C.W. Chou, H. Deng, K.S. Choi, H. de Riedmatten, J. Laurat, S. van Enk, H.J. Kimble

1
Quantum Networks with Atomic Ensembles
Daniel Felinto dfelinto_at_df.ufpe.br

• C.W. Chou, H. Deng, K.S. Choi, H. de Riedmatten,
  J. Laurat, S. van Enk, H.J.
  Kimble
• Caltech Quantum Optics
• Presently at Departamento de Física, UFPE
• International Workshop on Quantum Information
• Paraty, August 14, 2007

2
 Quantum Networking  Fundamental scientific
questions and Diverse experimental challenges
Quantum channel transport / distribute
quantum entanglement
Quantum node generate, process, store quantum
information
Goal develop the ressources that enable quantum
repeaters, thereby allowing entanglement-based
communication tasks on distance scales larger
than set by the attenuation length of fibers
3
Quantum Repeaters Principles

1. Divide into segments and generate entanglement

Fidelity close to 1, long distance But time
exponentially large with the distance
.
.
L0
L0
L0
L
Entanglement (often) and purification (always)
are probabilistic each step ends at different
times.
.
.

1. Connect the pairs

4
Quantum Repeaters Principles

1. Divide into segments and generate entanglement

Fidelity close to 1, long distance But time
exponentially large with the distance
.
.
L0
L0
L0
L
Entanglement (often) and purification (always)
are probabilistic each step ends at different
times.
.
.
Scalability requires the storage of
heralded entanglement

1. Connect the pairs

Quantum Memories
5
One Approach DLCZ
Atomic ensembles in the single excitation regime
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Capabilities Enabled by DLCZ Roadmap

• Beyond the original protocols of DLCZ
• Implementation of quantum memory
• Realization of fully controllable
• source for single photons
• A source for entangled photon pairs
• Universal quantum computation via
• the protocol of Knill, LaFlamme, Milburn

• Scalable long-distance
• quantum communication via
• quantum repeater architecture
• Distribution of entanglement
• over quantum networks

7
Outline

•  DLCZ building block 
  writing, reading, memory time
• Number-state entanglement between two ensembles
• Polarization entanglement between two nodes
  (4 ensembles)
• Towards entanglement swapping

8
Building Block (DLCZ)
Duan, Lukin, Cirac and Zoller, Long-distance
quantum communication with atomic ensembles and
linear optics, Nature 414, 413 (2001)
9
Creating a Single Atomic Excitation
10
Retrieving the Single Excitation
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Experimental Setup
Counter-propagating and off-axis configuration
H
Field 2
Read V
Write H
Field 1
V
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Conditional Field-2
Retrieval efficiency of the stored excitation
J. Laurat et al., Efficient retrieval of a
single excitation stored in an atomic ensemble,
Opt. Express 14, 6912 (2006)
13
Storage Time of the Single Excitation
Writing
Reading
Field 2
Read
H. De Riedmatten et al., Direct measurement of
decoherence for entanglement between a photon and
a stored excitation, PRL 97, 113603 (2006)
D. Felinto et al., Control of decoherence in the
generation of photon pairs from atomic
ensembles, Phys. Rev. A 72, 053809 (2005)
14
Outline

•  DLCZ building block 
  writing, reading, memory time
• Number-state entanglement between two ensembles
• Polarization entanglement between two nodes
  (4 ensembles)
• Towards entaglement swapping

C.W. Chou, H. de Riedmatten, D. Felinto, S.V.
Polyakov, S. van Enk, H.J. Kimble,
Measurement-induced entanglement for excitation
stored in remote atomic ensembles, Nature 438,
828 (2005)
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Entanglement between Two Ensembles
entangled
Atoms
Light
50/50 Beam splitter
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Entanglement between Two Ensembles
1 photon detected ? 1 atom transferred
50/50 Beam splitter
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Entanglement between Two Ensembles
1 photon detected ? 1 atom transferred
L
Entangled
R

18
How to Verify the Entanglement ?

• Tomography

19
Experimental Density Matrix
Populations
Coherence
D1c
D1b
lt1, suppression of 2-photon events relative to
single-excitation events
p9.10-4 160 Hz preparation rate
J. Laurat et al., Heralded Entanglement between
Atomic Ensembles Preparation, Decoherence, and
Scaling, arXiv0706.0528
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Scaling with Excitation Probability
Decreasing excitation probability
J. Laurat et al., Heralded Entanglement between
Atomic Ensembles Preparation, Decoherence, and
Scaling, arXiv0706.0528
21
Outline

