Towards practical quantum repeaters

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Towards practical quantum repeaters (and quantum
experiments with human eye detectors)
Christoph Simon
Group of Applied Physics, University of Geneva
Institute for Quantum Information
Science, University of Calgary
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Overview
Quantum repeaters motivation and
principle Repeaters with atomic
ensembles Multimode quantum memories CRIB and
AFC principles and experiments Requirements for
practical repeaters Quantum experiments with
human eyes Micro-macro entanglement?
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Long-distance quantum communication
Fundamental motivation (how far can one stretch
entanglement?), but also important for quantum
cryptography, quantum networks. Big problem
transmission losses 1000 km of fiber at optimal
wavelength transmission 10-17. 3 years to
distribute one entangled pair with 1 GHz
source! Straightforward amplification impossible
(no-cloning theorem). Potential solution
quantum repeaters.
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Quantum Repeater - Principle
A
Z
Create entanglement independently for each link.
Extend by swapping. n links with transmission t
each Direct transmission Repeater Requires
heralded creation, storage and swapping of
entanglement.
H.-J. Briegel et al., PRL 81, 5932 (1998)
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The DLCZ scheme atomic ensembles and linear
optics
DLCZ Duan, Lukin, Cirac, Zoller
Entanglement creation through single photon
detection. Entanglement swapping through
efficient reconversion of memory excitation into
photon
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Improved repeaters with ensembles and linear
optics
Entanglement swapping via two-photon
detections Jiang, Taylor, Lukin, PRA 76, 012301
(2007) Entanglement generation via two-photon
detections Zhao et al, PRL 98, 240502 (2007)
Chen et al, PRA 76, 022329 (2007) Spatial
multiplexing Collins et al, PRL 98, 060502
(2007). Photon pair sources and temporal
multimode memories Simon et al, PRL 98, 190503
(2007). Single-photon source based
protocol Sangouard et al, PRA 76, 050301(R)
(2007) Local generation of entangled pairs plus
two-photon detections Sangouard et al, PRA 77,
062301 (2008). Review N. Sangouard, C. Simon,
H. de Riedmatten, N. Gisin, arXiv0906.2699
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Comparison of repeaters with ensembles and linear
optics
N. Sangouard, C. Simon, H. de Riedmatten, N.
Gisin, arXiv0906.2699
Assuming 90 memory and detection
efficiency. Multiplexing is very attractive.
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Repeaters with photon pair sources and multi-mode
memories
Memories that are able to store and recall trains
of N pulses. N entanglement creation attempts
per waiting time interval L0 /c. Speedup by
factor of N
C. Simon et al, PRL 98, 190503 (2007).
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Temporal multi-mode memories

• Standard Photon Echo thousands of modes, but not
  a quantum memory!
• J. Ruggiero, J.L. Le Gouet, C. Simon, T.
  Chaneliere, PRA 79, 053851 (2009)
• Stopped light (EIT) M.D. Lukin, RMP 75, 457
  (2003)
• Controlled Reversible Inhomogeneous Broadening
  (CRIB)
• B. Kraus et al., PRA 73, 020302 (2006) C. Simon
  et al, PRL 98, 190503 (2007)
• Atomic frequency comb protocol N independent of
  d.
• M. Afzelius, C. Simon, H. de Riedmatten, N.
  Gisin, PRA 79, 052329 (2009).

CRIB and AFC exploit static inhomogeneous
broadening (e.g. in rare-earth doped solids)
AFC promises efficient storage of hundreds of
temporal modes on seconds timescale.
AFC
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CRIB Controlled Reversible Inhomogeneous
Broadening
STEP 1
Light Storage !
Moiseev and Kröll, PRL 87, 173601 (2001), B.
Kraus et al., PRA 73, 020302 (2006), N.
Sangouard, et al, PRA 75, 032327 (2007)
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Atomic Frequency Comb (AFC) Quantum Memory
Ensemble of inhomogeneously broadened atoms
Atomic density
D
Atomic detuning ?
M.Afzelius, C.Simon, H. de Riedmatten and
N.Gisin, PRA 79, 052329 (2009).
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CRIB vs AFC
CRIB
AFC
Absorption
Absorption
w
w
More atoms for the same bandwidth using an AFC ?
Higher efficiency!
N2
N
Many atoms are lost in the preparation step ? Low
efficiency!
AFC
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CRIB at telecom wavelength weak pulse storage
at the single photon level
Transmitted pulse
0.6 photons per pulse
Efficiency ? 0.5
CRIB echo
B. Lauritzen et al, arXiv0908.2348
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CRIB at telecom wavelength
Why is the efficiency low ?

