A quantum optical beam

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A quantum optical beam

• Classically an optical beam can have well defined
  amplitude AND phase simultaneously.
• Quantum mechanics however imposes an uncertainty
  principle.
• The deterministic classical beam is blurred out
  by quantum noise.

Uncertainty principle
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Coherent state Squeezed state

• VV-1
• Ideal output of a low-noise laser
• Same quantum noise as vacuum

• V or V- lt 1
• Very fragile in the presence of loss

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Sideband squeezing

• Laser outputs are typically very noisy at low
  frequency.
• Measure squeezing of the beat of the carrier with
  frequencies outside this noise bandwidth.

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Production of squeezing

• Produce squeezing in a below threshold optical
  parametric amplifier (OPA)

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Production of squeezing

• Produce squeezing in a below threshold optical
  parametric amplifier (OPA)

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Production of squeezing

• Produce squeezing in a below threshold optical
  parametric amplifier (OPA)

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Comparison of OPAs and OPOs

• OPAs are seeded with a bright beam whereas OPOs
  are vacuum seeded.
• Advantages of OPAs
• Can lock the length of the resonator.
• Bright squeezed output that can be controlled in
  downstream applications.
• Advantage of OPOs
• No classical noise coupled from the laser into
  the squeezed beam.

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Recovering buried squeezing

• In our two OPAs this noise is correlated and can
  be cancelled by optical or electronic means.

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The Poincaré sphere
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The Poincaré sphere
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The Poincaré sphere
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The Poincaré sphere
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Polarisation squeezing

• A Stokes parameter is squeezed if its variance is
  below the shot-noise of a coherent beam of equal
  power.

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Squeezed beam
OPA
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OPA I
Squeezed beam II
Squeezed beam I
OPA II
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Summary

• We have produced two reliable strongly quadrature
  squeezed sources.
• We produce new quantum polarisation states and
  investigate their properties.
• We cancel the classical noise of our input laser
  beam to produce squeezing at low frequencies.
• We have produced EPR entanglement and are
  presently characterising it.