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
11
Apr
2023
Hamiltonian Switching Control of Noisy Bipartite Qubit Systems
We develop a Hamiltonian switching ansatz for bipartite control that is inspired by the Quantum Approximate Optimization Algorithm (QAOA), to mitigate environmental noise on qubits.
We illustrate the approach with application to the protection of quantum gates performed on i) a central spin qubit coupling to bath spins through isotropic Heisenberg interactions, ii) superconducting transmon qubits coupling to environmental two-level-systems (TLS) through dipole-dipole interactions, and iii) qubits coupled to both TLS and a Lindblad bath. The control field is classical and acts only on the system qubits. We use reinforcement learning with policy gradient (PG) to optimize the Hamiltonian switching control protocols, using a fidelity objective defined with respect to specific target quantum gates. We use this approach to demonstrate effective suppression of both coherent and dissipative noise, with numerical studies achieving target gate implementations with fidelities over 0.9999 (four nines) in the majority of our test cases and showing improvement beyond this to values of 0.999999999 (nine nines) upon a subsequent optimization by Gradient Ascent Pulse Engineering (GRAPE). We analyze how the control depth, total evolution time, number of environmental TLS, and choice of optimization method affect the fidelity achieved by the optimal protocols and reveal some critical behaviors of bipartite control of quantum gates.
04
Apr
2023
A GKP qubit protected by dissipation in a high-impedance superconducting circuit driven by a microwave frequency comb
We propose a novel approach to generate, protect and control GKP qubits. It employs a microwave frequency comb parametrically modulating a Josephson circuit to enforce a dissipative
dynamics of a high impedance circuit mode, autonomously stabilizing the finite-energy GKP code. The encoded GKP qubit is robustly protected against all dominant decoherence channels plaguing superconducting circuits but quasi-particle poisoning. In particular, noise from ancillary modes leveraged for dissipation engineering does not propagate at the logical level. In a state-of-the-art experimental setup, we estimate that the encoded qubit lifetime could extend two orders of magnitude beyond the break-even point, with substantial margin for improvement through progress in fabrication and control electronics. Qubit initialization, readout and control via Clifford gates can be performed while maintaining the code stabilization, paving the way toward the assembly of GKP qubits in a fault-tolerant quantum computing architecture.
Quantum heat diode versus light emission in circuit quantum electrodynamical system
Precisely controlling heat transfer in a quantum mechanical system is particularly significant for designing quantum thermodynamical devices. With the technology of experiment advances,
circuit quantum electrodynamics (circuit QED) has become a promising system due to controllable light matter interactions as well as flexible coupling strengths. In this paper, we design a thermal diode in terms of the two-photon Rabi model of the circuit QED system. We find that the thermal diode can not only be realized in the resonant coupling but also achieve better performance, especially for the detuned qubit-photon ultrastrong coupling. We also study the photonic detection rates and their nonreciprocity, which indicates similar behaviors with the nonreciprocal heat transport. This provides the potential to understand thermal diode behavior from the quantum optical perspective and could shed new insight into the relevant research on thermodynamical devices.
03
Apr
2023
Optimizing resetting of superconducting qubits
Many quantum algorithms demand a large number of repetitions to obtain reliable statistical results. Thus, at each repetition it is necessary to reset the qubits efficiently and precisely
in the shortest possible time, so that quantum computers actually have advantages over classical ones. In this work, we perform a detailed analysis on three different models for information resetting in superconducting qubits. Our experimental setup consists of a main qubit coupled to different auxiliary dissipative systems, that are employed in order to perform the erasing of the information of the main qubit. Our analysis shows that it is not enough to increase the coupling and the dissipation rate associated with the auxiliary systems to decrease the resetting time of the main qubit, a fact that motivates us to find the optimal set of parameters for each studied approach, allowing a significant decrease in the reset time of the three models analyzed.
Microwave quantum diode
The fragile nature of quantum circuits is a major bottleneck to scalable quantum applications. Operating at cryogenic temperatures, quantum circuits are highly vulnerable to amplifier
backaction and external noise. Non-reciprocal microwave devices such as circulators and isolators are used for this purpose. These devices have a considerable footprint in cryostats, limiting the scalability of quantum circuits. We present a compact microwave diode architecture, which exploits the non-linearity of a superconducting flux qubit. At the qubit degeneracy point we experimentally demonstrate a significant difference between the power levels transmitted in opposite directions. The observations align with the proposed theoretical model. At -99 dBm input power, and near the qubit-resonator avoided crossing region, we report the transmission rectification ratio exceeding 90% for a 50 MHz wide frequency range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67 GHz to 6.91 GHz. The presented architecture is compact, and easily scalable towards multiple readout channels, potentially opening up diverse opportunities in quantum information, microwave read-out and optomechanics.
