Ziel
When the NOROS Action began, quantum noise reduction in light, also called "squeezing", had reached factors of 20-60%. For serious applications much higher factors are required. This Action aimed to:
-improve the noise reduction factor of existing squeezing systems, whose operation is still far from optimal
-investigate noise reduction in existing systems in regimes of operation other than the usual ones, and to search for new squeezing systems
-explore materials which would be relevant for applications, anticipating the advent of a new device, the "squeezer"
-assess the potential of reduced noise light-beams for high-sensitivity measurements and information technology applications.
Optical systems with reduced noise were investigated. It is now possible to reduce the noise in optics below the 'shot noise' which is due to the corpuscular nature of light. Work focussed on systems which can efficiently reduce noise, such as optoelectronic and nonlinear optical devices, and assessed their potential for future practical use. Optical communication and information technology applications are foreseen.
Most results have been obtained on squeezing in parametric and second harmonic generation, namely:
a monolithic device for second harmonic generation yielding 40% intensity squeezing;
improved operation of the parametric oscillator (86% noise reduction on twin beams);
a throrough theoretical understanding of the quantum fluctuations in the parametric oscillator, in both the 3-wave and 4-wave mixing configuration;
a new theoretical method of modelling the propagation of nonclassical light;
low light power parametric generation in single pass providing up to 60% quantum noise reduction on twin beams (demonstration of secure communication based on quantum properties of photon pairs, and of a system for quantum cryptography).
As far as optical bistability is concerned, theoretical studies and assessments of nonlinear materials are well underway. The main outcomes so far are:
new theories for 1-photon and 2-photon processes that clarifies the differences between 1-photon and 2-photon nonlinearities for squeezing and the role of atomic noise;
an overall evaluation of available nonlinear materials;
a quantum nondemolition (QND) measurement scheme which opens the way to noiseless optical tapping.
APPROACH AND METHODS
Quantum noise reduction, or squeezing, can be achieved by various kinds of systems involving non-linear optical processes or optical sources excited by low-noise pumping processes. The Action examined three different methods of producing squeezed light: -Parametric generation and second harmonic generation. The processes which involve the frequency conversion of an incoming light-beam also have a profound effect on its fluctuations and produce various kinds of squeezed light: with reduced intensity nois e, with reduced phase noise, or "twin beams", ie beams exhibiting quantum intensity correlation.
-Optical bistability. Non-linear optical cavities can switch between high and low light transmission when monitored by small optical signals. The quantum noise is also strongly modified and can be either phase-squeezed or intensity-squeezed at the output of the cavity.
-In some optoelectronic devices, such as laser diodes, the flow of emitted photons can be controlled by regulating the pumping current. In such cases intensity-squeezed light is emitted.
PROGRESS AND RESULTS
Most results have been obtained on squeezing in parametric and second harmonic generation, namely:
-a monolithic device for second harmonic generation yielding 40% intensity squeezing
-improved operation of the parametric oscillator (86% noise reduction on twin beams)
-a thorough theoretical understanding of the quantum fluctuations in the parametric oscillator, in both the three-wave and four-wave mixing configuration
-a new theoretical method of modelling the propagation of non-classical light
-low light power parametric generation in single pass providing up to 60% quantum noise reduction on twin beams. Demonstration of secure communication based on quantum properties of photon pairs, and of a system for quantum cryptography (patented).
As far as optical bistability is concerned, theoretical studies and assessments of non-linear materials are well underway. The main outcomes so far are:
-New theories for one and two-photon processes that clarifies the differences between one- and two-photon non-linearities for squeezing and the role of atomic noise.
-an overall evaluation of available non-linear materials
-a quantum non demolition (QND) measurement scheme which opens the way to noiseless optical tapping.
POTENTIAL
With the rapid advances in the theoretical understanding of squeezed light and its generation, and with the prospect of getting squeezing factors of over 90% within a few years, the application of squeezed light to practical purposes, such as high-precision measurements, optical communication and information technology, now lies within reach.
Further progress in squeezing factors and the implementation of communication schemes using various kinds of squeezed light strongly depend on the development of optical nonlinear materials and devices. The two fields could very well support each other through cross-fertilisation.
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