Kind: captions
Language: en
the following is a conversation with
Scott Aaronson a professor UT Austin
director of its quantum information
center and previously a professor at MIT
his research interest center around the
capabilities and limits of quantum
computers and computational complexity
theory more generally he is an excellent
writer and one of my favorite
communicators of computer science in the
world we only had about an hour and a
half for this conversation so I decided
to focus on quantum computing but I can
see us talking again in the future on
this podcast at some point about
computational complexity theory and all
the complexity classes that Scott
catalogs and his amazing complexity Zoo
wiki as a quick aside based on questions
and comments I've received my goal with
these conversations is to try to be in
the background without ego and do three
things one let the guest shine and try
to discover together the most beautiful
insights in their work and in their mind
to try to play devil's advocate just
enough to provide a creative tension in
exploring ideas to conversation and
three to ask very basic questions about
terminology about concepts about ideas
many of the topics we talk about in the
podcast I've been studying for years as
a grad student as a researcher and
generally as a curious human who loves
to read but frankly I see myself in
these conversations as the main
character for one of my favorite novels
badesti husky
called the idiot I enjoy playing dumb
clearly it comes naturally but the basic
questions don't come from my ignorance
of the subject but from an instinct that
the fundamentals are simple and if we
linger on them from almost a naive
perspective we can draw an insightful
thread from computer science to
neuroscience to physics the philosophy
and the artificial intelligence
this is the artificial intelligence
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enjoy and keep up to date with the
latest tech news and now here's my
conversation with Scott Aaronson
sometimes get criticism from a listener
here and there that while having a
conversation with a world-class
mathematician physicist neurobiologist
aerospace engineer or the theoretical
computer scientists like yourself I
waste time by asking philosophical
questions about freewill consciousness
mortality love nature of truth super
intelligence weather time travel as
possible weather space-time as emergent
fundamental even the crazy questions
like whether aliens exist what their
language might look like what their math
might look like whether Malthus
inventors discovered and of course
whether we live in a simulation or not
so I try with it out with it
I try to dance back and forth from the
deep technical to the philosophical so
I've done that quite a bit so you're a
world-class computer scientist and yet
you've written about this very point the
philosophy is important for experts in
any technical discipline though they
somehow seem to avoid this so I thought
it'd be really interesting to talk to
you about this point why should we
computer scientists mathematicians
physicists care about philosophy do you
think well I would reframe the question
a little bit I mean philosophy almost by
definition is the subject that's
concerned with the biggest questions
that you could possibly ask all right so
you know the ones you mentioned right
are are we living in a simulation you
know are we alone in the universe how
should we even think about such
questions you know is the future
determined and what you know what do we
even mean by being determined why are we
alive at the time we are and not at some
other time you know and and and you know
when you when you sort of contemplate
the enormity of those questions I think
you know you could ask well then why why
be concerned with anything else all
right why why not spend your whole life
on those questions you know I think I
think in some sense that is the the
right way to phrase the question and you
know and and and actually you know what
what we learned you know I mean
throughout history but really starting
with the Scientific Revolution
we've got you know Galileo and so on is
that there is a good reason to you know
focus on narrower questions you know
more technical you know mathematical or
empirical questions and that is that you
can actually make progress on them right
and you can actually often answer them
and sometimes they actually tell you
something about the philosophical
questions that sort of you know may be
motivated your curiosity as a child
right you know they don't necessarily
resolve the philosophical questions but
sometimes they reframe your whole
understanding of them right and so for
me philosophy is just the thing that you
have in the background from the very
beginning that you want to you know the
you know these are these are sort of the
reasons why you went into intellectual
life in the first place
at least the reasons why I did right but
you know math and science are tools that
we have for you know actually making
progress and you know hopefully even you
know changing our understanding of these
philosophical questions sometimes even
more than philosophy itself does what do
you think computer scientists avoid
these questions will run away from them
a little bit at least in technical
scientific discourse well I'm not I'm
not sure if they do so more than any
other scientists though I mean I mean I
mean I mean I mean Alan Turing was
famously you know interested and you
know is his most famous one of his two
most famous papers was in a philosophy
journal mind you know it was the one
where he proposed the Doering test he
took a Vidkun Stein's course at
Cambridge you know argued with him I
just recently learned that a little bit
and it's actually fascinating I I was I
was trying to look for resources in
trying to understand where the sources
of disagreement and debates between
