We have incorporated a single crystal silicon shunt capacitor into a Josephson phase qubit. The capacitor is derived from a commercial silicon-on-insulator wafer. Bosch reactive ion etching is used to create a suspended silicon membrane; subsequent metallization on both sides is used to form the capacitor. The superior dielectric loss of the crystalline silicon leads to a significant increase in qubit energy relaxation times. T1 times up to 1.6 μs were measured, more than a factor of two greater than those seen in amorphous phase qubits. The design is readily scalable to larger integrated circuits incorporating multiple qubits and resonators.
We have incorporated a single crystal silicon shunt capacitor into a Josephson phase qubit. The capacitor is derived from a commercial silicon-on-insulator wafer. Bosch reactive ion etching is used to create a suspended silicon membrane; subsequent metallization on both sides is used to form the capacitor. The superior dielectric loss of the crystalline silicon leads to a significant increase in qubit energy relaxation times. T1 times up to 1.6 μs were measured, more than a factor of two greater than those seen in amorphous phase qubits. The design is readily scalable to larger integrated circuits incorporating multiple qubits and resonators.
Superconducting devices as quantum bits (qubits) are one of the most promising candidates for implementing a practical quantum computer. Over the last decade, quantum coherence times of superconducting qubits have been continuously improved. Operating qubits in noise insensitive points, eliminating the participation of the lossy materials, and better isolation from environment increased the decoherence times from nanoseconds to tens of microseconds. One recent approach on the qubit design is to shunt the Josephson junctions with large capacitors to reduce the sensitivity to noise and to redistribute the stored electromagnetic energy in a well-engineered area. This approach was first applied on charge qubits and later adopted to other types of superconducting qubits. \n \nIn this thesis, we present the design and the characterization of a three-Josephson junction superconducting circuit with three large shunt capacitors. We present experiments on decoherence in the circuit used as a qubit. The qubit relaxation time T₁ is reaching as high as 47 μs and the spin-echo dephasing time T₂ = 9.4 μs. In addition, we present spectroscopy experiments and decoherence characterization in the qutrit subspace formed by the lowest three energy levels. The spectroscopy data is in excellent agreement with the complete circuit model based on the system capacitance matrix. At the flux symmetry point, the circuit has a large anharmonicity, defined as the difference between the 1-2 and 0-1 transition frequencies, approaching 2π × 3.7 GHz, which is enabling fast single qubit operations. We performed randomized benchmarking with the qubit gates with a duration of 1.62 ns and an average gate fidelity of 99.9%. \n \nMoreover, we present experiments with multi-level control in qutrit subspace, formed by the lowest three energy levels. We demonstrate a single step experimental implementation of a generalized Walsh-Hadamard gate. We use a decomposition of the quantum gate into two unitary operations, one implemented by an off-diagonal Hamiltonian and the other implemented by a diagonal Hamiltonian. The off-diagonal Hamiltonian is obtained by the simultaneous driving of the transitions between levels 0-1, 1-2, and 0-2, with the latter being a two-photon process. The diagonal Hamiltonian is effectively implemented by appropriately shifting the phases of adjacent pulses. The gate is characterized using tomography process and the average fidelity exceeds 90%, in good agreement with numerical simulations that take into account the multi-level structure of the system.
We developed transmon qubits based on epitaxial tunnel junctions and interdigitated capacitors. This multileveled qubit, patterned by use of all-optical lithography, is a step towards scalable qubits with a high integration density. The relaxation time T1 is 0.72−0.86 μs and the ensemble dephasing time T2* is slightly larger than T1. The dephasing time T2 (1.36 μs) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker level splitting than observed in qubits with amorphous barriers in equivalent-size junctions. The qubit’s inferred microwave loss closely matches the weighted losses of the individual elements (junction, wiring dielectric, and interdigitated capacitor), determined by independent resonator measurements.
A dc SQUID phase qubit consists of two Josephson junctions in a loop. One junction acts as a qubit with two lowest energy levels forming the |0> and |1> status. The second junction and the loop inductance act to isolate the qubit junction from noise. In this thesis, I report on the improvement of the relaxation time and the coherence time in a dc SQUID phase qubit that used an LC filter. I also report the measurement of anomalous switching curves. In order to improve the relaxation and coherence times, I used two isolation networks, an LC isolation network and an inductive isolation network, to decouple the device from the current bias lines. This produced a very large total effective resistance of the input leads that increases the relaxation time of the qubit. In addition, I connected a low-loss SiNx shunting capacitor across the qubit junction to reduce dielectric losses. I measured two dc SQUID phase qubits. Device DS6 had a 4 (μm)2 Al/AlOx/Al qubit junction with a critical current of 0.5 μA and a 1 pF shunting capacitor. It used an LC filter made from a 10 nH inductor and a 145 pF capacitor. The capacitors contained N-H rich SiNx which produced a loss tangent of about 7×10 -4 . Device DS8 had a 2 (μm)2 Al/AlOx/Al qubit junction with a critical current of 77 nA and an LC filter similar to the first one. The shunting capacitor contained Si-H rich SiNx. Using a pulse readout technique, I measured the characteristics of the qubits, including the transition spectrum, Rabi oscillations, relaxation, Ramsey fringes and state tomography. The best relaxation time T1 for device DS6 was 32 ns and 280 ns for device DS8. The best Rabi decay time T' for DS6 was 42 ns while for device DS8 it was 120 ns. From these and other data I obtained estimates for the best coherence time T2 in device DS6 of 61 ns and 76 ns in device DS8. In DS8, I observed anomalous switching curves; i.e. switching curves which were qualitatively different from conventional switching curves. In the conventional case, the switching curve for the superposition state is the weighted sum of the |0> and |1> curves, but it was not in device DS8. Instead, the switching curve shifted along the current axis as the exited state probability increased. I present a model for understanding the behavior and use this model to extract the probability to be in the excited state.
As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity. Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo> 40 μs) in the presence of silicon posts as near as 350 μm to the qubit.
We have fabricated and tested an Al/AlOx/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area 0.17 μm2and critical current I0,J1= 1.4 μA. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction with area 2 μm2and critical current I0,J2= 29 μA. Additional high-frequency filtering is provided by an on-chip LC filter, which consists of square spiral inductors and parallel plate SiNxcapacitors. From detuned Rabi oscillation measurements, we estimate a relaxation time T1≈ 400 ns and dephasing time TΦ1700 ns. Additionally, we observe that the qubit transition spectrum has multiple equally spaced subpeaks. These subpeaks are caused by coupling between the phase qubit and the LC filter, forming a Jaynes-Cummings system. Each individual subpeak corresponds to transitions with different photon numbers in the LC filter.
Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson junctions. They operate in the single-excitation quantum regime, photons of 27μeV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]Müller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)