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
13
Feb
2022
Fast readout and reset of a superconducting qubit coupled to a resonator with an intrinsic Purcell filter
Coupling a resonator to a superconducting qubit enables various operations on the qubit including dispersive readout and unconditional reset. The speed of these operations is limited
by the external decay rate of the resonator. However, increasing the decay rate also increases the rate of qubit decay via the resonator, limiting the qubit lifetime. Here, we demonstrate that the resonator-mediated qubit decay can be suppressed by utilizing the distributed-element, multi-mode nature of the resonator. We show that the suppression exceeds two orders of magnitude over a bandwidth of 600 MHz. We use this „intrinsic Purcell filter“ to demonstrate a 40-ns readout with 99.1% fidelity and a 100-ns reset with residual excitation of less than 1.7%.
11
Feb
2022
Bell’s inequality violation by dynamical Casimir photons in superconducting microwave circuit
We study the Bell’s inequality violation by dynamical Casimir radiation with pseudospin measurement. We consider a circuit quantum electrodynamical set-up where a relativistically
moving mirror is simulated by variable external magnetic flux in a SQUID terminating a superconducting-microwave waveguide. We analytically obtain expectation values of the Bell operator optimized with respect to channel orientations, in terms of the system parameters. We consider the effects of local noise in the microwave field modes, asymmetry between the field modes resulting from nonzero detuning, and signal loss. Our analysis provides ranges of the above experimental parameters for which Bell violation can be observed. We show that Bell violation can be observed in this set-up up to 40 mK temperature as well as up to 65 % signal loss.
09
Feb
2022
Controlled-NOT gates for fluxonium qubits via selective darkening of transitions
We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses
at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-NOT operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below 10−4 is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.
Markovian Modelling and Calibration of IBMQ Transmon Qubits
In the design of quantum devices, it is crucial to account for the interaction between qubits and their environment to understand and improve the coherence and stability of the quantum
states. This is especially prevalent in Noisy Intermediate Scale Quantum (NISQ) devices in which the qubit states quickly decay through processes of relaxation and decoherence. Ideal quantum states evolve according to Markovian dynamics, which modern devices are not always capable of maintaining. NISQ devices, such as those offered by IBM through their cloud-based open-access IBM Quantum Experience (IBM QE) are regularly calibrated to provide the users with data about the qubit dynamics which can be accounted for in the design of experiments on the quantum devices. In this paper we demonstrate a method of verifying the Markovianity of the IBMQ devices while extracting multiple calibration parameters simultaneously through a simplified process which can be modified for more complex modelling of qubit dynamics.
07
Feb
2022
Methods and Results for Quantum Optimal Pulse Control on Superconducting Qubit Systems
The effective use of current Noisy Intermediate-Scale Quantum (NISQ) devices is often limited by the noise which is caused by interaction with the environment and affects the fidelity
of quantum gates. In transmon qubit systems, the quantum gate fidelity can be improved by applying control pulses that can minimize the effects of the environmental noise. In this work, we employ physics-guided quantum optimal control strategies to design optimal pulses driving quantum gates on superconducting qubit systems. We test our results by conducting experiments on the IBM Q hardware using their OpenPulse API. We compare the performance of our pulse-optimized quantum gates against the default quantum gates and show that the optimized pulses improve the fidelity of the quantum gates, in particular the single-qubit gates. We discuss the challenges we encountered in our work and point to possible future improvements.
04
Feb
2022
Observation of wave-packet branching through an engineered conical intersection
In chemical reactions, the interplay between coherent evolution and dissipation is central to determining key properties such as the rate and yield. Of particular interest are cases
where two potential energy surfaces cross at features known as conical intersections (CIs), resulting in nonadiabatic dynamics that may promote ultrafast and highly efficient reactions when rovibrational damping is present. A prominent chemical reaction that involves a CI is the cis-trans isomerization reaction in rhodopsin, which is crucial to vision. CIs in real molecular systems are typically investigated via optical pump-probe spectroscopy, which has demanding spectral bandwidth and temporal resolution requirements, and where precise control of the environment is challenging. A complementary approach for understanding chemical reactions is to use quantum simulators that can provide access to a wider range of observables, though thus far combining strongly interacting linear (rovibrational) and nonlinear (electronic) degrees of freedom with engineered dissipation has yet to be demonstrated. Here, we create a tunable CI in a hybrid qubit-oscillator circuit QED processor and simultaneously track both a reactive wave-packet and electronic qubit in the time-domain. We identify dephasing of the electronic qubit as the mechanism that drives wave-packet branching along the reactive coordinate in our model. Furthermore, we directly observe enhanced branching when the wave-packet passes through the CI. Thus, the forces that influence a chemical reaction can be viewed as an effective measurement induced dephasing rate that depends on the position of the wave-packet relative to the CI. Our results set the groundwork for more complex simulations of chemical dynamics, offering deeper insight into the role of dissipation in determining macroscopic quantities of interest such as the quantum yield of a chemical reaction.
