arXiv subjects

Hong X. Tang

Publications and source records attributed to Hong X. Tang.

At least 37 records · Page 2Linked to original sources

arXiv2024

High-efficiency, cryogenic-compatible grating couplers on an AlN-on-sapphire platform through bottom-side coupling

Sapphire is a commonly used substrate for wide-bandgap III-nitride photonic materials. However, its relatively high refractive index results in low transmission efficiency in grating couplers. Here, we propose and demonstrate that the transmission efficiency can be significantly enhanced by bottom-side coupling. A metal reflector is deposited on the top side of the chip, and the fiber array is glued to the bottom side of the substrate. We experimentally achieve a transmission efficiency as high as 42% per coupler on an aluminum nitride (AlN) on sapphire platform at the telecom wavelength. In addition, the grating couplers show a robust performance at a cryogenic temperature as low as 3~K for both transverse-electric (TE) and transverse-magnetic (TM) modes. Our results can be useful to a wide range of sapphire-based applications that require low coupling loss and cryogenic operation.

physics.optics↗

arXiv2024

Fluorescence enabled phonon counting in an erbium doped piezo-optomechanical microcavity

Converting phonons to photons with optomechanical interaction provides a pathway to realize single phonon counting, which is instrumental in the quantum applications of mechanical systems such as entanglement generation, thermometry, and study of macroscopic quantum phenomenon. In this process, the key requirement is high-extinction, narrowbandwidth, and stable filtering of the parametric optical pump. Here, we propose to lift this necessity by counting fluorescence emission from a rare earth embedded optomechanical cavity. By doing so, we show that an equivalent filtering effect can be achieved due to spectral hole burning and cavity Purcell effect. To demonstrate this, we designed, fabricated, and characterized an integrated piezo-optomechanical FabryPerot cavity on the erbium doped thin-film lithium niobate platform. By collecting fluorescence from the optomechanical sideband, we show that 93dB suppression of the pump can be achieved with 10dB loss of signal, resulting in an increase of 83dB in sidebandpump ratio. Our results facilitate a route to realize f ilterless single phonon counting and also create new opportunities to study the interaction between solid state emitters and mechanical systems.

physics.optics↗

arXiv2024

Unveiling the Pockels Coefficient of Ferroelectric Nitride ScAlN

Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength ($K^2$) comparable to that of lithium niobate (LN), making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked interest in its use for nanophotonic devices, chiefly due to its large bandgap facilitating operation in blue wavelengths coupled with promises of enhanced nonlinear optical properties such as a large second-order susceptibility ($χ^{(2)}$). It is still an open question whether ScAlN can outperform oxide ferroelectrics concerning the Pockels effect -- an electro-optic coupling extensively utilized in optical communications devices. In this paper, we present a comprehensive theoretical analysis and experimental demonstration of ScAlN's Pockels effect. Our findings reveal that the electro-optic coupling of ScAlN, despite being weak at low Sc concentration, may be significantly enhanced and exceed LiNbO$_3$ at high levels of Sc doping, which points the direction of continued research efforts to unlock the full potential of ScAlN.

cond-mat.mtrl-sci↗

arXiv2024

Integrated photonic Galton board and its application for photon counting

The Galton board is a desktop probability machine traditionally used to visualize the principles of statistical physics with classical particles. Here, we demonstrate a photonic Galton board that enables on-chip observation of single-photon interference. The photonic Galton board, which can be considered as a simplified Boson sampler, consists of a directional coupler matrix terminated by an array of superconducting nanowire detectors to provide spatiotemporal resolution. This design also allows for photon-number-resolving capability, making it suitable for high-speed photon counting. Our results demonstrate the compatibility between single-photon detector array and photonic integrated circuits, paving the way for implementing on-chip large-scale quantum optics experiments and photonic quantum computing.

quant-ph↗

arXiv2024

Symmetric Second-Harmonic Generation in Sub-wavelength Periodically Poled Thin Film Lithium Niobate

