S. W. Nam

Detection-Loophole-Free Test of Quantum Nonlocality, and Applications

We present a source of entangled photons that violates a Bell inequality free of the fair-sampling assumption, by over 7 standard deviations. This violation is the first reported experiment with photons to close the detection loophole, and we demonstrate enough efficiency overhead to eventually perform a fully loophole-free test of local realism. The entanglement quality is verified by maximally violating additional Bell tests, testing the upper limit of quantum correlations. Finally, we use the source to generate device-independent private quantum random numbers at rates over 4 orders of magnitude beyond previous experiments.

A three-dimensional, polarization-insensitive superconducting nanowire avalanche photodetector

We measure a peak system detection efficiency (SDE) of 87.7 ± 0.5 % and a polarization dependence of less than 2 % using vertically-stacked superconducting nanowire single-photon detectors connected electrically in parallel.

Demonstration of Einstein-Podolsky-Rosen Steering Using Single-Photon Path Entanglement and Displacement-Based Detection

We demonstrate the violation of an Einstein-Podolsky-Rosen steering inequality developed for single-photon path entanglement with displacement-based detection. We use a high-rate source of heralded single-photon path-entangled states, combined with high-efficiency superconducting-based detectors, in a scheme that is free of any postselection and thus immune to the detection loophole. This result conclusively demonstrates single-photon entanglement in a one-sided device-independent scenario, and opens the way towards implementations of device-independent quantum technologies within the paradigm of path entanglement.

High‐Efficiency Photon‐Number‐Resolving Detectors based on Hafnium Transition‐Edge Sensors

Generation of nonclassical states of light is at the foundation of numerous quantum optics experiments and optical quantum information processing implementations. One such non‐Gaussian optical quantum state can be obtained by photon subtraction from a squeezed optical state. Photon‐number‐resolving detectors with high efficiency and low dark counts are needed for heralding the subtraction of one, two or more photons. Transition‐edge sensors (TES) optimized for high detection efficiency at 850 nm, seem to be ideal heralding detectors for such quantum optics experiments. In this work, we describe the fabrication and characterization of hafnium TESs embedded in an optical structure for optimal absorption at 850 nm. Accurate measurements of optical constants for all materials and fine control of layer thicknesses in the optical cavity should increase the detector efficiency to values higher than 95%.

UV superconducting nanowire single-photon detectors with high efficiency, low noise, and 4 K operating temperature

For photon-counting applications at ultraviolet wavelengths, there are currently no detectors that combine high efficiency (> 50%), sub-nanosecond timing resolution, and sub-Hz dark count rates. Superconducting nanowire single-photon detectors (SNSPDs) have seen success over the past decade for photon-counting applications in the near-infrared, but little work has been done to optimize SNSPDs for wavelengths below 400 nm. Here, we describe the design, fabrication, and characterization of UV SNSPDs operating at wavelengths between 250 and 370 nm. The detectors have active areas up to 56 μm in diameter, 70 - 80% efficiency at temperatures up to 4.2 K, timing resolution down to 60 ps FWHM, blindness to visible and infrared photons, and dark count rates of ∼ 0.25 counts/hr for a 56 μm diameter pixel. These performance metrics make UV SNSPDs ideal for applications in trapped-ion quantum information processing, lidar studies of the upper atmosphere, UV fluorescent-lifetime imaging microscopy, and photon-starved UV astronomy.

Experimental quantum state engineering with time-separated heraldings from a continuous-wave light source: A temporal-mode analysis

Conditional preparation is a well-established technique for quantum state engineering of light. A general trend is to increase the number of heralding detection events in such realization to reach larger photon-number states or their arbitrary superpositions. In contrast to pulsed implementations, where detections only occur within the pulse window, for continuous-wave light the temporal separation of the conditioning detections is an additional degree of freedom and a critical parameter. Based on the theoretical study by A.E.B. Nielsen and K. Molmer and on a continuous-wave two-mode squeezed vacuum from a nondegenerate optical parametric oscillator, we experimentally investigate the generation of two-photon state with tunable delay between the heralding events. The present work illustrates the temporal multimode features in play for conditional state generation based on continuous-wave light sources.

