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Please give us a warm round of applause
for Christophe.
Oh, excellent.
OK, I'm pissed off that I'm a physicist,
and today I would like to talk about
long distance quantum cryptography,
communication.
It's the same.
So at first I would like to give a small
introduction.
Then I would like to talk about quantum
communication, then about
extending the reach and finally about the
quantum repeater.
So the introduction,
um, let's assume we would
like to talk would like to communicate
in secrecy.
And it would be preferable
if you can use a provable
security and for approvable
security, we need one at the one
time. Pett the one
time. It is a very simple
symmetric encryption algorithm
and it was invented in 1882
by Frank Miller and later I think in 1917
and 1980, reinvented,
um, as you can see here,
as you can see,
as you can see, as you can see here,
there is a plaintext, there is a key
module of the alphabet and
the alphabet here is two
because we are using zeros and ones.
And if we just apply this machine,
we're getting a cipher text,
which is not decrypted
for a secure application.
We have got some
requirements.
We need a random
secret key. We need a secret as
long as the data and we need
a one time secret key.
So in the end, we Single-Use
long, random secret keys.
The problem is the key distribution,
the key must be printed, crafted
or saved on a physical object
and then the key could be copied
during mechanical transport.
And transport is a logistical
expensive and
without a quantum regime in the clerical
regime, the solution
relies on trust.
But you would like go to the
quantum regime.
Over here, you can see some photons.
And I would
like to introduce, introduce, to
introduce to the
quantum effect of entanglement
first in the quantum regime,
when you measure things,
you don't measure deterministic
but probabilistic.
So when you measure the photon, one in
the state, you measure it with the
probability of alpha.
The same goes with the full time
to which is higher probability
of better and also
of photon three.
But if the photon three
is is part of an entangled photon
pair and the
measurement of the photon four
depends on the previous measurement of
photons. Three Einstein
called this book after found vehicle
spooky action at a distance.
OK, so now we have
got some some
introduction. We come to the quantum
communication and we can
think of a simplified quantum
communication system.
It goes like this.
There is a quantum entanglement
chomper,
um, Eliz purpose of
prepress, not the state of the Fortran
one, but then
copies to state a fortune to
two to four, two and three,
then measures and then measures
for two and three.
And as it is probabilistic,
Bob, repeat steps two and three until
the issue about his measurement.
So, yeah, he
he's sure about his measurement.
The only problem is this
scheme is oversimplified
and it is possible
with this if if this
system would be possible
for information distribution would
be faster than light.
And that contradicts the theory of
relativity, which is
bad.
OK, and you can
see the original author, although
I don't think he
talked about entangled photocopiers.
So, um, it's
not only contradicts
the theory of relativity,
but there is also the no colonial
theory since no clue,
there's no cloning theory was just
rented to counter the possibility
of faster than light information
communication, faster than light
communication.
And the proof depends on the linearity
of quantum mechanics.
Therefore, it's not very
sophisticated.
It is sophisticated.
Anyway, um,
what we have to keep in mind is that a
single photon cannot be cloned.
While this destroys or oversimplified
quantum communications team, it
also gives some kind of copy protections
for single photons, single Quantum's.
In our case, we would like to talk about
photons and
the important the.
Yeah, it's important to remember
that this copy protection is
only for single photons.
So when we have when we have got a
thousand photons and measure 100
of them, then we have got still
nine hundred photons that are not
disturbed by the measurement and
nobody will miss the hundred.
So we need a single photon
sources.
What are single photon sources?
Single photon sources
can be
can be characterized by
an experiment you see down below.
So you
take the photons of your light source,
put it here in the beam splitter, a
beam symmetry. If it's perfect
that 50 percent of the photons
to this Avalon's fortitude
and the other 50 percent to this
forward to the old avalanched,
40 odd, the coincidence
contours and measure
the time difference between the events
on this day or
between the events on this, the old
and the old.
So
as you can see here, there is
a graph to see
different sources.
The one the first one is the light bulb
of on top here,
which is a chaotic, chaotic source.
And in in a certain
time interval, it's very
probable that when we have
measured one event that we measured
a second the wind.