Second harmonic generation (SHG) extensively employs periodically poled nonlinear crystals through forward quasi-phase-matching to achieve efficient frequency conversion. As poling periods approach sub-micrometers, backward quasi-phase-matching has also been demonstrated, albeit by utilizing pulsed laser drives. The realization of symmetric second harmonic generation, characterized by counterpropagating pumps, however, has remained elusive despite theoretical predictions. The main challenge lies in achieving strong nonlinear coupling with poling period below half the wavelength of the second-harmonic light. The recent emergence of high-quality ferroelectric lithium niobate thin films provides an opportunity for achieving precise domain control at submicron dimensions. In this article, we demonstrate reliable control of ferroelectric domains in thin film lithium niobate waveguide with a poling period down to 370nm, thereby realizing highly efficient continuous-wave pumped symmetric SHG. This demonstration not only validates the feasibility of achieving subwavelength periodic poling on waveguides but also opens new avenues for leveraging submicron ferroelectric domain structures in integrated photonics and nonlinear optics research.

physics.optics↗

arXiv2024

Non-contact excitation of multi-GHz lithium niobate electromechanical resonators

The demand for high-performance electromechanical resonators is ever-growing across diverse applications, ranging from sensing and time-keeping to advanced communication devices. Among the electromechanical materials being explored, thin-film lithium niobate stands out for its strong piezoelectric properties and low acoustic loss. However, in nearly all existing lithium niobate electromechanical devices, the configuration is such that the electrodes are in direct contact with the mechanical resonator. This configuration introduces an undesirable mass-loading effect, giving rise to spurious modes and additional damping. Here, we present an electromechanical platform that mitigates this challenge by leveraging a flip-chip bonding technique to separate the electrodes from the mechanical resonator. By offloading the electrodes from the resonator, our approach yields a substantial increase in the quality factor of these resonators, paving the way for enhanced performance and reliability for their device applications.

physics.app-ph↗

arXiv2024

Heterogeneous sapphire-supported low-loss photonics platform

Sapphire is a promising wideband substrate material for visible photonics. It is a common growth substrate for III-nitride light-emitting diodes and laser structures. Doped sapphires are important gain media foundational to the development of titanium-sapphire and ruby lasers. For lasers operating at visible and near-infrared wavelengths, a photonic platform that minimizes loss while maximizing gain material overlap is crucial. Here, we introduce a novel low-loss waveguiding strategy that establishes high-performance integrated photonics on sapphire substrates. This platform achieves a high intrinsic quality factor of 5.6 million near 780 nm and features direct compatibility with a range of solid-state laser gain media.

physics.optics↗

arXiv2024

Sub-terahertz optomechanics

We demonstrate optomechanics in the sub-terahertz regime. An optical racetrack resonator, patterned from thin-film lithium niobate, is suspended to support mechanical structures oscillating at these extremely high frequencies, which are read out through cavity optomechanical coupling. Our hybrid platform paves the way for advancing mechanical systems in the quantum regime at elevated temperatures.

physics.optics↗

arXiv2024

Junction-free microwave two-mode radiation from a kinetic inductance nanowire

Parametric down-conversion is a widely exploited technique in optics to produce entangled states of photons for quantum information processing and quantum sensing. In the microwave domain, devices based on Josephson junctions, such as Josephson parametric amplifiers (JPAs) and voltage-biased Josephson junctions, have been successfully utilized to generate such states. However, their high susceptibility to magnetic fields has posed challenges in many applications. Here we demonstrate the generation of two-mode squeezed states via four-wave-mixing in a superconducting nanowire resonator patterned from NbN. The NbN nanowire exhibits a strong Kerr nonlinearity, resulting in the emission of a signal-idler pair with a cross-correlation of $g^{(2)}(0) = 11.9$. Owing to the magnetic resilience and high critical temperature ($T_c$) of NbN, our microwave parametric sources based on kinetic inductance promise an expanded range of potential applications.

