introduction into quantum optomechanics

Introduction into QuantumOptomechanics

Part 2 of 2

F.Ya.Khalili

March 20, 2013

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1 Standard Quantum Limit

2 Quantum speedmeter

3 Negative optical inertia

4 Some metaphysics

5 Non-gaussian optomechanics

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4 Some metaphysics


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LENGTHY EQUATIONS AHEAD!

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Detection of classical force

Freemass

x(t)

F fl(t)

Meter“x(t)” = x(t) + xfl(t)F sign(t)

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Freemass

x(t)

F fl(t)


−mΩ2x(Ω) = F sign(Ω) + F fl(Ω)

“x(Ω)” = x(Ω) + xfl(Ω) =F sign(Ω)

−mΩ2+

the sum noise︷︸︸︷F fl(Ω)

−mΩ2+ xfl(Ω)

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Freemass

x(t)

F fl(t)


−mΩ2x(Ω) = F sign(Ω) + F fl(Ω)

“x(Ω)” = x(Ω) + xfl(Ω) =F sign(Ω)

−mΩ2+

the sum noise︷︸︸︷F fl(Ω)

−mΩ2+ xfl(Ω)

Ssum(Ω) = Sx +2SxF−mΩ2

+SF

m2Ω4

=~2/4 + S2

xF

SF+

2SxF−mΩ2

+SF

m2Ω4

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Freemass

x(t)

F fl(t)



+SF

m2Ω4

=~2/4 + S2

xF

SF+

2SxF−mΩ2

+SF

m2Ω4

Classical optimization: damn the back action!

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Freemass

x(t)

F fl(t)



+SF

m2Ω4

=~2/4 + S2

xF

SF+

2SxF−mΩ2

+SF

m2Ω4

Classical optimization: damn the back action!

SxF = 0 ⇒ Ssum(Ω) = Sx +SF

m2Ω4=

~2m

(1

Ω2q

+Ω2

q

Ω4

)

Ωq =

(SF

m2Sx

)1/4

∝√〈I 〉m

e2r

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Sum noise spectral densityS

(Ω)

Ω

Sx

SFm2Ω4

Ssum(Ω) =~

2m

(1

Ω2q

+Ω2

q

Ω4

)

Ω = Ωq

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Sum noise spectral densityS

(Ω)

Ω

Ω (1)q

<

Ω (2)q

<

Ω (3)q

<

Ω (4)q

<

Ω (5)q

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Standard Quantum LimitS

(Ω)

Ω

SSQL (Ω) = ~

mΩ 2

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(Ω)

Ω

“Quantum”

“Classical”

SSQL (Ω) = ~

mΩ 2

m = 40 kg , Ω = 2π × 100 s−1 ⇒√SSQL ≈ 2.5× 10−21 m/

√Hz

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(Ω)

Ω

“Quantum”

“Classical”

SSQL (Ω) = ~

mΩ 2

m = 40 kg , Ω = 2π × 100 s−1 ⇒√SSQL ≈ 2.5× 10−21 m/

√Hz

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How to overcome the SQL

Implicit assumptions that have been made:

1 The scheme is stationary (invariant with respect totime shift).

2 The noises are Markovian.

3 The noise are mutually uncorrelated.

4 The test object is a free mass.

Number of schemes based on violation of some of theseassumptions has been proposed.

Only one successful experiment was performed / (fornow).

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How to overcome the SQL

Implicit assumptions that have been made:

1 The scheme is stationary (invariant with respect totime shift).

2 The noises are Markovian.

3 The noise are mutually uncorrelated.

4 The test object is a free mass.

Number of schemes based on violation of some of theseassumptions has been proposed.

Only one successful experiment was performed / (fornow).

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Questions?

