Our research is currently focusing on three directions:
1. Reservoir engineering — Shaping the environment for quantum error correction and quantum control
2. Qubit coherence and gates — Elucidating decoherence mechanisms and improving quantum gates
3. Non-reciprocal circuit QED — Adding an arrow of time to quantum dynamics
Our research is funded by the Department of Energy (DOE), Army Research Office (ARO), Air Force Office of Scientific Research (AFOSR) and the National Science Foundation (NSF).
Reservoir engineering
— Shaping the environment for quantum error correction and controlling complex quantum systems
Recent advances in our ability to tailor-make quantum systems has allowed us to not only engineer their Hamiltonian but also their dissipation operators. The dynamics of an open quantum system is even more complex and fascinating than for closed quantum systems. We can harness the effect of a specifically-designed environment reservoir to control quantum systems in ways that are not available with only unitary operations.
Stabilization and Characterization of Pair Coherent State
The pair-coherent state (PCS) is a theoretical extension of the Glauber coherent state to two harmonic oscillators. It is an interesting class of non-Gaussian continuous-variable entangled state and it is also at the heart of a promising quantum error-correction code: the pair-cat code. Here, we report an experimental demonstration of the pair-coherent state of microwave photons in two superconducting cavities. We implement a cross-cavity pair-photon driven-dissipation process, which conserves the photon-number difference between cavities and stabilizes the state to a specific complex amplitude. We further introduce a technique of quantum subspace tomography, which enables direct measurements of individual coherence elements of a high-dimensional quantum state without global tomographic reconstruction. Please see PRX Quantum 4, 020319 (2023).
Protecting a Bosonic Qubit with Autonomous Quantum Error Correction
To build a universal quantum computer from fragile physical qubits, effective implementation of quantum error correction (QEC) is an essential requirement and a central challenge. Existing demonstrations of QEC use active routines that are hardware intensive, prone to introducing and propagating errors, and expected to consume a vast majority of the processing power in a large-scale quantum computer. In principle, QEC can be realized autonomously and continuously by tailoring dissipation within the quantum system. Here we encode a logical qubit in Schrödinger cat-like multiphoton states of a superconducting cavity, and demonstrate a corrective dissipation process that directly stabilizes an error syndrome operator: the photon number parity. Implemented with continuous-wave control fields only, this passive protocol realizes autonomous correction against single-photon loss and boosts the coherence time of the multiphoton qubit by over a factor of two. Compatible with other error suppression and phase stabilization techniques, our experiment suggests reservoir engineering as a resource-efficient alternative or supplement to active QEC in future quantum computing architectures. Please see Nature 590, 243-248 (2021).
Autonomous quantum state transfer by dissipation engineering
Quantum state transfer from an information-carrying qubit to a receiving qubit is ubiquitous for quantum information technology. In a closed quantum system, this task requires precisely timed control of coherent qubit-qubit interactions that are intrinsically reciprocal. Here, breaking reciprocity by tailoring dissipation in an open system, we show that it is possible to transfer a quantum state between stationary qubits autonomously without time-dependent control. We present the general requirements for this directional transfer process and show that the minimum system dimension for transferring one qubit of information is 3×2 (between one physical qutrit and one physical qubit) plus one auxiliary reservoir. We propose realistic implementations in present-day superconducting circuit QED experiments and further propose schemes compatible with long-distance state transfer using impedance-matched dissipation engineering. Please see Phys. Rev. Research 1, 033198 (2019).
Qubit coherence and gates
— Elucidating decoherence mechanisms in superconducting qubits and developing quantum gates with new qubit designs
The transmon qubit has seen enormous success since its invention in 2007 and has become nearly synonymous with superconducting qubits in industrial-scale quantum computing efforts. The transmon is arguably the easiest qubit to model and to produce consistently, but it is by no means optimal and is unlikely to remain the best option in the long run. We are part of a team together with Manucharyan Group at Maryland, Huard group at ENS Lyon, and Vavilov Group at Wisconsin to advance the coherence times and develop quantum gates for fluxonium — a Josephson-junction-chain-based qubit that we believe will deliver better performance.
We are also part of a large team working on designing and characterizing superconducting qubits with improved materials in the C2QA center. Our center has a grand goal of improving the physical qubit performance by 10x and improving the logical qubit performance (via quantum error correction) by another 10x on top of that in the next 5 years. We work together with other superconducting device groups at C2QA to translate the material science advances from the center research into the next generation of superconducting qubits.