You as you can see, the photons are
bunching here.
OK, then we have got a laser.
The laser is a coherent
photon source and
the.
Event of an event
that the event is
an event is unrelated of
any, if any event has been
previous fiercely measured.
So if,
as you can see, this line
is quite straight, this photo
needs a bunch, not a..
Bunch a
single photo.
Well, the truth single
photon source, which is perfect
in theory,
does a.. Bunching.
So you as you can see here, he has a
certain
is a certain time interval.
And it is
the probability that a second event is
registered in this time interval
is nearly zero.
So but for
real significance also,
we say that the second autocorrelation
below zero point five is
a single photon source and
therefore
it is in its eye.
It is in discussion if if attenuated
lasers can also be single
photon sources, because
as you can see here, when you drop
every second photon off of this
measurement, you might
get something similar to this
to the screen graph.
OK, so
what are the possible sources
of the possible photon sources
are quantum notes, which are atomic
structures
on a normal scale,
and
they are somehow
engineered out of semiconductors, but
they are also not in semiconductor
quantum dots.
And then there are optical parametric
oscillators.
This is a very interesting source because
as this opio
generates untenured photon pairs
and you can then measure one photon
and know that there is a second photon
flying around and then in a different
state, then called a
heralded photon
source.
You can use single photons and you can
use Nigerien vacancy centers, diamonds,
um, just arm.
It's a five year old that is a diamond
made out of carbon and one of the carbon
of one of the carbon opto.
One of the carbon atoms in
this crystal is not is
not carbon, but it's nitrogen.
And these are very stable and very
popular right now.
Yeah. So, um,
it's completely anti bunching behavior.
Correct. Desire's characterizes
our single photon source, and
the non single photon sources
would allow non disturbing measurement of
the state, which would
in the end harm our security
of our device.
OK, we,
um, here is a
quantum key concept, quantum
key distribution concept.
And you have over here,
over here Ellas a
photon and a certain polarization.
Then send this photon to the electoral
optical modulator which
shifts the polarization at random.
Uh, the photons then send over the
quantum channel to the BOP
where BOP
modulates the incoming phone to at
random and then measure it with a
Paula popularizing beam
splitter
and detect it was the property of the
polarizing beams, which is different from
the beam spectrum as
it transmits
or reflects photon.
This is here.
It reflects
a photon based on
on its polarization.
So, um,
now Ellerson bobbleheads have
done some measurement and
then Alice and Bob
compare.
They are random modulation
of the um, of
a verified, verifiable
channel.
Um, this channel
can be public but must not.
But it is must be verifiable.
Um, just
in Beebee, 1884,
there are two to two states for encoding
in two states for randomizing,
so you can use two states
and here you are using also two states.
And as an important they're also
two states you can use.
OK, there are various protocols
you can use. But I just
mentioned it was the first point of
protocol and is intended
for for states to enact
and ninety one is used
for users and on transmission
and zerk protocol
users is used in some commercial
and available quantum key distribution
devices.
Of course, the security
depends on on the implementation.
And you might have seen the nice
talk in 2007.
It's a college communication device
where they show some practical
attacks on
quantum cryptography.
So we have got to
we have got some quantum communication
going on in the laboratory, which is
awesome, but we would like to go
beyond and therefore we need
to extend the range.
One thing we could do is we can use
the free space like the
the air, which is
has an advantage that is very flexible
because it's nearly everywhere.
And
the disadvantage is that
there's background light like the sun or
streetlamps or street lamps.
And when you have got
a photon count in your APD,
the SPDR burns
out or just is separated and doesn't
detect any events.
You can handle others.
But it is a, doesn't it?
It is a disadvantage.
And other disadvantage is that the
distribution medium, the air is not
very stable.
That's, for example, the fog or heavy
rain or something like this.
And this would degrade your
your
key rate.
And yeah, it's
not what we would like to have.
And there has been
some experiments and they reached, I
think, about between 50 and 100
kilometers a
free space, quantum communication.
The main problem of them for
for for reaches beyond
100 kilometers.
It's a problem that the earth is not
flat.
