Qubits

Summary

The Qubits Project develops new geometries for superconducting quantum binary devices as fundamental building blocks for quantum computing.

Description

We have been developing superconducting devices using novel approaches to making tunnel barriers. Traditionally, tunnel junctions use amorphous, thermal oxide barriers because their thickness is controllable and they tend to be pin-hole free. However, they have energy-absorbing defects at random frequencies due to their amorphous nature. We developed a process to grow epitaxial tunnel junctions. This is challenging because the tunnel current depends exponentially on the barrier thickness, and epitaxial barriers require high temperatures to crystallize. We worked around these challenges using precision, ultra-high vacuum molecular-beam epitaxy (MBE) growth and lattice-matched, superconducting rhenium films on sapphire substrates. We are able to fabricate, characterize, and successfully integrate them into devices, the performance of which devices have improved significantly. We recently developed a new type of tunnel barrier: the overlap tunnel junction. It is superior to junctions made by shadow evaporation because it is more scalable to smaller dimensions. We are making readout resonators using Nb-Ti-N and Ti-N on Si.

Major Accomplishments

High Coherence Plane Breaking Packaging for Superconducting Qubits, N. T. Bronn, V. P. Adiga, S. B. Olivadese, X. Wu, J. M. Chow, D. P. Pappas
We demonstrate a pogo pin package for a superconducting quantum processor specifically designed with a nontrivial layout topology (e.g., a center qubit that cannot be accessed from the sides of the chip). Two experiments on two nominally identical superconducting quantum processors in pogo packages, which use commercially available parts and require modest machining tolerances, are performed at low temperature (10 mK) in a dilution refrigerator and both found to behave typical as standard planar packages with wirebonds where control and readout signals come in from the edges. Single- and two-qubit gate errors are also characterized, and occur at roughly the same rates as in standard packaging. More detailed crosstalk measurements indicate levels of crosstalk less than -40 dB at the qubit frequencies, opening the possibility of integration with extensible qubit architectures.

Pogo Package Assembly. (a) The base/pedestal and boss extruder attached with the alignment dowels. (b) The spacer board holds the quantum processor in place. (c) A wirebonded qubit device. (d) The interposer is attached and aligned by the boss extruder (one can observe the capture pads on the quantum processor through the large holes). (e) Dielectric plugs pushed into holes and resting on the qubit device. (f) Trimmed dielectric plugs populated with pogo pins.

Overlap Junctions for High Coherence Superconducting Qubits, X. Wu, J. L. Long, H. S. Ku, R. E. Lake, M. Bal, D. P. Pappas
Fabrication of sub-micron Josephson junctions is demonstrated using standard processing techniques for high-coherence, superconducting qubits. These junctions are made in two separate lithography steps with normal-angle evaporation. Most significantly, this work demonstrates that it is possible to achieve high coherence with junctions formed on aluminum surfaces cleaned in situ with Ar milling before the junction oxidation. This method eliminates the angle-dependent shadow masks typically used for small junctions. Therefore, this is conducive to the implementation of typical methods for improving margins and yield using conventional CMOS processing. The current method uses electron-beam lithography and an additive process to define the top and bottom electrodes. Extension of this work to optical lithography and subtractive processes is discussed.

Fabrication process flow of overlap junction. (a) 3D view of the device after the first evaporation of Al (bottom electrode). (b) After the metal liftoff, native oxide immediately forms on Al. (c) The second lithography defines the pattern of top electrode. Ar RF plasma cleaning is performed to remove native oxide on the surface of bottom electrode. (d) Low pressure, room temperature oxidation is used to form tunnel barrier. (e) Second evaporation of Al to complete the tunnel junction. (f) Completed overlap junction after the metal-liftoff process.

Circuit schematic and micrographs of the qubit device. (a) Circuit schematic showing the concentric tranmon qubit, coupled to a single resonator. (b) Optical and SEM images of our transmon qubit device. The resonator is realized in microstrip geometry with a measured frequency 6.48 GHz and line width 1.1 MHz.

Measured qubit coherence times. (a) Excited state probability as a function of measurement delay. (b) Excited state probability as a function of time between two microwave pulses.

Electromagnetically Induced Transparency in Circuit QED with Nested Polariton States, J. Long, H. S. Ku, X. Wu, X. Gu, R. E. Lake, M. Bal, Y.-X. Liu, D. P. Pappas
Electromagnetically induced transparency (EIT) is a signature of quantum interference in an atomic three-level system. By driving the dressed cavity-qubit states of a two-dimensional circuit QED system, we generate a set of polariton states in the nesting regime. The lowest three energy levels are utilized to form the Λ-type system. EIT is observed and verified by Akaike’s information criterion based testing. Negative group velocities up to −0.52 ± 0.09 km/s are obtained based on the dispersion relation in the EIT transmission spectrum.

Transmission magnitude (a) and phase (b) of electromagnetically induced transparency with varying coupling strength. (c) and (d) are line cuts on (a) and (b) with Ω_c/2π = 0.04 MHz, 0.2 MHz, 0.4 MHz, and 0.82 MHz, respectively. The black dash-dotted lines on (a) and (b) denote the boundary of EIT set by Ω_c/2π = γ_c/2π = 0.82 MHz.

Qubit Gates Using Hyperbolic Secant Pulses, H. S. Ku, J. L. Long, X. Wu, M. Bal, R. E. Lake, E. Barnes, S. E. Economou, D. P. Pappas
It has been known since the early days of quantum mechanics that hyperbolic secant pulses possess the unique property that they can perform cyclic evolution on two-level quantum systems independently of the pulse detuning. More recently, it was realized that they induce detuning-controlled phases without changing state populations. Here, we experimentally demonstrate the properties of hyperbolic secant pulses on superconducting transmon qubits and contrast them with the more commonly used Gaussian and square waves. We further show that these properties can be exploited to implement phase gates, nominally without exiting the computational subspace. This enables us to demonstrate the first microwave-driven Z-gates with a single control parameter, the detuning.

The excited state probability as a function of pulse amplitude (vertical axis) and detuning (horizontal) for (a) sech, (b) Gaussian, and (c) square pulses. The left and right panels compare experimental and theoretical simulations.

Superconducting Qubit Development and Spectroscopy
Superconducting qubits and their applications is one of our main focuses. In this area, we have developed high quality factor materials and radiation suppression techniques that allow for reliable T1 times on the order of 30 – 40 us. We have developed measurement infrastructure of large arrays of qubit and are currently testing adiabatic state transfer, dark state transfer, and novel gates. These devices are designed using combinations of readout-resonating circuits, capacitors, inductors, and superconducting Josephson junctions. The interactions between these elements is important because it allows us to extract the quantum information from the qubits. Below is an image of the susceptibility of a qubit that is strongly coupled to a resonant cavity. From these spectra we were able to completely describe the system in terms of it's total quantum state, successfully predicting the transition frequencies. For example the energy difference associated with the transitions 4-7, 1-3, 2-4, and 0-1 are indicated in the figure.

Radiation-suppressed superconducting quantum bit in a planar geometry, M. Sandberg, M. R. Vissers, T. A. Ohki, Jiansong Gao, J. Aumentado, M. Weides, D. P. Pappas, Appl. Phys. Lett. 102, 072601 (2013).

Detailed modelling of the susceptibility of a thermally populated, strongly driven circuit-QED system, A. F. Kockum, M. Sandberg, M. R Vissers, J. Gao, G. Johansson, D. P Pappas, J. Phys. B: At. Mol. Opt. Phys. 46, 224014 (2013), article featured on the cover.