talk
about that to do that we have to talk
about quantum physics though I'm going
to try to teach you quantum physics in
about five minutes and and you shouldn't
laugh because the the precepts of
quantum physics are very simple there's
the problem is that there's two of them
and you can't derive one from the other
now in physics we like to think of
ourselves as the king of science so we
have to have one law the very top
everything derives from that law even if
it's very hard to to actually do that to
derive how a baseball will fly through
the air based on particle physics we're
comfortable in the fact that a baseball
is made of particles we could do that if
we had to so the principle at the very
top there's the standard model or in
Newtonian physics force equals mass
times acceleration you can derive almost
everything
indeed everything in classical mechanics
using that very simple law well the
problem with quantum mechanics and this
is why physicists hate it is that there
are two laws there's like two peaks and
you can't get from one to the other
there are two separate rules but once
you accept those two rules or you're
comfortable using them everything's okay
just it's it's more of a sociological
thing I think well why people are afraid
of quantum physics so there are two
rules of quantum physics and I would say
these are they going to give this
opportunity that Fineman preface long
time ago
what are they the golden rules of
quantum mechanics there's two of them
the first one you've probably heard this
is that everything is a wave well
quantum physics is it quantum mechanics
is a wave theory we're familiar with
lots of wave theories mechanical waves
sound waves water waves and we
understand that a wave is sort of an
oscillation that travels in space but
also in time if you if you take a
picture of an of a wave like if you take
a picture of water waves you'll see sort
of a ripple but if you also stay at a
particular point on the surface of the
ocean the water will also ripple up and
down in time and space and there are
differential equations to describe waves
they can get nasty but that math doesn't
necessarily help you understand
everything just the concept of a wave we
know it when we see it so one of the
properties of waves that we're very
comfortable with is that they can be in
super positions when you play two notes
on a piano your ear can experience both
of those sound waves at the same time no
problem and your brain can resolve those
tones in general water waves can have
many different can be very complicated
and you can see many different
structures on the surface of the ocean
for instance so the idea when you throw
when you throw a rock into a pond we we
know that we'll have these circular
waves emanating from this from the core
where is the wave well it's everywhere
it's delocalized is this technical term
so waves can be in superposition now I'm
going to apply this to information
remember
bit these zero or one well if we apply
it to information we should if we're
going to say everything's a wave well we
can have information in wave form we can
have both zero and one at the same time
so this is a little jargon and I don't
want to avoid the jargon of quantum
mechanics is if you ever see that line
and a little bracket next to it that
means this is this is a quantum thing
it's a quantum state we use that all the
time and this is what we might call a
qubit a quantum bit it's a superposition
of something that can be in one state
and another state and I've chosen 0 and
1 just as labels you can call it up and
down left and right heads or tails
doesn't matter and the plus sign is not
your usual plus sign it means that
there's they're both there together you
can't just add these like like
arithmetic they're both there like maybe
one one note on a piano and another note
on a piano
and these parameters a and B are the
waiting's of how much 1 in how much 0
there are it could be 50 50 or 90/10 so
this is the simplest superposition we
can have just of to two levels now
here's a pick a depiction of a pretty
poor depiction of an atom that has one
electron on it orbiting the core that
one electron is in two orbits at the
same time and atoms can do that because
the electrons are waves and we have to
take it a little bit of a leap that that
matter can behave as a wave but we
should be comfortable with the idea of
this wave-like phenomenon now here's
where the math comes in but again don't
be bogged down by the math a and B
follow a wave equation called the
Schrodinger wave equation I'm not going
to talk about it at all it can get very
messy but all waves have equations it
doesn't have to be just quantum this is
no more complicated than water waves
yeah
so there's math behind a and B and you
can change the waiting's a and B if you
if you poke the system just right and
I'm talking about water waves a lot this
is of course if the the famous Great
Wave off Kanagawa painting this great
wave and we all know what's going to
happen to these poor folks in that boat
they're the mathematics behind this wave
it's very complicated I would say it's
even more complicated
the quantum wave and you know I don't
even know how to solemn know exactly
what these all mean I think they have to
do with viscosity and what the waves
going to do over space and time that
math doesn't give us any more insight in
this wave at least not me I know what's
gonna happen I think we all know what's
going to happen when waves come into the
shore at the beach the the water gets
shallower and the waves actually get
higher and they get slower we just you
just understand that we don't need a
wave equation understand that so that's
that's my advice you don't get bothered
by the math okay that was rule number
one I spent a little too long on it
but what about rule number two what does
it mean to have a superposition well a
quantum superposition means that the the
existence of something is in two states
at the same time this Cup in principle
if it's quantum it can be in two places
at the same time that's okay according
to quantum mechanics but it's not okay
according to real-world experience so we
have to invent another rule and this is
what drives people crazy is that we have
this other rule and this is a little bit
of a joke the way I state it but the
rule number two says yeah that
