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
18
Okt
2018
CUBIT: Capacitive qUantum BIT
In this letter, it is proposed that cryogenic quantum bits can operate based on the nonlinearity due to the quantum capacitance of two-dimensional Dirac materials, and in particular
graphene. The anharmonicity of a typical superconducting quantum bit is calculated, and the sensitivity of quantum bit frequency and anharmonicity with respect to temperature are found. Reasonable estimates reveal that a careful fabrication process can reveal expected properties, putting the context of quantum computing hardware into new perspectives.
16
Okt
2018
Active protection of a superconducting qubit with an interferometric Josephson isolator
Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional
routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.
15
Okt
2018
Antibunched photons emitted by a dc biased Josephson junction
We show experimentally that a dc biased Josephson junction in series with a high-enough impedance microwave resonator emits antibunched photons. Our resonator is made of a simple micro-fabricated
spiral coil that resonates at 4.4 GHz and reaches a 1.97 kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6 10^7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain.
13
Okt
2018
Quantum simulation of clustered photosynthetic light harvesting in a superconducting quantum circuit
We propose a scheme to simulate the exciton energy transfer (EET) of photosynthetic complexes in a quantum superconducting circuit system. Our system is composed of two pairs of superconducting
charge qubits coupled to two separated high-Q superconducting transmission line resonators (TLRs) connected by a capacitance. When the frequencies of the qubits are largely detuned with those of the TLRs, we simulate the process of the EET from the first qubit to the fourth qubit. By tuning the couplings between the qubits and the TLRs, and the coupling between the two TLRs, we can modify the effective coupling strengths between the qubits and thus demonstrate the geometric effects on the EET. It is shown that a moderate clustered geometry supports optimal EET by using exciton delocalization and energy matching condition. And the population loss during the EET has been trapped in the two TLRs.
10
Okt
2018
Three-dimensional superconducting resonators at T<20 mK with the photon lifetime up to τ=2 seconds
Very high quality factor superconducting radio frequency cavities developed for accelerators can offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored
quantum states in the 3D circuit QED architecture. Here we report the first measurements of several accelerator cavities of f_0=1.3, 2.6, 5 GHz resonant frequencies down to temperatures of about 10~mK and field levels down to a few photons, which reveal record high photon lifetimes up to 2 seconds, while also further exposing the role of the two level systems (TLS) in the niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by the special vacuum heat treatment.
09
Okt
2018
Non-thermal quantum engine in transmon qubits
The design and implementation of quantum technologies necessitates the understanding of thermodynamic processes in the quantum domain. In stark contrast to macroscopic thermodynamics,
at the quantum scale processes generically operate far from equilibrium and are governed by fluctuations. Thus, experimental insight and empirical findings are indispensable in developing a comprehensive framework. To this end, we theoretically propose an experimentally realistic quantum engine, that utilizes transmon qubits as working substance. We solve the dynamics analytically and calculate its efficiency, that reaches a maximum value of 35%.
Suppression of Qubit Crosstalk in a Tunable Coupling Superconducting Circuit
We report the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the ZZ interaction between the two qubits can be tuned to near zero. Characterization
of qubit crosstalk is performed using randomized benchmarking and a two-qubit iSWAP gate is implemented using parametric modulation. We observe the dependence of single-qubit gate fidelity on ZZ interaction strength and identify effective thermalization of the tunable coupler as a crucial prerequisite for high fidelity two-qubit gates.
08
Okt
2018
Left-handed superlattice metamaterials for circuit-QED
Quantum simulations is a promising field where a controllable system is used to mimic another system of interest, whose properties one wants to investigate. One of the key issues for
such simulations is the ability to control the environment the system couples to, be it to isolate the system or to engineer a tailored environment of interest. One strategy recently put forward for environment engineering is the use of metamaterials with negative index of refraction. Here we build on this concept and propose a circuit-QED simulation of many-body Hamiltonians using superlattice metamaterials. We give a detailed description of a superlattice transmission line coupled to an embedded qubit, and show how this system can be used to simulate the spin-boson model in regimes where analytical and numerical methods usually fail, e.g. the strong coupling regime.
05
Okt
2018
Quantum estimation of parameter in circuit QED by continuous quantum measurement
Designing high-precision and efficient schemes is of crucial importance for quantum parameter estimation in practice. The estimation scheme based on continuous quantum measurement is
one possible type of this, which looks also the most natural choice in case such as continuous dynamical process. In this work we specify the study to the stat-of-the-art superconducting circuit quantum-electrodynamics (QED) system, where the high-quality continuous measurement has been extensively exploited in the past decade. Within the framework of Bayesian estimation and particularly using the quantum Bayesian rule in circuit QED, we numerically simulate the likelihood function as estimator for the Rabi frequency of qubit oscillation. We find that, by proper design of the interaction strength of measurement, the estimate precision can scale with the measurement time beyond the standard quantum limit, which is usually assumed for this type of continuous measurement since no more special quantum resource is involved. We understand this remarkable result by quantum correlation in time between the output signals, and simulate the effect of quantum efficiency of the measurement on the precision scaling behavior.
04
Okt
2018
Cryogenic Microwave Filter Cavity with a Tunability Greater than 5 GHz
A wide variety of applications of microwave cavities, such as measurement and control of superconducting qubits, magnonic resonators, and phase noise filters, would be well served by
having a highly tunable microwave resonance. Often this tunability is desired in situ at low temperatures, where one can take advantage of superconducting cavities. To date, such cryogenic tuning while maintaining a high quality factor has been limited to ∼500MHz. Here we demonstrate a three-dimensional superconducting microwave cavity that shares one wall with a pressurized volume of helium. Upon pressurization of the helium chamber the microwave cavity is deformed, which results in in situ tuning of its resonant frequency by more than 5 GHz, greater than 60% of the original 8 GHz resonant frequency. The quality factor of the cavity remains approximately constant at ≈7×10^3 over the entire range of tuning. As a demonstration of its usefulness, we implement a tunable cryogenic phase noise filter, which reduces the phase noise of our source by approximately 10 dB above 400 kHz.