Superconducting quantum devices

Abstract:

Quantum computing is considered an emerging technology with promising applications in chemistry, materials, medicine, and cryptography. Superconducting circuits are a leading candidate hardware platform for the realisation of quantum computing, and superconducting devices have now been demonstrated at a scale of hundreds of qubits. Further scale-up faces challenges in wiring, frequency crowding, and the high cost of control electronics. Complementary to increasing the number of qubits, using qutrits (3-level systems) or qudits (d-level systems, d>3) as the basic building block for quantum processors can also increase their computational capability. A commonly used superconducting qubit design, the transmon, has more than two levels. It is a good candidate for a qutrit or qudit processor. Variational quantum algorithms are a type of quantum algorithm that can be implemented on near-term devices. They have been proposed to have a higher tolerance to noise in near-term devices, making them promising for near-term applications of quantum computing. The difference between qubits and qudits makes it non-trivial to translate a variational algorithm designed for qubits onto a qudit quantum processor. The algorithm needs to be either rewritten into a qudit version or an emulator needs to be developed to emulate a qubit processor with a qudit processor.

This thesis describes research on the implementation of variational quantum algorithms, with a particular focus on utilising more than two computational levels of transmons. The work comprises building a two-qubit transmon device and a multi-level transmon device that is used as a qutrit or a qudit (d = 4). We fully benchmarked the two-qubit and the single qudit devices with randomised benchmarking and gate-set tomography, and found good agreement between the two approaches. The qutrit Hadamard gate is reported to have an infidelity of 3.22 ± 0.11 × 10⁻³, which is comparable to state-of-the-art results. We use the qudit to implement a two-qubit emulator and report that the two-qubit Clifford gate randomised benchmarking result on the emulator (infidelity 9.5 ± 0.7 × 10⁻²) is worse than the physical two-qubit (infidelity 4.0 ± 0.3 × 10⁻²) result. We also implemented active reset for the qudit transmon to demonstrate preparing high-fidelity initial states with active feedback. We found the initial state fidelity improved from 0.900 ± 0.011 to 0.9932 ± 0.0013 from gate set tomography.

We finally utilised the single qudit device to implement quantum algorithms. First, a single qutrit classifier for the iris dataset was implemented. We report a successful demonstration of the iris classifier, which yields the training accuracy of the qutrit classifier as 0.96 ± 0.03 and the testing accuracy as 0.94 ± 0.04 among multiple trials. Second, we implemented a two-qubit emulator with a 4-level qudit and used the emulator to demonstrate a variational quantum eigensolver for hydrogen molecules. The solved energy versus the hydrogen bond distance is within 1.5 × 10⁻² Hartree, below the chemical accuracy threshold.

From the characterisation, benchmarking results, and successful demonstration of two quantum algorithms, we conclude that higher levels of a transmon can be used to increase the size of the Hilbert space used for quantum computation with minimal extra cost.

Abstract:

One major challenge that arises from quantum computing is to implement fast, high-accuracy quantum state readout. For superconducting circuits, this problem reduces to a time series classification problem on readout signals. We propose that using path signature methods to extract features can enhance existing techniques for quantum state discrimination. We demonstrate the superior performance of our proposed approach over conventional methods in distinguishing three different quantum states on real experimental data from a superconducting transmon qubit.

Abstract:

Superconducting circuits are a leading platform for quantum information processing, partly due to the great freedom of tailoring circuit parameters which enables the implementation of a wide variety of Hamiltonians. Many simple superconducting circuits commonly employed as qubits contain a variation of superconducting quantum interference device (SQUID) that adds in-situ magnetic flux tunability to the system and further increases its flexibility. At the same time, this control parameter requires additional dedicated circuitry and can introduce flux noise which is detrimental to qubit performance. This increases hardware complexity and hinders the scaling to large qubit numbers. In this thesis, we develop and test a simple 3D-integrated architecture for individual flux control of tileable, coaxial, gradiometric superconducting qubits, achieving highly selective flux bias (low crosstalk) and incorporating both charge and flux control into a single off-chip element. The addition of flux tunability fully retains the simplicity of the fabrication and packaging process of the original, fixed-frequency coaxial architecture. We then use this experimental platform to study the inductively shunted transmon (IST), an interesting qubit species based on a radio-frequency (RF) SQUID loop with a small linear shunting inductance. Its qualitative behavior is similar to a transmon qubit but with a positive instead of negative anharmonicity. We design, simulate, fabricate and experimentally characterize gradiometric IST qubits, showing that they can be easily integrated into an existing transmon-based quantum processor architecture. Further, by directly coupling an IST to a transmon via a mutual capacitance, we demonstrate how the opposite signs of anharmonicity can be exploited to effectively reduce the undesired static longitudinal coupling (ZZ interaction) to zero. We also investigate microwave sideband transitions in this two-qubit system and benchmark a controlled-Z (CZ) entangling gate. This work paves the way towards hardware-efficient, crosstalk-suppressed superconducting quantum processors based on multi-species qubit lattices.