Copyright © 2006 Keio University

Assume a Quantum Computer
Like This...

I want to Build a Distributed
Quantum System Like This

Laboratory-sized quantum multicomputer ortranscontinental network, either one!

Repeater Protocol Stack
Application
Purification Control (PC)‫‏‬
Entang. Swapping Ctl (ESC)‫‏‬
Purification Control (PC)‫‏‬
Entanglement Control (EC)‫‏‬
Physical Entanglement (PE)‫‏‬
Van Meter et al., IEEE/ACM Trans. on Networking,Aug. 20094 (to appear), quant-ph:0705.4128}}}

End-to-End
Repeated at
Different
Distances
Distance=1
Only quantum!

Outline• Two types of quantum networks• IPsec with QKD• IPsec with QKD• US & European efforts• Open problems & plans• Repeaters• Basic concepts• Our recent results• Open problems & plans
5Two Types of Quantum Networks
Unentangled
Networks

Entangled
Networks
CAGB

EH

6

Quantum Key Distribution (QKD)• Creates a shared, random secretbetween two nodes• Uses physical effects to guarantee that key
has not been observed• Requires authenticated classical channel• Limited to <150km per hop

7IPsec with QKD (ORF2008)‫‏‬

The DARPA Quantum Network

slide from Elliott, BBN

SECOQC Prototype - principle layout

81 m
LMU

FOR

Slide fromM. Peev, 2008

A Trusted repeater QKD-Network: Abstract
Architecture (SECOQC, Europe)
VPN-yellow
Site 1

VPN-yellow
Site 2

VPN-green
Site 1

Data Plane
QKDAccess
Node

QKD
Core Node

Secrets Plane
QKD
AccessNode

QKD
Core Node

Quantum Plane

Slide fromM. Peev, 2008

QKD with IPsec Plans• Test over raw fiber, Yagami<->K2• Use key for one-time pad• Work w/ NEC, BBN & ITU to standardize• Write experimental I-D on IKE changes• Take to IETF in Hiroshima?

12Outline• Two types of quantum networks• IPsec with QKD• IPsec with QKD• US & European efforts• Open problems & plans• Repeaters

A• Basic concepts• Our recent results• Open problems & plans
13CG
B

EH

Network Link Technology (Qubus)‫‏‬millimeters to kilometers

coherent
optical source(laser)‫‏‬waveguidetransceiver
qubit innode 1transceiver
qubit innode 2

Munro, Nemoto, Spiller, New J. Phys. 7, 137 (2005)‫‏‬14 Ladd et al., NJP 8, 184 (2006)‫‏‬homodyne
detector

Quantum Repeater Operation:
Entanglement Swapping
Station 0Station 1Station 2

Bell State
Measurement
Results must be communicatedFidelity decreases; you must purify afterwards
15

Nested Entanglement Swapping

16

Purification
Station 0Station 2Results must be communicated(two-way?)17

Repeater Protocol Stack
Application
Purification Control (PC)‫‏‬
Entang. Swapping Ctl (ESC)‫‏‬
Purification Control (PC)‫‏‬
Entanglement Control (EC)‫‏‬
Physical Entanglement (PE)‫‏‬
18

Van Meter et al., IEEE/ACM Trans. on Networking,Aug. 2009 (to appear), quant-ph:0705.4128}}}

End-to-End
Repeated at
Different
Distances
Distance=1
Only quantum!

Four-Hop Protocol Interactions
App
PC
ESC
PC
ESC
PC
EC
PE
19ESC
PC
EC
PE

ESC
PCESCPCECPE

Van Meter et al., IEEE/ACM Trans. on Networking,Aug. 2009 (to appear)

ESCPCECPEAppPCESCPCESCPCECPE

The Repeaterʼs Jobs
Entanglement swapping & purification, which
require:• A little bit of quantum communication• Quantum memory• Local quantum operations(gates & measurements)• Lots of decision making(both local and distributed)• Lots of classical communication
20

Entanglement Pumping
0.638
0.638

0.72
0.638

0.75
0.638

0.77
0.638

0.79

Ineffective w/ large fidelity difference21Symmetric Purification
0.638
0.6380.72
0.797

0.638

0.72

0.638 X

22Problems:
Exact matching canrequire long waits.
Not realistic whenmemory effects(decoherence)‫‏‬considered.
Can deadlock if
resources are limited.

