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Register a new userOptical Direct Write of Dolan--Niemeyer-Bridge Junctions for Transmon Qubits
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We characterize highly coherent transmon qubits fabricated with a direct-write photolithography system. Multi-layer evaporation and oxidation allows us to change the critical current density by reducing the effective tunneling area and increasing the barrier thickness. Surface treatments before resist application and again before evaporation result in high coherence devices. With optimized surface treatments we achieve energy relaxation $T_1$ times in excess of $80 mu$s for three dimensional transmon qubits with Josephson junction lithographic areas of 2 $mumathrm{m}^2$.
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Silicon-Germanium (SiGe) is a material that possesses a multitude of applications ranging from transistors to eletro-optical modulators and quantum dots. The diverse properties of SiGe also make it attractive to implementations involving superconducting quantum computing. Here we demonstrate the fabrication of transmon quantum bits on SiGe layers and investigate the microwave loss properties of SiGe at cryogenic temperatures and single photon microwave powers. We find relaxation times of up to 100 $mu$s, corresponding to a quality factor Q above 4 M for large pad transmons. The high Q values obtained indicate that the SiGe/Si heterostructure is compatible with state of the art performance of superconducting quantum circuits.
Superconducting transmon qubits comprise one of the most promising platforms for quantum information processing due to their long coherence times and to their scalability into larger qubit networks. However, their weakly anharmonic spectrum leads to spectral crowding in multiqubit systems, making it challenging to implement fast, high-fidelity gates while avoiding leakage errors. To address this challenge, we use a protocol known as SWIPHT [Phys. Rev. B 91, 161405(R) (2015)], which yields smooth, simple microwave pulses designed to suppress leakage without sacrificing gate speed through spectral selectivity. Here, we determine the parameter regimes in which SWIPHT is effective and demonstrate that in these regimes it systematically produces two-qubit gate fidelities for cavity-coupled transmons in the range 99.6%-99.9% with gate times as fast as 23 ns. Our results are obtained from full numerical simulations that include current experimental levels of relaxation and dephasing. These high fidelities persist over a wide range of system parameters that encompass many current experimental setups and are insensitive to small parameter variations and pulse imperfections.
Qubit information processors are increasing in footprint but currently rely on e-beam lithography for patterning the required Josephson junctions (JJs). Advanced optical lithography is an alternative patterning method, and we report on the development of transmon qubits patterned solely with optical lithography. The lithography uses 193 nm wavelength exposure and 300-mm large silicon wafers. Qubits and arrays of evaluation JJs were patterned with process control which resulted in narrow feature distributions: a standard deviation of 0:78% for a 220 nm linewidth pattern realized across over half the width of the wafers. Room temperature evaluation found a 2.8-3.6% standard deviation in JJ resistance in completed chips. The qubits used aluminum and titanium nitride films on silicon substrates without substantial silicon etching. T1 times of the qubits were extracted at 26 - 27 microseconds, indicating a low level of material-based qubit defects. This study shows that large wafer optical lithography on silicon is adequate for high-quality transmon qubits, and shows a promising path for improving many-qubit processors.
Superconducting circuits are currently developed as a versatile platform for the exploration of many-body physics, both at the analog and digital levels. Their building blocks are often idealized as two-level qubits, drawing powerful analogies to quantum spin models. For a charge qubit that is capacitively coupled to a transmission line, this analogy leads to the celebrated spin-boson description of quantum dissipation. We put here into evidence a failure of the two-level paradigm for realistic superconducting devices, due to electrostatic constraints which limit the maximum strength of dissipation. These prevent the occurence of the spin-boson quantum phase transition for transmons, even up to relatively large non-linearities. A different picture for the many-body ground state describing strongly dissipative transmons is proposed, showing unusual zero point fluctuations.
Non-equilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits. Understanding the energy distribution of these quasiparticles will yield insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence. To probe this energy distribution, we systematically correlate qubit relaxation and excitation with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples. By itself, the observed quasiparticle distribution would limit $T_1$ to $approx200~mumathrm{s}$, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms. Furthermore, the measured rate of quasiparticle-induced excitation events is greater than that of relaxation events, which signifies that the quasiparticles are more energetic than would be predicted from a thermal distribution describing their apparent density.
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