Researchers from QuTech have
achieved a world’s first in quan­tum inter­net tech­nol­o­gy. A team led
by Pro­fes­sor Stephanie Wehn­er has devel­oped a so-called link lay­er
pro­to­col that brings the phe­nom­e­non of ‘quan­tum entan­gle­ment’
from a physics exper­i­ment towards a real world quan­tum net­work. This
brings clos­er the day when quan­tum inter­net can
become a real­i­ty, deliv­er­ing appli­ca­tions that are impos­si­ble to
achieve using a ‘clas­si­cal’ inter­net.
The work was pre­sent­ed today at ACM SIGCOMM.

Robust data trans­mis­sion

In clas­si­cal com­put­ing, a col­lec­tion of soft­ware lay­ers referred to
as the ‘net­work stack’ allows com­put­ers to com­mu­ni­cate with each
oth­er. Under­ly­ing the net­work stack are com­mu­ni­ca­tion pro­to­cols, such
as the Inter­net Pro­to­col or the HTTP pro­to­col. Stephanie Wehn­er
explained that one essen­tial pro­to­col used by a net­work is the ‘link
lay­er’ pro­to­col, which over­comes the prob­lems caused by imper­fect
hard­ware: “All of us use clas­si­cal link lay­er pro­to­cols
in every­day life. One exam­ple is Wi-Fi, which allows an unre­li­able
radio sig­nal – suf­fer­ing from inter­rup­tions and inter­fer­ence – to
be used to trans­mit data reli­ably between com­pat­i­ble devices.”

A
quan­tum net­work, based on trans­mis­sion of quan­tum bits or ‘qubits’,
requires the same lev­el of reli­a­bil­i­ty. Stephanie Wehn­er: “In our
work, we have pro­posed a quan­tum net­work stack, and have con­struct­ed
the world’s first link lay­er pro­to­col for a quan­tum net­work.”

Demand­ing
physics

It
turns out that exist­ing clas­si­cal pro­to­cols can­not help in the
quan­tum world. One chal­lenge is pre­sent­ed by dif­fer­ences between the
tech­nolo­gies used. Stephanie Wehn­er: “Cur­rent­ly, qubits can­not be
kept in mem­o­ry for very long. This means con­trol deci­sions on what to
do with them need to be tak­en very quick­ly. By cre­at­ing this link
lay­er pro­to­col, we have over­come obsta­cles pre­sent­ed by some very
demand­ing physics.”

There
are also some fun­da­men­tal dif­fer­ences between a future quan­tum
inter­net and the inter­net that we see today. Stephanie Wehn­er said
that two quan­tum bits can be ‘entan­gled’: “Such entan­gle­ment is
like a con­nec­tion. This is very dif­fer­ent to the sit­u­a­tion for
clas­si­cal link lay­er pro­to­cols where we typ­i­cal­ly just send sig­nals.
In that case, there is no sense of con­nec­tion built in at a
fun­da­men­tal lev­el.”

Quan­tum
inter­net

The
phe­nom­e­non of entan­gle­ment forms the basis of a quan­tum inter­net.
When two fun­da­men­tal par­ti­cles are entan­gled, they are con­nect­ed with
each oth­er in such a way that noth­ing else can have any share of this
con­nec­tion. Researcher Axel Dahlberg said that this enables a whole
new range of appli­ca­tions “Secu­ri­ty is one impor­tant appli­ca­tion.
It is phys­i­cal­ly impos­si­ble to eaves­drop on an entan­gled net­work
con­nec­tion between two users. To give anoth­er exam­ple, the tech­nol­o­gy
also allows improved clock syn­chro­niza­tion, or it can join up
astro­nom­i­cal tele­scopes that are a long way apart, so they act as a
huge sin­gle tele­scope.”

Researcher
Matthew Skrzypczyk said that an impor­tant fea­ture of the pro­posed
quan­tum net­work stack and the link lay­er pro­to­col is that it any
future soft­ware writ­ten using the pro­to­col will be com­pat­i­ble with
many quan­tum hard­ware plat­forms. “Some­one who makes use of our link
lay­er pro­to­col no longer needs to know what the under­ly­ing quan­tum
hard­ware is. In our paper, we study the protocol’s per­for­mance on
Nitro­gen-Vacan­cy cen­ters in dia­mond, which are essen­tial­ly small
quan­tum com­put­ers. How­ev­er, our pro­to­col can also be imple­ment­ed on
Ion Traps, for exam­ple. This also means our link lay­er pro­to­col can
be used in the future on many dif­fer­ent types of quan­tum hard­ware.”

Build­ing
a quan­tum net­work sys­tem

Stephanie
Wehn­er said that the next step will be to test and demon­strate a new
net­work lay­er pro­to­col using the link lay­er pro­to­col: “Our link
lay­er pro­to­col allows us to reli­ably gen­er­ate entan­gle­ment between
two net­work nodes con­nect­ed by a direct phys­i­cal link, such as a
tele­com fiber. The next step is to pro­duce entan­gle­ment between
net­work nodes which are not con­nect­ed direct­ly by a fiber, using the
help of an inter­me­di­ary node. In order to real­ize large scale quan­tum
net­works, it is impor­tant to go beyond a physics exper­i­ment, and move
towards build­ing a quan­tum net­work sys­tem. This is one of the
objec­tives EU-fund­ed Quan­tum Inter­net Alliance (QIA).”

Using
the link lay­er pro­to­col, high­er-lay­er soft­ware can request the
cre­ation of entan­gle­ment with­out need­ing to know which quan­tum
hard­ware sys­tem is in the box.
The image is free to use, image
cred­it QuTech/Scixel.

Pub­li­ca­tion

A
Link Lay­er Pro­to­col for Quan­tum Net­works, by Axel Dahlberg et
al.,
Free preprint: https://arxiv.org/abs/1903.09778.

Con­tact

Stephanie
Wehn­er, S.D.C.Wehner@tudelft.nl. Please con­tact the QuTech
sec­re­tari­at at +31 15 27 86 133 for sched­ul­ing calls.