You can't reach Tokyo from Hamburg
without a very high tower or a satellite.
So there was quantum condition
proposed by satellite and
by satellite. You sending, for example,
a single photon and tangled
photon power source up was the satellite.
And the satellite sends one photon to
the receiver, one and one for to receiver
to each receiver measures,
then
measure, then the
photon.
And through entanglement
of the photons, the measurements are
correlated.
Uh, you can see a paper
that has been published.
So that shows some
more, uh, satellite
seems to communicate.
OK, then the next
thing we could do is we couldn't do
quantum communication by fiber.
And the advantages here that the signal
in the fiber is a relatively independent
of the environment. I say relatively
because this has
shaken his fiber during an experiment,
knows how hard it can get to
get any signal out of it.
The fiber is, of course, flexible, but it
is definitely not mobile.
And the good thing is also that you can
use commercially available
fibers and
these fibers are also already in the
ground and can be used as Advil.
So the disadvantage is that
that you can't use every wavelength
because the most wavelengths are,
uh, have a high loss in the fiber.
The wavelengths that are used today
for network traffic around
Etemad are around one thousand
three ten and around
one thousand five hundred fifty.
But even the the last one has
zero zero point two DB
damping per kilometer and
the reach is limited to about 100
kilometers.
So so what we should
do now is find
a scheme to enhance this reach.
One possibility is that
node concepts here you
combine quantum transmission like
here one, two and three with classic
transmissions like and B,
the problem is obvious that you
need to trust the persons or the machines
or whatever works at this
place.
Here at the
transmission takes place.
It relies on trust that
we also already had before.
I was always might be interesting.
Interesting test case for some things.
Instead, you would like to reach
the quantum repeater.
The quantum computer looks like this.
It's, uh, consists out of a quantum
memorium. And like here at one
like here it has
got some entangled photon pairs
of Inspecteur and two photoed yours.
It is an idea published in this
paper Downbelow.
So let's take a look at
first. It's a beam.
Spitzer It's a
common swapping and
Tengblad swapping means.
That you got here two on
one and here 42, and
you sent those photons, which are part of
a entangled photon pass to the speed
superior when this
photons are indistinguishable,
just photons bunch and
end up. Is that this totally accurate
and distorted fortitude
when this photon bunch and
entanglement between these two
and the entanglement
between two and three is swapped
and afterwards you have got entanglement
between three and
four and
thereby you have swapped in internment
if it have have reached a greater
range.
So now we have got entanglement
and entanglement swapping
and we would like to have also
memory quantum memory element because
it enlightens
networks and synchronization
and it might be possible
to connect this quantum memory event
to other devices you would like
to use.
OK, the quantum memory element
there has been defined some pretty for
memory elements.
As I said before, the wavelength is very
important.
The bandwidth is important, which means
that it's a frequency
interval in which photons
would be still stored in
the quantum memory the field entity
describes as a quality
of the photon. So if I send in a four
photon instead, are
the fortunate that the fortunate
comes out of the quantum memory element
should also be a state and
not something else, because then
it would be a random generator, but not a
memory element.
And then the efficiency, that means
how often I have to send in a photon
to get a photon out.
And of course, the storage time and the
capacity is also important.
Again, we can use
quantum dots in recent years in
atomic gases and atoms
for for quantum memory.
OK, quantum dots, you
can see them here on the right hand side.
I think it's an atomic force, microscopy
and other sizes.
And this is out of semiconductors.
So you've got a semiconductor
of a certain lattice and
other semiconductors with similar
but not the same letters on top
and a bunch because they thereby
reduce the energy
in the surface energy.
OK, yeah, we have got
the the conducting band
as the band structure of the
quantum dot and the upper left.
On the other side is the conducting
band, which is an
electron ensemble of electrons
far away from the forms, the atomic
core and Downbelow.
You've got to balance electrons which are
electrons near the atomic core.
And then there are
some some
some states in this this
this band, when
a photon comes into
into the quantum dot, it is absorbed
and
it generates an exit on an electron
is an electron holper.
So because quantum
dot is biased,
um, the, the
whole as you can see in the middle,
is moved out of the quantum dot
and the electron can't
recombine, at
least for a certain time.
