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Non-classical multi-photon light from clusters of colloidal quantum dots

Qi Luo

Published Jan 1, 2019• 0 citations
OpenAlex

Abstract

In this thesis, an experiment for testing the non-classicality of the light emitted by CdSe/CdS clusters is described. Different criteria are used, which are sufficient conditions for the non-classicality. They are based on the measurement of normalized Glauber’s correlation functions of different orders or multi-photon probabilities using the Hanbury Brown–Twiss (HBT) setup. After measuring the light emitted by CdSe/CdS clusters consisting of different numbers of quantum dots, the non-classicality of their emission has been verified with using the criteria mentioned above. In particular, the fourth-order anti-bunching and the non-classicality witnessed by the Klyshko criteria of the third-order have been observed in this experiment, which have never been reported on a triggered multi-photon source before, to the best of my knowledge. In this thesis, an experiment of calibrating the QE of the intensified CCD (ICCD) camera in the single-photon detection regime with using Klyshko’s absolute method has been described. Different groups of pixels in the same camera have been used as both the reference and calibrated detectors, and therefore this is an autonomous absolute calibration. The QE of the ICCD camera as a function of the threshold value, as well as the wavelength dependence of the QE at a chosen threshold value, have been measured. The QE value in the single-photon regime is much lower than the one of the photocathode of the camera. To the best of my knowledge, this is the first experiment to calibrate the QE of an ICCD camera in the single-photon regime. The experimental results reported in this thesis are useful in the field of non-classical multi-photon sources as well as the field of multiplexed single-photon detection techniques.

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Multi-Photon Interferences of Independent Light Sources

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Generation and detection of non-classical photon states

Katarina Stensson

This thesis intends to familiarize the reader with the concepts of photon statistics and correlations in quantum optics. Developing light sources that emit quantum states is central for the realization of quantum technologies. One important step in characterizing these sources is the measurement of field fluctuations and correlations, by coincidence measurements. The expectation value of a coincidence measurement, a simultaneous measurement of two intensities (or, more general, four fields), is represented by the fourth-order correlation function. The value of the correlation function, at zero delay between the detection of two photons, reveals important properties of the state to which they belonged, for example the fluctuations of the photon number. Since predictability is important for many applications, light sources emitting single photons are also characterized by the indistinguishability of consecutively emitted photons, or of two photons from separate emitters. In paper I we investigate blinking behaviour in quantum emitters, and its effect on the interference pattern and photon statistics with photons from two separate emitters. Blinking refers to an emitters transition into a non-emitting state, and subsequent transition back to an emitting state. We show that blinking can not be treated as linear loss, when measuring the fourth-order correlation function for two emitters in a Hong-Ou-Mandel setup. In general, a measurement of the fourth-order correlation function is robust to loss, which makes it a very practical tool. However, the relation between recorded coincidence counts and the correlation function is only direct in the limit of zero detection efficiency, and depends on the detection system. In paper II, we show that by adding a variable attenuation in the beam path, we can trace back to the ''true'' value of the correlation function at zero quantum efficiency. This method improves accuracy in correlation measurements by decreasing a systematic error at the expense of an increased statistical error, which is easier to handle, extending the use of coincidence methods to classical and non-classical multi-photon states.