Wittgenstein and touring war that's an
interesting that these two minds have
somehow met in the arc of history
yeah well the transcript you know of
their the course which was in 1939 right
is one of the more fascinating documents
that I've ever read because you know a
vit concern is trying to say well all of
these these four
systems are just a complete irrelevance
is right if a formal system is
irrelevant who cares you know why does
that matter in real life right and
touring is saying well look you know if
you use an inconsistent formal system to
design a bridge you know the bridge may
collapse right and you know soso touring
in some sense is thinking decades ahead
you know I think of where Vidkun Stein
is the way with formal systems are
actually going to be used you know in
computers right to actually do things in
the world you know and it's interesting
that touring actually dropped the course
halfway through why because he had to go
to Bletchley Park and you know work on
something of more immediate importance
that's fascinating
if you take a step from philosophy to
actual like the biggest possible step to
actual engineering the actual real
impact yeah and I would say more
generally right uh you know a lot of
scientists are you know interested in
philosophy but they're also busy right
and they have you know a lot on their
plate and there are a lot of sort of
very concrete questions that are already
you know not answered but you know look
like they might be answerable right and
so then you could say well then why you
know break your brain over these you
know metaphysically unanswerable
questions when there were all these
answerable ones instead so I think you
know for me I I enjoy talking about
philosophy I even go to philosophy
conferences sometimes such as the you
know fqx I conferences I enjoy
interacting with philosophers I would
not want to be a professional
philosopher because I like being in a
field where I feel like you know uh you
know if I get too confused about the
sort of eternal questions then I can
actually make progress on something can
you maybe a link on that for just a
little longer yeah what do you think is
the difference so like the corollary of
the criticism that I mentioned
previously that why ask the
philosophical questions of the
mathematician is if you want to ask for
softball questions then invite a real
philosopher on and ask that
so what's the difference between the way
a computer scientist and mathematician
Ponder's a philosophical question and a
philosopher Ponder's the falafels
question well I mean I mean a lot of it
just depends on the individual all right
it's hard to make generalizations about
entire fields but you know I think I
think if we if we if we tried to we
tried to stereotype you know we would
say that uh you know as scientists very
often will be less careful in their use
of words you know I mean philosophers
are really experts in sort of you know
like when it when it when I talk to them
and they will just pounce if you know
use the wrong phrase for something
versus a very nice word you could say
cyclers yeah yeah where or you know they
will they will sort of interrogate my
word choices let's say to a much greater
extent than scientists would write and
and scientists you know will often if
you ask them about a philosophical
problem like the hard problem in in of
consciousness or free will or whatever
they will try to relate it back to you
know recent research right you know
research about about neurobiology or you
know but you know the best of all was
research that they personally are
involved with right right and you know
and and and and you know of course they
will want to talk about that you know
and it is what they will think of you
know and then of course you could have
an argument that maybe you know it's all
interesting as it goes but maybe none of
it touches the philosophical question
right but you know but maybe you know as
a science you know at least it it as I
said it does tell us concrete things and
you know even if like a deep dive into
neurobiology will not answer the hard
problem of consciousness you know maybe
it can take us about as far as we can
get toward you know expanding our minds
about it you know toward thinking about
it in a different way
well I mean I think neurobiology can do
that but you know with these profound
philosophical questions I mean also art
and literature do that right they're all
different ways of trying to approach
these questions that you know we don't
for which we don't even know really
but an answer would look like but and
yet somehow we can't help but keep
returning to the questions and you have
a kind of mathematical beautiful
mathematical way of discussing this with
the idea of Q Prime oh you're right they
usually the only way to make progress on
the big questions like the full of the
philosophical questions we're talking
about now is to pick off smaller sub
questions
ideally sub questions you can attack
using math empirical observation or both
you define the idea of a Q Prime
so given an unanswerable philosophical
riddle q replace it with a mirror leak
in quotes scientific or mathematical
question q prime which captures part of
what people have wanted to know when
they first asked q yes then with luck
once all q prime so you described some
examples of such Q prime sub questions
in your long essay titled white
philosophers should care about
computational complexity so you catalog
the various Q Prime's on which you think
of theoretical computer science has made
progress can you mention a few favorites
if any pop any pup to mind or boy yes so
I mean I would say some of the most
famous examples in history of that sort
of replacement well you know I mean I
mean to go back to Alan Turing right
what he did in his Computing Machinery
and intelligence paper was exactly you
know he explicitly started with the
question can machines think and then he
