Dr. Boon Kok Tan

Abstract:

Millimeter and sub-millimeter wavelength astronomical heterodyne receivers operating in the simplest doublesideband (DSB) mode down-convert both the upper and lower sidebands (USB/LSB) near the local oscillator (LO) frequency to the same intermediate-frequency (IF) band, causing spectral confusion. The down-converted USB and LSB signals can be separated using a sideband-separating (2SB) scheme, which requires passive components such as hybrid couplers, in-phase power splitters, and directional couplers, in addition to a pair of near-identical DSB mixers. Traditionally, these passive components are implemented using waveguide technology, making them bulky and increasingly difficult to fabricate with high precision at higher frequencies.

In this manuscript, we present a compact 2SB receiver concept. By rearranging the pair of DSB mixers and replacing these waveguide-based passive components with superconducting planar-circuit counterparts, we aim to substantially reduce the receiver size while potentially minimizing transmission losses. We demonstrate this approach by presenting the design of a compact 2SB receiver that continuously covers the Atacama Large Mm/sub-mm Array (ALMA) Band 6 and Band 7 frequency ranges, intended as a second-generation instrument for the Africa Millimetre Telescope (AMT).

Abstract:

The development of next-generation astronomical receivers operating at millimetre (mm), sub-mm, and terahertz frequencies is essential to meet the increasing demand for wide-field, high-spectral-resolution observations. In this work, we examine the prospects and challenges associated with advancing heterodyne receiver technologies, with particular emphasis on superconductor–insulator–superconductor (SIS) mixers. We present a series of technological pathways aimed at enhancing receiver performance, including ultra-broadband RF and IF SIS mixer designs, compact sideband-separating (2SB) architectures enabled by planar superconducting circuit integration, and scalable focal plane array (FPA) concepts capable of supporting hundreds to thousands of pixels. These developments are motivated by the need to significantly improve mapping speed and survey efficiency for future facilities such as ALMA upgrades, AtLAST, LST, and next-generation space missions. Another key contribution of this work is the introduction of simultaneous observing multi-band receivers (SOMBRs), which enable concurrent multi-band 2SB observations through minimal additional hardware by reconfiguring conventional receiver architectures. This approach allows continuous spectral coverage while preserving phase information required for interferometric applications. We discuss the principal challenges in realising these systems, including bandwidth optimisation, impedance matching, local oscillator distribution, and scaling to large-format arrays. Overall, this work outlines a viable pathway towards highly integrated, ultra-broadband, and scalable heterodyne receiver systems, which are expected to play a critical role in enabling the next generation of astronomical discoveries.

Abstract:

At microwave frequencies, Josephson junction arrays have been widely employed to create metamaterials exhibiting a third-order χ⁽³⁾) nonlinearity, analogous to the Kerr effect in optics. These nonlinear metamaterials enable parametric amplification, as in Josephson travelling-wave parametric amplifiers (JTWPAs), which achieve quantum-limited noise performance over multigigahertz bandwidths. The exceptional properties of JTWPAs make them ideal for the sensitive readout of weak microwave signals, with applications in quantum computing, astrophysics, and fundamental physics experiments. Extending JTWPAs to higher frequencies, such as the W-band (70–110 GHz), holds promise for first-stage amplification in astronomical receivers, lowering system noise; as well as for reading out emerging superconducting qubit architectures at these frequencies. In this work, we investigate the χ⁽³⁾ nonlinear properties of Josephson arrays operating in the W-band as a step toward realizing parametric gain at these frequencies. We designed and fabricated an array composed of 704 Nb/Al-AlO_x/Nb tunnel junctions and experimentally demonstrated four-wave mixing via idler tone generation, providing clear evidence of third-order nonlinearity. These results mark an important step toward novel millimetre-wave and submillimetre-wave parametric-amplifier-based receiver technologies.

Abstract:

We describe a resonant cavity search apparatus for axion dark matter constructed by the quantum sensors for the hidden sector collaboration. The apparatus is configured to search for QCD axion dark matter, though also has the capability to detect axion-like particles, dark photons, and some other forms of wave-like dark matter. Initially, a tuneable cylindrical oxygen-free copper cavity is read out using a low noise microwave amplifier feeding a heterodyne receiver. The cavity is housed in a dilution refrigerator (DF) and threaded by a solenoidal magnetic field, nominally 8 T. The apparatus also houses a magnetic field shield for housing superconducting electronics, and several other fixed-frequency resonators for use in testing and commissioning various prototype quantum electronic devices sensitive at a range of axion masses in the range 2.0– 40μeVc−2. The apparatus as currently configured is intended as a test stand for electronics over the relatively wide frequency band attainable with the TM010 cavity mode used for axion searches. We present performance data for the resonator, DF, and magnet, and plans for the first science run.

Abstract:

Superconducting Parametric Amplifiers (SPAs) with near-quantum-limited added noise are crucial for weak signal detection applications such as astronomical receivers, quantum computation, and fundamental physics experiments. Commercially available SPAs include Josephson Parametric Amplifiers (JPAs), which offer high gain but narrow bandwidth performance; and Josephson-junction Travelling Wave Parametric Amplifiers (JTWPAs), which provide broader bandwidth at the cost of a complicated fabrication procedure, lower fabrication yield, and larger footprint area. In this paper, we investigate the parametric amplification of microwave signals in a Josephson array embedded in a low-Q Fabry-Pérot cavity. We fabricated a 500-junction array device and measured >15 dB phase-preserving gain over a ∼350 MHz bandwidth, while offering almost two orders of magnitude improvement in compression point (P1dB = −106.2 dBm) compared to standard JPAs. Furthermore, using a novel measurement technique, we configured our device to operate in the phase-sensitive mode, measuring a phase-sensitive extinction ratio (PSER) of 42.3 ± 2.81 dB, in line with state-of-the-art values for JPAs. These promising performances, combined with the ease of fabrication and improved yield compared with JTWPAs, underscore the potential of these devices for applications in advanced detection schemes.