Thesis defense

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Towards a Quantum Memory for Non-Classical Light With Cold Atoms

Thesis Director: Elisabeth GiacobinoThesis Co-director: Julien LauratQuantum Optics GroupLaboratoire Kastler-BrosselUniversité Pierre et Marie Curie, Paris

Sidney BurksOctober 13, 2010

http://213.251.135.217/ifraf/

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From Classical Bits to Quantum Bits

•Classical information is based on the bit▫Discrete values of 1 or 0

•Photonic bits

•Quantum information introduces the qubit▫Superposition of states

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Desideratum : Storage without measurement, on-demand retrieval i.e. a coherent and reversible transfer between light and matter.

General Strategy: Transfer the quantum superposition of light onto a superposition of states in a storage medium

Photonic qubit

A Quantum Memory

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Desideratum : Storage without measurement, on-demand retrieval i.e. a coherent and reversible transfer between light and matter.

General Strategy: Transfer the quantum superposition of light onto a superposition of states in a storage medium

The states |a> and |b> are typically ground states in order to avoid a rapid decoherence

General Recipe: Two ground states are connected via an excited state by a control field

Photonic qubit

A Quantum Memory

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Cavity Quantum Electrodynamics (strong coupling)

“Dynamic” EIT

Rephasing protocols- CRIB and AFC -

Rare earth elements in solids at cryogenic temperatures

Sin

gle

Ato

mA

tom

ic E

nse

mb

le:

Colle

ctiv

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xcit

ati

on

Long lifetime

A review of Quantum Memories

Experiments at LKB

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Applications of Quantum Memories

• Most photon sources are probabilistic

• We know however, how to create twin photon sources

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• Most photon sources are probabilistic

• We know however, how to create twin photon sources

• Memory loaded with a photon

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Deterministic “Photon Gun”

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Synchronization of photon emissions

•Two-photon interference

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Synchronization of photon emissions

•Two-photon interference

•Quantum gates

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Quantum Networks•Distribution of

entanglement throughout a network

•Propagation of entanglement in complex quantum systems

•Simulation of collective phenomenon

H.J. Kimble, The Quantum Internet, Nature 453, 1023 (2008)

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Long-distance Quantum Communication•Quantum states are

fragile

•Impossible to clone arbitrary quantum states

•Amplification impossible!

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Long-distance Quantum Communication

100 km, telecom fiber: 99.5 % losses

For 1000 km, and with a 10GHz qubit source, it would take 300000 years to transmit 1 qubit

Connection time increases exponentially with distance

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Long-distance Quantum Communication

100 km, telecom fiber: 99.5 % losses

For 1000 km, and with a 10GHz qubit source, it would take 300000 years to transmit 1 qubit

Connection time increases exponentially with distance

Quantum repeaters

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1) Divide into segments and Generate Entanglement

. .. .. .L0 L0 L0

L

2) Entanglement Swapping. .. .. .. . ..

Fidelity is close to 1 at long distances, but… the time increases exponentially with distance

Entanglement of the segments is probabilistic: each step occurs at a different moment.

Quantum repeaters

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1) Divide into segments and Generate Entanglement

. .. .. .L0 L0 L0

L

2) Entanglement Swapping. .. .. .. . ..“Scalability” : requires quantum memories, which allow an asynchronous preparation of the network

Fidelity is close to 1 at long distances, but… the time increases exponentially with distance

Entanglement of the segments is probabilistic: each step occurs at a different moment.

Quantum repeaters

Quantum Memories

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How do we entangle two memories?

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Probabilistic Entanglement: DLCZ Protocol

Collective Excitation

|e>

|s>

|g>

field 1write

L.M. Duan et al., Nature 414, 413 (2001)

• Experimental demonstration of first quantum repeater segment in 2007

1) Creation of a collective excitation

2) Entanglement of two ensembles

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Deterministic entanglement: Single photon and electromagnetically induced transparency (EIT)

• Mapping of a delocalized single photon

K.S. Choi et al., “Mapping photonic entanglement into and out of a quantum memory”, Nature 452, 7183 (2008)

Writing Storage Retrieval

ControlField

QuantumField

Re-emission of quantum field

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Continuous Variable Entanglement

• Deterministic entanglement source• Uses variables with continuous degrees of

freedom - quadratures of an electromagnetic field

• Characterized by homodyne detection

Y

X

Y

X

Y

X

Y

X

Coherent State Squeezed State

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Current results with EIT in continuous variables

•Delay of a squeezed state

•Storage of a single-sideband▫Storage without excess noise▫Coherent state

•Storage of squeezed vacuum▫−0.16 ± 0.01 dB ~4%▫−0.21 ± 0.04 dB

K. Honda et al., Phys. Rev. Lett. 100, 093601 (2008)

J. Appel et al., Phys. Rev. Lett. 100, 093602 (2008)

G. Hétet et al., Phys. Rev. A 74, 033809 (2005)E. Figueroa et al., New J. Phys. 11, 013044 (2009)

J. Cviklinski et al., Phys. Rev. Lett. 101, 133601 (2008)

LKB

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Our system for continuous variable entanglement storage