said uh sorry I think that question is
too meaningless but here's a different
question you know could you program a
computer so that you couldn't tell the
difference between it and a human right
and you know yeah in the very first few
sentences he in fact just yeah I miss
the Q prime precise he does precisely
that or you know we could look at at
girdle right where you know you had
these philosophers arguing for centuries
about the limits of mathematical
reasoning right in the limits of formal
systems and you know then by the early
20th century logicians you know starting
with you know frag a rustle and then you
know most spectacularly girdle you know
manage
to reframe those questions as look we
have these formal systems they have
these definite rules are there questions
that we can phrase within the rules of
these systems that are not provable
within the rules of the systems and can
we prove that fact right and so that
would be another example you know III
had this essay called the ghost in the
quantum Turing machine it's you know one
of the crazier things I've written but I
I tried to do something or you know to
to advocate doing something similar
there for free will where you know
instead of talking about is free will
you know real where we get hung up on
the meaning of you know what exactly do
we mean by freedom and can you have can
you be you know or do we mean
compatibilist free will libertarian free
will what are these things mean you know
I suggested just asking the question how
well in principle consistently with the
laws of physics could a person's
behavior be predicted
you know without so let's say destroying
the person's brain you know taking it
apart in the process of trying to
predict them and you know and that
actually asking that question gets you
into all sorts of meaty and interesting
issues you know issues of what is the
computational substrate of the brain you
know or can you understand the brain you
know just at the sort of level of the
neurons you know it sort of the
abstraction of a neural network or do
you need to go deeper to the you know
molecular level ultimately even to the
quantum level right and of course that
would put limits on predictability if
you did so you need to reduce you need
to reduce the mind to a computational
device like formalize it so then you can
make predictions about what you know
whether you could predict a B if you
were trying to predict a person yeah
then presumably you would need some
model of their brain all right and now
the question becomes one of how accurate
can such a model become can you make a
model that will be accurate enough to
really seriously threaten people's sense
of free will you know not just
metaphysically but like really I've
written in this envelope what you were
going to say next is you see the right
term here so
it's also a level of abstraction has to
be right so if your yeah if you're
accurate at the somehow at the quantum
level hmm
that may not be convincing to us at the
human level well right but the question
is what accuracy at the sort of level of
the underlying mechanisms do you need in
order to predict the behavior right at
the end of the day the test is just can
you you know foresee what the person is
going to do right I am you know and and
and and you know and and and and in
discussions of freewill you know it
seems like both sides want to you know
very quickly dismiss that question is
irrelevant well to me it's totally
relevant okay because you know if
someone says oh well you know I will
applause demon that knew the complete
state of the universe you know could
predict everything you're going to do
therefore you don't have free will you
know that it doesn't trouble me that
much because well you know I've never
met such a demon hey you know uh you
know and we you know we even have some
reasons the thing you know maybe it you
know it could not exist this part of our
world you know it was only an
abstraction a thought experiment on the
other hand if someone said well you know
I have this brain scanning machine you
know you step into it and then you know
every paper that you will ever write it
will write you know every thought that
you will have you know even right now
about the machine itself it will force a
you know a well if you can actually
demonstrate that then I think you know
that that you know that that sort of
threatens my internal sense of having
free will in a much more visceral way
you know but now you notice that we're
asking in a much more empirical question
we're asking is such a machine possible
or isn't it I mean if it's not possible
then what in the woes of physics or what
about the behavior of the brain you know
prevents it from existing so if you
could philosophize a little bit within
this empirical question at where do you
think would enter the the by which
mechanism would enter the possibility
that we can't predict the outcome so
there would be something they'll be akin
to a free will yeah well you could say
the the sort of obvious possibility
which was you know
kick knives by Addington and many others
about as soon as quantum mechanics was
discovered in the 1920s was that if you
know let's say a sodium ion channel you
know in the in the in in in the brain
right hey today you know it's its
behavior is chaotic right it sort of
it's governed by these hodja hodja ly
hot skin equations in neuroscience right
which are differential equations that
have a stochastic component right now
where does you know and this ultimately
governs let's say whether a neuron will
fire or not that's a basic chemical
process or electrical process by which
signals are sent in the brain exactly
exactly and and you know and so you
could ask well well where does the
randomness in the process you know that
that neuroscientists you're but what
neuroscientists would would treat is
randomness where does it come from
you know ultimately it's thermal noise
