Showing papers by "John F. O'Hara published in 2018"

High Purcell factor generation of indistinguishable on-chip single photons

Abstract: On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission, but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is to use the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7 ps) in a waveguide-coupled quantum dot–photonic crystal cavity system. This leads to near-lifetime-limited single-photon emission that retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under π-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates.

Comment on the Veracity of the ITU-R Recommendation for Atmospheric Attenuation at Terahertz Frequencies

Abstract: The Radiocommunication Sector of the International Telecommunication Union (ITU-R) produces numerous global standards on the use and management of radiocommunication systems. Among these is Recommendation ITU-R P.676-11 (09/2016), which provides methods to estimate the attenuation by atmospheric gases for electromagnetic waves in the 1–1000 GHz frequency range. In this letter, we comment on the veracity of that recommendation in the 450–1000 GHz range. In particular, we compare ITU water vapor absorption estimates to the same data available from other sources: HITRAN and previously reported experimental measurements. We find that ITU estimates increasingly disagree with both sources as frequency and water vapor density increase, demanding a closer inspection as to the cause. This discrepancy is attributed to the method of inclusion of the summed contribution of resonance wings from absorption lines located in the ${>}{\text{1}}$ THz range, and the method to account for continuum absorption.

Terahertz atmospheric propagation studies in support of wireless remote sensing

Abstract: Next generation sensors will all share in common the requirement to move increasingly massive amounts of data. As such, an infrastructure problem becomes apparent. Even if instruments can produce quality data, it is not necessarily feasible to collect and move it. With the rapidly growing number of sensors, basic data movement becomes an integral system-engineering problem. Wireless networks are being increasingly employed as part of that infrastructure, but may be rapidly overwhelmed, particularly in currently regulated frequency bands. These facts compel the development of terahertz wireless systems, which if implemented correctly could support the massive flow of `cloud', IoT, and distributed data. While such terahertz systems are continually growing closer to practical reality, they are still very immature. From a system-engineering perspective, it is apparent that there are even many fundamental gaps in knowledge that prevent reliable operations. Indeed, terahertz absorption through the atmosphere is still not fully understood, nor even correctly tabulated in some cases. We present new studies on terahertz propagation using comparisons to previous data and the international standards that commonly underpin system-level engineering of wireless systems. In particular, we examine the role of continuum absorption and high frequency absorption wings, and the method by which they are accounted for in engineering standards between 0-1 THz. These studies reveal a need for greater accuracy in atmospheric measurements.