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Surface acoustic waves (SAWs) coupled to quantum dots (QDs), trapped atoms and ions, and point defects have been proposed as quantum transduction platforms, yet the requisite coupling rates and cavity lifetimes have not been experimentally established. Although the interaction mechanism varies, small acoustic cavities with large zero-point motion are required for high efficiencies. We experimentally establish the feasibility of this platform through electro- and opto-mechanical characterization of tightly focusing, single-mode Gaussian SAW cavities at $\sim$3.6 GHz on GaAs. We explore the performance limits of the platform by fabricating SAW cavities with mode volumes approaching 6$\lambda^3$ and linewidths $\leq$1 MHz. Employing strain-coupled single InAs QDs as optomechanical intermediaries, we measure single-phonon optomechanical coupling rates $g_0 \approx 2\pi \times 1.2$ MHz. Sideband scattering rates thus exceed intrinsic phonon loss, indicating the potential for quantum optical readout and transduction of cavity phonon states. To demonstrate the feasibility of this platform for low-noise ground-state quantum transduction, we develop a fiber-based confocal microscope in a dilution refrigerator and perform single-QD resonance fluorescence sideband spectroscopy at mK temperatures. These measurements show conversion between microwave phonons and optical photons with sub-natural linewidths., Comment: 15 pages, 10 figures

Improvements in temporal resolution of single-photon detectors enable increased data rates and transmission distances for both classical and quantum optical communication systems, higher spatial resolution in laser ranging, and observation of shorter-lived fluorophores in biomedical imaging. In recent years, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the most efficient time-resolving single-photon-counting detectors available in the near-infrared, but understanding of the fundamental limits of timing resolution in these devices has been limited due to a lack of investigations into the timescales involved in the detection process. We introduce an experimental technique to probe the detection latency in SNSPDs and show that the key to achieving low timing jitter is the use of materials with low latency. By using a specialized niobium nitride SNSPD we demonstrate that the system temporal resolution can be as good as 2.6 ± 0.2 ps for visible wavelengths and 4.3 ± 0.2 ps at 1,550 nm. Knowledge about detection latency provides a guideline to reduce the timing jitter of niobium nitride superconducting nanowire single-photon detectors. A timing jitter of 2.6 ps at visible wavelength and 4.3 ps at 1,550 nm is achieved.

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer ${\mathrm{WSe}}_{2}$ reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

We demonstrate cryogenic, electrically-injected, waveguide-coupled Si light-emitting diodes (LEDs) operating at 1.22 $\mu$m. The active region of the LED consists of W centers implanted in the intrinsic region of a $p$-$i$-$n$ diode. The LEDs are integrated on waveguides with superconducting nanowire single-photon detectors (SNSPDs). We demonstrate the scalability of this platform with an LED coupled to eleven SNSPDs in a single integrated photonic device. Such on-chip optical links may be useful for quantum information or neuromorphic computing applications.

Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information and photonics research. Nature sets the ultimate limit, however, given by the strength of the system's coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the optical coherence time of the charged exciton transition in quantum dots embedded in a photonic waveguide. By manipulating the electronic wave functions through an applied lateral electric field, we increase the coherence time from $\ensuremath{\sim}1.4$ to $\ensuremath{\sim}2.7\text{ }\text{ }\mathrm{ns}$. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to $\ensuremath{\sim}6\text{ }\text{ }\mathrm{nm}$, which leads to a 30% weaker transition dipole moment. The ability to electronically control the coherence time opens new avenues for quantum communication and novel coupling schemes between distant qubits.

In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, \gamma, in an ensemble of InAs/GaAs quantum dots. We find that the dephasing rate is sensitive to the strength of the hyperfine interaction, which can be controlled through the application of an external magnetic field in the Faraday configuration. At zero applied field, strong electron-nuclear coupling induces additional dephasing beyond the radiative limit and \gamma = 230 MHz (0.95 \mu eV). Screening of the hyperfine interaction is achieved for an external field of ~1 T, resulting in \gamma = 172 MHz (0.71 \mu eV) limited only by spontaneous recombination. On the other hand, application of a Voigt magnetic field mixes the spin eigenstates, which increases the dephasing rate by up to 75%. These results are reproduced with a simple and intuitive model that captures the essential features of the electron hyperfine interaction and its influence on \gamma., Comment: 9 pages, 6 figures

We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ∼10(6) photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from10(7) laser pulses, we also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.