superposition stuff's fine as long as
you don't look and so it this cut
nothing wrong with a cut being in two
places we don't experience it but it
only works if you don't look at it but
actually that if you think about quantum
mechanics from that perspective you can
go a long way and I'm going to define
what it means to look and I'm not going
to define consciousness either believe
it or not because looking at something
doesn't have to involve a living body
let me let me explain what I mean there
what happens when you do look and of
course if we're gonna build a computer
based on these bits we're gonna want to
measure the answer we have to look so
what happens when we look at a quantum
superposition well that's where the
waiting's play a role because when we do
look the system randomly pops into one
or the other definite state with a
probability attached to it and the
probability is given by a and B it's not
exactly a and B but you can think of it
that way so if we have a 50-50
superposition than half the
we're going to see zero half the time
we're gonna see one and if we repeat it
over and over again it's like flipping a
coin it's going to be random and noisy
this should really bother you this rule
number two by the way this is it that's
quantum physics done I think I took like
seven or eight minutes sorry but this
rule number two is very strange I think
of it as being strange on two accounts
number one it appears that by looking at
something you change it and that's
that's a totally foreign concept to all
of science we expect if there's an
experiment running it's going to do the
same thing whether we look or not but
quantum physics says that by extracting
information that's a little more
technical but by looking at a system it
appears to change it another way to
think of this rule number two is weird
is that where did the probabilities come
from anyway what are probabilities well
you all know what probabilities are we
use them all the time when we're
ignorant when we're ignorant of
something or we don't want we're too
lazy to characterize everything we don't
want to calculate the trajectory of
every molecule on the air to predict the
weather in principle some would argue we
can't anyway but I think in principle we
could it might be a chaotic system and
my maryland colleague michelle Gervin
will tell you more about that next month
about chaos but probabilities like
flipping a coin I could calculate the
results of any coin toss if I know
precisely how hard I toss it precisely
the value of gravity and so forth
humidity temperature all that stuff but
I don't want to lazy I'm ignorant so we
use probabilities we're comfortable
using
we're not going to quantum physics is
the only theory in all of nature where
we can't do that we have to use
probabilities and that's weird
whenever probabilities are there you
want to wonder well who's cooking the
books who decides in a measurement what
what you get it's a good question
and it's really why Einstein didn't
believe in this theory you know many as
famous lines one of them God does not
play dice so there's something in the
background that seems to be playing dice
here so what about consciousness what
about looking eye when you look what if
there's no human or no consciousness to
observe something well we have an answer
for that too or at least we we we think
of things in a very funny way in in
quantum superpositions remember my cup
in two places well let's say I have that
cup in two places but that's tricky
because now the air in the room has to
has to reorganize itself depending on
where the cup is if the cups over here
there's definitely no air where the cup
is
is the cups there and there's air over
here but if the cups over here there
reverses through so the air has to sort
of get involved well let's get rid of
the air pump out all the air and let's
say there's one air molecule one
nitrogen molecule it's zipping on the
stage here and it's going to hit this
Cup only when it's here but if the cups
here it's going to go right through well
if the cups here the the air the
nitrogen molecule is going to bounce off
this cup say and hit that wall but if
the cups here the air is gonna go
straight through so the air that
molecule has to make a decision but it
doesn't have a consciousness it doesn't
have a free will we don't think it's
just it's just a nitrogen molecule so
what how do we deal with this well if we
made the trouble of having this Cup in
two places we can add one more molecule
to the system so let's have the molecule
also being in a superposition of going
that way
correlated with the cup here and going
that way correlator the cup here but
we're not done because this molecule
hits that wall and that wall so now the
the walls are part of the system yeah
invite them they're part the whole
building shakes one way and the other
way the building yes come on we're all
here
well the earth moves one way or the
other so you see the problem is these
super positions tend to blow up and we
have to like predicting the weather we
have to know the state of everything in
the universe and in practice we can't do
that so what we do and here's how we
deal with it we box up this cup and we
say it's perfectly isolated and nobody's
looking there's no light there's no air
there's no information as to where this
cup is leaking out so you can see right
away there's there's a there's a very
deep connection between quantum
measurement and information so maybe no
surprise it should be used for computing
but it's such a fun topic you've
probably heard of Schrodinger's cat
which is another of the of the
rebellions against quantum physics back
in 1935 Schrodinger made up this this he
even called it a ridiculous case of what
quantum might predict here we have a
single atom and the way he posed it it's
a radioactive atom and if you wait one
half-life of the radioactive atom it's
in a superposition of having decayed and
having not decayed that's how we treat
the single atom quantum mechanically a
radioactive one well he hooks a single
atom up to a Geiger counter
that's perfect it registers a click if
it decays and that Geiger counters
hooked to a hammer that smashes a flask
of poison cyanide and so there's a cat
in the box by the way and the cat
therefore is both alive and dead at the
same time and he says quite ridiculous
case it is ridiculous because we don't I