right where does thermal noise come from
but ultimately you know there were some
quantum mechanical events at the
molecular level that are getting sort of
chaotically amplified but you know a
sort of butterfly effect and so you know
even if you knew the complete quantum
state of someone's brain you know at
best you could predict the probabilities
that they would do one thing or do
another thing right I think that part is
actually relatively uncontroversial
right the the controversial question is
whether any of it matters for the sort
of philosophical questions that we care
about because you could say if all it's
doing is just injecting some randomness
into an otherwise completely mechanistic
process well then who cares right and
more concretely if you could build a
machine that you know could just
calculate the even just up the
probabilities of all of the possible
things that you would do all right and
you know um you know if all the things
that said you had a 10% chance of doing
you did exactly a tenth of them you know
and and and and and and so on that
somehow also takes away the feeling of
freedom
exactly I mean I mean to me it seems
essentially just as bad as if the
Machine deterministically predicted you
it seems you know hardly different from
that
so that so then but a more a more subtle
question is could you even learn enough
about someone's brain to do that okay
because you know another central fact
about quantum mechanics is that making a
measurement on a quantum state is an
inherently destructive operation okay so
you know if I want to measure the you
know position of a particle right it was
well before I measure it it had a
superposition over many different
positions as soon as I measure I
localized it right so now I know the
position but I've also fundamentally
changed the state and so so you could
say well maybe in trying to build a
model of someone's brain that was
accurate enough to actually you know
make let's say even even well calibrated
probabilistic predictions of their
future behavior maybe you would have to
make measurements that we're just so
accurate that you were just
fundamentally alter their brain okay or
or or or maybe not maybe you only you
know you it would suffice to just make
some nano robots that just measured some
sort of much larger scale you know
macroscopic behavior like you know is
that you know what is this neuron doing
what is that neuron doing maybe that
would be enough see but now you know III
but what I claim is that we're now
asking a question you know in which you
know it is it is it is possible to
envision what progress on it would look
like yeah but just as you said that
question may be slightly detached from
the philosophical question in the sense
if consciousness somehow has a role to
the experience of free will because
ultimately what we're talking about free
will we're also talking about not just
the predictability of our actions but
somehow the experience of them
predictability yeah well I mean a lot of
philosophical questions ultimately like
feed back to the hard problem of
consciousness you know and as much as
you can try to sort of talk around it or
not right and you know and then and and
there is a reason why people try to talk
around it which is that you know
Democritus talked about the hard problem
of consciousness you know in 400 BC in
terms
that would be totally recognizable to us
today right and it's really not clear if
there's been progress since or what
progress could possibly consist of is
there a Q prime type of sub question
that could help us get it consciousness
it's something about cars oh well I mean
well I mean there is the whole question
of you know of AI right of you know can
you build a human level or superhuman
level AI and you know can it can it work
in a completely different substrate from
the brain I mean there's you know of
course that was Alan Turing's point and
you know and and and even if that was
done it's you know maybe people would
still argue about the hard problem of
consciousness right and yet you know my
claim is a little different my claim is
that in a world where you know there
were you know human-level AI is where we
had been even overtaken by such a eyes
the entire discussion of the hard
problem of consciousness would have a
different character right it would take
place in different terms in such a world
even if we hadn't answered the question
and and my claim about free will would
be similar right that if there if this
prediction machine that I was talking
about could actually be built
well now the entire discussion of the
you know a free will is sort of
transformed by that you know even if in
some sense the the metaphysical question
hasn't been answered yeah exactly
transforms it fundamentally because say
that machine does tell you that it can
predict perfectly and yet there is this
deep experience of free will and then
that changes the question completely
yeah and it starts actually getting to
the question of the a the a GI the
touring questions if the demonstration
of free will the demonstration of
intelligence the demonstration of
consciousness does that equal
nauseousness intelligence and free will
but see elects if every time I was
contemplating a decision you know this
machine had printed out an envelope you
know where I could open it and see that
it knew my decision I think that
actually would change my subjective
experience of making decisions you might
mean doesn't knowledge change your
subjective experience well well you know
I mean I mean the knowledge that this
machine had pretty
did everything I would do I mean it
might drive me completely insane right
but at any rate it would change my
experience to act you know to not just
discuss such a machine as a thought
experiment but to actually see it yeah I
mean I mean you know you could say at
that point you know you could say you
know what why not simply call this
