I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
26
Jan
2024
Superconducting flux qubit operating at zero magnetic field
The operation of a conventional superconducting flux qubit requires the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the
qubit loop, which makes the scaling of quantum circuits based on this type of qubits difficult. It has been proposed that, by inducing a pi phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as pi-junction, it is possible to realize a flux qubit operating at zero magnetic flux. We report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic pi-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero-flux-biased flux qubits can become a promising platform for quantum computing.
24
Jan
2024
Entanglement assisted probe of the non-Markovian to Markovian transition in open quantum system dynamics
We utilize a superconducting qubit processor to experimentally probe the transition from non-Markovian to Markovian dynamics of an entangled qubit pair. We prepare an entangled state
between two qubits and monitor the evolution of entanglement over time as one of the qubits interacts with a small quantum environment consisting of an auxiliary transmon qubit coupled to its readout cavity. We observe the collapse and revival of the entanglement as a signature of quantum memory effects in the environment. We then engineer the non-Markovianity of the environment by populating its readout cavity with thermal photons to show a transition from non-Markovian to Markovian dynamics, reaching a regime where the quantum Zeno effect creates a decoherence-free subspace that effectively stabilizes the entanglement between the qubits.
22
Jan
2024
Observation of discrete charge states of a coherent two-level system in a superconducting qubit
We report observations of discrete charge states of a coherent dielectric two-level system (TLS) that is strongly coupled to an offset-charge-sensitive superconducting transmon qubit.
We measure an offset charge of 0.072e associated with the two TLS eigenstates, which have a transition frequency of 2.9 GHz and a relaxation time exceeding 3 ms. Combining measurements in the strong dispersive and resonant regime, we quantify both transverse and longitudinal couplings of the TLS-qubit interaction. We further perform joint tracking of TLS transitions and quasiparticle tunneling dynamics but find no intrinsic correlations. This study demonstrates microwave-frequency TLS as a source of low-frequency charge noise.
Geometric Phase of a Transmon in a Dissipative Quantum Circuit
Superconducting circuits reveal themselves as promising physical devices with multiple uses. Within those uses, the fundamental concept of the geometric phase accumulated by the state
of a system shows up recurrently, as, for example, in the construction of geometric gates. Given this framework, we study the geometric phases acquired by a paradigmatic setup: a transmon coupled to a superconductor resonating cavity. We do so both for the case in which the evolution is unitary and when it is subjected to dissipative effects. These models offer a comprehensive quantum description of an anharmonic system interacting with a single mode of the electromagnetic field within a perfect or dissipative cavity, respectively. In the dissipative model, the non-unitary effects arise from dephasing, relaxation, and decay of the transmon coupled to its environment. Our approach enables a comparison of the geometric phases obtained in these models, leading to a thorough understanding of the corrections introduced by the presence of the environment.
19
Jan
2024
Spectral signatures of non-trivial topology in a superconducting circuit
Topology, like symmetry, is a fundamental concept in understanding general properties of physical systems. In condensed matter systems, non-trivial topology may manifest itself as singular
features in the energy spectrum or the quantization of observable quantities such as electrical conductance and magnetic flux. Using microwave spectroscopy, we show that a superconducting circuit with three Josephson tunnel junctions in parallel can possess energy degeneracies indicative of \emph{intrinsic} non-trivial topology. We identify three topological invariants, one of which is related to a hidden quantum mechanical supersymmetry. Depending on fabrication parameters, devices are gapless or not, and fall on a simple phase diagram which is shown to be robust to perturbations including junction imperfections, asymmetry, and inductance. Josephson tunnel junction circuits, which are readily fabricated with conventional microlithography techniques, allow access to a wide range of topological systems which have no condensed matter analog. Notable spectral features of these circuits, such as degeneracies and flat bands, may be leveraged for quantum information applications, whereas quantized transport properties could be useful for metrology applications.
