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arXiv:1512.09195 · Physical Review Letters

Gatemon Benchmarking and Two-Qubit Operations

Lucas Casparis, T. W. Larsen, M. S. Olsen, Ferdinand Kuemmeth, Peter Krogstrup, Jesper Nygård, Karl D. Petersson, Charles M. Marcus

Published Apr 15, 2016• 92 citations
arXivHTMLPDF

Abstract

Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic of semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors.

DOI
10.1103/physrevlett.116.150505

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Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic of semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors.
The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building innovative superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit; however, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times compared to conventional transmons fabricated from aluminum-oxide-based Josephson junctions. Here, we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt-capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1-MHz precision over a range of several gigahertz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.
Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.
We analyze the coupling of two qubits via an epitaxial semiconducting junction. In particular, we consider three configurations that include pairs of transmons or gatemons as well as gatemonlike two qubits formed by an epitaxial four-terminal junction. These three configurations provide electrical control of the interaction between the qubits by applying voltage to a metallic gate near the semiconductor junction and can be utilized to naturally realize a controlled-Z gate (CZ). We calculate the fidelity and timing for such a CZ gate. We demonstrate that in the absence of decoherence, the CZ gate can be performed under 50 ns with gate error below ${10}^{\ensuremath{-}4}$.
Superconducting transmon qubits based on hybrid superconductor-semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.
Gate-tunable transmon qubits are based on quantum conductors used as weak links within hybrid Josephson junctions. These gatemons have been implemented in just a handful of systems, all relying on extended conductors, namely epitaxial semiconductors or exfoliated graphene. Here we present the coherent control of a gatemon based on a single molecule, a one-dimensional carbon nanotube, which is integrated into a circuit quantum electrodynamics architecture. The measured qubit spectrum can be tuned with a gate voltage and reflects the quantum dot behavior of the nanotube. Our ultraclean integration, using a hexagonal boron nitride substrate, results in record coherence times of 200 ns for carbon nanotube-based qubits. Furthermore, we investigate its decoherence mechanisms, thus revealing a strong gate dependence and identifying charge noise as a limiting factor. On top of positioning carbon nanotubes as contenders for future quantum technologies, our work paves the way for studying microscopic fermionic processes in low-dimensional quantum conductors. Gatemons, or gate-tunable transmons, are superconducting qubits based on hybrid Josephson junctions, which typically use extended quantum conductors as weak links. Here the authors report a gatemon made with a carbon-nanotube-based junction, showing improved coherence time compared to graphene-based devices.
Gatemon qubits are the electrically tunable cousins of superconducting transmon qubits. In this work, we demonstrate the full coherent control of a gatemon qubit based on hole carriers in a Ge/Si core/shell nanowire, with the longest coherence times in group IV material gatemons to date. The key to these results is a high-quality Josephson junction obtained using a straightforward and reproducible annealing technique. We demonstrate that the transport through the narrow junction is dominated by only two quantum channels, with transparencies up to unity. This novel qubit platform holds great promise for quantum information applications, not only because it incorporates technologically relevant materials, but also because it provides new opportunities, like an ultrastrong spin-orbit coupling in the few-channel regime of Josephson junctions.