Jelmer J. Renema

Entanglement-assisted atomic clock beyond the projection noise limit

We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 105 cold cesium atoms. For the first time we operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clock-interrogation time of 10 μs, the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.

Detection mechanism of superconducting nanowire single-photon detectors

In this paper we intend to give a comprehensive description of the current understanding of the detection mechanism in superconducting nanowire single-photon detectors. We will review key experimental results related to the detection mechanism, e.g. the variations of the detection probability as a function of bias current, temperature or magnetic field. Commonly used detection models will be introduced and we will analyze their predictions in view of the experimental observations. Although none of the proposed detection models is able to describe all experimental data, it is becoming increasingly clear that vortices are essential for the formation of the initial normal-conducting domain that triggers a detection event.

Position-dependent local detection efficiency in a nanowire superconducting single-photon detector

We probe the local detection efficiency in a nanowire superconducting single-photon detector along the cross-section of the wire with a far subwavelength resolution. We experimentally find a strong variation in the local detection efficiency of the device. We demonstrate that this effect explains previously observed variations in NbN detector efficiency as a function of device geometry.

Modified detector tomography technique applied to a superconducting multiphoton nanodetector.

We present an experimental method to characterize multi-photon detectors with a small overall detection efficiency. We do this by separating the nonlinear action of the multiphoton detection event from linear losses in the detector. Such a characterization is a necessary step for quantum information protocols with single and multiphoton detectors and can provide quantitative information to understand the underlying physics of a given detector. This characterization is applied to a superconducting multiphoton nanodetector, consisting of an NbN nanowire with a bowtie-shaped subwavelength constriction. Depending on the bias current, this detector has regimes with single and multiphoton sensitivity. We present the first full experimental characterization of such a detector.

Universal response curve for nanowire superconducting single-photon detectors

Using detector tomography, we investigate the detection mechanism in NbN-based superconducting single photon detectors (SSPDs). We demonstrate that the detection probability uniquely depends on a particular linear combination of bias current and energy, for a large variation of bias currents, input energies and detection probabilities, producing a universal detection curve. We obtain this result by studying multiphoton excitations in a nanodetector with a sparsity-based tomographic method that allows factoring out of the optical absorption. We discuss the implication of our model system for the understanding of meander-type SSPDs.

Tomography and state reconstruction with superconducting single-photon detectors

We investigate the performance of a single-element superconducting single-photon detector (SSPD) for quantum state reconstruction. We perform quantum state reconstruction, using the measured photon counting behavior of the detector. Standard quantum state reconstruction assumes a linear response; this simple model fails for SSPDs, which are known to show a nonlinear response intrinsic to the detection mechanism. We quantify the photon counting behavior of the SSPD by a sparsity-based detector tomography technique and use this to perform quantum state reconstruction of both thermal and coherent states. We find that the nonlinearities inherent in the detection process enhance the ability of the detector to do state reconstruction compared to a linear detector with similar efficiency for detecting single photons.

Experimental investigation of the detection mechanism in WSi nanowire superconducting single photon detectors

We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from

Optically probing the detection mechanism in a molybdenum silicide superconducting nanowire single-photon detector

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Local detection efficiency of a NbN superconducting single photon detector explored by a scattering scanning near-field optical microscope

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How noise affects quantum detector tomography

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