17
Jan
2024
Application of the Schwinger Oscillator Construct of Angular Momentum to an Interpretation of the Superconducting Transmon Qubit
The Schwinger oscillator construct of angular momentum, applied to the superconducting transmon and its transmission-line readout, modeled as capacitvely coupled quantum oscillators,
provides a natural and robust description of a qubit. The construct defines quantum-entangled, two-photon states that form an angular-momentum-like basis, with symmetry corresponding to physical conservation of total photon number, with respect to the combined transmon and readout. This basis provides a convenient starting point from which to study error-inducing effects of transmon anharmonicity, surrounding-environment decoherence, and random stray fields on qubit state and gate operations. Employing a Lindblad master equation to model dissipation to the surrounding environment, and incorporating the effect of weak transmon anharmonicity, we present examples of the utility of the construct. First, we calculate the frequency response associated with exciting the ground state to a Rabi resonance with the lowest-lying spin-1/2 moment, via a driving external voltage. Second, we calculate the frequency response between the three lowest two-photon states, within a ladder-type excitation scheme. The generality of the Schwinger angular-momentum construct allows it to be applied to other superconducting charge qubits.
16
Jan
2024
Flux-charge symmetric theory of superconducting circuits
The quantum mechanics of superconducting circuits is derived by starting from a classical Hamiltonian dynamical system describing a dissipationless circuit, usually made of capacitive
and inductive elements. However, standard approaches to circuit quantization treat fluxes and charges, which end up as the canonically conjugate degrees of freedom on phase space, asymmetrically. By combining intuition from topological graph theory with a recent symplectic geometry approach to circuit quantization, we present a theory of circuit quantization that treats charges and fluxes on a manifestly symmetric footing. For planar circuits, known circuit dualities are a natural canonical transformation on the classical phase space. We discuss the extent to which such circuit dualities generalize to non-planar circuits.
The Floquet Fluxonium Molecule: Driving Down Dephasing in Coupled Superconducting Qubits
High-coherence qubits, which can store and manipulate quantum states for long times with low error rates, are necessary building blocks for quantum computers. We propose a superconducting
qubit architecture that uses a Floquet flux drive to modify the spectrum of a static fluxonium molecule. The computational eigenstates have two key properties: disjoint support to minimize bit flips, along with first- and second-order insensitivity to flux noise dephasing. The rates of the three main error types are estimated through numerical simulations, with predicted coherence times of approximately 50 ms in the computational subspace and erasure lifetimes of about 500 μs. We give a protocol for high-fidelity single qubit rotation gates via additional flux modulation on timescales of roughly 500 ns. Our results indicate that driven qubits are able to outperform some of their static counterparts.
15
Jan
2024
Alternating Bias Assisted Annealing of Amorphous Oxide Tunnel Junctions
We demonstrate a transformational technique for controllably tuning the electrical properties of fabricated thermally oxidized amorphous aluminum-oxide tunnel junctions. Using conventional
test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature-dependent, and is independent of junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of ≈2×10−6, along with a significant reduction in resonant- and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, as well as the many other metal-insulator-metal structures used in modern electronics.
14
Jan
2024
Quantum information processing with superconducting circuits: realizing and characterizing quantum gates and algorithms in open quantum systems
This thesis focuses on quantum information processing using the superconducting device, especially, on realizing quantum gates and algorithms in open quantum systems. Such a device
is constructed by transmon-type superconducting qubits coupled to a superconducting resonator. For the realization of quantum gates and algorithms, a one-step approach is used. We suggest faster and more efficient schemes for realizing X-rotation and entangling gates for two and three qubits. During these operations, the resonator photon number is canceled owing to the strong microwave field added. They do not require the resonator to be initially prepared in the vacuum state and the scheme is insensitive to resonator decay. Furthermore, the robustness of these operations is demonstrated by including the effect of the decoherence of transmon systems and the resonator decay in a master equation, and as a result, high fidelity will be achieved in quantum simulation. In addition, using the implemented x-rotation gates as well as the phase gates, we present an alternative way for implementing Grover’s algorithm for two and three qubits, which does not require a series of single gates. As well, we also demonstrate by a numerical simulation the use of quantum process tomography to fully characterize the performance of a single-shot entangling gate for two and three qubits and obtain process fidelities greater than 0.93. These gates are used to create Bell and Greenberger-Horne-Zeilinger (GHZ) entangled states.