quant-ph↗

arXiv2023

Photonic link from single flux quantum circuits to room temperature

Broadband, energy-efficient signal transfer between cryogenic and room-temperature environment has been a major bottleneck for superconducting quantum and classical logic circuits. Photonic links promise to overcome this challenge by offering simultaneous high bandwidth and low thermal load. However, the development of cryogenic electro-optic modulators -- a key component for photonic readout of electrical signals -- has been stifled by the stringent requirements of superconducting circuits. Rapid single flux quantum circuits (RSFQ), for example, operate with a tiny signal amplitude of only a few millivolts (mV), far below the volt-level signal used in conventional circuits. Here, we demonstrate the first direct optical readout of an RSFQ circuit without additional electrical amplification enabled by a novel superconducting electro-optic modulator (SEOM) featuring a record-low half-wave voltage Vπ of 42 mV on a 1 m-long SEOM. Leveraging the low ohmic loss of superconductors, we break the fundamental Vπ-bandwidth trade-off and demonstrate electro-optic bandwidth up to 17 GHz on a 0.2 m-long SEOM at cryogenic temperatures. Our work presents a viable solution toward high-bandwidth signal transfer between future large-scale superconducting circuits and room-temperature electronics.

quant-ph↗

arXiv2023

Light-Induced Microwave Noise in Superconducting Microwave-Optical Transducers

Microwave-to-optical transducers are integral to the future of superconducting quantum computing, as they would enable scaling and long-distance communication of superconducting quantum processors through optical fiber links. However, optically-induced microwave noise poses a significant challenge in achieving quantum transduction between microwave and optical frequencies. In this work, we study light-induced microwave noise in an integrated electro-optical transducer harnessing Pockels effect of thin film lithium niobate. We reveal three sources of added noise with distinctive time constants ranging from sub-100 nanoseconds to milliseconds. Our results gain insights into the mechanisms and corresponding mitigation strategies for light-induced microwave noise in superconducting microwave-optical transducers, and pave the way towards realizing the ultimate goal of quantum transduction.

quant-ph↗

arXiv2023

Ultra-high Q alumina optical microresonators in the UV and blue bands

UV and visible photonics enable applications ranging from spectroscopic sensing to communication and quantum information processing. Photonics structures in these wavelength regimes, however, tend to experience higher loss than their IR counterpart. Particularly in the near-UV band, on-chip optical microresonators have not yet achieved a quality factor beyond 1 million. Here we report ultra-low-loss photonic waveguides and resonators patterned from alumina thin films prepared by a highly scalable atomic layer deposition process. We demonstrate ultra high Q factor of 1.5$\,\times\,$10$^6$ at 390nm, a record value at UV bands, and 1.9$\,\times\,$10$^6$ at 488.5nm.

physics.optics↗

arXiv2023

Radiatively-cooled quantum microwave amplifiers

Superconducting microwave amplifiers are essential for sensitive signal readout in superconducting quantum processors. Typically based on Josephson Junctions, these amplifiers require operation at milli-Kelvin temperatures to achieve quantum-limited performance. Here we demonstrate a quantum microwave amplifier that employs radiative cooling to operate at elevated temperatures. This kinetic-inductance-based parametric amplifier, patterned from a single layer of high-$T_\mathrm{c}$ NbN thin film\cmt{in the form of a nanobridge}, maintains a high gain and meanwhile enables low added noise of 1.3 quanta when operated at 1.5 Kelvin. Remarkably, this represents only a 0.2 quanta increase compared to the performance at a base temperature of 0.1 Kelvin. By uplifting the parametric amplifiers from the mixing chamber without compromising readout efficiency, this work represents an important step for realizing scalable microwave quantum technologies.

quant-ph↗

arXiv2023

Two-colour dissipative solitons and breathers in microresonator second-harmonic generation

Frequency conversion of dissipative solitons associated with the generation of broadband optical frequency combs having a tooth spacing of hundreds of giga-hertz is a topical challenge holding the key to practical applications in precision spectroscopy and data processing. The work in this direction is underpinned by fundamental problems in nonlinear and quantum optics. Here, we present the dissipative two-colour bright-bright and dark-dark solitons in a quasi-phase-matched microresonator pumped for the second-harmonic generation in the near-infrared spectral range. We also found the breather states associated with the pulse front motion and collisions. The soliton regime is found to be typical in slightly phase-mismatched resonators, while the phase-matched ones reveal broader but incoherent spectra and higher-order harmonic generation. Soliton and breather effects reported here exist for the negative tilt of the resonance line, which is possible only via the dominant contribution of second-order nonlinearity.

