perfect okay let's start now um some i
will talking about uh quantum
communication and one of the big goals
of quantum communication is to produce
quantum networks so what are quantum
networks quantum networks consist out of
nodes and these nodes can process or
store quantum information and they also
have a light matter interface so using
this light matter interface and using
quantum channels they can send photons
to each other and they can distribute
entanglement between the nodes so in the
end you could use a you could think
about applications
such as distributed or blind quantum
computing or what is important for this
talk is you can use these networks
for
cryptography um this talk will be about
the the building block of such a quantum
network which is a quantum network link
and the realization that we have is we
use single rubidium atoms which are
trapped in an optical
dipole trap and we can generate
entanglement between
the siemens state of the atom and the
polarization of a single photon
this we do in two of our traps we
collect the photons send them to a bell
state measurement setup and by
entanglement swapping we can generate
entanglement between the two atoms
so this is a top view of our experiment
so here you see the lmu campus in munich
where you see the locations of our two
labs so we have one lab at the shelling
stress and the other lab where trap two
is located is at the shock strata
roughly 400 meters away the two labs are
connected via a 700 meter fibre channel
which we can use for the photons
to exchange them between the two labs
so we're not the only ones that do a
long distance
yeah entanglement distribution so over
the recent years there are actually
quite some interesting and really nice
experiments
so for example and not only using single
or medium atoms so here they use atomic
ensembles to entangle them
or at the terampo group they use
rubidium atoms single rubidium atoms
coupled to two cavities
or
in delft they now for the first time
realized a three node network using np
centers in diamond
i will be talking about two recent
results in our group and one is about
quantum key distribution so we already
had a nice introduction in the previous
talk and this will be a special or more
advanced way of qkd which is device
independent qcd
the second part of the talk was actually
nicely introduced also by andreas reiser
and that is that when you work with
rubidium
if you send the photons at 780
nanometers through optical fibers then
you have a lot of losses so here you see
the event rate of our atom atom
entanglement over the fiber length and
if you go to for example 10 kilometer of
fiber length then at 780 nanometer 780
nanometers you have no
events left or no photons left at the
end of the fiber so we need to come up
with
frequency conversion techniques which i
will present to you as well
okay this is the outline of the talk so
first i will talk to you about some
fundamentals of our experiment so how do
we generate
the atom atom entitlement then i will
talk about the device independent qkd
results
and i will end with the telecom
conversion
okay how do we trap a single atom so we
have our
high vacuum setup which we create a mod
of rubidium atoms then we shoot in an
optical dipole trap laser which we focus
down narrowly with the microscope
objective up to roughly two microns the
focus of this trap is exactly at the
position of the mod and then we can load
one atom from the mud into the optical
dipole trap the reason that we only get
one is because the focus of our dipole
trap is so small
then the nice thing about this setup is
that the atom strap exactly in the focus
of this microscope objective so the
microscope objective also collects the
fluorescence from the atom so we collect
the fluorescence use a dichroic mirror
to couple it out and then we can couple
the fluorescence into single mode fibers
the probability if the atom
sends out a photon to couple it into the
single mode fiber is in our sperm
experiment around one percent
uh if you then look uh here at the
single photon detector then you can
clearly over time see two levels so you
see a low level when there's no atom in
the trap and a high level when we have
an atom trapped
then the next step is to generate atom
photon entanglement so for this we first
prepare our atom
in the mf zero f equals one state we
shoot in an excitation pulse and then
there are three decay channels of the
atom so the atom can go to this this and
this state however
by the orientation of our collection
setup we do not collect photons that are
of pi emission and therefore we generate
the following entangled atom photon
state
this we do in two of our traps so we
generate two pairs of atom photon
entanglement and then we shoot the atom
to a bell state measurement setup
um
for this to work for good entanglement
swapping what we need to have is
indistinguishable photons so they need
to be in distinguishable spatially so
this we do by using a fiber beam
splitter to combine them on temporarily
they need to be the same and this we do
by
synchronizing the two excitation pulses
to the two atoms up to
400 picoseconds and specially the
photons need to be the same and this we
do by two times using a rubidium atom
and zeroing all the fields during the
atom photon entanglement generation
process
so the moment that we get a coincidence
detection here at the bell state
measurement then we know that
entanglement generation
succeeded um the success probability of
this process is around one in a million
uh however we can
repeat the tries uh quite fast here at
52 kilohertz so we expect the rate of
this atom atom entanglement of around
one success in six seconds
okay after having this herald after
generating entanglement between the two
atoms we can read out our atomic states
this we do by a
state selective ionization scheme so by