•Jan 1, 2018•0 citations
In this thesis, an experiment for testing the non-classicality of the light emitted by CdSe/CdS clusters is described. Different criteria are used, which are sufficient conditions for the non-classicality. They are based on the measurement of normalized Glauber’s correlation functions of different orders or multi-photon probabilities using the Hanbury Brown–Twiss (HBT) setup. After measuring the light emitted by CdSe/CdS clusters consisting of different numbers of quantum dots, the non-classicality of their emission has been verified with using the criteria mentioned above. In particular, the fourth-order anti-bunching and the non-classicality witnessed by the Klyshko criteria of the third-order have been observed in this experiment, which have never been reported on a triggered multi-photon source before, to the best of my knowledge. In this thesis, an experiment of calibrating the QE of the intensified CCD (ICCD) camera in the single-photon detection regime with using Klyshko’s absolute method has been described. Different groups of pixels in the same camera have been used as both the reference and calibrated detectors, and therefore this is an autonomous absolute calibration. The QE of the ICCD camera as a function of the threshold value, as well as the wavelength dependence of the QE at a chosen threshold value, have been measured. The QE value in the single-photon regime is much lower than the one of the photocathode of the camera. To the best of my knowledge, this is the first experiment to calibrate the QE of an ICCD camera in the single-photon regime. The experimental results reported in this thesis are useful in the field of non-classical multi-photon sources as well as the field of multiplexed single-photon detection techniques.
We study nonclassical features of multiphoton light emitted by clusters of single-photon emitters. As signatures of nonclassicality, we use violation of inequalities for normalized correlation functions of different orders or the probabilities of multiphoton detection. In particular, for clusters of 2–14 colloidal CdSe/CdS dot-in-rods we observe antibunching and nonclassicality of up to the fourth-order. Surprisingly, violation of certain classical inequalities gets even more pronounced for larger clusters.
We consider nonclassical features of light emitted by clusters of room-temperature single-photon emitters. As signatures of nonclassicality, we discuss violation of inequalities for normalized correlation functions of orders higher than two. These conditions are stronger than higher-order antibunching, i.e., higher-order normalized correlation functions being less than unity. In experiment, we observe third-order nonclassicality as well as second-, third-, and fourth-order antibunching for clusters of 1 to 20 colloidal CdSe/CdS dot-in-rods.
In this thesis we discuss an extension of the historical Hanbury Brown and Twiss experiment and investigate spatial intensity correlations of higher-orders in the radiation field of statistically independent light sources. The light sources can be non-classical like single-photon emitters or classical, obeying thermal or coherent statistics. Although the light sources are assumed to emit the photons in an incoherent manner, we find spatial photon correlations in the far field which can be explained by the quantum mechanical concept of multi-photon interferences. We demonstrate that spatial correlation measurements between indistinguishable photons based on linear optical detection techniques can be used to overcome the resolution limit of classical optics (Abbe/Rayleigh limit) and also to manipulate the spatial radiation characteristic of the light sources. Both remarkable properties of higher-order intensity correlations have been experimentally verified using statistically independent pseudothermal light sources. Due to the fact that the reported correlation measurements require neither special quantum tailoring of light nor N-photon absorbing media, they are of particular interest for the field of quantum imaging and quantum information processing. The thesis is divided into three main parts: In the first part (Chapter 2 and 3) we introduce a quantum path formalism which allows to describe multi-photon interferences arising in both classical and nonclassical light fields. This quantum path approach explains the origin of the underlying interference mechanism of the higher-order spatial intensity correlation functions in a transparent and simple manner. Moreover, it enables us to compare the multi-photon interferences obtained by nonclassical single-photon emitters with those generated by classical light sources. In the course of this discussion it appears that Dirac's famous statement “Each photon interferes only with itself. Interference between two different photons never occurs.” should better read “A multi-photon state of independent photons only interferes with itself. Interference between different multi-photon states never occurs.” This general statement summarizes the physics behind all multi-photon interference phenomena investigated in this thesis. In the second part of the thesis (Chapter 4) we investigate particular multi-photon interference measurements of independent light sources in the context of quantum imaging. We demonstrate that the Nth-order spatial intensity correlation functions display at specific ‘magic’ detector positions the ability to filter sub-wavelength information out of the light fields arising from N incoherently radiating light sources. These multi-photon interference patterns achieve a higher resolution for N>2 than the classical Abbe/Rayleigh limit for imaging the light source. In case of N single-photon emitters we theoretically show that the super-resolving interference patterns obtained for the ‘magic’ detector positions are identical to the ones generated by NOON states with N-1 photons. Quite unexpected, the same is true for N thermal light sources, except for a reduced visibility. Experimental results with up to eight independent thermal light sources confirm this approach to improve the spatial resolution in imaging. In the last part of this thesis (Chapter 5) we finally demonstrate that multi-photon detections can lead to a strong angular focussing of incoherent photons emitted again by either uncorrelated single-photon sources or statistically independent classical light sources. For this effect the sources can be far away from each other and do not interact. The mth-order intensity correlation measurement causes source correlations which produce a heralded peaked emission pattern for the initially uncorrelated light sources. In particular, we show that if m-1 photons have been detected in a particular direction, the conditional probability to detect the mth photon in the same direction can be as high as 100 %. Owing to the great flexibility of detector positions for the correlation measurement, a wide variety of additional detection schemes exists which produces similar directional photon emission patterns. This theoretically predicted measurement-induced focussing was experimentally confirmed with up to eight pseudothermal light sources for three particular detection schemes.
Images of semiconductor "dot-in-rods" and their small clusters are studied by measuring the second-order correlation function with a spatially resolving intensified CCD camera. This measurement allows one to distinguish between a single dot and a cluster and, to a certain extent, to estimate the number of dots in a cluster. A more advanced measurement is proposed, based on higher-order correlations, enabling more accurate determination of the number of dots in a small cluster. Nonclassical features of the light emitted by such a cluster are analyzed.
Images of semiconductor "dot-in-rods" and their small clusters are studied by measuring the second-order correlation function with a spatially resolving intensified CCD camera. This measurement allows one to distinguish between a single dot and a cluster and, to a certain extent, to estimate the number of dots in a cluster. A more advanced measurement is proposed, based on higher-order correlations, enabling more accurate determination of the number of dots in a small cluster. Nonclassical features of the light emitted by such a cluster are analyzed.
This thesis intends to familiarize the reader with the concepts of photon statistics and correlations in quantum optics. Developing light sources that emit quantum states is central for the realization of quantum technologies. One important step in characterizing these sources is the measurement of field fluctuations and correlations, by coincidence measurements. The expectation value of a coincidence measurement, a simultaneous measurement of two intensities (or, more general, four fields), is represented by the fourth-order correlation function. The value of the correlation function, at zero delay between the detection of two photons, reveals important properties of the state to which they belonged, for example the fluctuations of the photon number. Since predictability is important for many applications, light sources emitting single photons are also characterized by the indistinguishability of consecutively emitted photons, or of two photons from separate emitters. In paper I we investigate blinking behaviour in quantum emitters, and its effect on the interference pattern and photon statistics with photons from two separate emitters. Blinking refers to an emitters transition into a non-emitting state, and subsequent transition back to an emitting state. We show that blinking can not be treated as linear loss, when measuring the fourth-order correlation function for two emitters in a Hong-Ou-Mandel setup. In general, a measurement of the fourth-order correlation function is robust to loss, which makes it a very practical tool. However, the relation between recorded coincidence counts and the correlation function is only direct in the limit of zero detection efficiency, and depends on the detection system. In paper II, we show that by adding a variable attenuation in the beam path, we can trace back to the ''true'' value of the correlation function at zero quantum efficiency. This method improves accuracy in correlation measurements by decreasing a systematic error at the expense of an increased statistical error, which is easier to handle, extending the use of coincidence methods to classical and non-classical multi-photon states.