mean a cat has lots of atoms that's 10
to the 22 atoms in there and we it's
very hard to think of a cat as a quantum
system it's too many too much stuff
there so it's very hard to apply just
like just like the air hitting the walls
and the building getting involved it's
very hard to isolate such a big system
and when I look at this thought
experiment Schrodinger's cat you know
the hardest part of the experiment it's
not the
or the Geiger counter over the cat it's
actually the box if you have to isolate
it from everything so it brings you know
there's lots of thought experiments like
that and we can if we ascribe quantum
mechanics to the system we have an atom
that's in one state correlated with the
cat alive and it's in the other state
cat's dead this I'm gonna comment a
little more on this kind of a quantum
state it's just a superposition of two
situations it's also called entangled
and I'll get to that in a minute
so here you can sort of go off the deep
end and in quantum physics there are
lots of interpretations as to what
really happens
the problem with interpretations is that
they all predict the same answer in any
conceivable experiment so the
differences between interpretations to
me is not necessarily science it's very
interesting
I love reading about those here's one
interpretation you've probably heard of
this and it's a very neat one it says
that that rule number two remember that
that when you measure you get one or the
other well let's just say the universe
bifurcates at that point and if we say
that everything is neatly tied up
because we only see a definite answer in
our universe and our alter ego sees the
other the other answer in the other
universe so I guess that works that
works for some people to me it to me it
has a big expense because now we have to
think about all these universes and
there's not just two of them there's a
gazillion universes every time a quantum
system gets resolved how to keep track
of that until we can travel between
those universes it's very hard to think
of this as a as a I don't know how to
test this so it's hard to call it
science but some people say well it's
the only thing it did buy it you know if
you eliminate a huh if you eliminate
everything else that's the only thing
that makes sense
Einstein in the same year 1935 hit this
title his papers are wonderful a very
very easy to read them his title here is
basically his quantum mechanics even
right it seems like somebody's cooking
the books there's other stuff going on
and he for the first time
introduced the idea of an entangled
state what does that mean
well here now we have two qubits this is
the simplest version of an entangled
State two qubits a red one and a blue
one and they're prepared you can think
of two coins if you want both heads and
both tails so it's a superposition if
we're going to allow super positions
this is not a big deal here right we're
allowing a superposition of these two
situations what's interesting about this
particular superposition is that the red
of the blue system are different they're
separate in fact they can be spatially
separated they can be you can take the
red one on the moon if you want and
leave the blue one here and what's neat
there is that if if I look at the blue
one here on earth whatever I measure I
know exactly what the one on the moon
has if I measure is zero remember this
is a 50-50 superposition if I measure
zero I know that the moon qubit is also
zero and what bothered Einstein in
particular is that I know that
information faster than light could
could have beamed the information to me
so that's strange he called it another
one of his famous quotes he called it
spooky action at a distance and
therefore it must be wrong there must be
something going on it turns out I think
his heart was in the right place it's
just super weird but quantum mechanics
as far as we know it's correct we can
make entangled States and make
measurements on them faster than
information could be communicated the
trick is there if you think about this
from information theory point of view
and information theory was not really
invented until the early 40s the
resolution of this so-called EPR
einstein-podolsky-rosen paradox is that
if you do this many times and you should
imagine I have a bunch of coin numbered
and you have a bunch of coins numbered
and they're all entangled they're all
the same value but each one of them is
random if we repeat it we're just gonna
get random numbers
I'm not going to learn any information
that you could encode in your system by
measuring mine all I know is I have the
same random bit you do and random bit
streams contain no information so the
resolution of EPR paradox
from information theory actually so it
doesn't violate any physics principles
this idea of entangling but it's still
kind of cool there's some kind of a
connection between these qubits I think
of a tangle man as wires without wires
wiring without wires and these
correlations as will maybe get a hint at
their behind all the power of quantum
computing ok so there's only one analogy
I know of entanglement and it's based on
a visual illusion and so let me just go
through that quickly so you all gone
through the exercise of trying to draw a
cube on 2d surface on a chalkboard
in fact I've drawn this incorrectly I've
drawn this so all the lines are exactly
parallel so it has ambiguous perspective
where's the front face is it this one or
is it that one is it up and to the right
or down to the left that's a little like
a superposition you can sort of see it
flipping back and forth it has both but
then when you sort of lock on to one
perspective it stays that way
that's sort of like a measurement and
the beauty of this there here the two
definite perspectives of course the
beauty of this this analogy is it works
for entanglement here are two qubits
both zeros and both ones and I'll bet
you when you resolve the perspective
they're the same you can try to trick it
but it's very hard to do that it's only
an analogy but the fact that these two
sort of shimmer back and forth together
is the essence of entanglement and
they're connected even though they're
not connected they're connected by our
consciousness I guess but it's a little
bit like the mystery of entanglement and
there are the two definite perspectives
and unfortunately this analogy falls