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4 Some metaphysics


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SQL: simplified consideration

Position measurement

x(t) = x +pt

m+

1

m

∫ t

0

(t − t ′)F (t ′) dt ′

[x(0), x(t)] =i~tm⇒ ∆x(0) = ∆x(t) >

√~t2m

Ssum >(∆x)2

Ω∼ ~

mΩ2

Momentum measurement

p(t) = p(0) +

∫ t

0

F (t ′) dt ′ ⇒ [p(0), p(t)] = 0

⇒ no SQL

Velocity measurement

We can not measure momentum?Let us measure velocity instead!

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x(t) = x +pt

m+

1

m

∫ t

0

(t − t ′)F (t ′) dt ′

[x(0), x(t)] =i~tm⇒ ∆x(0) = ∆x(t) >

√~t2m

Ssum >(∆x)2

Ω∼ ~

mΩ2


p(t) = p(0) +

∫ t

0

F (t ′) dt ′ ⇒ [p(0), p(t)] = 0

⇒ no SQL



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x(t) = x +pt

m+

1

m

∫ t

0

(t − t ′)F (t ′) dt ′

[x(0), x(t)] =i~tm⇒ ∆x(0) = ∆x(t) >

√~t2m

Ssum >(∆x)2

Ω∼ ~

mΩ2


p(t) = p(0) +

∫ t

0

F (t ′) dt ′ ⇒ [p(0), p(t)] = 0

⇒ no SQL



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The idea

Light interacts with the probe twice:

V.B.Braginsky, F.Ya.Khalili, Phys.Lett.A 147, 251 (1990)22 / 70

The idea

Light interacts with the probe twice:

V.B.Braginsky, F.Ya.Khalili, Phys.Lett.A 147, 251 (1990)23 / 70

Sagnac interferometerDelay lines

aRN

bRN

bLN

aLN

aRE

bRE

aLE

bLEBS

SRM

PRM

z

Laser

p q

PD

ITM ETM

ITM

ETM

Ring cavities

aRN bRN

bLN

aLN

aRE

bRE

aLE

bLE

BS

SRM

PRM

ETM

ETM

z

Laser

p q

PD

Yanbei Chen, Phys.Rev.D 67, 122004 (2003)

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Sagnac interferometerDelay lines

aRN

bRN

bLN

aLN

aRE

bRE

aLE

bLEBS

SRM

PRM

z

Laser

p q

PD

ITM ETM

ITM

ETM

Ring cavities

aRN bRN

bLN

aLN

aRE

bRE

aLE

bLE

BS

SRM

PRM

ETM

ETM

z

Laser

p q

PD

δφ(t) ∝ xnorth(t − τ) + xeast(t)

δφ(t) ∝ xeast(t − τ) + xnorth(t)

δφ(t)− δφ(t) ∝ τd

dt[xeast(t)− xnorth(t)]

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Overview of 1m speed meter experiment

!  1g mirrors suspended in monolithic fused silica suspensions.

!  1kW of circulating power. Arm cavities with finesse of 10000. 100ppm loss per roundtrip.

!  Sophisticated seismic isolation + double pendulums with one vertical stage.

!  Large beams to reduce coating noise.

!  Armlength = 1m. Target better than 10-18m/sqrt(Hz) at 1kHz.

!  No recycling, no squeezing, but plan to use homodyne detection.

!  LOTS OF CHALLENGES! (let me know if you want to help…)

S.Hild et al, LSC QNWG telecon, January 30, 201326 / 70

Questions?

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4 Some metaphysics


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The idea

SSQL =~

mΩ2

It is assumed that minert = mgrav = m. But what if not?

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The idea

SSQL =~

mΩ2


SSQL(Ω) =~minert

m2gravΩ4

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The idea

SSQL =~

mΩ2


SSQL(Ω) =~minert

m2gravΩ4

minert

mgrav→ 0 ⇒ SSQL(Ω)→ 0

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The idea

SSQL =~

mΩ2


SSQL(Ω) =~minert

m2gravΩ4

minert

mgrav→ 0 ⇒ SSQL(Ω)→ 0

Impossible? But...

