We have demonstrated the reduction of photon-number fluctuations below the shot-noise level (photon-number squeezing) by two-photon absorption in an inorganic semiconductor material consisting of a thin film of TOPO-decorated CdSe nanocrystals embedded in a PMMA polymer host. The photon-number squeezing was measured at 800 nm, where the linear absorption of the material was negligible. The amount of photon-number squeezing from the CdSe system was ∼5% for a moderate value of nonlinear transmission. This observation was possible due to the large nonlinear absorption exhibited by the system of CdSe NCs embedded in PMMA and also due to the reduced linear scattering losses in thin films. The CdSe squeezing data are compared to results previously obtained for the first time in a nonlinear optical polymeric material. The achievement of photon-number squeezing in these novel materials gives the opportunity for future application in the improvement of the signal-to-noise ratio in optical communications.
We have demonstrated the reduction of photon-number fluctuations below the shot-noise level (photon-number squeezing) by two-photon absorption in an inorganic semiconductor material consisting of a thin film of TOPO-decorated CdSe nanocrystals embedded in a PMMA polymer host. The photon-number squeezing was measured at 800 nm, where the linear absorption of the material was negligible. The amount of photon-number squeezing from the CdSe system was ∼5% for a moderate value of nonlinear transmission. This observation was possible due to the large nonlinear absorption exhibited by the system of CdSe NCs embedded in PMMA and also due to the reduced linear scattering losses in thin films. The CdSe squeezing data are compared to results previously obtained for the first time in a nonlinear optical polymeric material. The achievement of photon-number squeezing in these novel materials gives the opportunity for future application in the improvement of the signal-to-noise ratio in optical communications.
We present experimental studies based on all three methods by which the generation of squeezed light in semiconductors has thus far been demonstrated experimentally: Fourwave mixing, multi-photon absorption and direct generation at the source. Four-wave mixing was used to generate femtosecond-pulsed quadrature squeezed light by cross-phase modulation in single-crystal hexagonal CdSe at wavelengths between 1.42 and 1.55 μm. We measured 0.4 dB squeezing (1.1 dB is inferred at the crystal) using 100 fs pulses. The wavelength and the intensity dependence, as well as variations in the local oscillator configuration were investigated. At higher intensities squeezing was shown to deteriorate owing to competing nonlinear processes. We also characterised the nonlinear optical properties of CdSe in this wavelengths range using an interferometric autocorrelator. In addition, we studied the feasibility of extending this technique to AlGaAs waveguides. The key problems are addressed and solutions are proposed. In a different experiment we used an AlGaAs waveguide to demonstrate for the first time photon-number squeezing by multi-photon absorption. By tuning the pump energy through the half bandgap energy we could effectively select two- or three-photon absorption as the dominant mechanism. Squeezing by these two mechanisms could be clearly distinguished and was found to be in good agreement with longstanding theoretical predictions. We also established the generality of the effect, by demonstrating the same mechanism in organic semiconductors, where it led to the first ever observation of squeezed light in an organic material. Finally, we present our measurements of photon-number squeezing in high-efficiency double heterojunction AlGaAs light-emitting diodes. We measured squeezing of up to 2.0 dB. In addition, we observed quantum noise correlations when several of these devices were connected in series.
The signal to noise ratio of optical systems is ultimately limited by the shot noise due to the Poissonian statistics of the light field. In recent years there has been a considerable effort to find ways to overcome this limitation by generating non-classical light with sub-shot noise statistics. Several groups have demonstrated non-classical light generation in diode lasers or light emitting diodes, and the amplitude fluctuations have been reduced substantially below the shot noise level [1,2]. However, for phase-sensitive measurements it is necessary to generate the more general quadrature squeezed states. Such quadrature squeezed light can be generated either by second-order or third-order nonlinear processes provided the nonlinear material has very low losses at the optical wavelength. To date, the successful generation schemes for quadrature squeezed light have been restricted to atomic vapours, optical fibres, or second-order nonlinear crystals [3]. The possibility of using the large χ(3) nonlinearities of semiconductors at frequencies below the band gap where the optical absorption is small has been discussed in the literature, but so far there have been no successful experiments [4-6]. In this paper we report measurements on GaAs/Al0.3Ga0.7As multiple quantum well waveguides and bulk ZnS in the wavelength range 750 - 890nm. Although we measure much larger nonlinearities in the quantum wells, the strong two-photon absorption is detrimental for squeezed light generation. However, in the ZnS where the laser frequency is below the two- photon absorption edge, we have succeeded in generating quadrature squeezed light in a semiconductor for the first time. On the basis of our results, we would expect that squeezed light could be generated in many other semiconductor systems provided the photon energy is less than half the band-gap energy.
