Kind: captions
Language: en
as you said mono 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 and 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 ok 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 the D sort of
let's say the possibilities for you know
what a system could be doing are
described using numbers called
amplitudes ok 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 ok and if you've heard
of a quantum superposition
this just means the some 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
they're 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't 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 and 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 close off
one of the possibilities then the
amplitude is positive or its negative
and the other 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 as 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
one 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 lets or let's say in the
memory of a conventional computer if I
had to write down two to the 1,000
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 do the 1,000 parameters
you know amplitudes to describe what is
going on now know now I can tell you
know where all the popular articles you
know about
I'm computing 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 okay 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 would
take a brief step back and when 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
called 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 in us
manipulated in a superposition of zero
in one states now you know sometimes
people ask well but-but-but what is a
qubit physically right and there are all
these different you know uh 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 zero
another representing the one 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 hmm does not
does not interfere with the that next
level of abstraction that you can
program over it so it truly 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
they are very noisy right and so sort of
the the the you know the qubits are very
far from from perfect and so the lower
level sort of does 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 suit
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 Monica yes can you just
maybe describe what does it mean for
there to be noise in the system
absolutely so yes 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
here 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 cubit was a 0 or a 1 serve
that causes the 0 ness or the oneness of
the cubit to be recorded in 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 ok 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 ok 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 ok 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 ok 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 in 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 ok
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
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 qubits
leak 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 right 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 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
which is you know maybe you know the
most famous application people talk
about right let's say two-factor
huge numbers and thereby break the RSA
cryptosystem well what what that would
take would be thousands of several
thousand logical qubits 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 mean
time 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 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
we'll 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 yes just
entering and I'm just entering Wow okay
so yeah how do we escape the vacuum so
how do 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 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
you