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Optics Letters

Phase-sensitive tomography of the joint spectral amplitude of photon pair sources

Iman Jizan, Bryn A. Bell, L. G. Helt, Alvaro Casas‐Bedoya, Chunle Xiong, Benjamin J. Eggleton

Published Oct 11, 2016• 42 citations
Publisher

Abstract

We present a novel measurement technique to perform full phase-sensitive tomography on the joint spectrum of photon pair sources, using stimulated four-wave mixing and phase-sensitive amplification. Applying this method to an integrated silicon nanowire source with a frequency chirped pump laser, we are able to observe a corresponding phase change in the spectral amplitude that would otherwise be hidden in standard intensity measurements. With a highly nonlinear fiber source, we show that phase-sensitive measurements have superior sensitivity to small spectral features when compared to intensity measurements. This technique enables more complete characterization of photon pair sources based on nonlinear photonics.

DOI
10.1364/ol.41.004803

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We present a novel measurement technique to perform full phase-sensitive tomography on the joint spectrum of photon pair sources, using stimulated four-wave mixing and phase-sensitive amplification. Applying this method to an integrated silicon nanowire source with a frequency chirped pump laser, we are able to observe a corresponding phase change in the spectral amplitude that would otherwise be hidden in standard intensity measurements. With a highly nonlinear fiber source, we show that phase-sensitive measurements have superior sensitivity to small spectral features when compared to intensity measurements. This technique enables more complete characterization of photon pair sources based on nonlinear photonics.
In quantum photonics, the requirement for photon pairs with specific quantum states has led to a demand for a fast, high resolution and accurate characterisation of photon pair sources. However, current quantum methods of characterisation suffer from limited accuracy and resolution, and only consist of intensity measurements that prevent access to phase-sensitive measurement photon pairs. A promising tool that addresses these challenges, uses the classical analogue of nonlinear processes to stimulate photon generation, yielding much higher count rates that allows for a higher resolution and accurate photon pair source characterisation. Furthermore, this classical measurement allows for an innovative method to perform full phase-sensitive quantum tomography of photon pair sources that was previous thought to be experimentally challenging to obtain. This thesis examines and compares the quantum and classical method of characterisation of spectral correlations in χ^3 nonlinear devices; namely two integrated silicon nanowires, and a highly nonlinear fibre. In the first study, we use stimulated nonlinear process to confirm the speed-up of characterisation of photon pairs and demonstrate that additional resolution is gained when compared to the traditional coincidence measurements with no increase in measurement time. By applying this technique with phase-sensitive amplification to another identical silicon nanowire, the first phase sensitive measurements are presented showing details that are otherwise hidden in traditional intensity measurements. Furthermore, phase-sensitive measurement of a highly nonlinear fibre shows that phase-sensitive measurements have excellent sensitivity to small features when compared to the traditional intensity measurements.
The exponential growth of photonic quantum technologies is driving the demand for tools to measure the quality of their information carriers. One of the most prominent is stimulated emission tomography (SET), which uses classical coherent fields to measure the joint spectral amplitude (JSA) of photon pairs with high speed and resolution. While the modulus of the JSA can be directly addressed from a single intensity measurement, the retrieval of the joint spectral phase (JSP) is far more challenging and received minor attention. However, a wide class of spontaneous sources of technological relevance, as chip integrated micro-resonators, have a JSP with a rich structure that carries correlations hidden in the intensity domain. Here, using a compact and reconfigurable silicon photonic chip, the complex JSA of a micro-ring resonator photon pair source is measured for the first time. The photonic circuit coherently excites the ring and a reference waveguide, and the interferogram formed by their stimulated fields is used to map the ring JSP through a novel phase reconstruction technique. This tool complements the traditionally bulky and sophisticated methods implemented so far, simultaneously minimizing the set of required resources.
Photon pair sources have wide ranging applications in a variety of quantum photonic experiments and protocols. Many of these protocols require well controlled spectral correlations between the two output photons. However, due to low cross-sections, measuring the joint spectral properties of photon pair sources has historically been a challenging and time-consuming task. Here, we present an approach for the real-time measurement of the joint spectral properties of a fiber-based four wave mixing source. We seed the four wave mixing process using a broadband chirped pulse, studying the stimulated process to extract information regarding the spontaneous process. In addition, we compare stimulated emission measurements with the spontaneous process to confirm the technique's validity. Joint spectral measurements have taken many hours historically and several minutes with recent techniques. Here, measurements have been demonstrated in 5-30 s depending on resolution, offering substantial improvement. Additional benefits of this approach include flexible resolution, large measurement bandwidth, and reduced experimental overhead.
The development of new quantum light sources requires robust and convenient methods of characterizing their joint spectral properties. Measuring the joint spectral intensity between a photon pair ignores any correlations in spectral phase which may be responsible for degrading the quality of quantum interference. A fully phase-sensitive characterization tends to require significantly more experimental complexity. Here, we investigate the sensitivity of the frequency-resolved double-pair emission to spectral phase correlations, in particular to the presence of a simple form of correlated phase which can be generated by a chirped pump laser pulse. We observe interference fringes in the four photon coincidences which depend on the frequencies of all four photons, with a period which depends on the strength of their correlation. We also show that phase correlations in the JSA induce spectral intensity correlations between two signal photons, even when the corresponding idler photons are not detected, and link this correlation pattern to the purity of a single signal photon. These effects will be useful in assessing new photon-pair sources for quantum technologies, especially since we require little additional complexity compared to a joint spectral intensity measurement - essentially just the ability to detect at least two photons in each output port.
Stimulated Emission Tomography (SET) is a technique which maps the Joint Spectral Amplitude (JSA) of photon pairs with extremely high signal to noise ratio and low integration times. While the modulus of the JSA is easy to access, the retrieval of the Joint Spectral Phase (JSP) is far more challenging and requires advanced interferometric techniques. Here we present and validate a scheme to measure the complex JSA by using a reconfigurable silicon chip. In particular, we map the rich structure of the JSP of a silicon microresonator.