Research – Quantum Wave-Matter Group

Graphene-based single-photon detector

In quantum technology, Josephson junction is the new transistor, forming the foundational building block of quantum processors. However, its potential extends beyond retaining and processing quantum information. By hybridizing with graphene, we have demonstrated that it can detect a single infrared photon. This artistic illustration depicts how photons can break Cooper pairs within the Josephson junction, generating voltage pulses. Our invention could function as an optical interconnect for quantum processors, bridging the gap between light and superconducting quantum circuits.
Hydrodynamics of the Dirac fluid in graphene

This is an artistic rendering of the massless Dirac fermions exhibiting the hydrodynamic property because of the strong many-body interaction. We discover the experimental signature of the Dirac fluid by its violation of the Wiedemann-Franz law near the Dirac point, suggesting a non-Fermi liquid behavior. Theory predicts this is a nearly perfect fluid such that the viscosity-to-entropy ratio can approach the Kovtun-Son-Starinets bound. If graphene is disorder-free, the Dirac fluid is analogous to the quark-gluon plasma that forms in high energy collider and shortly after the Big Bang. This is a exemplary topic that we love to study in Quantum Wave-Matter group, showing how some big scientific ideas manifest in a table-top experiment!
Instrumentation strategies of QCD axion haloscope

While Peccei-Quinn mechanism sets the axion-photon coupling strength, less is known about the mass and frequency of the dark matter. Thus the search of dark matter axions spans a wide electromagnetic spectrum that demands various experimental instrumentations and strategies. As the frequency increases, single-photon detector could become more favorable than heterodyne detector. Survey by Lentz and Fong with insight from Marsh. [Link]
Superconducting transmon qubit

Optical micrograph of a nano-fabricated, superconducting quantum bit (qubit).The qubit forms by a superconductor-insulator-superconductor Josephson junction (in the middle between the wire at the center that is barely visible) and a capacitor (the two big rectangles). It is capacitively coupled to other qubits or outside world via the superconducting microwave cavities (coplanar waveguides). This circuit-QED design is one of the basis of modern superconductor-based quantum processor.
Our experiments at ultralow temperatures

This is the base plate of a dilution refrigerator that can bring our setup to about one-hundredth of a degree from the absolute zero so that we can perform our quantum physics experiments. Behind the microwave circuit is the mixing chamber (silver cylinder) that contains a mixture of helium-3 and -4 isotopes during the operation. Cooling to about 0.01 Kelvin is achieved by pumping the helium-3 from the concentrated to dilute phase of the mixture. Coppers are plated with gold to enhance the thermal conductivity which is dominated by the electronic heat diffusion at the ultra-low temperature.
Pleasure of Finding Things Out (together)

By sunrise, after filling and erasing the blackboard countless times, analyzing and interpreting data from every angle, and engaging in spirited debates and discussions, we knew we had experienced a truly remarkable day, and gained a fresh appreciation for the beauty of Mother Nature. In picture, Andy was explaining hydrodynamics and AdS/CMT to me, sitting on the lower right, while Jesse took this picture. Heartfelt thanks to my collaborators for adding so much color and joy to my journey!
Graphene-based microwave bolometer

Electrons in graphene has extremely small heat capacity and thermal conductance because of its pseudo-relativistic band structure. It is a promising material for bolometry and single-photon calorimetry. This pseudo-colored image is the optical micrograph of the graphene bolometer that we invented, designed, fabricated, and studied. Detector physics is intimately related to fundamental questions of quantum measurements. In this case, our bolometer reaches the fundamental thermodynamic limit due to intrinsic fluctuation of temperature. Sensors with such a ultra-high sensitivity are very useful in creating remote entangled quantum states for quantum information and detecting the cosmic infrared background for radio-astronomy. This detector is scheduled to be deployed in the IBS's dark matter experiment in 2025.
Kinetic inductance and superfluid stiffness

Inspired by techniques used to probe qubit states, we developed resonator methods to measure the kinetic inductance of novel superconductors. This approach provides insights into superfluid stiffness, revealing the Cooper pairing symmetry and the mechanisms behind unconventional superconductivity. The optical micrograph shows half-wave resonators coupled to a small piece of the Weyl superconductor MoTe₂, positioned at the end of the resonator (lower right).
Kinetic inductance traveling wave amplifier (KIT)

This optical micrograph of KIT shows a roughly 2 meters long coplanar waveguide of about 1 micrometer wide, bending around to form the Yin-Yang pattern within a 10 by 10 mm2 footprint. It is micro-fabricated using niobium nitride by our collaborators in NIST and JPL, NASA. When properly biased and pumped, it can amplify a microwave signal by three-wave or four-wave mixing due to the non-linearity in the kinetic inductance of the superconductor. This amplifier can have a wide-band, high gain, and low noise amplification that will only be limited by the quantum principle. We uses KIT amplifier to simultaneously measure multiple qubits with high-fidelity.

Graphene-based single-photon detector

Hydrodynamics of the Dirac fluid in graphene

Instrumentation strategies of QCD axion haloscope

Superconducting transmon qubit

Our experiments at ultralow temperatures

Pleasure of Finding Things Out (together)

Graphene-based microwave bolometer

Kinetic inductance and superfluid stiffness

Kinetic inductance traveling wave amplifier (KIT)