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
05
Aug
2020
High-Fidelity Measurement of a Superconducting Qubit using an On-Chip Microwave Photon Counter
We describe an approach to the high-fidelity measurement of a superconducting qubit using an on-chip microwave photon counter. The protocol relies on the transient response of a dispersively
coupled measurement resonator to map the state of the qubit to „bright“ and „dark“ cavity pointer states that are characterized by a large differential photon occupation. Following this mapping, we photodetect the resonator using the Josephson Photomultipler (JPM), which transitions between classically distinguishable flux states when cavity photon occupation exceeds a certain threshold. Our technique provides access to the binary outcome of projective quantum measurement at the millikelvin stage without the need for quantum-limited preamplification and thresholding at room temperature. We achieve raw single-shot measurement fidelity in excess of 98% across multiple samples using this approach in total measurement times under 500 ns. In addition, we show that the backaction and crosstalk associated with our measurement protocol can be mitigated by exploiting the intrinsic damping of the JPM itself.
04
Aug
2020
Microwave Quantum Link between Superconducting Circuits Housed in Spatially Separated Cryogenic Systems
Superconducting circuits are a strong contender for realizing quantum computing systems, and are also successfully used to study quantum optics and hybrid quantum systems. However,
their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks either spanning different cryogenics systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on-demand with average transfer and target state fidelities of 85.8 % and 79.5 %, respectively, between the two nodes of this elementary network. Cryogenic microwave links do provide an opportunity to scale up systems for quantum computing and create local area quantum communication networks over length scales of at least tens of meters.
03
Aug
2020
Characterization of multi-level dynamics and decoherence in a high-anharmonicity capacitively shunted flux circuit
We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. The circuit used as a qubit shows long energy
relaxation times, of the order of 40 μs, and a spin-echo dephasing time of 9.4 μs. The circuit has high anharmonicity, of 2π×3.69 GHz. We extract the multi-level relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with 99.92% average gate fidelity, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond time scale two-qubit gates and multi-level quantum logic
30
Jul
2020
Noise reduction in qubit readout with a two-mode squeezed interferometer
Fault-tolerant quantum information processing with flawed qubits and gates requires highly efficient, quantum non-demolition (QND) qubit readout. In superconducting circuits, qubit
readout using coherent light with fidelity above 99% has been achieved by using quantum-limited parametric amplifiers such as the Josephson Parametric Converter (JPC). However, further improvement of such measurement is fundamentally limited by the vacuum fluctuations of the coherent light used for readout. In this work we measure a transmon qubit/cavity system with an unbalanced two-mode squeezed light interferometer formed from two JPCs. The first amplifier generates two-mode squeezed vacuum at its output, which is coherently recombined by the second amplifier after one branch is shifted and displaced by the transmon’s state after it interacts with the qubit/cavity system on one arm of the interferometer. We have observed a 44% improvement in power Signal-to-Noise Ratio (SNR) of projective readout compared to that of coherent light readout in the same system. To investigate the quantum properties of the two-mode squeezed light in the system, we also studied weak measurement and found, surprisingly, that tuning the interferometer to be as unprojective as possible was associated with an increase in the quantum efficiency of our readout relative to the optimum setting for projective measurement. These enhancements may enable remote entanglement with lower efficiency components in a system with qubits in both arms of the interferometer.
27
Jul
2020
Floquet-engineered enhancement of coherence times in a driven fluxonium qubit
vWe use the quasienergy structure that emerges when a fluxonium superconducting circuit is driven periodically to encode quantum information with dynamically induced flux-insensitive
sweet spots. The framework of Floquet theory provides an intuitive description of these high-coherence working points located away from the half-flux symmetry point of the undriven qubit. This approach offers flexibility in choosing the flux bias point and the energy of the logical qubit states as shown in [\textit{Huang et al., 2020}]. We characterize the response of the system to noise in the modulation amplitude and DC flux bias, and experimentally demonstrate an optimal working point which is simultaneously insensitive against fluctuations in both. We observe a 40-fold enhancement of the qubit coherence times measured with Ramsey-type interferometry at the dynamical sweet spot compared with static operation at the same bias point.