setting the polarization of this readout
beam
we can
excite a certain qubit state and this
excited state we then directly ionized
by at the same time shooting in an
ionization
um so by tuning yeah exactly so this uh
this readout is then completed by or
directly detecting the ionization
fragments so this is a picture of our of
our setup and you see channel electron
multipliers so what we can do is we put
five kilovolt
uh voltage
over here and then we can directly
detect the ion or the
end electron or we can just use
fluorescence detection and check is the
atom still in the trap yes or no and
that gives our final result
let me come to the next part and this is
about the implementation of device
independent qkd with this setup
so yeah let's first take a step back we
go to qkd actually the previous talk was
quite uh gave a quite good of an
introduction
so why we want to have qkd
and qkd systems currently are
commercially available so this is just a
figure from
toshiba having two quantum units which
are in different cities and are
connected via 10 kilometer optical fiber
and users can use this system to
distribute keys
however for qkd to be secure there are
always some assumptions
that you need to make so the first one
is that quantum theory needs to be
correct
but there are more assumptions for
example that the users need to have
freedom and secrecy of the measurement
settings and also the outcomes of the
measurements need to be secure
so if this was the only one then if
these were the only assumptions then it
would have not actually been a big
problem however for normal qkd there are
more assumptions so for example in the
security proof
you assume things about you make
assumptions about the states for example
that you use or you make assumptions
about the inner workings of the devices
and therefore the users actually need to
check their devices if they really
behave as specified in this proof
so the users need to have trust in their
devices another thing is is to make the
the people that make the devices can
make some mistakes so they can uh
not see some security breaches for
example it has been shown that the
detector
dark times
can open up
implementation flaws or cannot can open
up possibilities to eavesdropper key
so you need to really
the good thing here is is that
most probably it's always really easy to
come up with a solution for these
problems but you never know
if you're protected against all those
implementation vulnerabilities
so then in the
1990s there was actually an idea to
overcome all these issues and that was
device independent quantum key
distribution and this gives a relaxation
of the assumptions that you need to make
about the devices so for example you do
not need to you don't make any
assumptions on how the devices work
themselves
how can i then still be secure so if you
if the two devices share an entangled
state
then you can observe non-local
correlations
by
violating a bell inequality and by only
looking at the input and output of the
devices you can evaluate this bell
inequality
and you can do two things so you can end
you can
secure or you can make sure that the
devices behave
properly and secure and the other thing
is that you can directly bound or upper
bound the maximum information an
eavesdropper can have about the key so
how do we implement this
experimentally so for this we need to
have entanglement distribution between
two remote locations however we need to
do this with really high fidelity we
need to have high detection efficiency
of our state
to close the
detection loophole and we need to do
this with a high rate
then
that was
these ideas came up in the 1990s and
then end of last year within one week
they were like the first three
proof of concept implementations of this
uh so besides our experiment there was
also an experiment done at the
university of oxford where they use
entanglement of strontium ions
and there was another implementation
in fae where they used entangled photon
pairs to verify that you could implement
the iqkd with this setup
so then about our setup so what was the
protocol that we implemented so for
normal the iq
for the original diqd
what you do is you generate an entangled
state and the two users alice and bob
measured this state
so alice has three different settings
and bob has two different settings to to
measure the state of which setting two
and three for alice and zero and one for
bob are chosen such that they
maximize the violation of a bell
inequality
and they're also used to to evaluate the
spell inequality so then there's one
more pair of states so l is zero and bob
zero and these measurements are chosen
to have maximum maximally correlated or
anti-correlated outcomes and the
measurement outcomes here can then be
used to generate
the key
so for this
we actually worked together with renee
fonek and charles lim from the national
university of singapore and they
added an extra setting or they
they improved a bit on this on this
protocol and by adding an extra setting
they could make the product more robust
against noise so how does this work so
in the normal secure in the normal case
when you have three settings then you
always assume a worst case and the worst
case is that eve knows which setting you
use
for the key generation however if you
now randomly choose between two settings
then eve doesn't have this and and these
settings are orthogonal to each other
then eve doesn't have this information
advantage anymore so in fifty percent of
the chase uh chant uh fifty percent uh
of the of the times eve will not measure
in the correct
basis and will not gain maximal
information so therefore
it's harder for eve to get information
and therefore
that this
scheme
can handle a bit more noise
okay then what do we see here so here we
see the
s value so the chsh s value and the
quantum bit error rate so this is how