03
Feb
2022
Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit
Mesoscopic Josephson junctions (JJs), consisting of overlapping superconducting electrodes separated by a nanometer thin oxide layer, provide a precious source of nonlinearity for superconducting
quantum circuits and are at the heart of state-of-the-art qubits, such as the transmon and fluxonium. Here, we show that in a fluxonium qubit the role of the JJ can also be played by a lithographically defined, self-structured granular aluminum (grAl) nano-junction: a superconductor-insulator-superconductor (SIS) JJ obtained in a single layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large grAl nano-junction charging energy in the range of 10−100GHz, comparable to its Josephson energy EJ. We measure average energy relaxation times of T1=10μs and Hahn echo coherence times of Techo2=9μs. The exponential sensitivity of the gralmonium to the EJ of the grAl nano-junction provides a highly susceptible detector. Indeed, we observe spontaneous jumps of the value of EJ on timescales from milliseconds to days, which offer a powerful diagnostics tool for microscopic defects in superconducting materials.
Engineering superconducting qubits to reduce quasiparticles and charge noise
In any physical realization of a qubit, identifying, quantifying, and suppressing mechanisms of decoherence are important steps towards the goal of engineering a universal quantum computeror a quantum simulator. Superconducting circuits based on Josephson junctions offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs) whose density, as observed in various systems, is considerably higher than that expected in thermal equilibrium. A full understanding of the generation mechanism and a mitigation strategy that is compatible with scalable, high-coherence devices are therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit structure, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. We achieve record low charge-parity switching rate (<1Hz) in our aluminium devices. At the same time, the devices display improved stability with respect to discrete charging events. Our findings support the hypothesis that the generation of quasiparticles is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption mediated by the antenna-like qubit structure. We thus demonstrate a convenient approach to shape the electromagnetic environment of superconducting circuits in the sub-terahertz regime, inhibiting decoherence from quasiparticle poisoning.[/expand]
25
Jan
2022
Entanglement purification and protection in a superconducting quantum network
High-fidelity quantum entanglement is a key resource for quantum communication and distributed quantum computing, enabling quantum state teleportation, dense coding, and quantum encryption.
Any sources of decoherence in the communication channel however degrade entanglement fidelity, thereby increasing the error rates of entangled state protocols. Entanglement purification provides a method to alleviate these non-idealities, by distilling impure states into higher-fidelity entangled states. Here we demonstrate the entanglement purification of Bell pairs shared between two remote superconducting quantum nodes connected by a moderately lossy, 1-meter long superconducting communication cable. We use a purification process to correct the dominant amplitude damping errors caused by transmission through the cable, with fractional increases in fidelity as large as 25%, achieved for higher damping errors. The best final fidelity the purification achieves is 94.09±0.98%. In addition, we use both dynamical decoupling and Rabi driving to protect the entangled states from local noise, increasing the effective qubit dephasing time by a factor of 4, from 3 μs to 12 μs. These methods demonstrate the potential for the generation and preservation of very high-fidelity entanglement in a superconducting quantum communication network.
Qubit-compatible substrates with superconducting through-silicon vias
We fabricate and characterize superconducting through-silicon vias and electrodes suitable for superconducting quantum processors. We measure internal quality factors of a million for
test resonators excited at single-photon levels, when vias are used to stitch ground planes on the front and back sides of the wafer. This resonator performance is on par with the state of the art for silicon-based planar solutions, despite the presence of vias. Via stitching of ground planes is an important enabling technology for increasing the physical size of quantum processor chips, and is a first step toward more complex quantum devices with three-dimensional integration.