Ultralow power artificial synapses using nanotextured magnetic Josephson junctions.

Neuromorphic computing promises to markedly improve the efficiency of certain computational tasks, such as perception and decision-making. Although software and specialized hardware implementations of neural networks have made tremendous accomplishments, both implementations are still many orders of magnitude less energy efficient than the human brain. We demonstrate a new form of artificial synapse based on dynamically reconfigurable superconducting Josephson junctions with magnetic nanoclusters in the barrier. The spiking energy per pulse varies with the magnetic configuration, but in our demonstration devices, the spiking energy is always less than 1 aJ. This compares very favorably with the roughly 10 fJ per synaptic event in the human brain. Each artificial synapse is composed of a Si barrier containing Mn nanoclusters with superconducting Nb electrodes. The critical current of each synapse junction, which is analogous to the synaptic weight, can be tuned using input voltage spikes that change the spin alignment of Mn nanoclusters. We demonstrate synaptic weight training with electrical pulses as small as 3 aJ. Further, the Josephson plasma frequencies of the devices, which determine the dynamical time scales, all exceed 100 GHz. These new artificial synapses provide a significant step toward a neuromorphic platform that is faster, more energy-efficient, and thus can attain far greater complexity than has been demonstrated with other technologies.

Hybrid superconducting-magnetic memory device using competing order parameters.

In a hybrid superconducting-magnetic device, two order parameters compete, with one type of order suppressing the other. Recent interest in ultra-low-power, high-density cryogenic memories has spurred new efforts to simultaneously exploit superconducting and magnetic properties so as to create novel switching elements having these two competing orders. Here we describe a reconfigurable two-layer magnetic spin valve integrated within a Josephson junction. Our measurements separate the suppression in the superconducting coupling due to the exchange field in the magnetic layers, which causes depairing of the supercurrent, from the suppression due to the stray magnetic field. The exchange field suppression of the superconducting order parameter is a tunable and switchable behaviour that is also scalable to nanometer device dimensions. These devices demonstrate non-volatile, size-independent switching of Josephson coupling, in magnitude as well as phase, and they may enable practical nanoscale superconducting memory devices.

Multiexciton dynamics in infrared-emitting colloidal nanostructures probed by a superconducting nanowire single-photon detector.

Carrier multiplication (CM) is the process in which absorption of a single photon produces multiple electron-hole pairs. Here, we evaluate the effect of particle shape on CM efficiency by conducting a comparative study of spherical nanocrystal quantum dots (NQDs) and elongated nanorods (NRs) of PbSe using a time-resolved technique that is based on photon counting in the infrared using a superconducting nanowire single-photon photodetector (SNSPD). Due to its high sensitivity and low noise levels, this technique allows for accurate determination of CM yields, even with the small excitation intensities required for quantitative measurements, and the fairly low emission quantum yields of elongated NR samples. Our measurements indicate an up to ∼60% increase in multiexciton yields in NRs versus NQDs, which is attributed primarily to a decrease in the electron-hole pair creation energy. These findings suggest that shape control is a promising approach for enhancing the CM process. Further, our work demonstrates the effectiveness of the SNSPD technique for the rapid screening of CM performance in infrared nanomaterials.

Analysis of a distributed fiber-optic temperature sensor using single-photon detectors.

We demonstrate a high-accuracy distributed fiber-optic temperature sensor using superconducting nanowire single-photon detectors and single-photon counting techniques. Our demonstration uses inexpensive single-mode fiber at standard telecommunications wavelengths as the sensing fiber, which enables extremely low-loss experiments and compatibility with existing fiber networks. We show that the uncertainty of the temperature measurement decreases with longer integration periods, but is ultimately limited by the calibration uncertainty. Temperature uncertainty on the order of 3 K is possible with spatial resolution of the order of 1 cm and integration period as small as 60 seconds. Also, we show that the measurement is subject to systematic uncertainties, such as polarization fading, which can be reduced with a polarization diversity receiver.

Generation of degenerate, factorizable, pulsed squeezed light at telecom wavelengths.

We characterize a periodically poled KTP crystal that produces an entangled, two-mode, squeezed state with orthogonal polarizations, nearly identical, factorizable frequency modes, and few photons in unwanted frequency modes. We focus the pump beam to create a nearly circular joint spectral probability distribution between the two modes. After disentangling the two modes, we observe Hong-Ou-Mandel interference with a raw (background corrected) visibility of 86% (95%) when an 8.6 nm bandwidth spectral filter is applied. We measure second order photon correlations of the entangled and disentangled squeezed states with both superconducting nanowire single-photon detectors and photon-number-resolving transition-edge sensors. Both methods agree and verify that the detected modes contain the desired photon number distributions.

