Coherent Control of a Few-Channel Hole Type Gatemon Qubit
Han Zheng, Luk Yi Cheung, Nikunj Sangwan, A. Kononov, R. Haller, Joost Ridderbos, Carlo Ciaccia, Jann Hinnerk Ungerer, Ang Li, Erik P. A. M. Bakkers, Andreas Baumgartner, Christian Schönenberger
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.
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.
Abstract We demonstrate that superconducting gatemon qubits based on superconductor-semiconductor-superconductor Josephson junctions can be constructed on hole-type Ge/Si core/shell nanowires. The frequency of the qubit can be set firstly by controlling the diffusion of Al in the nanowire via thermal annealing, which yields a roughly suitable critical supercurrent for the junction, and then by fine tuning of a gate voltage, by which an accurate adjustment of the frequency can be realized. On the resulted qubit, Rabi oscillation with an energy relaxation time T1~180 ns was observed in the time domain, and the gate voltage dependence of both T1 and the dephasing time T2 was investigated. Such a hole-type superconducting gatemon qubit, based on materials with strong spin-orbit coupling and the absence of hyperfine interaction, could be used for exploring the quantum coherence phenomena of hole-gas and potentially also Majorana physics in Ge-based quantum devices.
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.
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.
Superconducting qubit based on superconducting circuits consist of a superconducting capacitor and a Josephson junction with the transmon geometry is extensively used in advanced quantum processors, pursuing scalable quantum computing. The tuning of the qubit frequency of the transmon relies on magnetic flux-dependent interference between the supercurrents of two superconductor-insulator-superconductor (S-I-S) Josephson junctions in a superconducting loop. Josephson junction based on superconductor-semiconductor-superconductor (S-Sm-S) materials opens up a possibility to the gate-tunable transmon, referred to as the "gatemon", in which the qubit frequency can be tuned by electrostatic mean. Recent realizations of gatemons on III-V material platforms show impressive development on the alternative to the transmon, yet still leave a big question on the scalability. The silicon-germanium (SiGe) heterostructure is one of the potential platforms to host hybrid devices due to its high hole mobility and the low Schottky barrier at the Ge-metal interface. Additionally, the compatibility with the silicon-based semiconductor industry is a capable advantage for the scaling-up qubit platform.In this thesis, we develop gatemons based on the Al-Ge-Al Josephson junction in the SiGe heterostructure. Firstly, the robust fabrication recipe, found on a top-down approach, for Josephson Field Effect Transistors (JoFETs) is established. We perform measurements exhaustively on the JoFETs to study their properties as a function of the gate voltage, temperature, and magnetic field. The devices show gate-tunability of the critical current and the normal state resistance . The devices are estimated to have a high-transparency superconductor-semiconductor interface, as demonstrated by the high product on the SiGe heterostructure. In the finite-voltage regime, the features corresponded to multiple Andreev reflections (MARs) are observed. Then, we fabricate and characterize niobium nitride (NbN) superconducting resonators on SiGe heterostructure. We measure the resonators in the transmission mode and extract the resonant frequency , internal quality factor, and coupling quality factor from the transmission coefficient. Following that, we develop the fabrication process to integrate Al-Ge-Al junctions shunted with capacitors, or, in other words, gatemon, into the resonator scheme and perform the fabrication according to the design. We demonstrate the anticrossing feature in one of the fabricated gatemons. The resonant frequency of the gatemon is mapped using the two-tone spectroscopy technique and is found to be gate-tunable. The qubit has large spectral linewidth, implying a low coherence time. Additionally, we conduct the current-phase relationship (CPR) measurement on the junctions in the Superconducting QUantum Interference Device (SQUID) geometry. We can demonstrate that the junctions pose non-sinusoidal CPR. Further, the integer and half-integer Shapiro steps are observed in the current-voltage characteristic curves of irradiated junctions. This indicates that our junctions have the element, which can open up another possibility toward protected qubits.
Ultrastrong light-matter coupling (USC) gives access to exotic quantum phenomena and promises faster quantum gates, yet coherent time-domain control in this regime remains largely unexplored. Here, we realize USC in a hybrid system consisting of an InAs nanowire-based gatemon qubit coupled to a superconducting resonator. Spectroscopy reveals an avoided crossing that cannot be captured by the Jaynes-Cummings (JC) model, as well as photon-number-dependent transitions whose energies deviate markedly from the JC ladder expected in the strong coupling regime. Beyond demonstrating USC, we achieve time-resolved coherent control of the qubit and measure coherence times comparable to gatemons operating outside the USC regime. These results establish that hybrid semiconductor-superconductor qubits can retain coherent control in USC and provide a platform for exploring quantum dynamics and device concepts in this regime.
Gatemons are superconducting qubits resembling transmons, with a gate-tunable semiconducting weak link as the Josephson element. Here, we report a gatemon device featuring an aluminum microwave circuit on a Ge/SiGe heterostructure embedding a Ge quantum well. Owing to the superconducting proximity effect, the high-mobility two-dimensional hole gas confined in this well provides a gate-tunable superconducting weak link between two Al contacts. We perform Rabi oscillation and Ramsey interference measurements, demonstrate the gate-voltage dependence of the qubit frequency, and measure the qubit anharmonicity. We find relaxation times T 1 up to 119 ns, and Ramsey coherence times T 2 * up to 70 ns, and a qubit frequency gate-tunable over 3.5 GHz. The reported proof-of-concept reproduces the results of a very recent work [Sagi et al. Nat. Commun. 2024, 15, 6400] using similar Ge/SiGe heterostructures, thereby validating a novel platform for the development of gatemons and parity-protected cos(2ϕ) qubits.