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. Overview. Quantum repeaters: motivation and principle

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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

Z

A

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, arXiv:0906.2699


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Comparison of repeaters with ensembles and linear optics

N. Sangouard,

C. Simon,

H. de Riedmatten,

N. Gisin, arXiv:0906.2699

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).


Temporal multi mode memories l.jpg
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 exploitstatic

inhomogeneous broadening

(e.g. in rare-earth doped solids)

AFC promises efficient storage ofhundreds of temporal modes

on seconds timescale.

AFC


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CRIB: Controlled Reversible Inhomogeneous Broadening

Natural broadening

STEP 2

« Burn a hole »

Absorption

w

Absorption

w

Optical pumping

STEP 3

STEP 4

Controlled broadening

Trigger re-emission

Absorption

Absorption

w

w

Linear Stark shifts by

EXTERNAL ELECTRIC FIELD

Mirror broadening by changing

the POLARITY of the E-field

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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State after absorption

Control fields

Output mode

Input mode

Dephasing

Backward readout

Storage state

Avoid re-absorption by

constructive interference

Periodic structure =>

Rephasing after a time

Output

mode

Input

mode

Input

mode

Output

mode

Intensity

Control fields

kin

Intensity

k

k

Time

kout

Time

Collective emission in the forward mode. Photon echo like emission

Atomic Frequency Comb (AFC) Quantum Memory

Ensemble of inhomogeneously broadened atoms

D

Atomic density

Atomic detuning 

M.Afzelius, C.Simon, H. de Riedmatten and N.Gisin, PRA 79, 052329 (2009).


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APPLY BROADENING

Absorption

w

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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+V

-V

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, arXiv:0908.2348


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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).


2 level afc in various places ions and crystals l.jpg

Nd:YVO in Geneva: =1%

Tu:YAG in Orsay/Paris: =9%

Input

Transmitted

input

Nd:YSO in Geneva: =5%

Pr:YSO in Lund: =19%

Output 19%

2-level AFC in various places, ions and crystals


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0.10

Transmitted light

0.08

0.06

Intensity (au)

0.04

Internal Efficiency ≈ 0.5 %

0.02

Echo

0.00

0

100

200

300

Time (ns)

… and one more that I got this morning! (W. Tittel, Calgary)


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Complete 3-level AFC storage experiment in Pr3+:Y2SiO5

5/2

3/2

Ts=15.6 µs

1/2

Ts=10.6 µs

Ts=7.6 µs

Control field

Input mode

Ts=5.6 µs

1/2

Decay of coherence

due to inhomogeneous

spin dephasing.

Fitted spin distribution

Gaussian FWHM: 26 kHz

3/2

5/2

M. Afzelius et al., arXiv:0908.2309

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, arXiv:0906.2699


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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


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