Tsung-Ju Lu

A scalable multi-photon coincidence detector based on superconducting nanowires

Coincidence detection of single photons is crucial in numerous quantum technologies and usually requires multiple time-resolved single-photon detectors. However, the electronic readout becomes a major challenge when the measurement basis scales to large numbers of spatial modes. Here, we address this problem by introducing a two-terminal coincidence detector that enables scalable readout of an array of detector segments based on superconducting nanowire microstrip transmission line. Exploiting timing logic, we demonstrate a sixteen-element detector that resolves all 136 possible single-photon and two-photon coincidence events. We further explore the pulse shapes of the detector output and resolve up to four-photon events in a four-element device, giving the detector photon-number-resolving capability. This new detector architecture and operating scheme will be particularly useful for multi-photon coincidence detection in large-scale photonic integrated circuits.Superconducting nanowires can be engineered to realize scalable coincidence detectors of single photons.

Ultraviolet integrated photonics (Conference Presentation)

Quantum information processing relies on the fundamental property of quantum interference, where the quality of the interference directly correlates to the indistinguishability of the interacting particles. The creation of these indistinguishable particles, photons in this case, has conventionally been accomplished with nonlinear crystals and optical filters to remove spectral distinguishability, albeit sacrificing the number of photons. This research describes the use of an integrated aluminum nitride microring resonator circuit to selectively generate photon pairs at the narrow cavity transmissions, thereby producing spectrally indistinguishable photons in the ultraviolet regime to interact with trapped ion quantum memories. The spectral characteristics of these photons must be carefully controlled for two reasons: (i) interference quality depends on the spectral indistinguishability, and (ii) the wavelength must be strictly controlled to interact with atomic transitions. The specific ion of interest for these trapped ion quantum memories is Ytterbium which has a transition at 369.5 nm with 12.5 GHz offset levels. Ytterbium ions serve as very long lived and stable quantum memories with storage times on the order of 10’s of minutes, compared with photonic quantum memories which are limited to 10-6 to 10-3 seconds. The combination of the long lived atomic memory, integrated photonic circuitry, and the photonic quantum bits are necessary to produce the first quantum information processors. In this article, I will present results on wavelength operation, dispersion analysis, and second harmonic generation in aluminum nitride waveguides.

Wide-bandgap integrated photonic circuits for interfacing with quantum memories (Conference Presentation)

Quantum information processing relies on the fundamental property of quantum interference, where the quality of the interference directly correlates to the indistinguishability of the interacting particles. The creation of these indistinguishable particles, photons in this case, has conventionally been accomplished with nonlinear crystals and optical filters to remove spectral distinguishability, albeit sacrificing the number of photons. This research describes the use of an integrated aluminum nitride microring resonator circuit to selectively generate photon pairs at the narrow cavity transmissions, thereby producing spectrally indistinguishable photons in the ultraviolet regime to interact with trapped ion quantum memories. The spectral characteristics of these photons must be carefully controlled for two reasons: (i) interference quality depends on the spectral indistinguishability, and (ii) the wavelength must be strictly controlled to interact with atomic transitions. The specific ion of interest for these trapped ion quantum memories is Ytterbium which has a transition at 369.5 nm with 12.5 GHz offset levels. Ytterbium ions serve as very long lived and stable quantum memories with storage times on the order of 10’s of minutes, compared with photonic quantum memories which are limited to 10-6 to 10-3 seconds. The combination of the long lived atomic memory, integrated photonic circuitry, and the photonic quantum bits are necessary to produce the first quantum information processors. In this seminar, I will present results on ultraviolet wavelength operation, dispersion analysis, and propagation loss in aluminum nitride waveguides.

Quantum emission from atomic defects in wide-bandgap semiconductors

Non-classical light sources, such as atoms and atom-like emitters play central roles in many areas of quantum information processing with applications as single photon generators, sources for nonlinearity and quantum memories. Solid-state quantum emitters have attracted growing interest due to the promise of combining remarkable optical properties with the convenience of scalability [1]. In recent years, there has been tremendous progress in developing quantum emitter systems based on crystallographic defects in wide-bandgap semiconductors. Nitrogen vacancies (NV) in diamond were among the first studied systems due to the well-defined optical transitions as well as electronic spin states that can be controlled optically. Quantum spins in diamond are among the most advanced systems in solid state for quantum based technologies such as quantum computing or quantum sensing [2]. Nevertheless, solid-state quantum emitters are not only limited to diamond and efforts to engineer single photon emitters (SPE) based on atom-like defects in scalable system have expanded beyond NV centers in diamond. Similar quantum emitters have been discovered in many other wide-bandgap host materials, including silicon carbide (SiC), III-nitride semiconductors such as gallium nitride (GaN) and aluminum nitride (AlN), and layered materials such as hexagonal boron nitride (hBN) [1]. Here, we will review our recent progress in developing and characterizing new quantum emitters in wide-bandgap semiconductors, and consider their applications as quantum light sources and sensors.

High-purity single photon emitter in aluminum nitride photonic integrated circuit

Efficient, on-demand, and robust single photon emitters (SPEs) are important to a wide varity of applications in quantum information processing [1]. Over the past decade, color centers in solid-state systems have emerged as excellent SPEs [2] and have also been shown to provide optical access to internal spin states at room and cyogenic temperatures. Color centers in diamond [3] and silicon carbide [4] are among the most intensively studied SPEs. Recently, other cost-efficient wide-bandgap materials have become attractive as potential host materials. Theoretical calculations show that aluminum nitride (AlN) with a bandgap of 6.015 eV can serve as a stable environment for well isolated SPEs with optically accessible spin states [5].

Ultraviolet integrated photonic circuits (Conference Presentation)

Quantum information processing relies on the fundamental property of quantum interference, where the quality of the interference directly correlates to the indistinguishability of the interacting particles. The creation of these indistinguishable particles, photons in this case, has conventionally been accomplished with nonlinear crystals and optical filters to remove spectral distinguishability, albeit sacrificing the number of photons. This research describes the use of an integrated aluminum nitride microring resonator circuit to selectively generate photon pairs at the narrow cavity transmissions, thereby producing spectrally indistinguishable photons. These spectrally indistinguishable photons can then be routed through optical waveguide circuitry, concatenated interferometers, to manipulate and entangle the photons into the desired quantum states. Photon sources and circuitry are only two of the three required pieces of the puzzle. The final piece which this research is aimed at interfacing with are trapped ion quantum memories, based on trapped Ytterbium ions. These ions serve as very long lived and stable quantum memories with storage times on the order of 10’s of minutes, compared with photonic quantum memories which are limited to 10-6 to 10-3 seconds. The caveat with trapped ions is the interaction wavelength of the photons is 369.5nm and therefore the goal of this research is to develop entangled photon sources and circuitry in that wavelength regime to interact directly with the trapped ions and bypass the need for frequency conversion.

NV-based quantum memories coupled to photonic integrated circuits

The negatively charged nitrogen vacancy (NV) center in diamond is a promising solid-state quantum memory. However, developing networks comprising such quantum memories is limited by the fabrication yield of the quantum nodes and the collection efficiency of indistinguishable photons. In this letter, we report on advances on a hybrid quantum system that allows for scalable production of networks, even with low-yield node fabrication. Moreover, an NV center in a simple single mode diamond waveguide is shown in simulation and experiment to couple well to a single mode SiN waveguide with a simple adiabatic taper for optimal mode transfer. In addition, cavity enhancement of the zero phonon line of the NV center with a resonance coupled to the waveguide mode allows a simulated <1800 fold increase in the collection of photon states coherent with the state of the NV center into a single frequency and spatial mode.