Two-fluxonium Cross-Resonance Gate
The superconducting fluxonium qubit has a great potential for high-fidelity quantum gates with its long coherence times and strong anharmonicity at the half-flux-quantum sweet spot. However, current implementations of two-qubit gates compromise fluxonium’s coherence properties by requiring either a temporary population of the noncomputational states or tuning the magnetic flux off the sweet spot. Here we realize a fast all-microwave cross-resonance gate between two capacitively coupled fluxoniums with the qubit dynamics well confined to the computational space. We demonstrate a direct CNOT gate in 70 ns with fidelity up to F=0.9949(6) despite the limitations of a suboptimal measurement setup and device coherence. Our results project a possible pathway towards reducing the two-qubit error rate near to or below 1E-4 with present-day technologies. Please see Phys. Rev. Applied 20, 024011 (2023).
Non-reciprocal circuit QED
— Adding an arrow of time to quantum dynamics
Non-reciprocity, a property related to broken time reversal symmetry that breaks the symmetry between the source and the receiver in an information system, has important implications for controlling the dynamics of a quantum system. So far, most studies on non-reciprocity, both theoretical and experiment, have focused on linear systems that can be described by a non-Hermitian effective Hamiltonian. In this research, we use a circuit QED device integrated with ferrite oscillators to demonstrate non-reciprocal interactions between superconducting qubits and cavities. We currently collaborate with the theory group of Aash Clerk at U Chicago on this project.
Dispersive non-reciprocity between a qubit and a cavity
The dispersive interaction between a qubit and a cavity is ubiquitous in circuit and cavity quantum electrodynamics. It describes the frequency shift of one quantum mode in response to excitations in the other, and in closed systems is necessarily bidirectional, i.e. reciprocal. Here, we present an experimental study of a nonreciprocal dispersive-type interaction between a transmon qubit and a superconducting cavity, arising from a common coupling to dissipative intermediary modes with broken time reversal symmetry. We characterize the qubit-cavity dynamics, including asymmetric frequency pulls and photon shot-noise dephasing, under varying degrees of nonreciprocity by tuning the magnetic field bias of a ferrite component in situ. We introduce a general master-equation model for dissipative interactions in the dispersive regime, which provides a compact description of the observed qubit-cavity dynamics without invoking the complexity of the intermediary system. Our result provides an example of quantum nonreciprocal phenomena beyond the typical paradigms of non-Hermitian Hamiltonians and cascaded systems. See arXiv: 2307.05298.
Past Research Highlights:
A Schrodinger cat in two boxes
Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as “cat states,” have been an elegant demonstration of Schrödinger’s famous cat paradox. Here, we realize a two-mode cat state of electromagnetic fields in two microwave cavities bridged by a superconducting artificial atom, which can also be viewed two as two Schrödinger’s cat that are entangled with each other (simultaneous dead or simultaneous alive). We present full quantum state tomography of this complex cat state over a Hilbert space exceeding 100 dimensions via quantum non-demolition measurements of the joint photon number parity. The ability to manipulate such multicavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication. Please see Science 352, 1087 (2016).
Surface dielectric loss in superconducting qubits
We study the energy relaxation times (T1) of superconducting transmon qubits in 3D cavities as a function of dielectric participation ratios of material surfaces. This surface participation ratio, representing the fraction of electric field energy stored in a dissipative surface layer, is computed by a two-step finite-element simulation and experimentally varied by qubit geometry. With a clean electromagnetic environment and suppressed non-equilibrium quasiparticle density, we find an approximately proportional relation between the transmon relaxation rates and surface participation ratios. These results suggest dielectric dissipation arising from material interfaces is the major limiting factor for the T1 of transmons in 3D circuit QED architecture. Please see Appl. Phys. Lett.107, 162601 (2015).
Quasiparticle dynamics and in superconducting qubits
Extensive efforts have always been taken to completely shield superconducting quantum circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, we show vortices can improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we quantitatively distinguish between recombination and trapping mechanisms in controlling the dynamics of residual quasiparticles, and show quantized changes in quasiparticle trapping rate due to individual vortices. See Nature Communications. 5, 5836 (2014).