apart when you get them too far apart
now because they're too far apart the
visual illusion disappears okay so I
want to talk to her a few minutes on
what a quantum computer is well it's a
collection of qubits clearly we're going
to be storing we're going to be storing
superpositions of numbers and so there's
a few numbers on this next slide but the
high level is pretty clear we're going
to put a lot of qubits together and see
what happens and I think of it as a good
news
bad news good news story and the last
piece of good news is only recent the
last decade or two the last couple of
decades what's the good news
well when we put lots of qubits together
the system sort of blows up
exponentially
why because one qubit can store two
values you're on one two qubits can
store four values right heads heads
heads tails tails heads tails tails
three qubits can store eight numbers and
qubits can store two to the N binary
numbers all right so this is an
illustration of three qubits and we have
eight numbers and the shading of grey
tells you sort of how much zero there is
how much to there is how much seven
there is and so forth so this is really
good news because you can do parallel
processing in a way that you only have
one input but the input has all the
numbers at the same time as long as you
don't look that's it so you have to
compute this function in the dark no
information leaking out and so forth but
in principle we can do that and we can
get all these answers here's a quantum
state of three qubits
it has eight wait waiting's and they
should all add up to one because the
probabilities have to add up well
eight is not very big so three qubits is
a pretty small system but if I put four
cubits in here we get sixteen five
qubits is 32 so we have exponential
growth if we put three hundred qubits
together that's two to the three hundred
states we can represent and I picked
that number because it's huge it's more
than the number of atoms in the universe
so even if every atom in the universe is
part of a regular PC like this or some
conventional computer it wouldn't have
enough space to store information merely
in three hundred atoms or three hundred
electrons in a quantum computer so this
is the really good news it's explosive
good news this exponential growth we're
tempted to say well there's a solution
to Moore's law we have this exponential
growth well what's the bad news we have
to look at it we have to measure the
quantum computer and when you when you
make a measurement you only
one answer you could have two to the
three had 10 to the 90 inputs you only
get one answer and it's random
it's totally noisy so it's almost like
it's really bad news it's devastating
news because it seems like there's
nothing good that can come from this you
don't know what the output was because
it's probabilistic you have to reverse
the function to find out what input
corresponding to that output so why not
just run it serially with every one of
the inputs so at first glance that
doesn't seem to be anything to be gained
here but at second glance and this took
30 or 40 years after fineman's original
observation of this quantum opportunity
David Deutsch a computer scientist
physicist mathematician at Oxford
pointed out that well before you make a
measurement you can take all of these
inputs all these qubits they're maybe
two to the three hundred of them sorry
two to the three hundred pieces of
information you can have them interfere
in superposition that's another
wave-like phenomenon interference well
if these if if these this is sort of be
sort of like time now I feel left to
right and these red dots they're called
quantum gates just like in classical
computing we combine information in ways
to do computation we can do that with
quantum gates in quantum states as well
there are some algorithms where if you
allow them to interfere in just the
right way only one answer appears at the
end or just a few answers before you
measure and so now that that answer can
in some cases depend on all of these
inputs and that's something you could
never do with conventional computers if
the inputs are sufficiently large and
now when you make a measurement
there's rule number two is no problem
now when you make a measurement there's
only one answer you're going to get and
it can depend on all the inputs in an
important way I'm being kind of vague
here because quantum computers are not a
panacea they're not they won't solve
every problem this is a so-called one to
one problem where every input gives a
unique output cronic computers are bad
for that for the obvious reason that as
I said this bad news will kill you but
there are problems that are not
one-to-one where some output depends on
lots of inputs and in in the mid-90s a
killer application emerged that really
gave birth to this field as we know
today and it's a actually a simple
recipe in in in number theory factoring
factoring numbers 39 is 3 times 13
that's easy it's a small number but when
you make the number big that you want to
factor into its primes it becomes
exponentially hard there's no known fast
algorithm to factor big numbers so with
with a thousand digits you just can't
factor it now factoring seems sort of
esoteric except the inability to factor
big numbers is the basis of all modern
encryption standards so if you can
factor big numbers you can break codes
and there are lots of three-letter
agencies that want to do that well
actually it's interesting they they not
just want to break codes they want to
know can somebody else break our codes
when will a quantum computer exist it's
powerful enough to factor and well the
good news in a sense is that I think
we're at a conversation last night with
my sister who is a computer scientist at
Los Alamos here in Santa Fe and she said
well it's always 20 years away 20 years
away all the time
well factoring is a really hard problem
you need millions of qubits billions of
operations
isn't it and as I'll tell you in a
little bit the state of the art now is
dozens and hundreds not millions and
billions so we're a long way away but
the way factoring works is if you
imagine we want to factor the number 39
well we store all of those numbers at
the same time and how would you factor
classically one not so efficient way is
to test every number smaller than
smaller than you don't have to go above
