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
16
Jan
2024
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.
09
Jan
2024
Long-lived topological time-crystalline order on a quantum processor
Topologically ordered phases of matter elude Landau’s symmetry-breaking theory, featuring a variety of intriguing properties such as long-range entanglement and intrinsic robustness
against local perturbations. Their extension to periodically driven systems gives rise to exotic new phenomena that are forbidden in thermal equilibrium. Here, we report the observation of signatures of such a phenomenon — a prethermal topologically ordered time crystal — with programmable superconducting qubits arranged on a square lattice. By periodically driving the superconducting qubits with a surface-code Hamiltonian, we observe discrete time-translation symmetry breaking dynamics that is only manifested in the subharmonic temporal response of nonlocal logical operators. We further connect the observed dynamics to the underlying topological order by measuring a nonzero topological entanglement entropy and studying its subsequent dynamics. Our results demonstrate the potential to explore exotic topologically ordered nonequilibrium phases of matter with noisy intermediate-scale quantum processors.
A parametrically programmable delay line for microwave photons
Delay lines capable of storing quantum information are crucial for advancing quantum repeaters and hardware efficient quantum computers. Traditionally, they are physically realized
as extended systems that support wave propagation, such as waveguides. But such delay lines typically provide limited control over the propagating fields. Here, we introduce a parametrically addressed delay line (PADL) for microwave photons that provides a high level of control over the dynamics of stored pulses, enabling us to arbitrarily delay or even swap pulses. By parametrically driving a three-waving mixing superconducting circuit element that is weakly hybridized with an ensemble of resonators, we engineer a spectral response that simulates that of a physical delay line, while providing fast control over the delay line’s properties and granting access to its internal modes. We illustrate the main features of the PADL, operating on pulses with energies on the order of a single photon, through a series of experiments, which include choosing which photon echo to emit, translating pulses in time, and swapping two pulses. We also measure the noise added to the delay line from our parametric interactions and find that the added noise is much less than one photon.
08
Jan
2024
Design of Fully Integrated 45 nm CMOS System-on-Chip Receiver for Readout of Transmon Qubit
This study unveils a comprehensive design strategy, intricately addressing the realization of transmon qubits, the design of Josephson parametric amplifiers, and the development of
an innovative fully integrated receiver dedicated to sensing ultra-low-level quantum signals. Quantum theory takes center stage, leveraging the Lindblad master and quantum Langevin equations to design the transmon qubit and Josephson parametric amplifier as open quantum systems. The mentioned quantum devices engineering integrated with the design of a fully integrated 45 nm CMOS system-on-chip receiver, weaves together a nuanced tapestry of quantum and classical elements. On one hand, for the transmon qubit and parametric amplifier operating at 10 mK, critical quantum metrics including entanglement, Stoke projector probabilities, and parametric amplifier gain are calculated. On the other hand, the resulting receiver is a symphony of high-performance elements, featuring a wide-band low-noise amplifier with a 0.8 dB noise figure and ~37 dB gains, a sweepable 5.0 GHz sinusoidal wave generator via the voltage-controlled oscillator, and a purpose-designed mixer achieving C-band to zero-IF conversion. Intermediate frequency amplifier, with a flat gain of around 26 dB, and their low-pass filters, generate a pure sinusoidal wave at zero-IF, ready for subsequent processing at room temperature. This design achieves an impressive balance, with low power consumption (~122 mW), a noise figure of ~0.9 dB, high gain (~130 dB), a wide bandwidth of 3.6 GHz, and compact dimensions (0.54*0.4 mm^2). The fully integrated receiver capability to read out at least 90 qubits positions this design for potential applications in quantum computing. Validation through post-simulations at room temperature underscores the promising and innovative nature of this design.
07
Jan
2024
Tantalum airbridges for scalable superconducting quantum processors
The unique property of tantalum (Ta), particularly its long coherent lifetime in superconducting qubits and its exceptional resistance to both acid and alkali, makes it promising for
superconducting quantum processors. It is a notable advantage to achieve high-performance quantum processors with neat and unified fabrication of all circuit elements, including coplanar waveguides (CPW), qubits, and airbridges, on the tantalum film-based platform. Here, we propose a reliable tantalum airbridges with separate or fully-capped structure fabricated via a novel lift-off method, where a barrier layer with aluminium (Al) film is first introduced to separate two layers of photoresist and then etched away before the deposition of tantalum film, followed by cleaning with piranha solution to remove the residual photoresist on the chip. We characterize such tantalum airbridges as the control line jumpers, the ground plane crossovers and even coupling elements. They exhibit excellent connectivity, minimal capacitive loss, effectively suppress microwave and flux crosstalk and offer high freedom of coupling. Besides, by presenting a surface-13 tunable coupling superconducting quantum processor with median T1 reaching above 100 μs, the overall adaptability of tantalum airbridges is verified. The median single-qubit gate fidelity shows a tiny decrease from about 99.95% for the isolated Randomized Benchmarking to 99.94% for the simultaneous one. This fabrication method, compatible with all known superconducting materials, requires mild conditions of film deposition compared with the commonly used etching and grayscale lithography. Meanwhile, the experimental achievement of non-local coupling with controlled-Z (CZ) gate fidelity exceeding 99.2% may further facilitate qLDPC codes, laying a foundation for scalable quantum computation and quantum error correction with entirely tantalum elements.
05
Jan
2024
Analytical Quantum Full-Wave Solutions for a 3D Circuit Quantum Electrodynamics System
High-fidelity general-purpose numerical methods are increasingly needed to improve superconducting circuit quantum information processor performance. One challenge in developing such
numerical methods is the lack of reference data to validate them. To address this, we have designed a 3D system where all electromagnetic properties needed in a quantum analysis can be evaluated using analytical techniques from classical electromagnetic theory. Here, we review the basics of our field-based quantization method and then use these techniques to create the first-ever analytical quantum full-wave solution for a superconducting circuit quantum device. Specifically, we analyze a coaxial-fed 3D waveguide cavity with and without transmon quantum bits inside the cavity. We validate our analytical solutions by comparing them to numerical methods in evaluating single photon interference and computing key system parameters related to controlling quantum bits. In the future, our analytical solutions can be used to validate numerical methods, as well as build intuition about important quantum effects in realistic 3D devices.
04
Jan
2024
Measurement-induced bistability in the excited states of a transmon
High power measurement-induced cavity response is investigated in the |g>, |e>, and |f> states of a transmon. All the states exhibit photon blockades above a certain critical value,
a phenomenon that has previously been understood based on the bistability of semiclassical Duffing oscillators. The measurement-induced state transition (MIST) to high-level transmon states is expected to be one contributor to the bistability; however, the critical values measured in the |e> and |f> states are not coincident with the MIST. To understand this discrepancy, we utilize the recently developed semiclassical dynamics model of a cavity photon state. The appearance of dim and bright cavity states obtained from the model’s steady-state solution leads to the photon blockades at lower critical photon numbers, and this can explain the response of the bistable region in the |e> and |f> states.
One-step implementation of nonadiabatic holonomic fSim gate in superconducting circuits
Due to its significant application in reducing algorithm depth, fSim gates have attracted a lot of attention, while one-step implementation of fSim gates remains an unresolved issue.
In this manuscript, we propose a one-step implementation of holonomic fSim gates in a tunable superconducting circuit based on the three lowest energy levels. Numerical simulations demonstrate the feasibility of our scheme. This scheme may provide a promising path toward quantum computation and simulation.