21
Jul
2020
Coupling a Superconducting Qubit to a Left-Handed Metamaterial Resonator
Metamaterial resonant structures made from arrays of superconducting lumped circuit elements can exhibit microwave mode spectra with left-handed dispersion, resulting in a high density
of modes in the same frequency range where superconducting qubits are typically operated, as well as a bandgap at lower frequencies that extends down to dc. Using this novel regime for multi-mode circuit quantum electrodynamics, we have performed a series of measurements of such a superconducting metamaterial resonator coupled to a flux-tunable transmon qubit. Through microwave measurements of the metamaterial, we have observed the coupling of the qubit to each of the modes that it passes through. Using a separate readout resonator, we have probed the qubit dispersively and characterized the qubit energy relaxation as a function of frequency, which is strongly affected by the Purcell effect in the presence of the dense mode spectrum. Additionally, we have investigated the ac Stark shift of the qubit as the photon number in the various metamaterial modes is varied. The ability to tailor the dense mode spectrum through the choice of circuit parameters and manipulate the photonic state of the metamaterial through interactions with qubits makes this a promising platform for analog quantum simulation and quantum memories.
The Rayleigh-Lorentz Invariant and Optimal Adiabatic Qubit-Information Detection for Superconducting Qubit Resonators
Dynamical properties of a resonator can be analyzed using the Rayleigh-Lorentz invariant which is not an exact constant but varies more or less over time. We investigate the time behavior
of this invariant for a flux qubit resonator in order for better understanding of qubit-information detection with the resonator. Flux qubit resonators can be utilized in implementing diverse next generation nano-optic and nano-electronic devices such as quantum computing systems. Through the analyses of the temporal evolution of the invariant, we derive a condition for optimal adiabatic qubit-information detection with the resonator. This condition is helpful for controlling the dynamics of qubit resonators over long periods of time. It is necessary to consider it when designing a nano-resonator used for quantum nondemolition readouts of qubit states, crucial in quantum computation.
19
Jul
2020
Probing the current-phase relation of graphene Josephson junctions using microwave measurements
We perform extensive analysis of graphene Josephson junctions embedded in microwave circuits. By comparing a diffusive junction at 15 mK with a ballistic one at 15 mK and 1 K, we are
able to reconstruct the current-phase relation.
17
Jul
2020
Observation of Bloch Oscillations and Wannier-Stark Localization on a Superconducting Processor
In a crystal lattice system, a conduction electron can exhibit Bloch oscillations and Wannier-Stark localization (WSL) under a constant force, which has been observed in semiconductor
superlattice, photonic waveguide array and cold atom systems. Here, we experimentally investigate the Bloch oscillations on a 5-qubit superconducting processor. We simulate the electron movement with spin (or photon) propagation. We find, in the presence of a linear potential, the propagation of a single spin charge is constrained. It tends to oscillate near the neighborhood of initial positions, which is a strong signature of Bloch oscillations and WSL. In addition, we use the maximum probability that a spin charge can propagate from one boundary to another boundary to represent the WSL length, and it is verified that the localization length is inversely correlated to the potential gradient. Remarkably, benefiting from the precise simultaneous readout of the all qubits, we can also study the thermal transport of this system. The experimental results show that, similar to the spin charges, the thermal transport is also blocked under a linear potential. Our work demonstrates possibilities for further simulation and exploration of the Bloch oscillation phenomena and other quantum physics using multiqubit superconducting quantum processor.
15
Jul
2020
The Energy of an Arbitrary Electrical Circuit, Classical and Quantum
In this Note, I show an algorithmic method to find the energy and, thus, the Hamiltonian of an arbitrary electrical circuit based on the so-called incidence matrix and the circuit’s
total power. This method does not require to find any Lagrangian; instead, it is based on the concept of generalized linear momenta for the kinetic and co-kinetic energy of a circuit. The method can account for superconducting loops by a simple extension of Faraday-Henry-Neumann’s law. Auxiliary (i.e., parasitic) circuit elements are required to deal with circuits with an incomplete set of generalized velocities resulting in an incomplete set of canonical coordinates. This method can be readily automatized to obtain the Hamiltonian of arbitrarily complicated circuits. I also show how to quantize the circuit associated with a resonator capacitively coupled with a qubit.