• Low efficiency due to
• Low optical depth
• Poor optical pumping

Both points can be improved (other crystals,
cavities higher B field, hyperfine states).
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2-level AFC in various places, ions and crystals
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and one more that I got this morning! (W.
Tittel, Calgary)
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Complete 3-level AFC storage experiment in
Pr3Y2SiO5
M. Afzelius et al., arXiv0908.2309
See also poster by H. De Riedmatten!
Solution Spin echo ? 1 s spin coherence !
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How good do memories and detectors have to be for
repeaters?
1 percent drop in efficiency costs 7-19 percent
in repeater rate Status quo Memory
efficiencies up to 50 for DLCZ (q) and EIT (cl)
in atomic gases, 60 for CRIB (cl) in RE doped
solid. Overall detection efficiency 95
(transition edge sensors) A.E. Lita, A.J. Miller,
S.W. Nam, Opt. Exp. 16, 3032 (2008) Reducing
coupling losses is essential!
N. Sangouard, C. Simon, H. de Riedmatten, N.
Gisin, arXiv0906.2699
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What about intercontinental entanglement?
Geneva Calgary still seems out of reach. Can
we do better than multi-mode memories plus linear
optics?
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Repeaters with deterministic entanglement swapping
Repeaters with trapped ions. N. Sangouard, R.
Dubessy, C. Simon, PRA 79, 042340 (2009)
Great gain in performance!
Quantum gates with over 99 fidelity (R.
Blatt) Ion-ion entanglement (C.
Monroe) Efficient coupling to cavity (R. Blatt)
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Quantum experiments with human eye
detectors based on quantum cloning via stimulated
emission
Cloning by stimulated parametric
down-conversion. Simon, Weihs, Zeilinger, PRL
00, De Martini, Mussi, Bovino, Opt. Comm. 00
P. Sekatski et al., PRL 103, 113601 (2009)
Realistic model of the eye as a threshold
detector with losses Hecht, Shlaer, Pirenne, J.
Gen. Physiol. 25, 819 (1942), Rieke and Baylor,
RMP 98
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Micro-macro experiment following De Martini et al.
Can violate Bell inequality with human eye
detectors! Would prove micro-micro entanglement
(amplifier part of detector) Different from
usual detection (amplification before choice of
basis). Brings observer closer to quantum
phenomenon... But wait. Is the micro-macro state
entangled?
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Micro-macro entanglement?
Human-eye experiment does not prove micro-macro
entanglement!
Separable model
This also applies to De Martini group experiments.
P. Sekatski et al., PRL 103, 113601 (2009)
Micro-macro entanglement criterion
for all separable states
for micro-macro experiment
etc., Stokes parameters
Micro-macro entanglement is present, even with
loss. Requires precise counting of large photon
numbers.
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Conclusions
Multimode memories are attractive for quantum
repeaters. First quantum memory at telecom
wavelength via CRIB. First complete
implementation of AFC memory protocol. Quantified
importance of memory efficiency. Thousands of
km distances will require deterministic
swapping. Quantum experiments with human eye
detectors seem realistic. Clarified the role of
micro-macro entanglement. Graduate student
applications welcome!

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Thanks to
Thierry Chaneliere Romain Dubessy Jean-Louis Le
Gouet Nicolas Sangouard
Hugues de Riedmatten Pavel Sekatski Matthias
Staudt Imam Usmani Hugo Zbinden
Mikael Afzelius Cyril Branciard Nicolas
Brunner Nicolas Gisin Bjoern Lauritzen Jiri Minar