many mistakes you actually made when you
expect perfectly correlated
outcomes and in the color coded you see
the asymptotic key rate so
for higher fidelity entangled state your
s value goes up your cuber goes down and
you come here at the highest point of at
the point where you have the highest
asymptotic key rate
so if you compare this to the original
protocol uh the original protocol the
zero key rate boundary
is at a bit higher fidelity so this is
actually the gain
of adding this extra setting
so then where were we so five years ago
we did a loop of free bell test
and there we showed an s value which was
where we actually were really happy with
of s equals 2.2 however if you now look
for the diq d
applications then this is not enough so
this is roughly a fidelity of 80
and this is not enough to do anything
with the iqkd so we had to improve our
entangled state and so we made several
improvements here and i actually want to
highlight one
so in our diagonal generation process
there's a high probability that the atom
decays back to the
f equals one mf equals zero state so
this happens with the probability of one
third
and since the
since the excitation post that we shoot
in is roughly 20 nanoseconds and the
lifetime of the atomic state is
comparable around 26 nanoseconds it can
happen that this decay
occurs when the excitation pulse is
still on if this happens there's a
possibility of a second excitation and a
second
decay if we now look at the atom photon
state that we get out here it's perfect
right this is exactly the state that we
wanted to have the photon that we can
detect
is entangled with the atomic state
however the temporal shape here is
different and for our two-photon
interference the temporal shape of our
photons is also important so while
having the correct atom photon state we
still get the reduced
atom atom state fidelity so what you see
here is the
simulation of this process where you see
the detection time of the photons and
you see here this bump and this is
yeah this second excitation which
happens in four percent of the cases
roughly which reduces the atom atom
state so what we did is we somehow took
a brute force uh uh our approach here
and what we said it's a bit
counterintuitive but we said we do not
accept photons that are emitted in the
beginning so we set an acceptance window
and by setting this acceptance window we
could
increase the interference contrast from
88 percent to almost 99
and this increased the fidelity of the
obtained atom atom state
from 80 percent uh to 90 percent so if
you now look here so by changing the
start of the acceptance window so moving
the acceptance the start value here
you see that the number of events we get
goes down however we increase the
fidelity of the obtained at the atom
state
another
improvement that we made is we
implemented new microscope objectives
for two
atoms and thereby we could collect more
or more efficiently the fluorescence
over atoms and we increased the event
rate by a factor six
and therefore we could somehow make this
trade off of getting the
higher fidelity
okay then what was our
setup
so
alice is in lab one bob is in lab2 they
both have a device ls3 device is a
single atom trap together with the bell
state measurement setup
bob's device is a single atom trap a
beam splitter and a single photon
detector here
both devices can give a can get an input
this input sets the polarization of the
readout pulse and they give an output
and output is the number of fluorescence
counts if it's above or below a certain
threshold
then
this gives a binary output
then we start antenna generation tries
if we get a successful coincidence we
send a signal back to ls and bob and say
that the state is ready to be measured
then we implemented two quantum random
number generators and the other side two
random bits are generated
to select one out of uh four inputs and
at both sides one random bit is
generated to select one out of two
and then uh for this to work it's
important
that the in and outputs of the devices
do not leak to the weight and also the
in and outputs of course are stored in
the in the local storage then for this
to work it's important that the in and
outputs of the devices do not leak to
the environment for this we had to
implement a spectral filter and a
shutter at bob's side so the readout
here is nicely aligned such that it
leaves the laboratory and this we cannot
have because then the measurement
setting
or the input to the device would go to
the environment so therefore we
implemented the spectral filter here to
filter this out and also the
fluorescence detection
after a readout event there are 780
light also nicely leaving the laboratory
so therefore we close the shutter before
starting with the fluorescence readout
at bob's side or at lsi this is not
necessary because the photons do not
leave
the device actually because the bsm is
included in the device so then we let
our experiment run for 75 hours we get
more than 3 thousand entanglement
generation events
and then here you see the data that we
got so on the horizontally you see the
four settings from alice and the two
different plots are the settings from
bob
then in blue you see correlated outcomes
of the two devices and in red you see
the anticorrelated outcomes of the
devices we fit
the data and then we get visibilities
around 87 88
with which we can estimate our fidelity
of 89
then we use the green settings here to
evaluate the s value and the yellow
settings to
evaluate the cube
so this were the results that we got so
by improving the fidelity we now see
that we're nicely
in the asymptotic
positive key rate regime so the s value
measure was 2.6 with the qr cuber of
lower than 8 percent and then we can
generate 0.07 entanglement or secret
bits per entanglement generation event
normally you would do a finite key
analysis to really say okay now we have