the square root of 39 but every numbers
smaller than a number just test and see
if it divides it it's trial and error
and of course if this this number has a
hundred digits then you have to test 10
to the 100 numbers and you're out of
luck but we can store a superposition of
all these numbers in a quantum computer
again and here's where it's sort of the
magic happens we can we can apply
quantum gates in the system so that the
quantum state ends up being this it sort
of gets forced into the answer and that
number 39
encoded in this particular pattern of
these gates and that's the art of
quantum computing algorithms and again
this is a killer application it's a big
deal because it's it's it's it's a fast
algorithm on a quantum computer
exponentially faster than any known
classical algorithm so there there are
some other applications that over the
last few years have come to light and
I'm going to be very vague here they
have to do with optimization any kind of
problem where the answer depends on all
the inputs is something that quantum may
be good at this is a very spiky function
you can think of it as a topographical
map or something what's the minimum
value of this function as a function of
these two variables well it's clearly
right here you can see that but what if
we have what if we have 10,000 variables
we very hard to plot that since we only
have three dimensions so what's the
minimum value of a function that has
lots of inputs well you have to test
every one of them and that's a hard
problem well the minimum is a global
property of all the inputs and there are
quantum computer algorithms that might
allow us to approximate where that
minimum value is and this is going to
hit this could hit all kinds of
different walks of life any kind of
optimization problem there is there are
people thinking about how to apply
quantum computers to it that said it's a
very speculative game it's not clear how
how well quantum computers will do in
these problems one of my favorites is a
well known problem called the Traveling
Salesman problem if you specify a bunch
of cities on a map what's the what's the
path that hits every city once and only
once and covers the minimum distance
that's a really hard problem classically
in fact it's exponentially hard with the
number of cities quantum mechanically it
looks like a quantum computer might not
be able to solve it either but it might
get a better approximation than a
classical algorithm could and this is
sort of where a lot of the field is
right now this is an article just a
couple months ago on the Wall Street
Journal
it's nothing about quantum it's about
models for instance autonomous driving
we have models of how cars can recognize
certain things on the road
solving some of those models are really
difficult and that's I think maybe where
quantum will play play play a role so I
want I want to close by actually you
know I haven't talked anything about
experiment and I'm an experimentalist
and I've given you sort of an exotic
platform for computing and remember that
a quantum system to be to to work as a
quantum computer has to be isolated to
an extreme level and then when you're
ready to measure you need to look in
there so we have to have very
controllable hardware to do this and
it's really exotic stuff there exist a
few different types of systems that can
be built now for quantum computers and
in fact two platforms in particular
they're very exotic they're being built
right now
one is superconducting circuits and I
think of this as sort of a coil of wire
where the current flows without
resistance so it flows forever yeah
without bumping into any material it's a
fascinating physics phenomena phenomenon
that was discovered a long time ago
super conductivity you've heard the
terms well in this case we can run
currents in two directions at the same
time and nobody's looking and you know
even the matter in the wire is not
looking because the the the electricity
flows without resistance so that's an
interesting platform to store a qubit
and you can physically wire these things
up this is actually five cubits this is
an IBM chip here where they wire five
superconducting qubits and there's lots
of investment in this area from big
companies that you've heard of to build
quantum computers out of this there's
another platform that is really sticking
out as well this is even more exotic and
this is the one I work with
it's individual atoms and the strength
of individual atoms is that they're all
the same they're given to us they're all
the same same element same isotope so we
can scale up in a way that you could
never do if you have manmade manmade
qubits and so this is a picture of one
of our devices this is a chip it's a
silicon chip by a centimeter on the side
there's nothing quantum about this chip
what's quantum is the floating atoms
above the chip in fact I've expanded it
by maybe a factor of
50 these atoms sit right here above the
surface about 1/10 of a millimeter above
that surface and here you can see there
are 75 atoms in that chain each dot is a
single atom they're all the same and the
reason you can see them is that we're
shining laser light on them and that
laser is tuned to a particular
wavelength that this particular atom
will respond to if you don't know what
the atom is it's you terbium YB it
really doesn't matter so much that that
determines the lasers we're going to use
and so forth they're separated by a few
microns a few millionths of a meter
these are ions they're charged and
that's why they repel each other and
step and and form this crystal it's
anatomically perfect crystal and each
atom is a qubit why can you see single
atoms well because this is in a vacuum
there's nothing there there's nothing
else there of course you can see things
if there's no noise so these atoms
behave as wonderful qubits quantum bits
and we can we can actually shine lasers
on them to to to perform quantum
computations and this little animation
will kind of show you about that so this
is just a collection of five atoms we
initialize them using other lasers and
we're gonna poke lasers at these
individual atoms you should think of
them as like masses connected by Springs
and when we when we when we poke them
with lasers they move around a little
bit and that causes them to be coupled
that's how we wire together two atoms