F.Khalili et al, Phys. Rev. D 83, 062003 (2011)

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Optical springs to help

Mechanical responce function:

χ−1(Ω) = −mΩ2 + K

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χ−1(Ω) = −mΩ2 + K

Two optical springs (γ δ, just for simplicity):

K ≈ 4ωp〈I1〉cL

δ1

δ21 − Ω2

+4ωp〈I2〉cL

δ2

δ22 − Ω2

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χ−1(Ω) = −mΩ2 + K



δ1

δ21 − Ω2

+4ωp〈I2〉cL

δ2

δ22 − Ω2

〈I 〉1δ1

+〈I 〉2δ2

= 0 , Ω δ1,2 ⇒ K ≈ −µΩ2

µ =4ωp

cL

〈I1〉δ1 + 〈I2〉δ2

δ21 + δ2

2

: the optical inertia!

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χ−1(Ω) = −mΩ2 + K



δ1

δ21 − Ω2

+4ωp〈I2〉cL

δ2

δ22 − Ω2

〈I 〉1δ1

+〈I 〉2δ2

= 0 , Ω δ1,2 ⇒ K ≈ −µΩ2

µ =4ωp

cL

〈I1〉δ1 + 〈I2〉δ2

δ21 + δ2

2

: the optical inertia!

mgrav = m , minert = m + µ

µ < 0 ⇒ minert < mgrav36 / 70

Questions?

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4 Some metaphysics


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~F = m~a

(deterministic!)

Quantum-classical cut

i~d |ψ〉dt

= H |ψ〉

(deterministic!)

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~F = m~a (deterministic!)


i~d |ψ〉dt

= H |ψ〉

(deterministic!)

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i~d |ψ〉dt

= H |ψ〉 (deterministic!)

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Quantum reduction: THE randomness!

i~d |ψ〉dt

= H |ψ〉 (deterministic!)

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The root of all evil:the (in)famous Reduction Postulate:

|ψn〉 =En|ψ〉√

Pn

Pn = 〈ψ|En|ψ〉

which cuts off randomly brunchesof the World Wave Function.

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Where and when the reduction REALLY happens?

• Copenhagen interpretation: We do not know and donot want to know; shut up and calculate theprobabilities.

• David Bohm: Reduction is a human invention. Thereal world is classical (but non local!)

• Eugene Wigner: Reduction happens in the humanbrain (which does not obey the ordinary laws ofphysics)

• Hugh Everett: Reduction is a human invention. Thereal world is completely quantum.

• Most of the physicists (in their subconsciousness):Reduction happens somewhere in meso-scale (andtherefore open systems blah blah blah...)

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But what about cats?47 / 70







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The most consistent options


• David Bohm: Reduction is a human invention.The real world is classical (but non local!)

• Eugene Wigner: Reduction happens in the humanbrain (which does not obey the ordinary law ofphysics)

• Hugh Everett: Reduction is a human invention.The real world is completely quantum.

• Most of the physicists (in their subconsciousness):Reduction happens somewhere in meso-scale(and therefore open systems blah blah blah...)

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The options which can be tested


• David Bohm: Reduction is a human invention.The real world is classical (but non local!)

• Eugene Wigner: Reduction happens in the humanbrain (which does not obey the ordinary law ofphysics)

• Hugh Everett: Reduction is a human invention.The real world is completely quantum.

• Most of the physicists (in their subconsciousness):Reduction happens somewhere in meso-scale(and therefore open systems blah blah blah...)

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The paper to read tonight

Lev Vaidman, “On schizophrenic experiences of theneutron or why we should believe in the many-worldsinterpretation of quantum theory”,arXiv:quant-ph/9609006

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The Gaussianity problem

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

0.2

0.4

0.6

0.8

1.0

Quantum Wignerfunction W (x , p) of

|ψ〉 = |0〉.