really distributed the finite key
however there for that we need to have a
lot more events so we would need to let
our experiments run for a few months
even
so therefore we did a statistical
analysis how certain are we that we are
in the positive key rate regime and this
we can say or claim with 99
okay then i come to the second result
and that was about
quantum frequency conversion to increase
the range of our quantum network link so
for this we teamed up with
mathias bock and christophe becker from
the university of silent and what they
did for us is they developed really
efficient really nice quantum frequency
conversion devices
so what do we use here we use a pplan
waveguide and in the pplm waveguide
we have difference frequency generation
so we have for 780 nanometer photons
coming from uh from the rubidium atoms
we mix them with this bump field at six
or seven nanometers and then we convert
our photos to 15 17
nanometer so the process uh of this
conversion is actually polarization
dependent and this is a problem because
we have uh or um or or state is encoded
in the polarization of a photon so what
we did uh is uh putting the ppl and wave
guide in the synagogue interferometer by
placing here also a pbs and a half wave
plate and now what we get in so we split
or two arms let's say horizontal
vertically polarized light but then we
rotate uh one of the two arms such that
in the ppl and wave guide we have two
counter propagating beams however with
the same polarization so both of them
can be
converted with an efficiency up to 96
percent and then we rotate one of the
two back combine them on a pbs and after
filtering out the single photons we now
have a device that is polarization
preserving
which also has a external device
efficiency so this already includes the
filtering
of around 57 mainly limited by fiber
couplings
for this we need to spectrofilter with
27 megahertz and we get single to noise
ratios of more than 100 so this is a
pretty good device and also it was
stable over
weeks
okay
then
the original idea was to move one of our
nodes to the mpq and really have
entanglement distribution between the
lmu and mpq but it turned out to be hard
to get a fiber between
those two institutes so what we did is
note we put node 1 into our first lab
and node 2
into our second lab so they're still 400
meters apart
however we put in now these two quantum
frequency conversion devices
and then we could plug in really long
or really long we could plug in long
fibers on spools up to 33 kilometer
and then what we do is we convert the
photons we shoot them to the middle
station and now the middle station does
a bell state measurement on
in the telecom
to swap the entanglement to the atoms
and then what we also do is we yeah the
moment that we get the two photon
coincidence we send the heralding signal
back to the nodes and we implemented an
extra delay here to really simulate as
if the two nodes were separated
by the
6 11 23 or 33 kilometer
then again we needed we need to do the
entanglement swapping so we need to have
a good two-photon interference and now
you can think okay if you
converted the frequency of the photons
does it still work
so what we do here is
we use this the the the bump light for
the two qfc devices originates from the
same laser source and by originating
from the same laser source we pump with
exactly the same frequency and therefore
we're also certain that after conversion
the two photons still have the same
frequency in the telecom
here you see the photon shape and we you
see that we can nicely
overlap uh yeah or two photons and we
get here an interference contrast of
around 95 percent uh here we still have
some of these two photon events where i
talked about before so this reduces the
number and we have a tiny bit of
noise from the from the converters that
also reduces this number a bit so this
is not yet corrected for noise
okay
then the results
so here you see over distance so this is
the fiber length you see the fidelity
that we obtained so we first did
characterization measurements
for the atom photon entanglement
distribution for the two nodes and then
you see that they don't start at
the fidelity of one
because there are some errors in the
entanglement generation process or in
the readout
and they decay over time and this is
because
we have atomic decoherence
then we did the atom atom entanglement
measurements and you see
what we expect here to get is somehow
the uh yeah the multiplication of those
two
visibilities times the interference
contrast
and this is exactly what we observed and
to somehow verify or characterize that
this drop in fidelity really comes from
the atomic state the coherence we did
another set of measurements where we did
not plug in these long fibers but we
only delayed the atomic state according
to the distance that the photon should
have traveled and then we see a similar
decay
and yeah this is actually a nice day to
present these results because this
afternoon
they should come online or they will be
published
all right so what are the the next steps
that we're working on now uh so one of
the things that we want to do is we want
to get rid of this decay so we want to
get rid of the atomic decoherence we
want to make this slide straight so for
this we actually implemented some tools
recently that were developed in the
rampa group so initially our atomic
qubit is encoded in these two
mf states of the f equals one level
however if a magnetic field comes by
then these
magnetic states decohere so what we
could do
is we can
transfer a population here to the f
equals two level and those two states if
magnetic fields come by they behave a
lot more similar
when a magnetic
they behave more similar
to magnetic fields
so therefore we could actually reduce
the sensitivity
to magnetic fields by more than
effective 500.
