Transmon leakage from the computational subspace is a major obstacle (a headache!) for quantum error correction using codes such as surface code, which are designed to correct errors only within the computational subspace. We have implemented and extended the leakage reduction units (LRUs) proposed by our colleagues in the Terhal group [F. Battistel et al., PRX, 2, 030314 (2021)]. These LRUs induce seepage from leaked transmon states to the computational subspace using all-microwave pulses and only quantum hardware already available in our processors. We demonstrate the power of these LRUs in a building-bloc QEC experiment. Please check out our paper for more information!

We demonstrate a complete suite of logical-qubit operations for the distance-2 surface code, aka Surface-7, capable of quantum error detection. Our logical operations span arbitrary initialization on the logical Bloch sphere, measurement in all cardinal bases, and a universal set of single-logical-qubit gates. For each operation type, we quantify the increased performance of fault-tolerant variants over non-fault-tolerant ones. Please check out our manuscript for all the details!

We perform stabilization of two-qubit entanglement using repeated parity measurements performed using a third, ancilla qubit. We use hidden Markov models to detect leakage from the string of two-level ancilla outcomes, improving the entanglement by post-selection of inferred leakage events. We invite you to check out our paper here!

We have recently developed a new type of conditional-phase gate for transmon qubits providing several key improvements over standard flux-pulsing-based versions. The Net-Zero gate uses “leakage interference” to minimize leakage to non-computational states. The zero-average, bipolar shape of the pulse makes the gate robust to long timescale distortions in the flux control line and additionally provides an echo effect. We demonstrate a state-of-the-art conditional-phase gate of duration 40 ns achieving 99.1% fidelity and 0.1% leakage.

While the processing power in circuit QED chips is growing rapidly, it remains an open challenge to establish high-fidelity quantum links between qubits on different chips. We have achieved entanglement between transmon qubits on different cQED chips with 49% concurrence and 73% Bell-state fidelity. We engineer a half-parity measurement by successively reflecting a coherent microwave field from two nearly-identical transmon-resonator systems. By ensuring that the measured output field does not distinguish |01> from |10>, unentangled superposition states are probabilistically projected onto entangled states in the odd-parity subspace. We use in-situ tunability and an additional weakly coupled driving field on the second resonator to overcome imperfect matching due to fabrication variations. Please check out our manuscript here!

For our second-generation nanowire transmon qubits, we used InAs nanowires with an epitaxially grown Al shell, provided by our colleagues in Copenhagen. We etched away a section of this shell to define a superconducting-normal-superconducting (SNS) Josephson junction, giving the transmon its nonlinearity. Combining the hard induced superconducting gap of these wires with our established NbTiN fabrication recipes lead to unprecedented coherence times in SNS devices. This allowed us to study various noise sources affecting the coherence of these qubits. In addition, we showed that the devices can withstand magnetic fields of 80 mT, taking a step towards interesting applications of circuit quantum electrodynamics at field. We invite you to check out our manuscript here!

One of the most exciting application areas for future quantum computers is in quantum simulation, the task of using a well-controlled quantum system to mimic the behaviour of another target quantum system. Recently, using a dedicated circuit QED system, we have demonstrated a highly flexible and accurate digital quantum simulation of the quantum Rabi model, a cornerstone of modern quantum physics, in the exotic “deep-strong coupling” parameter regime, where traditional strong-coupling approximations break down. We accurately control dynamical evolution of a coupled qubit-resonator system over a broad range of parameters and demonstrate key dynamical signatures of deep-strong coupling physics. Our demonstration is one of the most advanced digital quantum simulations to date, showing important performance characteristics which have not previously been observed in the solid state. We invite you to read our manuscript here.

Quantum processors based on circuit QED can require a significant wait time after measurement to allow leftover photons to exit the readout resonators. We have just demonstrated two active photon depletion methods that minimize deadtime even when the resonator is driven nonlinearly to maximize readout fidelity. This speedup reduces the buildup of errors in a quantum error correction cycle. We invite you to read our manuscript.

Coplanar waveguide resonators are crucial elements in quantum integrated circuits based on circuit QED, as well as in photodetectors, parametric amplifiers, and hybrid systems. We have recently investigated two fabrication techniques (substrate surface treatment and deep reactive ion etching) to improve or displace interfaces where dissipation-inducing two-level systems lie. Our niobium titanium nitride resonators on silicon reach internal Q’s above 1 million in the quantum regime (low temperature and single-photon operation). We are now exploring the application of these techniques to the fabrication of superconducting qubits. We invite you read out manuscript recently posted on the arXiv.

Transmon qubits are weakly anharmonic oscillators. Packing more of them into the 4-8 GHz band typical of cQED processors presents a challenge: how to drive the logical transition (0-1) of a targeted transmon (Qa) without driving the leakage transition (1-2) of its nearest neighbor higher up in frequency (Qb)? The Wilhelm group has recently proposed Wah-Wah pulsing for this purpose. We have now implemented Wah-Wah control in a 2D processor operating at unity crosstalk: control pulses for all qubits are applied via one feedline coupling equally to all transmons. We show that Wah-Wah successfully avoids leakage in the unaddressed transmon at gate times where conventional DRAG pulsing cannot cope. We invite you to check out our manuscript on the arXiv!

Continuing our developmnet of ancilla-based indirect measurements, we have implemented a two-qubit parity meter on a versatile four-transmon processor. Our parity measurement occurs in two discrete steps: First, using unitary control, we associate the state of the ancilla qubit with data-qubit states of definite parity. Second, a high-fidelity projective measurement of the ancilla completes the protocol without dephasing the data qubits. We believe that this approach – combined with fabrication developments enabling more complex on-chip connectivity – is the way forward to error-correcting quantum circuits. Check out the details in our manuscript and stay tuned for more!

We have realized the indirect measurement of a transmon using another transmon as ancillary qubit. This is a two-step scheme where the qubit and ancilla first interact and the ancilla is then measured with a dedicated readout resonator in a 2D circuit QED architecture. We exercise full control over the interaction and the ancilla measurement basis to investigate the tradeoff between information gain and imposed disturbance on the qubit. Combining partial and projective measurements, we observe non-classical weak values and the corresponding violations of Leggett-Garg inequalities. We invite you to check out our manuscript for all the details!

We have just finished an in-depth study of spin dynamics in a P1-center ensemble in diamond using a superconducting resonator as a sensitive probe. A key feature in this work is that the resonator, patterned from a thin film of the highly-disordered superconductor NbTiN, can withstand in-plane magnetic field (~300 mT) needed to significantly polarize the P1 ensemble at 250 mK. We characterize the resonator-mode and temperature dependence of unprecedentedly strong coherent coupling of hyperfine-split sub-ensembles to the resonator, and perform measurements of spin linewidth, diffusion, and cross-relaxation. We believe P1-center ensembles have potential to realize a useful quantum memory for superconducting qubit circuits. We invite you to check out our manuscript. This research is done in collaboration with the Hanson and Klapwijk groups at TU Delft.

We have recently implemented a high-fidelity quantum nondemolition readout of 3D transmon qubits by boosting the sensitivity of linear dispersive readout with a Josephson parametric amplifier. We now routinely use this readout scheme to purify a two-qubit device from residual steady-state excitation, enhancing ground-state initialization by measurement and postselection. The nondemolition character of this readout will make it ideal for measurement-based quantum information protocols.

We invite you to check out our manuscript.