machine a second instantiation of me and
be done with it right what we know what
why even privilege the original me over
this perfect duplicate that that exists
in the machine yeah or yeah there could
be a religious experience with a Jew
it's kind of what God throughout the
generations is supposed to that God kind
of represents that perfect machine is
able to I guess actually well I I don't
even know what a work what are the
religious interpretations of freewill
yeah does so if God knows perfectly
everything in in religion in the various
religions were does freewill fit into
that do you know that has been one of
the big things that theologians have
argued about for thousands of years you
know I am I am NOT a theologian maybe I
shouldn't go there there's not a clear
answer in a book like I mean I mean this
is you know the Calvinists debated this
the you know this has been you know I
mean different religious movements have
taken different positions on that
question but that is how they think
about it you know meanwhile you know a
large part of sort of what what animates
you know theoretical computer science
you could say is you know we're asking
sort of what are the ultimate limits of
you know what you can know or you know
calculate or figure out by you know
entities that you can actually build in
the physical world right and if I were
trying to explain it to a theologian
maybe I would say you know we are
studying you know to what extent you
know gods can be made manifest in the
physical world I'm not sure my
colleagues would like that so let's talk
about quantum computers yeah sure sure
as you said modern computing at least in
the 1990s was a profound story at the
intersection of computer science physics
engineering math and philosophy
so the
there's this broad and deep aspect to
quantum computing that represents more
than just the quantum computer yes but
can we start at the very basics what is
quantum computing yeah so it's a
proposal for a new type of computation I
would say a new way to harness nature to
do computation that is based on the
principles of quantum mechanics
okay now the principles of quantum
mechanics have been in place since 1926
you know they haven't changed you know
what's new is you know how we want to
use them okay so what does quantum
mechanics say about the world you know
the the physicists I think over the
generations you know convinced people
that that is an unbelievably complicated
question and you know just give up on
trying to understand it I can let you in
not not being a physicist I can let you
in on a secret which is that it becomes
a lot simpler if you do what we do in
quantum information theory and sort of
take the physics out of it so the way
that we think about quantum mechanics is
sort of as a generalization of the rules
of probability themselves so you know
you might say there's a you know there
was a 30% chance that it was going to
snow today or something you would never
say that there was a negative 30% chance
right that would be nonsense much less
would you say that there was a you know
an I percent chance you know square root
of minus 1% chance now the central
discovery that sort of quantum mechanics
made is that fundamentally the world is
described by you know these are let's
say the possibilities for you know what
a system could be doing are described
using numbers called amplitudes okay
which are like probabilities in some
ways but they are not probabilities they
can be positive for one thing they can
be positive or negative in fact they can
even be complex numbers okay and if
you've heard of a quantum superposition
this just means the sum state of affairs
where you assign an amplitude one of
these complex numbers to every possible
configuration that you could see assist
them in on measuring it so for example
you might say that an electron has some
amplitude for being here and some other
amplitude for being there right now if
you look to see where it is
you will localize it right you will sort
of force the amplitudes to could be
converted into probabilities that
happens by taking their squared absolute
value okay and then and and then you
know you can say either the electron
will be here or it will be there and you
know knowing the amplitudes you can
predict the price the probabilities that
it will that you'll see each possible
outcome okay but while a system is
isolated from the whole rest of the
universe the rest of its environment the
amplitudes can change in time by rules
that are different from the the normal
rules of probability and that are you
know alien to our everyday experience so
any time anyone ever tells you anything
about the weirdness of the quantum world
you know or assuming that they're not
lying to you right they are telling you
you know and yet another consequence of
nature being described by these
amplitudes so most famously what
amplitudes can do is that they can
interfere with each other okay so in the
famous double slit experiment what
happens is that you shoot a particle
like an electron let's say at a screen
with two slits in it and you find that
there you know on a second screen now
there are certain places where that
electron will never end up you know
after it passes through the first screen
and yet if I close off one of the slits
then the electron can appear in that
place okay so by so by decreasing the
number of paths that the electron could
take to get somewhere you can increase
the chance that it gets there okay now
how is that possible
well it's because we you know as we
would say now the electron has a
superposition state okay it has some
amplitude for reaching this point by
going through the first slit it has some
other amplitude for reaching it by going
through the second slit but now if one
amplitude is
positive and the other one is negative
then note you know I have to add them
all up right I have to add the
amplitudes for every path that the