Generation of nonclassical lights, such as photon-number squeezed states and quadrature-phase squeezed state, is required to realize optical communications and accurate measurements at sub-shot-noise levels. Among a variety of schemes, the direct generation of sub-Poissonian photon fluxes from semiconductor light emitters has been attracting much attention because of the simplicity of experimental arrangement, the low energy consumption, the possibility of a large degree of noise suppression, and the compatibility with modern communication system. In this presentation, we demonstrate the experimental results of generation of sub-Poissonian photon fluxes in an integrated light-emitter/detector system. It is shown that the pump-noise-suppression owing to the macroscopic Coulomb blockade effect can work well even when a superlattice alloylike material is used.
In this dissertation measurements are presented that extend our understanding of the important noise processes involved in the semiconductor laser generation of photon-number squeezed light. Strong sub-shot noise correlations between photon-number fluctuations on orthogonal polarization axes are observed in the output of a low temperature injection-locked quantum-well laser. Such polarization dependent effects are accounted for by intrinsic correlations generated in the laser gain medium and polarization mixing due to birefringence. Using polarization preserving balanced homodyne detection, up to 4.5 dB of photon-number squeezing is reported, which is within 95% of the expected maximum squeezing based on the device quantum efficiency. The work in this dissertation demonstrates the application of such nonclassical fields to practical measurements that achieve sub-shot noise sensitivity. Coherent nonlinear optical spectroscopy with a photon-number squeezed probe field is used to measure the Urbach tail and exciton resonances in a GaAs/AlGaAs multiple quantum well with sensitivity below the semiclassical shot-noise limit. Direct drive current modulation of an injection-locked room temperature laser results in a nonclassical amplitude modulated field with a signal to noise ratio that exceeds the maximum ratio permitted for a classical source under equivalent modulation conditions.
Electromagnetic fields, with the noise on one quadrature component reduced to below the quantum mechanical zero-point fluctuation level and the noise on the other quadrature component enhanced to above it, are currently of great interest in quantum optics because of their potential applications to various precision measurements. Such squeezed states of light are usually produced by imposing nonlinear unitary evolution on coherent (or vacuum) states. On the other hand, squeezed states with reduced photon number noise and enhanced phase noise are generated directly by a constant current-driven semiconductor laser. This is the simplest scheme for the generation of nonclassical light, and so far it has yielded the largest quantum noise reduction. The mutual coupling between a lasing junction and an external electrical circuit provides opportunities for exploring the macroscopic and microscopic quantum effects in open systems.
Abstract Recent theoretical and experimental advances have shown that squeezed states of light can be engineered to enhance the resolution of optical measurements. Extending such quantum-enhanced techniques to nonlinear optical processes has therefore attracted considerable interest. In particular, two-photon absorption (TPA), which has wide-ranging applications in physics, chemistry, and biology, has emerged as a promising platform for exploiting these quantum advantages. In this work, we investigate whether and how correlations between light modes can be used to improve optical resolution in TPA. We analyze non-degenerate TPA transmission measurements using two-mode squeezed light and compare different measurement strategies. We find that intensity correlation measurements offer better performance in terms of the achievable estimation error compared to normalized intensity correlations or measurements of the noise reduction factor (NRF). The achievable enhancement from intensity and NRF measurements are degraded by competing photon losses. In contrast, the normalized intensity correlation approach remains robust to loss, albeit at the cost of forfeiting the scaling enhancement at large photon numbers achievable with nonclassical light fields. Thus, we establish a trade-off between robustness to loss and the achievable quantum advantage. We further compute the corresponding Cramér–Rao bound, which sets the ultimate lower limit on the achievable estimation error for the photon-counting-based measurement strategies investigated in this work.