31
Mä
2023
Using inductive Energy Participation Ratio for Superconducting Quantum Chip Characterization
We have developed an inductive energy participation ratio (iEPR) method and a concise procedure for superconducting quantum chip layout simulation and verification that is increasingly
indispensable in large-scale, fault-tolerant quantum computing. It can be utilized to extract the characteristic parameters and the bare Hamiltonian of the layout in an efficient way. In theory, iEPR sheds light on the deep-seated relationship between energy distribution and representation transformation. As a stirring application, we apply it to a typical quantum chip layout, obtaining all the crucial characteristic parameters in one step that would be extremely challenging through the existing methods. Our work is expected to significantly improve the simulation and verification techniques and takes an essential step toward quantum electronic design automation.
30
Mä
2023
Non-classical microwave-optical photon pair generation with a chip-scale transducer
Modern computing and communication technologies such as supercomputers and the internet are based on optically connected networks of microwave frequency information processors. In recent
years, an analogous architecture has emerged for quantum networks with optically distributed entanglement between remote superconducting quantum processors, a leading platform for quantum computing. Here we report an important milestone towards such networks by observing non-classical correlations between photons in an optical link and a superconducting electrical circuit. We generate such states of light through a spontaneous parametric down-conversion (SPDC) process in a chip-scale piezo-optomechanical transducer. The non-classical nature of the emitted light is verified by observing anti-bunching in the microwave state conditioned on detection of an optical photon. Such a transducer can be readily connected to a superconducting quantum processor, and serve as a key building block for optical quantum networks of microwave frequency qubits.
Semiclassical dynamics of a superconducting circuit: chaotic dynamics and fractal attractors
In this article, we study the semiclassical dynamics of a superconducting circuit constituted by two Josephson junctions in series, in the presence of a voltage bias. We show that the
equations of motion describing the superconducting phase correspond to those controlling the dynamics of a planar rotor with an oscillating pivot and, consequently, to those of a Kapitza pendulum in the absence of gravity. In addition, we show that the system exhibits a rich dynamical behavior with chaotic properties and provide insight into its attractor’s fractal nature.
Highly tunable NbTiN Josephson junctions fabricated with focused helium ion beam
We demonstrate a „direct writing“ method for the fabrication of planar Josephson junctions from high quality superconducting niobium titanium nitride (NbTiN) thin films
using focused He-ion beam irradiation. Compared to the materials previously used in such processing, YBCO and MgB2, NbTiN has much better mechanical and electrical properties, as well as good corrosion resistance. We show that we can control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, and that this controllable critical temperature suppression combined with the high spatial resolution and position control of the He-ion beam in a helium ion microscope enables us to successfully fabricate Josephson junctions with highly tunable weak links. Because of the continuous nature of the disorder-induced metal-insulator transition, this method allows the creation of barriers with wide range of resistivities ranging from the metallic to the insulating state, with the critical current and the junction resistance varying over two orders of magnitude. Electrical transport measurements show that junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages (0.2−1.4 mV) and show Shapiro steps up to very high orders. This suggests that these type of junctions are suitable for a wide range of applications in superconducting electronics and quantum information technology, with the bonus that a whole device can be fabricated from just a single thin film, with the excellent electrical and microwave characteristics offered by NbTiN.
29
Mä
2023
Control of the ZZ coupling between Kerr-cat qubits via transmon couplers
Kerr-cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of errors. The ZZ coupling between the qubits can be utilized for a two-qubit
entangling gate, but the residual coupling causes unnecessary always-on gates and crosstalk. In order to resolve this problem, we propose a tunable ZZ-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual ZZ couplings via the two couplers cancel each other out. We also apply our scheme to the Rzz(Θ) gate (ZZ rotation with angle Θ), one of the two-qubit entangling gates. We numerically show that the fidelity of the Rzz(−π/2) gate is higher than 99.9% in a case of 16 ns gate time and without decoherence.