That's not stable for hundreds of years.
So then you have got
up there and
the incoming incoming photon
and to the information
on the incoming photon and the
information on on the electron
are correlated
over a polarization to
spin correlation.
So then for a readout,
you would you would
bias the quantum lot again.
But this time to move in some
holes, as you can see down below,
then to enter the quantum
dot, then the electron
and one how we can combine and
and photon will be sent
out if the fidelity is perfect,
is the same
photon we have sent in.
So
in this paper, they mentioned that their
fidelity was around 80 percent plus
minus 10 and they
had only efficiency of
20 percent, but
others reach out to 97
percent.
So which is a very high
value. Of course, the start
time is about 100 microseconds.
And just recently I read
the paper or
at least the abstract where they thought
a photon for about 39
minutes. So it's a
sure enough probable
long, long enough storage time.
The bandwidth is one
one gigahertz with
which I think is also wide enough
as as
you would like to
entangles and swap this photons
and four and taking swopping as a mind
to remind you, you need to
indistinguishable photons.
And I'm
not not so sure where the
where you can't and where you about
photons are not indistinguishable
anymore. But I think
one gigahertz is good enough
for using it as a quantum theory.
So as we have got quantum dots,
um, the wavelength of the quantum
memory can be tuned as
as you wish and for the
capacity you can do,
of course, if can frequency multiplexing.
And on the other hand,
multi photon storage is not likely,
as they say in their paper.
OK, now
we have got the quantum repeater.
We have got here all quantum memory,
which is toss photons
for us for a certain time.
And over here, the entangled photon paths
which are useful
for the entanglement swapping.
If we combine if we combine
a long chain of this quantum repeaters,
we can reach any distance
we would like to have.
OK,
so coming to my conclusion is
talk to you something about the freespace
quantum communication, the quantum
communication by fiber and
quantum computer. Was it was it
was it was
there the main parts of
the bedside measurement and the quantum
memory, if you like,
if you like the quantum communication,
you also could take a look at quantum
dots, which are a very interesting
nanoscale structure and frequency
down, down and up conversion.
And yeah, I would like to thank you
for your attention.
Now, if you have questions, there are
mikes there, there, and
I think this one also works also,
if you're on the stream, you can use
your Twitter.
Any questions in the room?
OK, you go ahead.
Yeah, you
showed us with a repeater.
You have this beam splitter.
Yeah. And you just set the photons
can bunch.
How do they do that?
Um, I don't know how,
but the important point, um,
because it's quantum mechanics.
The important point is this is photons
come to the beam splitter.
OK, and if they're just indistinguishable
then they are.
This is the concept.
Why is that bunch. It's not part of
physics, but,
uh, it's it's.
Yeah, it's in the quantum mechanics.
OK, thank you.
Internet, please.
Audio, please.
Thanks. Um, so I only
have one question.
How do you fix the static magnetic
symmetry problem for the cube?
It does that problem
even exist?
It's called static magnetic.
The symmetry problem for
the cube. It
yes. Does that exist?
I'm not so sure.
OK, I think the
one who posed the question wasn't so sure
either. So never mind.
Then
you go ahead.
I don't know if I could follow.
Probably just wouldn't be possible
to have men in the middle attacks,
so if there is one untrusted
step in this whole chain,
would you detect it or is
it undetectable this
way, though?
You mean with a quantum key distribution?
Not if you have a chain, but not
normally in a bar you have to
do to amplify the photons.
This is done with some notation
now and classical way
with. Yes, some some
rough elements and something
like this.
But this is all classical and that use
stimulated emission to simply multiply
the photons.
So we would have one photon, which is
exactly the same quantum numbers but
hasn't been measured.
I never understood why this is not a
problem for for quantum
cryptography.
I think you're talking about the no
cloning theory.
You can't clone a single
photon state perfectly as
far as I could read.
It only applies for a
system. Professor Hamiltonian and
stimulated emission has a
completely different Hamiltonian,
which which is not.
This is not a play as simple as I think.
I don't think so.
As the no cloning theory
says that, you know, you can't
copy a perfect
perfectly copy quantum and
arbitrary state.
