Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013
ABSTRACT Proper grounding between different ground planes in coplanar superconducting qubit circu... more ABSTRACT Proper grounding between different ground planes in coplanar superconducting qubit circuits is important to avoid spurious resonances which increase decoherence. Here, the authors present a possible solution to suppress such undesired modes using superconducting aluminum air-bridges which have been fabricated on top of aluminum coplanar waveguide transmission lines. 3D electromagnetic simulations were done to guide the design of the air-bridges such that the input reflection (S11) of the bridges was kept at a minimum level. A fabrication method based on optical lithography techniques was developed and it resulted in air-bridges with a height of approximately 10 μm and lengths of up to 500 μm. The method can be generalized to arbitrary length air-bridge with heights even exceeding 15 μm.
The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctio... more The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. An amorphous layer with a thickness of ∼5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. The observations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions.
ABSTRACT We demonstrate a Josephson parametric amplifier, based on a coplanar waveguide quarter-w... more ABSTRACT We demonstrate a Josephson parametric amplifier, based on a coplanar waveguide quarter-wavelength resonator, connected to ground by means of a superconducting quantum interference device (SQUID). The fundamental resonant frequency is 1GHz, and we use the higher modes for our measurements. We investigate three different pumping schemes, involving one mode or a pair of modes. The device can be current pumped by applying the pump tone to the input port of the resonator. An on-chip tuning line also allows for magnetic flux pumping. We show that we can approach quantum-limited noise performance while employing the single-mode flux-pumping scheme. We report a gain in excess of 20dB and a saturation power of -133.5dBm. The gain-bandwidth product depends on the employed pumping scheme and was shown to be as high as 27.4MHz.
IEEE Transactions on Applied Superconductivity, 2000
ABSTRACT The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operat... more ABSTRACT The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operating in the microwave regime are described. Using microwave signals in microfabricated superconducting Al waveguides, the concept of an interferometer is transferred from optics to on-chip. Tuning of the path length of one of the interferometer arms is executed through the tunable inductance of a SQUID. By placing one or more SQUIDs in the waveguide structure and by varying the magnetic flux through the SQUID loop, the total SQUID inductance can be tuned. In this way, a phase difference leading to destructive or constructive interference at the interferometer output can be achieved. Thorough software simulations were performed to determine the different design parameters, assign a desired working frequency and provide a reference for comparison with experimental results. Measurements at 300 mK show an effective working frequency close to the simulations, with a deviation smaller than 0.05 GHz. The behavior of the interferometer is very similar to the simulations as well. The on-off ratio exceeds 40 dB.
We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupl... more We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupling it with a Josephson parametric oscillator. We employ a tunable quarter wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185±15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot read-out performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013
ABSTRACT Proper grounding between different ground planes in coplanar superconducting qubit circu... more ABSTRACT Proper grounding between different ground planes in coplanar superconducting qubit circuits is important to avoid spurious resonances which increase decoherence. Here, the authors present a possible solution to suppress such undesired modes using superconducting aluminum air-bridges which have been fabricated on top of aluminum coplanar waveguide transmission lines. 3D electromagnetic simulations were done to guide the design of the air-bridges such that the input reflection (S11) of the bridges was kept at a minimum level. A fabrication method based on optical lithography techniques was developed and it resulted in air-bridges with a height of approximately 10 μm and lengths of up to 500 μm. The method can be generalized to arbitrary length air-bridge with heights even exceeding 15 μm.
The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctio... more The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. An amorphous layer with a thickness of ∼5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. The observations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions.
ABSTRACT We demonstrate a Josephson parametric amplifier, based on a coplanar waveguide quarter-w... more ABSTRACT We demonstrate a Josephson parametric amplifier, based on a coplanar waveguide quarter-wavelength resonator, connected to ground by means of a superconducting quantum interference device (SQUID). The fundamental resonant frequency is 1GHz, and we use the higher modes for our measurements. We investigate three different pumping schemes, involving one mode or a pair of modes. The device can be current pumped by applying the pump tone to the input port of the resonator. An on-chip tuning line also allows for magnetic flux pumping. We show that we can approach quantum-limited noise performance while employing the single-mode flux-pumping scheme. We report a gain in excess of 20dB and a saturation power of -133.5dBm. The gain-bandwidth product depends on the employed pumping scheme and was shown to be as high as 27.4MHz.
IEEE Transactions on Applied Superconductivity, 2000
ABSTRACT The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operat... more ABSTRACT The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operating in the microwave regime are described. Using microwave signals in microfabricated superconducting Al waveguides, the concept of an interferometer is transferred from optics to on-chip. Tuning of the path length of one of the interferometer arms is executed through the tunable inductance of a SQUID. By placing one or more SQUIDs in the waveguide structure and by varying the magnetic flux through the SQUID loop, the total SQUID inductance can be tuned. In this way, a phase difference leading to destructive or constructive interference at the interferometer output can be achieved. Thorough software simulations were performed to determine the different design parameters, assign a desired working frequency and provide a reference for comparison with experimental results. Measurements at 300 mK show an effective working frequency close to the simulations, with a deviation smaller than 0.05 GHz. The behavior of the interferometer is very similar to the simulations as well. The on-off ratio exceeds 40 dB.
We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupl... more We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupling it with a Josephson parametric oscillator. We employ a tunable quarter wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185±15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot read-out performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing.
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