Christopher J. Chunnilall

Metrology of single-photon sources and detectors: a review

Abstract. The generation, measurement, and manipulation of light at the single- and few-photon levels underpin a rapidly expanding range of applications. These range from applications moving into the few-photon regime in order to achieve improved sensitivity and/or energy efficiency, as well as new applications that operate solely in this regime, such as quantum key distribution and physical quantum random number generation. There is intensive research to develop new quantum optical technologies, for example, quantum sensing, simulation, and computing. These applications rely on the performance of the single-photon sources and detectors they employ; this review article gives an overview of the methods, both conventional and recently developed, that are available for measuring the performance of these devices, with traceability to the SI system.

The quantum candela : a re-definition of the standard units for optical radiation

The candela, the SI (système internationale) unit for optical radiation, has been one of the base units since the inception of the system. The latest definition was in 1979, when it was linked to the derived unit, the watt. Advances in optical technology and the needs of the communication sector suggest that it is timely that consideration be given to redefining the candela in terms of fundamental quantum optical entities, i.e. photons. Validation of this approach will require comparison against the most accurate conventional technique, cryogenic radiometry. A definition in terms of photon number and the requirements for demonstrating equivalence with existing techniques is discussed, together with new possibilities which would result from further improvements in accuracy. Work being carried out at the National Physical Laboratory (NPL) towards these goals is described, drawing on developments of photon-counting calibration techniques and low temperature measurements, and research into single photon sources and detectors.

NPL scales for radiance factor and total diffuse reflectance

The National Reference Reflectometer has been developed by the National Physical Laboratory (NPL) to realize scales for radiance factor and total diffuse reflectance, the latter scale being obtained through the spatial integration of goniometric measurements of reflectance. Analysis of its performance in the spectral region from 400 nm to 1000 nm shows that for spectrally neutral white materials such as matte white tiles and SpectralonTM plaques the instrument is capable of realizing 0/45 radiance factor measurements with an uncertainty of 0.2% (k = 2) and 0/d total diffuse reflectance measurements with an uncertainty of approximately 0.25% (k = 2). The uncertainties depend on the variation of reflectance with angle, and there will therefore be some dependence on the material being measured. This instrument is now used to establish UK diffuse reflectance scales in the visible part of the spectrum.

The partial space qualification of a vertically aligned carbon nanotube coating on aluminium substrates for EO applications

The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrates melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications.

Field trial of a quantum secured 10 Gb/s DWDM transmission system over a single installed fiber

We present results from the first field-trial of a quantum-secured DWDM transmission system, in which quantum key distribution (QKD) is combined with 4 × 10 Gb/s encrypted data and transmitted simultaneously over 26 km of field installed fiber. QKD is used to frequently refresh the key for AES-256 encryption of the 10 Gb/s data traffic. Scalability to over 40 DWDM channels is analyzed.

Low optical power reference detector implemented in the validation of two independent techniques for calibrating photon-counting detectors.

We introduce a technique for measuring detection efficiency that is traceable to the primary standard, the cryogenic radiometer, through a reference silicon photodiode trap detector. The trap detector, used in conjunction with a switched integrator amplifier, can measure signals down to the 0.1 pW (3 x 10⁵ photons second-1) level with 0.1% uncertainty in a total integration time of 300 seconds. This provides a convenient calibration standard for measurements at these levels across the optical spectrum (UV - near IR). A second technique is also described, based on correlated photons produced via parametric down-conversion. This can be used to directly measure detection efficiency in the photon counting regime, and provides a route for expanding the formulation of the candela in terms of photon flux to enable it to address the needs of emerging quantum optical technologies and applications. The two independent techniques were cross-validated by a comparison carried out at 702.2 nm, which showed agreement to within 0.2%.

Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UV/visible Molecular Absorption Spectrophotometry

The transmittance minima of 18 absorption bands of a solution of 40 g/L holmium oxide in 10% (volume fraction) perchloric acid are certified as intrinsic traceable wavelength standards, by means of a multicenter measurement on material from a single source coupled with comparisons of a variety of preparations of the material evaluated on a single instrument. Fit-for-purpose artifact standards for the experimental calibration or validation of wavelength scales of chemical spectrophotometers can be carefully produced by end users themselves or by commercial standards producers. The intrinsic (data) standard confers traceability to the SI unit of length in place of costly transfer artifacts and repetitive calibration procedures. Certified values are provided for instrumental spectral bandwidths of 0.1–3.0 nm in 0.1 nm intervals, and information values are provided to a spectral bandwidth of 10 nm at wider intervals. Expanded uncertainties are typically less than ±0.1 nm for certified band positions.

Metrology for industrial quantum communications: the MIQC project

The ?Metrology for Industrial Quantum Communication Technologies? project (MIQC) is a metrology framework that fosters development and market take-up of quantum communication technologies and is aimed at achieving maximum impact for the European industry in this area.MIQC is focused on quantum key distribution (QKD) technologies, the most advanced quantum-based technology towards practical application. QKD is a way of sending cryptographic keys with absolute security. It does this by exploiting the ability to encode in a photons degree of freedom specific quantum states that are noticeably disturbed if an eavesdropper trying to decode it is present in the communication channel. The MIQC project has started the development of independent measurement standards and definitions for the optical components of QKD system, since one of the perceived barriers to QKD market success is the lack of standardization and quality assurance.

Characterization of the Linearity of Response and Spatial Uniformity of Response of Two InGaAsP/InP Geiger-Mode Avalanche Photodiodes

The linearity of response and spatial uniformity of response characteristics of two commercially available single-photon avalanche photodiode (SPAD) detection systems were experimentally investigated using a dedicated characterization facility that measures these parameters. Both SPAD detection systems are shown to exhibit a nonlinear response. Moreover, the responsivity of both SPAD systems is shown to be spatially nonuniform and the degree of nonuniformity depends on the single-photon detection probability setting of these systems. The experimentally observed spatial uniformity of response behavior of these detectors is explained in terms of the spatial nonuniformity in the breakdown voltage V_b across the active area of the device. The experimentally observed linearity of response characteristics of these detectors at high count rates can be explained in terms of “pulse collisions.” However, the origin of the experimentally observed linearity of response characteristics of the same detectors at low count rates is currently unknown.

Correlated photon metrology of detectors and sources

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.