and this is actually a quantum
computation a circuit here and we point
lasers at the atoms I won't go into
details don't have time to do that but
in the end we do a chronic computation
based on those gates and then we measure
them all if it's in one state they
fluoresce if it's in another state
they're dark and we collect that light
on detectors so this is a very
simplified version of what we do and
here's again that picture of these
individual atoms and these individual
atoms are on top of that chip and I
didn't show you the best part this chip
has about 100 electrodes and we have
controllers and lasers and the chip is
right in there now
all the action is here this is all
support but this is the this I don't
want to say it's noisy we think about it
but that that sorry one a little too
fast in that little cubic meter is where
all the action happens and we even have
visions to scale up those those boxes by
using optical fibers again this is this
is lurching into the science fiction
area of my research but we have working
prototypes that work with small numbers
of qubits and this is what you have to
think of course when you see that system
we're at the level where the things are
really really complicated still and and
you know these systems are very hard to
maintain but nobody's tried yet to
engineer these individual atoms and
we're trying to do that both that in my
university research group and also our
small company inq I'm happy to say you
may have noticed on a slide I said
before Honeywell Corporation is also
investing a lot in this this this
interesting platform but it's pretty
exotic and there are other platforms
that are more researching now anything
that sort of shows quantum coherence
individual atoms individual electrons if
you can isolate them
there's an interesting defect in diamond
that makes Diamond turn red when there's
a vacancy of the carbon atom and next
door there's a nitrogen atom defect
those two together make for a very
interesting type of qubit people think
about that it's called envy diamond so
the you know there are a lot of ideas
out there and what's what's what's the
coolest for me is that this field is
still in its infancy on the hardware
side and there's just lots of good ideas
out there and so what what happens in
the future well we turn to Tom Clancy of
course so he he he and his ghost writers
they like to read up on modern research
and put little zingers in their books
and actually they you know he talks
about quantum computing here and there
but his hero actually builds builds a
device that's stable and it's scalable
and it works really well and he's asked
well how did you manage to do that if
it's so hard he says because I'm smarter
than all the rest and that's the great
thing about this field there's so many
smart young people that may might not
know quantum physics the math behind it
but but they can understand the concepts
just well enough to maybe think about
new systems
it can be built you know there's
wonderful you know wonderful directions
that are out there it bridges physics
chemistry mathematics information theory
computer science of course electrical
engineering all forms of engineering and
I guess one of the challenge in this is
in this field and I I'm hoping that I
dispelled a little bit of that is that
most people are not comfortable with
quantum mechanics some of them some
people don't even believe it they just
think it's a you know it's an ivory
tower subject engineers especially older
generations of Engineers that build
airplanes we need these systems and huge
engineers to apply their skills to
quantum they won't touch it it's it's
it's just such a goofy theory to them
and I think this culture is starting to
change now but there's sort of a gap
between universities and industry in
this field I often say that universities
at universities are very comfortable
with applying quantum physics but we
don't build things we don't build things
that are useful for others to use like
computers you never build an airplane or
a computer at a university industry has
the opposite problem they build things
they have systems engineers that have
worked 15 years on one thing perfected
it prototype after prototype but they're
not comfortable with quantum mechanics
yet so there's sort of a gap now in in
this I think of it as a workforce issue
and for that reason actually the US
government is very interested in this
for many reasons one one is that the
countries of countries throughout the
world are putting lots of effort into
quantum computing u.s. has mighty
industry we can take your risks and so
forth but we don't have that workforce
in the middle so actually Congress this
year is the unanimous have the House
unanimously passed a law called the
National quantum initiative that would
that would direct agencies to sort of
bridge that middle ground and the Senate
will probably act by this year's end and
the White House is actually engaged in
this there's an expert in quantum
physics in the White House right now who
is sort of advising
and coordinating the agencies hopefully
in this field and again hopefully in 10
or 20 years you know this won't be
needed we'll have a culture of folks
that are at least comfortable with
quantum physics so don't worry about not
understanding it none of us do but just
get comfortable with it and then when
you go home you can read interesting
books and think about the mystical
nature of what could be the backbone of
high-performance computing in the future
thanks for your attention
so okay so if anyone has any questions
oh I have one right here the Brent okay
how does this relate to the Chinese
satellite that we heard had split a
photon and uses the positions of this
photon in two different locations to
transmit indecipherable code good
question
so I did mention that quantum computers
could break codes but actually quantum
information and you may have gathered
from just the background you can also
encode information in a very interesting
way because if somebody reads it they
destroy it
remember rule number two so there's
something called quantum cryptography
where you can actually send quantum bits
like photons through a fiber or through
space and if somebody reads it you know
in principle that they are eavesdropping
that's kind of neat so indeed the
Chinese launched a satellite and there