so this we implemented recently so what
you see here is in red we see the
visibility of our atom photon state
over the readout delay and then in red
you see our initial basis so that is the
basis where we see more magnetic field
noise and then after transferring the
atom
or one of the qubit states to the other
uh to the other mf state or other f
level you see that we increased the
coherence time of our atomic state up to
five milliseconds so what we could do by
imp by combining and the frequency
conversion and
end this new encoding of our atomic
state
we could also measure atom photon
entanglement after the photo propagated
over up to 101 kilometer fiber
all right let me summarize
so i talked
about
our
quantum network link in downtown munich
and i told you how the entangle are two
atoms i showed you a recent result of
one of the first proof of concept
experiments of di qkd
and i showed you that we can now
entangle two atoms
when the photons traveled through up to
33 kilometer fiber
and with that i want to thank you for
listening
[Applause]
okay thanks for the very nice talk we
have time for questions
ah there's one already
hi so um thanks again for this very nice
talk i actually have two questions and i
hope they're appropriate this setting
first of all with the whole quantum
cryptography aspect is there any good
reason to believe that there will nobody
will come up with a with just a
mathematical algorithm
that's both quantum and classically
secure because i think once you have
this there's no reason to invest
billions
in quantum cryptography if you're
already secure
and
question number two would be
does this type of protocol actually
protect against man the middle attacks
as you as you would expect them today so
instead of
eve listening in on a key exchange just
convince you there bob
so you do so you do your key exchange
with eve think you exchange the key with
bob it works great they do the same with
bob you think you have a secure
communication but you actually don't
like this is how you would do it today
is there any protection against this
type of attack as well in this scheme
thanks let's first go to the
the first question i think also in the
previous talk there was something about
this right so there are post uh crypto
or uh quantum crypto
algorithms that actually work uh quite
well uh and i think what we what we're
looking for and also uh go along with
the previous speaker is that uh
you can come up with some hybrid
solution here so you need to keep in
mind that you really play a different
game so you really you're not securing
your information by encrypting it uh
super well but you really can detect
uh if someone else is uh is
eavesdropping you so i think this is a
really uh vulnerable uh or a really
important
aspect as well
then to your i hope that that somehow
answers your question a bit so that if
the
future should
tell us how how this will go
then about the other attack
so for device independent quantum key
distribution i think this is secure
against the scenario that you uh that
you sketched
because alice and bob really work with a
device and if they really that device
needs needs to be in a secure location
so what you also always assume
for the iqkd is that the locations that
alice and buff have are secure and you
really
start doing a belt test between those
two locations that are secure
and with only the in and outputs of of
the device you can violate the bell
inequality and you can secure your
information so i think there's no way if
so
we need to have we need to assume those
secure locations and we need to make
sure that the device is really in there
and then
device independent qkd should be
secure against this scenario
but you have to have these secure
locations it's not not so easy right so
they're
is a challenge
good further questions there's one
question
hey
first of all thanks this was a really
nice talk and i have a question on the
part where you talked about the atom
photon entanglement so you showed that
there are these two decay paths with
orthogonal polarization
and my question would be is this
splitting ratio naturally 50 50 such
that you automatically get a maximally
entangled state or do you have to or
actually can control this in some way
good question so here we are lucky with
uh with rubidium uh so it's really those
three levels that we see here it's
really one third one third one third
probability that they decay there
so we really come up with a maximally
entangled state or we generate a
maximally integral state
great then let's take the speaker again