Observation of transparency of Erbium-doped silicon nitride in photonic crystal nanobeam cavities.

One dimensional nanobeam photonic crystal cavities are fabricated in an Er-doped amorphous silicon nitride layer. Photoluminescence from the cavities around 1.54 microm is studied at cryogenic and room temperatures at different optical pump powers. The resonators demonstrate Purcell enhanced absorption and emission rates, also confirmed by time resolved measurements. Resonances exhibit linewidth narrowing with pump power, signifying absorption bleaching and the onset of stimulated emission in the material at both 5.5 K and room temperature. We estimate from the cavity linewidths that Er has been pumped to transparency at the cavity resonance wavelength.

Linewidth narrowing and Purcell enhancement in photonic crystal cavities on an Er-doped silicon nitride platform.

Light emission at 1.54 microm from an Er-doped amorphous silicon nitride layer coupled to photonic crystal resonators at cryogenic and room temperatures and under varying optical pump powers has been studied. The results demonstrate that small mode volume, high quality factor resonators enhance Er absorption and emission rates at the cavity resonance. Time resolved measurements give 11- to 17-fold Purcell enhancement of spontaneous emission at cryogenic temperatures, and 2.4-fold enhancement at room temperature. Resonances exhibit linewidth narrowing with pump power, signifying absorption bleaching and partial inversion of the Er ions cryogenic temperatures. We estimate that 31% of Er ions are excited at the highest pump power.

High-order temporal coherences of chaotic and laser light.

We demonstrate a new approach to measuring high-order temporal coherences that uses a four-element superconducting nanowire single-photon detector. The four independent, interleaved single-photon-sensitive elements parse a single spatial mode of an optical beam over dimensions smaller than the minimum diffraction-limited spot size. Integrating this device with four-channel time-tagging electronics to generate multi-start, multi-stop histograms enables measurement of temporal coherences up to fourth order for a continuous range of all associated time delays. We observe high-order photon bunching from a chaotic, pseudo-thermal light source, measuring maximum third- and fourth-order coherence values of 5.87 +/- 0.17 and 23.1 +/- 1.8, respectively, in agreement with the theoretically predicted values of 3! = 6 and 4! = 24. Laser light, by contrast, is confirmed to have coherence values of approximately 1 for second, third and fourth orders at all time delays.

High-brightness, low-noise, all-fiber photon pair source.

We demonstrate an all-fiber photon pair source for the critical telecom C-band. We achieve high pair generation rates in excess of 10 MHz while maintaining coincidence-to-accidental ratios (CARs) greater than 100. This is one of the brightest and lowest-noise photon pair sources ever demonstrated. We achieve the high pair rate through CW-pumped spontaneous four-wave mixing in dispersion-shifted fiber. We achieve the high CAR by cooling the fiber to 4 K to suppress the Raman generation and detecting the photons with low jitter and low dark count superconducting single-photon detectors.

Ultra fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization.

We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78- 0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.

Distribution of time-energy entanglement over 100 km fiber using superconducting single-photon detectors.

In this letter, we report an experimental realization of distributing entangled photon pairs over 100 km of dispersion-shifted fiber. In the experiment, we used a periodically poled lithium niobate waveguide to generate the time-energy entanglement and superconducting single-photon detectors to detect the photon pairs after 100 km. We also demonstrate that the distributed photon pairs can still be useful for quantum key distribution and other quantum communication tasks.

High-efficiency, ultra low-noise all-fiber photon-pair source.

We demonstrate an all-fiber photon-pair source with the highest coincidence-to-accidental ratio (CAR) reported to date in the fiber-optic telecom C-band. We achieve this through careful optimization of pairproduction efficiency as well as careful characterization and minimization of all sources of background photons, including Raman generation in the nonlinear fiber, Raman generation in the single-mode fiber, and leakage of pump photons. We cool the nonlinear fiber to 4 K to eliminate most of the Raman scattering, and we reduce other noise photon counts through careful system design. This yields a CAR of 1300 at a pair generation rate of 2 kHz. This CAR is a factor of 12 higher than previously reported results in the C-band. Measured data agree well with theoretical predictions.