electron could have taken to reach this
point and those amplitudes if they're
pointing in different directions they
can cancel each other out that would
mean the total amplitude is zero and the
thing never happens at all I closed off
one of the possibilities then the
amplitude is positive or it's negative
and now the thing can happen okay so
that is sort of the one trick of quantum
mechanics and now I can tell you what a
quantum computer is okay a quantum
computer is a computer that tries to
exploit you know these exactly these
phenomena superposition amplitudes and
interference in order to solve certain
problems much faster than we know how to
solve them otherwise so is the basic
building block of a quantum computer is
what we call a quantum bit or a qubit
that just means a bit that has some
amplitude for being zero and some other
amplitude for being what so it's a
superposition of zero in one states
right but now the key point is that if
I've got let's say a thousand cubits the
rules of quantum mechanics are
completely unequivocal that I do not
just need one amp but you know I don't
just need amplitudes for each qubit
separately okay in general I need an
amplitude for every possible setting of
all thousand of those bits okay so that
what that means is two to the 1000 power
amplitudes okay if I if I had to write
those down
let's or let's say in the memory of a
conventional computer if I had to write
down two to the 1000 complex numbers
that would not fit within the entire
observable universe okay and yet you
know quantum mechanics is unequivocal
that if these qubits can all interact
with each other and in some sense I need
to to the 1000 parameters you know
amplitudes to describe what is going on
now you know now I can do you know where
all the popular articles you know about
quantum computer and go off the rails is
that they say you know they they sort of
sort of say what I just said and then
they say oh so the way a quantum
computer works is just by
trying every possible answer in parallel
okay you know you know that that sounds
too good to be true and unfortunately it
kind of is too good to be true that the
problem is I could make a superposition
over every possible answer to my problem
you know even if there were two to the
one thousand of them right I can I can
easily do that the trouble is for a
computer to be useful you've got at some
point you've got to look at it and see
and see an output right and if I just
measure a superposition over every
possible answer then the rules of
quantum mechanics tell me that all I'll
see will be a random answer you know if
I just wanted a random answer well I
could have picked one myself with a lot
less trouble right so the entire trick
with quantum computing with every
algorithm for a quantum computer is that
you try to choreograph a pattern of
interference of amplitudes and you try
to do it so that for each wrong answer
some of the paths leading to that wrong
answer have positive amplitudes and
others have negative amplitudes so on
the whole they cancel each other out
okay whereas all the paths leading to
the right answer should reinforce each
other you know should have amplitudes
pointing the same direction so the
design of algorithms in the space is the
choreography of the interferences
precisely that's precisely what it was
take a brief step back and write you
mentioned information yes so in which
part of this beautiful picture that
you've painted is information contained
oh well information is that the core of
everything that we've been talking about
right I mean the bit is you know the
basic unit of information since you know
Claude Shannon's paper in 1948 you know
and you know of course you know people
had the concept even before that you
know he popularized the name right but I
mean but a bit at zero or one that's
right basically that's right and what we
would say is that the basic unit of
quantum information is the qubit is you
know the object any object that can be
maintained tennis manipulated in a
superposition of 0 and 1 States now you
know sometimes people ask well but but
but what is a qubit physically
and there are all these different you
know proposals that are being pursued in
parallel for how you implement qubits
there is you know superconducting
quantum computing that was in the news
recently because of Google's the quantum
supremacy experiment right where you
would have some little coils where a
current can flow through them in two
different energy states one representing
a 0 another representing the 1 and if
you cool these coils to just slightly
above absolute zero like a hundredth of
a degree then they super conduct and
then the current can actually be in a
superposition of the two different
states so that's one kind of qubit
another kind would be you know just in
an individual atomic nucleus it has a
spin it could be spinning clockwise it
could be spinning counterclockwise or it
could be in a superposition of the two
spin States that is another qubit but
she's just like in the classical world
right you could be a virtuoso programmer
without having any idea of what a
transistor is right or how the bits are
physically represented inside the
machine even that the machine uses
electricity right you just care about
the logic it's sort of the same with
quantum computing right qubits could be
realized by many many different quantum
systems yet all of those systems will
lead to the same logic you know the
logic of qubits and and how you know how
you measure them how you change them
over time and so you know that the
subject of you know how qubits behave
and what you can do with qubits that is
quantum information so just a linger on
that short so does the physical design
implementation of a qubit mm-hmm
does not does not interfere with the
that next level of abstraction that you
can program over it so the true is the
idea of it is is the a is it okay well
to be honest with you today they do
