Getting rid of those leftover measurement photons
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.
Hybrid transmon-like circuits using SNS junctions
We have realized quantum microwave circuits using proximitized InAs nanowires as Josephson elements, replacing the ubiquitous Al/AlOx/Al SIS junctions. These SNS Josephson elements can be tuned electrically: a voltage applied on a nearby gate affects the carrier density in the nanowire, which in turn changes the Josephson energy. The transition frequencies in these circuits reveal signatures of non-sinusoidal current-phase relations, offering interesting applications. Using NbTiN as the contacting superconductor also makes these circuits potentially compatible with magnetic field. We invite you to read our paper, twin paper from Charlie Marcus’ group, and an excellent Viewpoint article written about them both.
Increasing the single-photon relaxation time in planar resonators by optimization of interfaces
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 titation 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.
Detecting bit-flip errors in a logical qubit
We have implemented the quantum error detection step of the textbook three-qubit repetition code, using ancilla-based parity measurements in the five-qubit quantum processor shown to the right. Two, two-qubit parity measurements performed in parallel can discretize and signal bit-flip errors on the three qubits used to encode a logical qubit in GHZ-type states. Importantly, this signalling happens without decoding the information, meeting a necessary condition for fault-tolerant quantum computing. We invite you to check out our manuscript on the arXiv.
Wah-Wah single-qubit control
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!
Reversing quantum trajectories with analog feedback
We have implemented an analog feedback scheme for qubit control to undo the backaction of dispersive measurement in real time. By correlating the homodyne measurement record with individual qubit trajectories, we are able to revert the stochastic phase imparted on the qubit, thus suppressing the measurement-induced dephasing. This type of analog feedback, combined with improved quantum efficiency, will be the basis of measurement-based protocols such as qubit-state stabilization and continuous-time error correction. We invite you to check out our manuscript on the arXiv!
Ancilla-based parity measurement on a 2D quantum processor
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!
Deterministic 2-qubit entanglement by parity measurement and feedback
We have realized a high-fidelity parity measurement of two superconducting qubits using the cavity in a 3D circuit QED architecture. This parity measurement generates entanglement between two non-interacting qubits starting from a maximal superposition state. Although the projection to even or odd parity is probabilistic, using a digital feedback loop
we can target the same odd-parity Bell state every time! Parity measurements and parity-conditioned qubit control as here developed are key ingredients for active error correction in quantum computing. We invite you to check out our Nature article for all experimental details and to browse the fun press coverage!
Ancilla-based indirect qubit measurement
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!
Quasiparticle tunneling and induced decoherence in transmons
Building on our work on high-fidelity readout and feedback control, we have just transformed a transmon qubit into a charge-parity detector with 6 microsecond resolution. Using this detector, we perform the first real-time observation of quasiparticle tunneling across the transmon Josephson junction, and investigate the contribution of this tunneling to qubit relaxation and pure dephasing. The observed millisecond timescale shows that this process does not currently bottleneck qubit coherence. As far as quasiparticles are concerned, the coast seems clear for another order of magnitude in coherence! You can find all details in our manuscript. Research done in collaboration with the Lehnert Lab at JILA-NIST, University of Colorado.
Hybrid superconductor/spin systems for quantum computing
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.
Feedback control of superconducting qubits
We have just realized digital feedback control of a superconducting qubit. That is, coherent control on the qubit that is conditioned in real time on the result of projective measurement. We use feedback to reset any qubit state on demand and fast compared to the qubit lifetime. This type of control will be at the heart of innovative measurement-based protocols in the solid state, such as quantum error correction and teleportation. Details available in our manuscript. This research is done in collaboration with the Lehnert Lab at JILA-NIST, University of Colorado.
High-fidelity readout of superconducting qubits
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.
The two dilution refrigerators in our main lab reach <12 mK base temperature in 18 hours. Each one allows for multiple experiments in parallel. In turn, the helium-3 system in our basement lab reaches 240 mK in under 2 hours, and is just big enough to fit a cryogenic amplifier and circulator. This fridge allows very fast characterization of devices with minimal expense of liquid helium. Following characterization, our good devices are promoted to a dilution fridge upstairs.
We have all the equipment necessary to fabricate devices from start to finish. The Van Leeuwenhoek Lab, a 3500 m2 state-of-the-art cleanroom facility established in 2009, features high-resolution electron-beam and optical lithography, mask fabrication, sputtering- and evaporation-based deposition, dry etching, wet processing, high-resolution optical and electron microscopy, and dicing, all in a class 10,000 or better environment staffed by 9 techs. Additionally, Quantum Transport has a dedicated Al e-beam evaporator for making Josephson junctions, a wirebonder and multiple probe stations.
We gratefully acknowledge support from the Foundation for Fundamental Research on Matter (FOM), IARPA, Intel, and Microsoft Research.