A crucial limitation in building a quantum information processor from superconducting qubits stems from the materials they are made out of. While the capacitor component of transmon qubits has been extensively studied, we still have limited information about the Josephson junction. We have developed a high coherence trilayer junction process that allows us to explore a much broader range of materials and synthesis techniques than is possible with traditional junctions. We couple transmission electron microscopy and x-ray measurements that let us characterize materials properties of junctions with qubit measurements to develop an understanding of what leads to two-level systems and how those contribute to decoherence.

When a charged particle encircles magnetic flux, it acquires an Aharonov-Bohm phase. Often cited as an odd consequence of quantum mechanics, this aspect of the magnetic field should perhaps be seen as its central feature. Photons, themselves linear electromagnetic fields, do not typically experience such a phase shift. However it is possible to build a meta-material in which photons acquire phase when encircling an area, resulting in dynamics mathematically equivalent to that of a charged particle in a magnetic field. We designed a circuit-based topological insulator in which a series of LC circuits are connected in such a way that they form two copies of a “spin-Hall” system, each of which behaves as an independent particle in a magnetic field. Operating at room temperature, this shows that the topological structure of such materials is classical in nature and realizable in a wide variety of systems. We then built a superconducting-cavity-based topological insulator, adding ferrimagnetic crystals to some of these microwave cavity resonators which respond to magnetic field and enforce an added phase on photons circling around the material. This “quantum Hall” system works at cryogenic temperatures, allowing us to add superconducting qubits which give the photons places to interact. A feature of topological materials with time-reversal symmetry breaking like this one is a set of backscatter-protected `edge channels’ at different energies in which photons are constrained to travel around the edge of the material in one direction or the other. We use two qubits coupled to this material to put a photon in an edge channel and send it in one direction to the second qubit. This ability to couple two (and more!) qubits to such a superconducting topological material, introducing additional single-photon interactions, opens avenues to realize a host of exotic quantum phases. This work is done in tight collaboration with Jon Simon's group here at Stanford.

A central challenge of modern quantum science is understanding strongly correlated quantum matter. Inspired by Feynman, one way to approach the problem is to recreate the physics of interest in pristine analog quantum simulators. In recent years, superconducting circuits have proven to be a rich testbed for modeling and probing many-body phenomena. We have built a chip with a chain of transmon qubits realizing a 1D Bose-Hubbard model, ie a system where particles (here microwave photons) can hop between lattice sites (qubits) at a rate given by inter-qubit capacitance and interact with a strength given by the qubit anharmonicity. We have used dissipative stabilization to prepare quantum solids (Mott insulator) and adiabatic control of relative lattice detuning to prepare quantum fluids. More generally, we are interested in creative and resource-efficient ways of preparing and characterizing complex many particle quantum states! Recent examples include interfering coherent superpositions of fluids to extract their thermodynamic properties, as well studying phonon transport in our synthetic material through driving and lattice potential shaping.

Superconducting qubits have made tremendous progress in the last decade, starting from barely measurable coherence measured in picoseconds, to coherence times approaching 1 ms. The rapid progress on individual and few qubits, has encouraged research into scaling superconducting quantum processors to larger numbers of qubits. Rather than scale the system by simply microfabricating many copies of a qubit, we are attempting to use multi-mode resonators as quantum memories to store larger numbers of qubits. It is straight forward to create high quality factor resonators with 10's of modes (bits) with their quantum state being controlled by a single Josephson junction circuit. We believe that this multi-plexed approach combined with brute force scaling can realize a medium sized quantum processor in the near term.

Quantum random access memory (QRAM) – which can deterministically address a superposition of memory cells and return a superposition of the data – is required for the implementation of numerous quantum algorithms, such as Grover’s search, matrix inversion, and various proposals in the field of quantum machine learning. We are working on a key building block for QRAM: the Q2 router, which routes an incoming quantum signal to a superposition of different outgoing ports depending on the superposition state of a quantum switch (determined for example by a quantum address bit). The Q2 router has quantum states for both signal and switch, as distinct from conventional routers with the signal and/or the switch being in some classical state. The Q2 router is a key enabling component for QRAM, that will also open up various other new applications.

What would superconducting circuits look like beyond the transmon? While great advances have been made with the weakly anharmonic transmon, there are other exciting parameter regimes that can be explored, especially those that are activated with large kinetic inductances. In particular, the fluxonium qubit has large anharmonicities and rich level structures, lending itself to novel high-fidelity single- and two-qubit gate schemes. Furthermore, the low frequencies (tens of MHz) that circuits in the heavy-fluxonium regime can achieve allow for very long coherence times. There are fascinating open questions about Many interesting ways of rethinking established superconducting circuit paradigm exist when you enter the weird, wacky world of fluxonium!

Large-scale quantum computer will inevitably need Quantum Error Correction (QEC) to protect information against decoherence. Compared to error correction in bit information, QEC is more challenging under additional no-go theorems. In Autonomous Quantum Error Correction (AQEC), the system is cooled via an appropriate set of drives and couplings to engineered thermal reservoirs, which is hardware efficient compared to measurement feedback based QEC. We propose and implement new AQEC code, the Star Code, that protects a single logical qubit against decay and dephasing with only two transmons and a linear coupler.

Over 80% of the universe’s matter content is hidden from us. Low mass bosons, such as axions and hidden photons, are compelling candidates for this dark matter. Under the right circumstances, these particles could, on rare occasion, convert to microwave light. We engineer novel high-Q cavities to capture and store the resulting signal. We use superconducting qubits to both enhance the conversion of dark matter to light and count the number of deposited photons in the cavity. These techniques significantly increase signal-to-noise ratio of the search and reduce the time required to search for a candidate.

Scaling up microwave superconducting devices to higher frequencies offers transformative possibilities for quantum experiments at temperatures above the millikelvin range because of the reduced sensitivity to thermal noise of higher-energy photons. Higher temperatures provide significantly more cooling power, which reduces cooling complexity and power dissipation constraints, enables new pathways for scalability, and facilitates direct integration with high-speed superconducting digital logic. Extending superconducting quantum device functionality to higher frequencies also offers new opportunities for detection and transduction and access to larger coupling strengths for hybrid experiments. To establish more robust quantum information systems, we explore new fabrication techniques to strive to understand and minimize decoherence in superconducting components. By designing various higher-frequency higher-temperature qubits we investigate superconducting devices in new frequency regimes, higher temperatures and even magnetic fields.

Electrons, ubiquitous elementary particles of non-zero charge, spin and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits through either motional or spin states depends critically on their material environment.We aspire to realize experimental realization of a qubit platform based on isolated single electrons trapped on an ultraclean solid neon/liquid Helium surface in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator, has been demonstrated. We are currency investigating to integrate the spin degree of freedom into our platform, which can potentially demonstrate a second-level of coherence time.