interfere with each other that's because
the all the quantum computers we can
build today are very noisy right and so
sort of the the the you know the qubits
are very far from perfect and so the
lower level sort of
affect the higher levels and we sort of
have to think about all of them at once
okay but eventually where we hope to get
is to what are called error corrected
quantum computers where the qubits
really do behave like perfect abstract
qubits for as long as we want them to
and in that future you know the you know
which you know a future that we can
already Street or sort of prove theorems
about or think about today but in that
future the the logic of it really does
become decoupled from the hardware
so if noise is currently like the
biggest problem for quantum computing
and then the dream is error correcting
modern computers can you just maybe
describe what does it mean for there to
be noise in the system
absolutely so yeah so the problem is
even a little more specific than noise
so that the fundamental problem if
you're trying to actually build a
quantum computer you know of any
appreciable size is something called
decoherence okay and this was recognized
from the very beginning you know when
people first started thinking about this
in the 1990s now what decoherence means
is sort of unwanted interaction between
you know your qubits you know the state
of your quantum computer and the
external environment okay and why is
that such a problem why I said talked
before about how you know when you
measure a quantum system so let's say if
I measure a qubit that's in a
superposition of 0 and 1 States to ask
it you know are you zero or are you one
well now I force it to make up its mind
right and now probabilistically it
chooses one or the other and now you
know it's no longer a superposition
there's no longer amplitudes there's
just there's some probability that I get
a zero and there's some that I get a one
and now the the the the the trouble is
that it doesn't have to be me who's
looking guy or in fact it doesn't have
to be any conscious entity any kind of
interaction with the external world that
leaks out the information about whether
this qubit was a 0 or a 1 sort of that
causes the zero Ness or the oneness of
the qubit to be recorded
you know the radiation in the room in
the molecules of the air in the wires
that are connected to my device any of
that as soon as the information leaks
out it is as if that qubit has been
measured okay it is you know the the the
state has now collapsed you know another
way to say it is that it's become
entangled with its environment okay but
you know from the perspective of someone
who's just looking at this qubit it is
as though it has lost its quantum state
and so what this means is that if I want
to do a quantum computation I have to
keep the qubits sort of fanatically well
isolated from their environment but then
at the same time they can't be perfectly
isolated because I need to tell them
what to do I need to make them interact
with each other for one thing and not
only that but in a precisely
choreographed way okay and you know that
is such a staggering problem right how
do i isolate these qubits from the whole
universe but then also tell them exactly
what to do I mean you know there were
distinguished physicists and computer
scientists in the 90s who said this is
fundamentally impossible you know the
laws of physics will just never let you
control qubits to the degree of accuracy
that you're talking about now what
changed the views of most of us was a
profound discovery in the mid to late
90s which was called the theory of
quantum error correction and quantum
fault tolerance okay and the upshot of
that theory is that if I want to build a
reliable quantum computer and scale it
up to you know an arbitrary number of as
many qubits as I want you know and doing
as much on them as I want I do not
actually have to get the cube it's
perfectly isolated from their
environment it is enough to get them
really really really well isolated okay
and even if every qubit is sort of
leaking you know it state into the
environment at some rate as long as that
rate is low enough okay I can sort of
encode the information that I care about
in very clever ways across the
collective states of multiple qubits
okay in such a way that even if you know
a small percentage of my cube it's
leaked well I'm constantly monitoring
them to see if that week happened I can
detect it and I can correct it I can
recover the information I care about
from the remaining qubits okay and so
you know you can build a reliable
quantum computer even out of unreliable
parts right now the the in some sense
you know that discovery is what set the
engineering agenda for quantum computing
research from the 1990s until the
present okay the goal has been you know
engineer qubits that are not perfectly
reliable but reliable enough that you
can then use these error correcting
codes to have them simulate qubits that
are even more reliable than they are
regarded the error correction becomes a
net win rather than a net loss right and
then once you reach that sort of
crossover point then you know your
simulated qubits could in turn simulate
qubits that are even more reliable and
so on until you've just you know
effectively you have arbitrarily
reliable cubans so long story short we
are not at that break-even point yet
we're a hell of a lot closer than we
were when people started doing this in
the 90s like orders of magnitude closer
but the key ingredient there is the more
qubits the butter because well the more
qubits the larger the computation you
can do right I mean I mean a qubit Tsar
what constitute the memory of your
quantum computer it also for the sorry
for the error correcting mechanism yes
so so so the way I would say it is that
error correction imposes an overhead in
the number of qubits and that it is