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Creation of two ensembles

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Plan: Towards a Quantum Memory

A. Source▫Squeezed Vacuum▫Characterization▫Interfacing

B. Memory

Quantum Memory

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B. Memory

Quantum Memory

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Generation of Squeezed Vacuum with an OPO

•Source of Squeezed Vacuum

•Compatible with a Cesium-based quantum memory

•Optical Parametric Oscillator (OPO)

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Coherent StateSqueezed Vacuum

Usage of nonlinear opticsSecond-harmonic Generation Parametric Down-Conversion

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Experimental Layout

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Experimental Layout

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Second-Harmonic Generation

•Ring cavity•Stabilization via Tilt-Locking•Temperature regulation

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Doubling CavitySecond-harmonic

Power

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Doubling CavitySecond-harmonic

Power

330 mW of blue

330 mW

50% conversion efficiency

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Experimental Layout

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OPO Cavity

Linear Quadratic

Balance between strong squeezing and experimental

stability

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OPO Cavity

•Output coupler T = 7%•Below-threshold operation•Stabilization by Pound-Drever-Hall•Counter-propagating lock beam

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Lock Beam

•Stray photons in the Squeezed Vacuum•Reduction of lock beam intensity•Antireflective treatment•Active Switch

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B. Memory

Quantum Memory

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Experimental Layout

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S. Burks et al., “Squeezed light at the D2 cesium line for atomic memories”, Opt. Express 17, 3777 (2008)

Squeezed Vacuum Generation

Analysis frequency: 1MHz

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S. Burks et al., “Squeezed light at the D2 cesium line for atomic memories”, Opt. Express 17, 3777 (2008)


Analysis frequency: 1MHz

- 3 dB of squeezing (50% reduction of quantum noise)

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Squeezed Vacuum GenerationCompatibility with the memory?

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•Will be used for EIT in Cesium

AbsorptionDispersion

Compatibility with the memory?

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•Will be used for EIT in Cesium

•Frequency fixed by linear region of the dispersion

AbsorptionDispersion

500 kHz

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Compatibility with bandwidth-limited EIT!

Squeezing starting at 30 kHz

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State Reconstruction

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State Reconstruction• Photon pairs for

Squeezed Vacuum

• Thermal state mixed with the vacuum state

Complete characterization of our state

Wigner function for 2 dB of squeezing

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B. Memory

Quantum Memory

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Creation of Pulses

•Temporal mode adapted to the memory

•Conversion of a continuous source into a pulsed source

•Very difficult due to the fragility of quantum states

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Pulses with an Optical Chopper

time

Acoustic noise suppression

Mechanical vibration attenuation

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Pulses with an Optical Chopper

• Optical losses~2%

• Pulses of 500 ns!

1 µs width

time

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Pulses via AOM

• Low optical losses: ~10%• Precise timing control: 25 ns

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A. Source

B.Memory

Quantum Memory

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Creation of Two Ensembles

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Necessary Elements

•Atoms▫Large and dense cloud

•EIT▫Lasers and transitions

•Magnetic field cancelation▫Avoid ground state decoherence

•Timing and Synchronization

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•Chamber

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•Chamber•MOT

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•Chamber•MOT•Lasers

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•Chamber•MOT•Lasers•Multiplexing

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•Chamber•MOT•Lasers•Multiplexing

How can we characterize this cloud?

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Optical density measurement

-10 MHz

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Optical density measurement

Optical density of 20

Memory efficiency of 25%

-10 MHz

Gorshkov et al., Phys. Rev. A 76, 033805 (2007)

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Necessary Elements•Atoms

▫Large and dense cloud




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Optical Phase Lock

Optical beat signal

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Phase Lock

• Rests locked for several hours

• sub-Hz frequency precision

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Necessary Elements•Atoms

▫Large and dense cloud




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Extinguishing the magnetic field

•Field due to MOT coils

•Residual fields

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•Cloud remains ~5 ms after cutting the field•Fields are difficult to cut quickly

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•Cloud remains ~5 ms after cutting the field

•Fields are difficult to cut quickly

Time constant 300 µs The cloud remains dense!

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Raman Spectroscopy

• Presence of parasite fields• milliGauss compensation in 3

dimensions

Field present

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Raman Spectroscopy

• milliGauss compensation in 3 dimensions

Field present

Memory time: 10-100 µs

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Necessary Elements

•Atoms▫Large and dense cloud




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Timing of Memory Lasers

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Timing of Memory Lasers•Simple Interface•Rapid Development•Scaleable

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Memory Optical Table

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Conclusion

•Entanglement of memory ensembles

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Conclusion

• Entanglement of memory ensembles

• Squeezed Vacuum generation with an ’OPO▫Strong squeezing: -3 dB▫Compatible with EIT▫ Interfaced with the memory

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Conclusion

• Entanglement of memory ensembles

• Squeezed Vacuum generation with an ’OPO▫ Strong squeezing: -3 dB▫ Compatible with EIT▫ Interfaced with the memory

• Characterization of Memory Elements

Memory storage time: 10-100 µs

Memory efficiency of 25%

Creation of two ensembles

Thesis defense

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