Many of the schemes under study for Quantum Information Processing technology based on photon states involve active and passive optical components as well as detectors. In order to able to establish fidelity levels for these schemes, the performance of the optical components and the quantum efficiency (q.e.) of the detectors require careful and accurate characterization. Correlated photons produced from spontaneous parametric downconversion, which are also the basis of entangled photon states, conveniently offer a direct means of measuring detector q.e. in the photon counting regime, while stimulated parametric downconversion can be used to measure source radiance. Detector and source calibration using correlated photon techniques therefore address some of the key issues critical to the development of QIP technology and the development of correlated/entangled photon metrology. This paper reports work being undertaken at NPL to establish the accuracy limitations of these correlated photon techniques. Significant sources of uncertainty are the need to measure losses due to any optical components used and the requirement to obtain and maintain good geometrical and spectral alignment.
Many of the schemes under study for Quantum Information Processing technology based on photon states involve active and passive optical components as well as detectors. In order to able to establish fidelity levels for these schemes, the performance of the optical components and the quantum efficiency (q.e.) of the detectors require careful and accurate characterization. Correlated photons produced from spontaneous parametric downconversion, which are also the basis of entangled photon states, conveniently offer a direct means of measuring detector q.e. in the photon counting regime, while stimulated parametric downconversion can be used to measure source radiance. Detector and source calibration using correlated photon techniques therefore address some of the key issues critical to the development of QIP technology and the development of correlated/entangled photon metrology. This paper reports work being undertaken at NPL to establish the accuracy limitations of these correlated photon techniques. Significant sources of uncertainty are the need to measure losses due to any optical components used and the requirement to obtain and maintain good geometrical and spectral alignment.
Many of the schemes under study for Quantum Information Processing technology based on photon states involve active and passive optical components as well as detectors. In order to able to establish fidelity levels for these schemes, the performance of the optical components and the quantum efficiency (q.e.) of the detectors require careful and accurate characterization. Correlated photons produced from spontaneous parametric downconversion, which are also the basis of entangled photon states, conveniently offer a direct means of measuring detector q.e. in the photon counting regime, while stimulated parametric downconversion can be used to measure source radiance. Detector and source calibration using correlated photon techniques therefore addresses some of the key issues critical to the development of QIP technology and the development of correlated/entangled photon metrology. This paper reports work being undertaken at NPL to establish the accuracy limitations of these correlated photon techniques. Significant sources of uncertainty are the need to measure losses due to any optical components used and the requirement to obtain and maintain good geometrical and spectral alignment.
Many of the schemes utilizing photon states under investigation for Quantum Information Processing (QIP) technology involve active and passive optical components. In order to be able to establish fidelity levels for these schemes, the performance of these optical components and their coupling efficiencies require careful and accurate characterization. Correlated photons, the basis of entangled photon states, offer a direct means of measuring detector quantum efficiency and source radiance in the photon counting regime. Detector and source calibration by correlated photon techniques therefore address some of the key factors critical to QIP technology and the developing techniques of correlated/entangled photon metrology. Work is being undertaken at NPL to establish the accuracy limitations of the correlated photon technique for detector and source calibration. This paper will report on investigations concerning the characterization of silicon avalanche photodiode detectors using the correlated photon technique.
We will present our current efforts on single photon quantum metrology using high-efficiency superconducting detectors and high-efficiency single-photon sources based on spontaneous parametric downconversion. Optical power measurements based on single photon counting could establish a quantum standard for optical power calibration in the future. At NIST we are pursuing the development of single photon sources and single photon detectors for metrology, quantum and classical applications. As part of these efforts, we are pursuing the establishment of a measurement service for the calibration of single photon detectors. We present how our calibration is tied to the calibration of our transfer standard optical fiber power meters. Using the beamsplitter method, we have implemented a fiber-coupled and free-space measurement system. Also, we have developed testbeds and measurement protocols for the characterization of single photon sources and single photon detectors. We will review several methods, which allow for the characterization of the spatial and spectral degree of freedom in spontaneous parametric downconversion.
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.
This portfolio of work contributes to the remit of the National Physical Laboratory (NPL) to develop the underpinning expertise and tools for validating nascent and future optical quantum technologies based on the discrete and quantum properties of photons. This requirement overlaps with the requirement to provide validation for devices operating in the photon-counting regime. A common theme running through the portfolio is photon pairs generated through spontaneous parametric downconversion (SPDC). \n \nA Hong-Ou-Mandel (HOM) interferometer sourced with visible wavelength photon pairs from an SPDC process in beta-barium borate (BBO) was designed, built and characterised. The visibility of the HOM interference is dependent on the indistinguishability of the interfering photons, but is also influenced by imperfections of the interferometer; therefore an investigation was carried out to quantify the effects of the interferometer imperfections on the measured visibility so that the true photon indistinguishability could be measured with a quantified uncertainty. \n \nA bright source of correlated pair photons in the telecoms band based upon a pump enhanced SPDC process in periodically-poled potassium titanyl phosphate (PPKTP) was designed, built and characterised. From the characterisation measurements the source brightness was estimated to be 6.2×10⁴ pairs/ s/ mw pump. The photon pairs were further characterised through their incorporation as a source in a HOM interference experiment. \n \nThe developed correlated photon pair source was at the heart of a novel scheme for the generation of polarisation entangled photon pairs, for which the design, build and characterisation work is presented. The source was demonstrated to produce two of the four maximally entangled Bell states with quantum interference visibilities of around 0.95. The generated states were also shown to break a form of Bell’s inequality by around six standard deviations. The polarisation entangled photon pair source was originally built at the University of St Andrews and was later transferred to the NPL where it will extend NPL’s capabilities to this key spectral region. \n \nFinally a study was carried out to investigate the possibility of a wavelength tuneable device for the absolute measurement of single photon detector quantum efficiencies based upon an established SPDC technique.
The aim of this review paper is to enlighten some recent progresses in quantum optical metrology in the part of quantum efficiency measurements of photo-detectors performed with bi-photon states. The intrinsic correlated nature of entangled photons from Spontaneous Parametric Down Conversion phenomenon has opened wide horizons to a new approach for the absolute measurement of photo-detector quantum efficiency, outgoing the requirement for conventional standards of optical radiation; in particular the simultaneous feature of the creation of conjugated photons led to a well known technique of coincidence measurement, deeply understood and implemented for standard uses. On the other hand, based on manipulation of entanglement developed for Quantum Information protocols implementations, a new method has been proposed for quantum efficiency measurement, exploiting polarisation entanglement in addition to energy-momentum and time ones, that is based on conditioned polarisation state manipulation. In this review, after a general discussion on absolute photo-detector calibration, we compare these different methods, in order to give an accurate operational sketch of the absolute quantum efficiency measurement state of the art.