Analysis of coincidence-time loopholes in experimental Bell tests

We apply a distance-based Bell-test analysis method [E. Knill et al., Phys. Rev. A. 91, 032105 (2015)] to three experimental data sets where conventional analyses failed or required additional assumptions. The first is produced from a new classical source exploiting a coincidence-time loophole for which standard analysis falsely shows a Bell violation. The second is from a source previously shown to violate a Bell inequality; the distance-based analysis agrees with the previous results but with fewer assumptions. The third data set does not show a violation with standard analysis despite the high source quality, but is shown to have a strong violation with the distance-based analysis method.

Heterogeneous HI-V/Si 3 N 4 integration for quantum photonic circuits

Photonic integration is as an enabling technology for photonic quantum science, providing great experimental scalability, stability, and functionality. Although the increasing complexity of quantum photonic circuits has allowed proof-of-principle demonstrations of quantum computation, simulation, and metrology[1], further development is severely limited by the on-chip photon flux that can be made available from external quantum light sources[2]. Overcoming such limitations would allow a significant scaling of quantum photonic experiments, and enable quantum-level investigation of many physical processes observable on-chip through nanophotonic and nanoplasmonic structures (e.g., Kerr, optomechanical, single-photon nonlinearities). Towards such goals, we have developed a scalable, heterogeneous III-V/Si3N4 integration platform for quantum photonic circuits based on passive Si3N4 waveguides which directly incorporate nanophotonic single-photon sources based on self-assembled InAs quantum dots (QDs)[3]. InAs quantum dots constitute the most promising solid-state triggered single-photon sources to date[4], while SÍ3N4 waveguides offer low-loss propagation, tailorable dispersion and high Kerr nonlinearities which can be used for linear and nonlinear optical signal processing down to the quantum level. In our platform, the building blocks of which are shown in Fig. 1(a), active GaAs waveguide-based geometries containing InAs QDs are designed to efficiently capture QD-emitted single-photons. Captured photons, confined within the GaAs core, are then transferred with high efficiency into a passive Si3N4 waveguide network via adiabatic mode transformers. Figure 1(b) shows an example device fabricated with our platform: a GaAs microring resonator containing InAs quantum dots, evanescently coupled to a GaAs bus waveguide, which is in turn coupled to an underlying Si3N4 waveguide through adiabatic mode-transformers. The photoluminescence spectrum for this device, in Fig. 1(b), shows that a single QD exciton near 1125 nm, coupled to a microring whispering-gallery mode, acts as a source of single-photons that are launched directly into the Si3N4 waveguide. This geometry also allows us to effectively control the QD spontaneous emission decay lifetime by spectrally detuning the WGM with respect to the QD, as shown in Fig. 1(d).

Heterogeneous III-V / Si 3 N 4 integration for scalable quantum photonic circuits

We develop a scalable heterogeneous integration platform for quantum photonic circuits based on Si3N4 waveguides and on-chip, self-assembled InAs quantum dot-based single-photon sources. Hybrid waveguides, photonic crystals, and microring resonators are demonstrated.

Verification of calibration methods for determining photon-counting detection efficiency using superconducting nano-wire single photon detectors

In recent years several ways to radiometrically calibrate optical fiber-coupled detectors have been developed. However, fiber-coupled calibration methods for single photon detectors have not been compared by national metrology institutes in order to validate their equivalence or traceability to the international systems of units yet.. Here, we present the comparison of radiometric calibration methods traceable to a NIST cryogenic radiometer at the few-photon level. The calibration methods are based on metrology grade optical power meters. The expanded (k = 2) relative standard uncertainties of the calibration methods for the detection efficiency are of the order of 0.5%. However, the results changed relatively by 10% with a different set of optical fibers and mating connectors. These results stress the importance of fiber-core dimensions and fiber-connector repeatability.