So you maybe generate
a photon in a
certain state
because they have been produced before.
So if I use the same process, I might get
the same photon again.
But you can't measure it perfectly
with a laser. But in my opinion, does
exactly this simply copy?
No, they they are not in the same state
and then they don't have
the same polarization
in the laser.
That's the answer.
OK, OK.
So can you
can you say anything about the loss rate
of the repeater or is it perfect.
It's perfect, of course.
Uh, uh,
well right now there is no quantum
repeater that it is an active
research topic.
And right
now there is a loss rates.
I mean,
the problem right now is that the crunch
memory is not not there.
So we can talk about a quantum repeater,
but the rates are right now are very
high.
So there's something that we have
which is actively research.
OK, thanks.
Please go ahead.
I am so usually when
people talk about entanglement, they
consider properties of particles, for
example, photons in two different
locations.
So the two particles have
essentially pretty applications.
And the Heisenberg's uncertainty
principle, of course, and the momentum
component of the particles, properties
moving kind of broad.
So the wavelength is not
very sharply defined for the photons,
which I think is fine in this case.
But I'm saying in
the way people normally talk about
entanglement, the space component is very
sharp and the momentum component isn't.
So my question is, in principle, I think
you could also have a sharp, um,
momentum component and a very undefined
space component. For example, if you look
at electrons and carbon nanotubes, they
might be delocalized
essentially, and one might be going this
way around the nanotube
government going the other way around.
So my question is, have there been any
experiments in using entanglement, not
in space, but in momentum space?
And if so, what could be practical
applications for encryption and
stuff?
Uh, because it would be possible
that Senator.
I can't remember people.
This was described so
I don't know every paper, but
it might be possible,
so. OK, thanks.
OK, do we have any more questions?
Yes, but
the entanglement of the polarizations of
two photons in two different locations.
Yeah. And you could also, for example,
two electrons in the same position
in the same nanotube and one is going
counterclockwise.
In the other one's going clockwise.
And that is two different locations in
momentum space where they overlap
in the direct conventional
space.
That's what I meant.
Hmm. OK.
Any more questions?
OK, go ahead.
I have only a very small
question, so I was wondering
what is the problem with
conserving the state
when you, for example, when
you use
when you try to to save
phantom states,
you just have that you can save
it up to one hundred
microseconds.
Also, what the problem
why can't I only save the state of
such a short time.
The.
And this is only one
from the memory, so there might be others
I just mentioned, there has
been published a paper in
November would say that
they can save the state for about 39
minutes at room temperature
and a sink for about three
hours, an
encouraging environment.
The problem here is that the occurrence
of the electron, you see
this electron is not stable for
arbitrary time.
And after
and after more time, they
I think they had problems
through the doubt.
Thank you very much.
I have a question concerning detector's
how do you actually detect a thought
on state and get the
on state out of the photon?
So it's,
uh, I can't really imagine
how to build and detect that can
catch these different states.
Do you apply to this with you, with
the beam splitter to do.
Yeah.
When when I talk about states,
I mean, just one certain property,
like for example, for example, the
polarization, of course you can use other
properties, but the polarization
you can catch with this
polarizing beam splitter, which
can differ between
two polarization and
when it's of one paralyzation, it goes
to the front
of your Downbelow and when it's off,
another polarization, it goes to the 40
year old on the other side.
But if you if you if you supported
old, you never have actually
one single photon detection
with an avalanche. What do you do to your
diet? You only can detect
a bunch of photons coming to the light.
You're not able to detect one thing
before because this will not cause
the avalanche effect.
How do you come across this?
No, I mean, you use
an avalanche dude for detecting this
photon, but you can't detect one single
photon with an avalanche.
You do know.
I mean, it did.
Yeah. But you don't have one photon done
because it does not have a single
thoughtful resolution.
This type of detector.
It depends on what kind of avalanche
you use.
But there are there
are avalanche Forteo that, uh,
that are used for a single photon
detection.
I'm very sure,
very sure.
OK.
Any more questions, anyone?
No. OK, then. Thank you.
And another round of applause.