Fluxonium qubit

We created the most coherent superconducting qubit to date. It is made of a chain of about 100 Josephson junctions, but that's ok. We got T2 > 3/2 ms (yes, over a millisecond), the gate fidelity exceeds 0.9999 (yes, four nines), and still have room to make this qubit even better.

Y.-H. Lin et al. (PRL 2018)

L. Nguyen et al. (PRX 2019)

A. Somoroff et al. (arxiv 2021)

High-fidelity two-qubit gates

We implemented fast microwave-activated c-Z and c-Phase gates on capacitively-coupled fluxoniums. The very first high-fidelity gates on non-transmon-type qubits. The average gate fidelity is already 0.992, on our way to the three nines!

Q. Ficheux, L. B. Nguyen et al. (PRX 2021)

H. Xiong, Q. Ficheux et al. (arxiv 2021)

Cavityless circuit QED

We found a way to readout fluxonium qubits without invoking a cavity mode. Seriously, no cavity! Instead one can detect the qubit state in a QND-manner using conditional fluorescence of a non-computational transition.

N. Cottet, H. Xiong et al. (arxiv 2020)

Blochnium qubit

Flux (fluxonium) and charge qubits are dual to each other. What is the dual of transmon? Normally, a Josephson junction shunted by a large capacitance behaves as an inductance. Paradoxically, the same junction can behave as a capacitance when shunted by a *very* large external inductance - hyperinductance. That's Blochnium, the fourth missing kind of superconducting artificial atoms.

I. Pechenezhskiy, R. Mencia et al. (Nature 2020)

Quantum electrodynamics of superconductor-insulator transitions

Instead of looking for scaling of resistance, we explore the finite-frequency dynamics of the collective phase-mode near the transition. We are focused at the Bose glass transition in weakly-coupled 1D Josephson junction chains and dissipative localization (Schmid-Bulgadaev) transition in a single weak junction connected to high-impedance leads. So far we have observed that a Bose glass may be an insulator at DC but it does have a stiff phase response at finite frequency/wavelength. You can compare this phenomenon to walking on water.

R. Kuzmin et al. (Nature Physics 2019)

Circuit QED meets quantum impurity physics

Can we go beyond quantum optics by simply coupling light to a single atom? We observed discrete many-body states of radiation, inelastic collisions between single photons ( = photon decay), and evidence for quantum thermalization transition. All these effects are manifestations of quantum impurity physics, normally considered in the context of strongly-correlated electrons or boundary quantum field theories.

A. Burshtein et al. (PRL 2021)

R. Kuzmin et al. (PRL 2021)

Transverse-field Ising model

We are building a quantum critical Ising chain using synthetic spin-1/2 circuits.

Metrology of direct current via Bloch oscillations

DC-voltage can be measured with an exceptional accuracy thanks to the AC-Josephson effect. We are working on stabilizing the dual Bloch oscillations in current-bias Josephson junctions based on our invention of Blochnium qubit.

Multiterminal Josephson effect

Junctions of more than two topological superconductors often come up in proposals for braiding of Majorana fermions. However, superconducting phase-coherence in a mesoscopic conductor connected to multiple superconducting terminals is an interesting subject already in the case of conventional superconductivity.

N. Pankratova, H. Lee et al. (PRX 2020)