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

0.2

0.4

0.6

0.8

1.0

Classical probabilitydistribution W (x , p) for

T =~Ω0

2kB

Find any difference!

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The Gaussianity problem

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

0.2

0.4

0.6

0.8

1.0

Quantum Wignerfunction W (x , p) of

|ψ〉 = |0〉.

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

0.2

0.4

0.6

0.8

1.0

Classical probabilitydistribution W (x , p) for

T =~Ω0

2kBFind any difference!

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An always positive Wigner function can serve as thehidden-variable probability distribution with respect tomeasurements corresponding to any linear combinationof x and p.

J. S. Bell, Speakable and Unspeakable in Quantum Mechanics,Cambridge Univ. Press, Cambridge, 1987

S. L. Braunstein, P. van Loock, RMP 77, 513 (2005)

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.0

−0.8

−0.6

−0.4

−0.2

0.0

0.2

0.4

The real tests ofquantumness requirenon-linear measurementsor non-Gaussian states.

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An always positive Wigner function can serve as thehidden-variable probability distribution with respect tomeasurements corresponding to any linear combinationof x and p.

J. S. Bell, Speakable and Unspeakable in Quantum Mechanics,Cambridge Univ. Press, Cambridge, 1987

S. L. Braunstein, P. van Loock, RMP 77, 513 (2005)

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

−1.0

−0.8

−0.6

−0.4

−0.2

0.0

0.2

0.4

The real tests ofquantumness requirenon-linear measurementsor non-Gaussian states.

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4 Some metaphysics


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Generic scheme of experiment

• Prepare some really macroscopic mechanical objectin a really non-classical (non-Gaussian) state.

• Show that this state can survive for considerabletime (no spontaneous reduction!)

• Unprepare the mechanical object back to its initialstate (crucial for Everett’s point of view)

It would be good to include into the last step also theguy who do this experiment (resolving thus the Wigner’sfriend paradox).

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Mechanical non-Gaussian preparation

Two ways:

• Direct use the mechanical non-linearity.

Impossible at the current technological level (?):mechanical systems are too linear and too noisy.

• Injection of an optical non-Gaussian state into themechanical resonator.

The way to go?

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Mechanical non-Gaussian preparation

Two ways:

• Direct use the mechanical non-linearity.Impossible at the current technological level (?):mechanical systems are too linear and too noisy.

• Injection of an optical non-Gaussian state into themechanical resonator. The way to go?

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Direct linear interaction: simple and effective

Ωmech ωe.m.

Λ

Hint = −~K (a + a†)x

Ωmech = ωe.m.

|φ〉|ψ〉 ⇐⇒t=π/Λ

|ψ〉|φ〉

(feasible in microwave band)64 / 70

Up-converting: effective as well

Ωmech ωe.m.− ωp

Λ

Hint = −2~G (Ae−iωpt a + A∗e iωpt a†)x

ωp = ωe.m. − Ωmech

|φ〉|ψ〉 ⇐⇒t=π/Λ

|ψ〉|φ〉

(feasible in both microwave and optical bands)65 / 70

The idea

The setup is identical tothe optical cooling in theresolved sideband regime.

The only difference:the input field ina non-Gaussian state,instead of the ground one.

J.Zhang, K.Peng, and S.L.Braunstein, PRA 68, 013808 (2003)

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The idea

The setup is identical tothe optical cooling in theresolved sideband regime.

The only difference:the input field ina non-Gaussian state,instead of the ground one.

J.Zhang, K.Peng, and S.L.Braunstein, PRA 68, 013808 (2003)

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(A bit) more real-world proposal

Adaption to LIGO and table-top interferometers.

F.Khalili et al, PRL 105, 070403 (2010)68 / 70

Quantum memory?

Storage times of:

optical cavities: . 10−3 s

MW resonators: . 1 s

mechanical pendulums: up to 108 s!

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Shrodinger cat in AdvLIGO?

M = 40 kg70 / 70

introduction into quantum optomechanics

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