actually one of the biggest practical
problems with building a scalable
quantum computer if you look at the
error correcting codes at least the ones
that we know about today and you look at
you know what would it take to actually
use a quantum computer to you know a I'm
hack your credit card number because you
know you know maybe you know the most
famous application people talk about
right let's say to factor huge numbers
and thereby break the RSA cryptosystem
well what what that would take would be
thousands of several thousand logical
cube
but now with the known error correcting
codes each of those logical qubits would
need to be encoded itself using
thousands of physical qubits so at that
point you're talking about millions of
physical qubits and in some sense that
is the reason why quantum computers are
not breaking cryptography already it's
because of this these immense overheads
involved so that overhead is additive or
multiplicative I mean it's like you take
the number of logical qubits that you
need in your abstract quantum circuit
you multiply it by a thousand or so so
you know there's a lot of work on you
know inventing better trying to invent
better error correcting codes okay that
is the situation right now in the
meantime we are now in what physicist
John Prescott called the noisy
intermediate scale quantum or NIST era
and this is the era you can think of it
as sort of like the vacuum you know
we're now entering the very early vacuum
tube era of quantum computers the
quantum computer analog of the
transistor has not been invented yet
right that would be like true error
correction right where you know we are
not or
or something else that would achieve the
same effect right we are not there yet
and but but but where we are now let's
say as of a few months ago you know as
of Google's announcement of quantum
supremacy you know we are now finally at
the point where even with a non error
corrected quantum computer with you know
these noisy devices we can do something
that is hard for classical computers to
simulate okay so we can eke out some
advantage now will we in this noisy era
be able to do something beyond what a
classical computer can do that is also
useful to someone that we still don't
know people are going to be racing over
the next decade to try to do that by
people I mean Google IBM you know a
bunch of startup companies or you know a
player's apps
yeah and in research labs and
governments and yeah you just mentioned
a million things well backtrack for a
sec yeah sure sure so we're in these
vacuum tube days yeah
just entering and I'm just entering Wow
okay so yeah how do we escape the vacuum
so
we get to how to get to where we are now
with the cpu is this a fundamental
engineering challenge is there is there
breakthroughs in on the physics side
they're needed on the computer science
side
what Oh is there an is it a financial
issue we're a much larger just sheer
investment and excitement is new so you
know those are excellent questions oh my
god well no no my my my guess would be
all of the above yeah I mean my my guess
you know I mean I mean you know you
could say fundamentally it is an
engineering issue right the theory has
been in place since the 90s you know at
least you know you know this is what you
know error correction what you know
would look like you know we we do not
have the hardware that is at that level
but at the same time you know so you
could just you know try to power through
you know maybe even like you know if
someone spent a trillion dollars on some
quantum computing Manhattan Project
right then conceivably they could just
you know build a an error corrected
quantum computer as it was envisioned
back in the 90s right I think the more
plausible thing to happen is that there
will be further theoretical
breakthroughs and there will be further
insights that will cut down the cost of
doing this so let's take good briefs
yeah to the faux soft goal I just
recently talked to Jim Keller who's a
sort of like the famed architect
and then in the microprocessor world
okay and he's been told for decades
every year that the Moore's law is going
going to die this year and he tried
tries to argue that the the Moore's law
is still alive and well and it'll be
alive for quite a long time to come how
long how long he's is the the main point
is it still alive but he thinks there's
still a thousand X improvement just on
shrinking a transition as possible
whatever the point is that the
exponential growth you see it is
actually a huge number of these s curves
just constant breakthroughs at the
philosophical level mm-hmm why do you
think we as a descendants of apes were
able to to just keep coming up with
these new breakthroughs on the CPU side
is this something unique to this
particular endeavor or will it be
possible to replicate in the quantum
computer space okay all right the other
there was a lot there too but didn't it
to to break off something I mean I think
we are in an extremely special period of
human history right I mean it's it is
you could say obviously special you know
in many ways right there you know you
know way more people alive than there
than there than there have been and you
know the you know the whole you know
future of the planet is in is in is in
question in a way that it it hasn't been
you know through for the rest of human
history but but you know in particular
you know we are in the era where you
know we we finally figured out how to
build you know Universal machines it's
that you know the things that we call
computers you know machines that you
program to simulate the behavior of
whatever machine you want and you know
and and and and and and and and and once
you've sort of crossed this threshold of
universality
you know you've built you could say you
know touring you've instantiated touring
machines in the physical world well then
the main questions are ones of numbers
there you know ones of how m