was a marvelous engineering task to send
single photons up to the satellite and
they could basically communicate it
wasn't a very fast rate of communication
but they could do it knowing that the
communication was secure so how does
this relate to that yes that's an
example of something called quantum
communication I will say on the other
hand the direct use of qubits for
encryption is not necessarily so
interesting because if you want to break
if you want to if you want to spy on the
person sending information from here to
the satellite I would not maybe not
break try to try to intercept it along
the line I would look behind the back of
the guy who typed in the information or
try to blackmail and maybe or or do
something on the satellite at the other
end when he gets read so you know those
problems are still there it does solve
the problem of the channel itself being
broken but there's always an element at
either end in fact this country is not
necessarily so interested in quantum
for that reason but it's a marvelous
engineering feat and I would I'll give I
will say that if you share photons with
say many parties there are protocols
that can take advantage of it this is
more in game theory so you could imagine
having an election system where nobody
trusts anybody else how can you be sure
there's a fair election there's a way to
use entanglement maybe to help in that
direction so it's very researchy
but the Chinese satellite I mean it's a
very expensive and beautiful engineering
project it also made a lot of noise and
I think that's the design they they
wanted to really hear the kind of make
make a lot of press out of that I have
another one over here in the aisle
I'm just curious it's it sounds as if
you're talking about it is what I would
consider the extremely low level of
hardware's if you're talking about okay
we're here we can take a quantum analogy
for a a computer gate and sort of sort
of logic gate but of course computers
you know you build the CPU out of
multiple gates and you have the memory
and you have then you have on top of
that you have software so it seems like
there's maybe I'm one of those slightly
older engineers who's just uncomfortable
with all this but it seems like there's
a huge gap between what you described
and what I would think of as as even
anything approaching software like how
does that where does that come in yeah
good point there's a huge gap so so when
I think of software I in quantum I think
of applying individual gates I mean that
is really low level I mean in classical
software we're I mean I remember doing a
little bit of assembling language code
in the 70s and that was very low level
but the gates behind it or even much
lower very hardware driven so you're
absolutely right there's a huge gap
between what we can do at hardware and
the highest level say the cloud user
interface however in these quantum
systems like any computer if you can
abstract away the hardware whether
whether we have superconducting loops
going one direction or another or atoms
that are that are in one state or
another if you can abstract even just to
the gate level
that's very powerful and there are
currently lots of software people even
Microsoft's by leading the charge
developing quantum software it's still
very low level and right now you can't
cheat you can't throw away a memory like
we look like we do these days you have
to really squeeze out every piece of
efficiency you can but you're absolutely
right and we're dying for for the
hardware to graduate so that we can add
more software layers to it that's that's
going to be absolutely necessary but yes
I'm sorry to say that you know I can
agree with you entirely that we're we
haven't even started
on the software side yet thanks I guess
I don't I've had trouble understanding
what the scaling dimension of this is
like in the ordinary in say the non
quantum computation we scale on space
that is we know we can get down to half
M levels and and we just do this kind of
arbitrary computation unit in a smaller
space right but what makes two quantum
computers different from each other in
terms of scale like once you get one of
these things working how do you get the
next step that is the 2o version how is
that going to be faster in the first one
yeah ok question has lots of lots of
directions so just the number of qubits
is in a sense you get to exponentiate
that number or take it to you know so 2
to the N is in a sense the naive power
of n qubits so every time you add one
cubit you've doubled the power of that
system in a sense the problem is every
time every time you add a queue
the system gets a little more messy and
you have to probably operate more gates
so as you make the system bigger you
have to also go deeper you have to have
deeper circuits and here's here's the
real challenge as I see it in the
laboratory is that quantum systems when
you make them big they become classical
they they get noticed it's very
challenging to scale up a system and
maintain quantum coherence and that's
the central challenge in the hope and
the entire endeavor now one thing I
didn't talk about I wish I had time is
the idea of quantum error correction and
it turns out if you make your quantum
system big enough it can be stable
against errors and you can scale it up
unfortunately the overhead to store a
really good qubit might require 10,000
plain Cubans classically we have error
correction as well but it's much more
efficient you can you can make the
errors go infinitesimally small just by
adding a tiny bit more redundancy and
how you encode but there's so many new
types of errors that can happen in
quantum you have to encode things in a
mass massive entangled state but so I'm
not sure I'm answering your question but
the scale up of quantum computing
depends on the algorithm it depends on
the system and again I sort of revert to
my earlier answer where it's such an
early stage we just want to build one
that can do something demonstrably
different than what we do classically
maybe that factoring algorithm applied
to a small number and think about
scaling that up or maybe some simulation
of some molecular dynamics and see if
you can scale that up so I hate to be
wishy-washy but boy we still don't know
what what quantum computers are good for
it's a slightly peripheral question but