•  DLCZ building block 
  writing, reading, memory time
• Number-state entanglement between two ensembles
• Polarization entanglement between two nodes
  (4 ensembles)
• Towards entaglement swapping

22
How Having one Click on Each Side ?
3 m
Entangled !
Node L
Node R
Entangled !
LU
RU
LD
RD
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Polarization Entanglement
3 m
Node L
Node R
2RU
2LU
2L
2R
LU
RU
2RD
2LD
LD
RD
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Results Preparation and Bell Violation
Asynchronous Preparation
C.W. Chou, J. Laurat, H. Deng, K.S. Choi, H. de
Riematten, D. Felinto, H.J. Kimble, Functional
Quantum Nodes for Entanglement Distribution over
a Scalable Quantum Networks, Science 316, 1316
(2007)
25
Results Preparation and Bell Violation
Asynchronous Preparation
Preparation x 35 Final state x 20
Duration that the first entanged pair is stored
before retrieval
D. Felinto, C.W. Chou, J. Laurat, H. de
Riedmatten, H. Kimble, Conditional control of
the quantum states of remote atomic memories for
Q. networking, Nature Physics 2, 844 (2006)
C.W. Chou, J. Laurat, H. Deng, K.S. Choi, H. de
Riematten, D. Felinto, H.J. Kimble, Functional
Quantum Nodes for Entanglement Distribution over
a Scalable Quantum Networks, Science 316, 1316
(2007)
26
Results Preparation and Bell Violation
Asynchronous Preparation
Preparation x 35 Final state x 20
Bell Violation (CHSH)
Large violation quantum key distribution with
security at minimum against individual attacks
Duration that the first entanged pair is stored
before retrieval
C.W. Chou, J. Laurat, H. Deng, K.S. Choi, H. de
Riematten, D. Felinto, H.J. Kimble, Functional
Quantum Nodes for Entanglement Distribution over
a Scalable Quantum Networks, Science 316, 1316
(2007)
27

• 2 nodes separated by 3m
• 2 ensembles per node
• Asynchronous preparation (memory) of 2 parallel
  number-state entangled pairs
• Polarization coding and passive phase stability
• ? Polarization entanglement distribution,
  violating Bell, in a scalable fashion

C.W. Chou, J. Laurat, H. Deng, K.S. Choi, H. de
Riematten, D. Felinto, H.J. Kimble, Functional
Quantum Nodes for Entanglement Distribution over
Scalable Quantum Networks, Science 316, 1316
(2007)
28
Outline

•  DLCZ building block 
  writing, reading, memory time
• Number-state entanglement between two ensembles
• Polarization entanglement between two nodes
  (4 ensembles)
• Towards entanglement swapping

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Towards Entanglement Swapping
3 m
Entangled !
Node L
Node R
Entangled !
LU
RU
LD
RD
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Towards Entanglement Swapping
Populations
Coherence

• From two entangled pairs with h(2)0.15 and 90
  vacuum

• The transfert succeeds only 50 of the time,
  while the weight of two-photon events stays the
  same.
• ? Overall, h(2) multiplied by 4

lt1, suppression of 2-photon events relative to
single-excitation events
J. Laurat et al., Towards entanglement swapping
with atomic ensembles in the single excitation
regime, arXiv0704.2246
31
In a Nutshell

• Q. Repeaters, DLCZ
• and Building Block

Writing
Reading
Field 2
Write

• Photon pair alt1
• Efficient retrieval 50
• Memory time 10 µs

Read

• Number-state entanglement

• Heralded and stored
• C0.90.3 for the atoms

• Polarization Entanglement

• 2 nodes, 4 ensembles
• Asynchronous preparation
• Bell violation

• Towards swapping

• Coherence transfert

32
Decoherence
1) MOT magnetic field
Each atom sees a different field ? Inhomogeneous
broadening of the ground states
B
z
t 100 ns
Solution Switching off the trapping field
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Storage Time of the Excitation
Timing and linewidth
Perspectives ?? Better cancellation of residual
fields
_at_ 40 Hz
MOT off 6 ms
34
Experimental Setup
Repumper
Write
PBS
BSW
Read
LU
RU
BSR
LD
RD
D2RV
D2LV
BS1
D2LH
D2RH
l/2
l/4
D1Va
D1Vb
Compensator
Beam displacer
D1Ha
D1Hb
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Experimental Setup
Interferometers Entangling the (U, D) Pairs
Repumper
Write
PBS
BSW
Read
LU
RU
BSR
LD
RD
D2RV
D2LV
BS1
D2LH
D2RH
l/2
l/4
D1Va
D1Vb
Compensator
Beam displacer
D1Ha
D1Hb