Evolution of Nanowire Transmon Qubits and Their Coherence in a Magnetic Field
Florian Luthi, Thijs Stavenga, Oscar Enzing, Alessandro Bruno, Christian Dickel, Nathan K. Langford, Michiel Adriaan Rol, Thomas Sand Jespersen, Jesper Nygård, Peter Krogstrup, L. DiCarlo
We present an experimental study of flux- and gate-tunable nanowire transmons with state-of-the-art relaxation time allowing quantitative extraction of flux and charge noise coupling to the Josephson energy. We evidence coherence sweet spots for charge, tuned by voltage on a proximal side gate, where first order sensitivity to switching two-level systems and background 1/f noise is minimized. Next, we investigate the evolution of a nanowire transmon in a parallel magnetic field up to 70 mT, the upper bound set by the closing of the induced gap. Several features observed in the field dependence of qubit energy relaxation and dephasing times are not fully understood. Using nanowires with a thinner, partially covering Al shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation and other applications.
We present an experimental study of flux- and gate-tunable nanowire transmons with state-of-the-art relaxation time allowing quantitative extraction of flux and charge noise coupling to the Josephson energy. We evidence coherence sweet spots for charge, tuned by voltage on a proximal side gate, where first order sensitivity to switching two-level systems and background 1/f noise is minimized. Next, we investigate the evolution of a nanowire transmon in a parallel magnetic field up to 70 mT, the upper bound set by the closing of the induced gap. Several features observed in the field dependence of qubit energy relaxation and dephasing times are not fully understood. Using nanowires with a thinner, partially covering Al shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation and other applications.
Quantum computers promise to solve certain problems such as quantum chemistry simulations much more efficiently than their classical counterparts. Although it is still unclear what material system will ultimately host large-scale quantum computers, solid-state systems are promising candidates due to their inherent scalability and advanced fabrication techniques that can be adapted from comparable technologies. Crucially, a future quantum computer will depend on the quality of its most fundamental building block, the quantum bit, or qubit. Qubits, although ideally insensitive to potential noise, are very susceptible to slight changes in their environment. Therefore, they do not only make the building block for quantum computers, but are also precise sensors. One of the most studied solid-state implementations of a qubit is the transmon, a weakly anharmonic oscillator based on superconducting capacitive and nonlinear inductive elements. Typically, Al-AlOx-Al superconductor-insulator-superconductor Josephson junctions are used for the latter. The interaction of the transmon with the control circuitry, typically superconducting resonators, is described by circuit quantum electrodynamics. In this PhD thesis, a more recently demonstrated type of qubit is further developed and studied in detail using circuit quantum electrodynamics. In these qubits, the Josephson element of the transmon is replaced with indium arsenide nanowires, forming a superconductor-normal metalsuperconductor junction. In addition to the standard flux tunability, these qubits can also be voltage tuned. Due to the compatibility of all the materials used with an applied magnetic field, this type of qubit is a good candidate to be used as a precise and accurate sensor in a magnetic field. The goal of this work is to introduce the in-plane magnetic field as a new tuning knob to the toolbox of circuit quantum electrodynamics. Advances in material science, especially the epitaxial growth of an aluminum shell directly on the indium arsenide nanowire, have enabled the fabrication of nanowire transmons with state-of-the-art coherence. An understanding of their workings in a zero-field environment is important before applying a magnetic field. Thus, we characterize the noise these qubits are subject to (Chapter 4) and find a strong coupling of charge two-level systems to their Josephson energy next to the expected weakly coupled flux and voltage noise. Applying a magnetic field reveals that coherence in these qubits can be observed up to 70 mT, substantially above the superconducting gap of bulk aluminum (Chapter 5). Effects limiting the performance include the thick and fully covering aluminum shell, and the alignment and stability of the magnetic field. The use of different nanowires, the installation of a persistent-current vector solenoid and additional magnetic shielding then enables the operation of voltage- and flux-tunable devices in a magnetic field (Chapter 6). This constitutes a good starting point for circuit quantum electrodynamics experiments in a magnetic field, such as the investigation of the microscopic origin of flux-noise.
We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmonlike device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (~0.8 μs) and dephasing times (~1 μs), exceeding gate operation times by 2 orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces cross talk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.
The technology of quantum computing is believed to solve certain computational problems significantly faster than classical computers, enabling classically inaccessible problems. However, the technology is still in its infancy and it is still too early to conclude which physical system(s) will form the basis of tomorrow's quantum computer. Small-scale quantum processors, built on the superconducting transmon qubit, demonstrated already the anticipated quantum speed-up. Despite this tremendous milestone, scaling up to a full-fledged quantum computer is far from trivial: with these qubits, the flux-control causes crosstalk between qubits and heating, the room-temperature microwave control is hardly scalable and comes with high energetic radiation, and most importantly, the loss of quantum information in time leads to computational errors. Recent advances in various hybrid semiconductor materials enabled novel voltage-controlled transmons, gatemons, which are less susceptible to heating and crosstalk. Even more exciting, gatemons can be designed to host Majorana zero modes in a way that renders the qubit inherently protected against decoherence. In order to induce Majorana zero modes in such nanowire systems strong magnetic fields are required. Problematically, magnetic fields destroy the superconductivity that these microwave circuits rely on. In this thesis we integrate three types of hybrid Josephson junctions in magnetic field compatible microwave devices. Chapter 4 demonstrates the first graphene-based transmon. Due to the mono-atomic thickness of graphene in combination with magnetic field resilient coplanar waveguide resonators, we are able to operate the transmon circuit at an in-plane magnetic field of 1 Tesla. Chapter 5 embeds an InAs-Al semiconducting nanowire Josephson junction in a high quality factor superconducting transmission line resonator, demonstrating on-chip microwave generation. The gate-controllable semiconducting platform lends itself for fast pulse control, providing a perspective for the coherent on-chip manipulation of artificial two-level systems, in particular superconducting qubits such as transmons. Chapter 6 continues the development of InAs-Al nanowire transmons. The offset-charge-sensitive regime, additional plunger gates and magnetic field compatibility prepares the platform for the detection of coherent coupling between Majorana zero modes, a phenomena which unfortunately still remains unobserved. Additionally, we realise the first InSb-Al gatemon. The higher spin-orbit coupling makes InSb to be a preferred material in the search for Majorana signatures. Chapter 7 reports on the dynamics of quasiparticle tunneling events in real-time across the InAs-Al nanowire junction in a transmon architecture. The magnetic field compatibility of our device up to 1 Tesla together with in-situ voltage-controlled quasiparticle trap engineering, allows us to measure the survival of the charge-parity lifetime up to strong magnetic fields. A result which is extremely important in the research field of topological quantum computing, where the qubit space is defined in the charge-parity.
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 anharmonicity of a transmon qubit, defined as the difference in energy level spacing, is a key design parameter. In transmons built from hybrid superconductor-semiconductor Josephson elements, the anharmonicity is tunable with gate voltages that control both the Josephson energy and the weak link transparency. In Sn-InAs nanowire transmons, we use two-tone microwave spectroscopy to extract anharmonicity ranging in absolute value from the transmon charging energy Ec to values smaller than Ec/10. This behavior contrasts with the predictions of the multi-channel short-junction model, which sets a lower limit on anharmonicity at Ec/4. Coherent operation of the qubit is still possible at the point of the lowest anharmonicity. These findings demonstrate the potential of quantum circuits that benefit from widely electrically tunable anharmonicity.
The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 μs even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.