say quantum computers are realized and
the killer app being factoring large
numbers is a solvable problem
the peripheral question is since so much
of our security computer security type
protocols are based on that can you say
anything about as quantum computing
develops in 20 years 50 years whatever
the number is can you say anything about
the directions that computer security is
going that is not based on that
breakable encoding yeah good good point
and just like quantum cryptography this
decryption application is a little
overstated because there are
cryptographic schemes that that can be
proven quantum can't break it's called
post quantum cryptography and and NIST's
the nationalistic standards that
technology is developing several of
those that the government use I'm sure
NSA is way ahead so indeed I find the
factoring problem you know not so
interesting I mean it's academically
interesting because it's a different
complexity class of a very classic
problem but the application of factoring
there's not much there because we cannot
we will change our cryptography
standards in the coming maybe in a
decade or something but it's still true
that people are listening in right now
and writing down ciphertext they can't
break and and maybe in 20 years they can
break it so if you want to keep a secret
more than 20 years you should be careful
right now sort of taking action now so
so indeed I'm not an expert in
cryptography but there are many
different many different forms of
cryptography that seem to be quantum
quantum secure
I'm not a techie but the other night I
was listening to a report on the various
currencies and they were discussing
Bitcoin and others so is the Bitcoin
what you mentioned just now is it felt I
mean what was that last expression you
mentioned just now crypto what I mean is
the Bitcoin crypto secure boy I wish I
knew more about that but the Bitcoin and
other cryptocurrencies are based on this
this blockchain protocol that as far as
I understand it's very much related to
the classic type of cryptography based
on the ability to factor and if you can
factor you can maybe mind bitcoins
I think that I think that's true but I
don't think it has to be true I think
you can adopt different standards for
for crypto currencies that are also a
quality cure but I'm actually not the
guy to ask it's one of those terms
everybody talks about it about and
nobody understands it I have a device
question for you I'm looking at the
space you had one picture of the IBM
system where the superconducting
circuits and it looks like those are
Josephson junctions can you kind of
describe the physics of those oh yeah
a Josephson junction so basic loop of
wire here it has inductance and
capacitance it's an oscillator so the
circuit oscillates if you had a
Josephson junction in there it becomes
nonlinear and therefore you can store it
you can make a qubit out of it an
oscillator has infinitely many levels
quantum levels but to store a qubit you
need to make the levels differently
spaced and the Josephson Junction does
that so it's a little it's the Josephson
junction is this this little thing there
it's a tiny gap where the electrons
tunnel through the gap and that gives
the non-linearity it's it's it's a
little bit technical but that
some people in that field would call
this thing an atom they call that big
it's a big atom it can be 1/10 of a
millimeter on the side
it's an atom and has a very simple
degree of freedom
everything else is frozen out because
it's it it's at nearly zero degrees
Kelvin it's it's it's it's in a dilution
refrigerator very low temperature and
everything freezes out except this one
degree of freedom so Josephson junctions
actually were popular in the 80s for
making a new type of conventional
computer classical computer based on
Joseph's no chance because there was
very little dissipation and it's thought
that was important turned out not to be
it's still looking got smaller and
smaller they just engineered the heck
out of it and you know silicon conquered
but I have to been involved in that
project act or make me did yeah yeah
it's it's no surprise that I should have
included Northrop Grumman here IBM and
Northrop Grumman now have sort of legacy
Josephson computing groups maybe you
were involved with something maybe older
TRW that went over to Northrop and these
same people are now making circuits for
quantum so it's a very yeah it's a very
active field very exciting
this may be a little mystical that
there's a Santa Fe can I give to me so
the human human mind is pretty good
pretty smart and there's a handful of
well-respected physicists and others
like Penrose and Hameroff who in fact
think that the human brain is a quantum
computer and I know there are issues
with how that can be but I'm just
wondering what your thought my gut
feeling is that something hot sticky and
wet is no place for quantum physics so I
find it hard to believe but I wish I
could say the same
say more to it there are many biological
effects that seem to have quantum
coherence at their core one is the one
is the rhodopsin in the back year I can
detect the olicity of single photons and
there's the magnetic field sensing of
certain birds that apparently they can
they can detect it at the single atom
level of magnetism and so their quantum
coherence and even superposition could
play a role the problem is rule number
two always gets in the way when you have
something hot or when you have something
that's super big how does how does how
can you think of quantum in that context
and you know they're really interesting
ideas out there I think it's fascinating
I think it's a little bit fringe which
is too bad I think there shouldn't be
Oster shouldn't be ostracized for for
thinking that way but I I just don't
know how to think that way I'm if you've
gathered the the platform I like our
individual atoms you know ten or twenty
that's a lot so I'm sort of a bottom-up
kind of kind of person and it's very
hard for me to think of applying quantum
to a really complex system with ten to
the twenty atoms in it so you know sorry
it's not mystical I think that's it's
really interesting and I think biology
is kind of the one of the frontiers of
science right now if we can only link it
with physics mark would be great
actually next month's lecture Michelle
will talk maybe a little in that
direction