physics.optics↗

arXiv2022

Architectures for Multinode Superconducting Quantum Computers

Many proposals to scale quantum technology rely on modular or distributed designs where individual quantum processors, called nodes, are linked together to form one large multinode quantum computer (MNQC). One scalable method to construct an MNQC is using superconducting quantum systems with optical interconnects. However, a limiting factor of these machines will be internode gates, which may be two to three orders of magnitude noisier and slower than local operations. Surmounting the limitations of internode gates will require a range of techniques, including improvements in entanglement generation, the use of entanglement distillation, and optimized software and compilers, and it remains unclear how improvements to these components interact to affect overall system performance, what performance from each is required, or even how to quantify the performance of each. In this paper, we employ a `co-design' inspired approach to quantify overall MNQC performance in terms of hardware models of internode links, entanglement distillation, and local architecture. In the case of superconducting MNQCs with microwave-to-optical links, we uncover a tradeoff between entanglement generation and distillation that threatens to degrade performance. We show how to navigate this tradeoff, lay out how compilers should optimize between local and internode gates, and discuss when noisy quantum links have an advantage over purely classical links. Using these results, we introduce a roadmap for the realization of early MNQCs which illustrates potential improvements to the hardware and software of MNQCs and outlines criteria for evaluating the landscape, from progress in entanglement generation and quantum memory to dedicated algorithms such as distributed quantum phase estimation. While we focus on superconducting devices with optical interconnects, our approach is general across MNQC implementations.

quant-ph↗

arXiv2022

Controlling single rare earth ion emission in an electro-optical nanocavity

Rare earth emitters enable critical quantum resources including spin qubits, single photon sources, and quantum memories. Yet, probing of single ions remains challenging due to low emission rate of their intra-4f optical transitions. One feasible approach is through Purcell enhanced emission in optical cavities. The ability to modulate cavity-ion coupling in real time will further elevate the capacity of such systems. Here, we demonstrate direct control of single ion emission by embedding erbium dopants in an electro-optically active photonic crystal cavity patterned from thin-film lithium niobate. Purcell factor over 170 enables single ion detection, which is verified by second-order autocorrelation measurement. Dynamic control of emission rate is realized by leveraging electro-optic tuning of resonance frequency. Using this feature, storage and retrieval of single ion excitation is further demonstrated, without perturbing the emission characteristics. These results promise new opportunities for controllable single photon sources and efficient spin-photon interfaces.

quant-ph↗

arXiv2022

Light-induced dynamic frequency shifting of microwave photons in a superconducting electro-optic converter

Hybrid superconducting-photonic microresonators are a promising platform for realizing microwave-to-optical transduction. However, the absorption of scattered photons by the superconductors leads to unintended microwave resonance frequency variation and linewidth broadening. Here, we experimentally study the dynamics of this effect and its impact on microwave-to-optics conversion in an integrated lithium niobate-superconductor hybrid resonator platform. We unveiled an adiabatic frequency shifting of the intracavity microwave photons induced by the fast photo-responses of the thin-film superconducting resonator. As a result, the temporal and spectral responses of electro-optics transduction are modified and well described by our theoretical model. This work provides important insights on the light-induced conversion dynamics which must be considered in future designs of hybrid superconducting-photonic system.

physics.app-ph↗

arXiv2022

Magnetic field-resilient quantum-limited parametric amplifier

Superconducting parametric amplifiers are crucial components in microwave quantum circuits for enabling quantum-limited signal readout. The best-performing such amplifiers are often based on Josephson junctions, which however are sensitive to magnetic fields. Therefore, they require magnetic shields and are not easily integratable with other quantum systems that operates within magnetic fields, such as spin ensemble quantum memories. To tackle this challenge, we have developed a kinetic inductance-based parametric amplifier featuring a NbN nanobridge instead of Josephson Junctions, which provides the desired nonlinearity for a strong parametric gain up to 42 dB. The added noise of this nanobridge kinetic-inductance parametric amplifier (hereby referred as NKPA) is calibrated and found to be $0.59\pm 0.03$ quanta for phase-preserving amplification, approaching the quantum limit of 0.5 quanta. Most importantly, we show that such excellent noise performance is preserved in an in-plane magnetic field up to 427 mT, the maximum field available in our experiment. This magnetic field-resilient parametric amplifier presents an opportunity towards addressing single electron-spin resonance and more efficient search for Axions as well as Majorana Fermions.

quant-ph↗