Greedy Purification
0.638
0.638

0.72
0.757

0.638
0.638 X

23

Doesn’t wait foranything, uses
whatever’s available.
Works well w/ large
number of qubits
per repeater.

Banded Purification
0.638
0.638

0.72
0.797

0.638
0.72

0.638 X

Large gains in throughput.
Moderate # qubits (5-50).
Avoids deadlock.
Realistic memory model.
Simple to implement inreal time (even in HW).
Probably not optimal,
but probably close.

Divide fidelity space
into multiple bandse.g., above & below 0.70
24Banded Purification Performance

25

Van Meter et al., IEEE/ACM Trans. on Networking,Aug. 2009 (to appear), quant-ph:0705.4128

Banded Purification Latency

26

Van Meter et al., IEEE/ACM Trans. on Networking,Aug. 2009 (to appear), quant-ph:0705.4128

Protocol Design27

RoutingCA

G
B

EDH

F
Simple: use Dijkstra’s Shortest Path First.
...but we don’t yet know the cost metric.
28

A Different Meaning of“Which Path?”
CA

29

GE
DH
F

3 hops: ACGB
4 hops: ACGHB
ACEHB
B
ADEHB
ADFHB
5 hops: ACEHGBADEHGBADECGB
ADFHGB
6 hops: ACECGHB
7 hops: ADFHECGBACEDCHGBBut What is Distance?
CAG What if hops are not homogeneous?

E
DH
F

B

Are 2n-1 hops,2n hops,
and 2n+1 hops
significantly different?

30

How Do We Order These?
• How does number oflinks matter?
• Does number of weaklinks matter?
• Does position of weaklink matter?
• Is cost additive?
• At this logical level,is this technologyindependent?
31

Other Problems• Defining swap points• Static or dynamic?
• Avoiding leapfrog• Avoiding deadlock• Minimizing waits forclassical messages

32Other Problems
Partial messaging
sequence
Can this be mademore efficient?
Due to memory
degradation, gains
will be better thanlinear

33

Leapfrog

Station 034

Station 1

Station 2

Station 3

Resource Management (QoS?)‫‏‬
A<->B & C<->D
want to talk.

CA

Remember, it’s a
distributed computation.

DWorse, fragile quantum memory means there
is a hard real time component.

==>requires circuit switching???
(bottleneck likely is memory per node)‫‏‬35B

Open Repeater Problems• Well, repeater HW doesn’t work yet...
‒ Sims of “weak links” mostly functional‒ Establishing swapping points‒ More dynamic behavior‒ Non-power-of-two hops
‒ Finish & publish protocol state machine

36

Open Complex Network Problems• Coding partially done
‒ Using graphviz file format‒ Routing not done
‒ Workload generator needs work
‒ QoS / resource allocation not implemented• Visualization of networks• Investigate graph states & quantum network coding• More detailed workload definition37

Milestones for JSPS
• Define a cost metric(figure out if it’s additive!)• Define a path selection algorithm• Define test cases• Simulate that set of test cases• Extend to topologically complex networks• Create static visualizations

38

Food for Thought• When will first Science or Nature paper
appear using a quantum computer, but notabout the quantum computer?
• That is, when will a quantum computer doscience, rather than be science?
• Answers from quantum researchers range
from “less than five years” to “more than fortyyears”
39Thanks
Thanks to Thaddeus Ladd, Bill Munro and KaeNemoto (coauthors on much of this work), aswell as Austin Fowler, Jim Harrington, Kohei
Itoh, Agung Trisetyarso, Byung-Soo Choi,
Shota Nagayama, and Takahiko SatohAnd funding from NICT, MEXT, NSF, the MoriFund at Keio, and now JSPS for funding.

Copyright © 2006 Keio University ｜40

AQUA: Advancing Quantum
Architecture