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
26
Okt
2017
The nature of the Lamb shift in weakly-anharmonic atoms: from normal mode splitting to quantum fluctuations
When a two level system (TLS) is coupled to an electromagnetic resonator, its transition frequency changes in response to the quantum vacuum fluctuations of the electromagnetic field,
a phenomenon known as the Lamb shift. Remarkably, by replacing the TLS by a harmonic oscillator, normal mode splitting leads to a similar shift, despite its completely classical origin. In a weakly-anharmonic system, lying in between the harmonic oscillator and a TLS, the origins of such shifts can be unclear. An example of such a system is the transmon qubit in a typical circuit quantum electrodynamics setting. Although often referred to as a Lamb shift, it cannot originate purely from vacuum fluctuations since in the limit of zero anharmonicity, the system becomes classical. Here, we treat normal-mode splitting separately from quantum effects in the Hamiltonian of a weakly-anharmonic system, providing a framework for understanding the extent to which the frequency shift can be attributed to quantum fluctuations.
1/f noise effect on dissipative dynamics of a LC-shunted qubit
We consider dissipative dynamics of a flux qubit caused by 1/f noises, which act both on the shunting LC-contour and on the SQUID loop, and modulate the level splitting and the tunnel
coupling, respectively. These classical Gaussian noises are partially correlated. The transient evolution of qubit is studied for the regimes: (a) the interwell incoherent tunneling, (b) the relaxation of interlevel population, and (c) the decoherence of the off-diagonal part of a density matrix. For all regimes, the relaxation rates and the frequency renormalization [for the case (c)] are analyzed versus the parameters of qubit and couplings to the noises applied through different channels. The fluctuation effect restricts an averaged description of evolution processes, so that the dissipative dynamics is not valid at tails of relaxation. The results obtained open a way for verification of relaxation parameters and demonstrate a possibility for minimization of coupling between qubit and environment. Under typical level of noises, the contributions considered here are comparable to recent experimental data on the population relaxation and the interwell tunneling.
24
Okt
2017
Controllable anisotropic quantum Rabi model beyond the ultrastrong coupling regime with circuit QED systems
By manipulating the flux qubits with bichromatic time-dependent magnetic fluxes in standard circuit QED systems, we propose an experimentally-accessible method to approach the physics
of the anisotropic quantum Rabi model (AQRM) in broad parameter ranges, where the rotating and counter-rotating interactions are governed by two different coupling constants. Assisted by theoretical derivations and numerical calculations, we show that our scheme not only allows for individual control of the parameters in the simulated AQRM but also reproduces the dynamics of the ultrastrong and deep strong coupling regimes. Therefore, our scheme advances the investigation of extremely strong interactions of the AQRM, which are usually experimentally unattainable. Furthermore, associated with the special case of the degenerate AQRM, we demonstrate that our setup may also find applications for protected quantum memory and quantum computation since it can be used to generate the Schr\“{o}dinger cat states and the quantum controlled phase gates when scaling up.
20
Okt
2017
Studying Light-Harvesting Models with Superconducting Circuits
The process of photosynthesis, the main source of energy in the animate world, converts sunlight into chemical energy. The surprisingly high efficiency of this process is believed to
be enabled by an intricate interplay between the quantum nature of molecular structures in photosynthetic complexes and their interaction with the environment. Investigating these effects in biological samples is challenging due to their complex and disordered structure. Here we experimentally demonstrate a new approach for studying photosynthetic models based on superconducting quantum circuits. In particular, we demonstrate the unprecedented versatility and control of our method in an engineered three-site model of a pigment protein complex with realistic parameters scaled down in energy by a factor of 105. With this system we show that the excitation transport between quantum coherent sites disordered in energy can be enabled through the interaction with environmental noise. We also show that the efficiency of the process is maximized for structured noise resembling intramolecular phononic environments found in photosynthetic complexes.
Cross-coupling effects in circuit-QED stimulated Raman adiabatic passage
Stimulated Raman adiabatic passage is a quantum protocol that can be used for robust state preparation in a three-level system. It has been commonly employed in quantum optics, but
recently this technique has drawn attention also in circuit quantum electrodynamics. The protocol relies on two slowly varying drive pulses that couple the initial and the target state via an intermediate state, which remains unpopulated. Here we study the detrimental effect of the parasitic couplings of the drives into transitions other than those required by the protocol. The effect is most prominent in systems with almost harmonic energy level structure, such as the transmon. We show that under these conditions in the presence of decoherence there exists an optimal STIRAP amplitude for population transfer.
17
Okt
2017
Itinerant microwave photon detector
The realization of a high-efficiency microwave single photon detector is a long-standing problem in the field of microwave quantum optics. Here we propose a quantum non-demolition,
high-efficiency photon detector that can readily be implemented in present state-of-the-art circuit quantum electrodynamics. This scheme works in a continuous fashion, gaining information about the arrival time of the photon as well as about its presence. The key insight that allows to circumvent the usual limitations imposed by measurement back-action is the use of long-lived dark states in a small ensemble of inhomogeneous artificial atoms to increase the interaction time between the photon and the measurement device. Using realistic system parameters, we show that large detection fidelities are possible.
Experimental demonstration of a two-dimensional phonon cavity in the quantum regime
The quantum regime in acoustic systems is a focus of recent fundamental research in the new field of Quantum Acoustodynamics (QAD). Systems based on surface acoustic waves having an
advantage of easy integration in two-dimensions are particularly promising for the demonstration of novel effects in QAD and development of novel devices of quantum acousto-electronics. We demonstrate the vacuum mode of the surface acoustic wave resonator by coupling it to a superconducting artificial atom. The artificial atom is implemented into the resonator formed by two Brag mirrors. The results are consistent with expectations supported by the system model and our calculations. This work opens the way to map analogues of quantum optical effects into acoustic systems.
14
Okt
2017
Multi-time correlators in continuous measurement of qubit observables
We consider multi-time correlators for output signals from linear detectors, continuously measuring several qubit observables at the same time. Using the quantum Bayesian formalism,
we show that for unital (symmetric) evolution in the absence of phase backaction, an N-time correlator can be expressed as a product of two-time correlators when N is even. For odd N, there is a similar factorization, which also includes a single-time average. Theoretical predictions agree well with experimental results for two detectors, which simultaneously measure non-commuting qubit observables.
12
Okt
2017
Mitigating coherent leakage of superconducting qubits in a large-scale quantum socket
A practical quantum computer requires quantum bit (qubit) operations with low error rates in extensible architectures. We study a packaging method that makes it possible to address
hundreds of superconducting qubits by means of three-dimensional wires: The large-scale quantum socket. A qubit chip is housed in a superconducting box, where both box and chip dimensions lead to unwanted modes that can interfere with qubit operations. We theoretically analyze these interference effects in the context of qubit coherent leakage. We propose two methods to mitigate the resulting errors by detuning the resonance frequency of the modes from the qubit frequency. We perform detailed electromagnetic field simulations indicating that the resonance frequency of the modes increases with the number of installed three-dimensional wires and can be engineered to be significantly higher than the highest qubit frequency. Finally, we show preliminary experimental results towards the implementation of a large-scale quantum socket.
Quantum–Classical Interface Based on Single Flux Quantum Digital Logic
We describe an approach to the integrated control and measurement of a large-scale superconducting multiqubit circuit using a proximal coprocessor based on the Single Flux Quantum (SFQ)
digital logic family. Coherent control is realized by irradiating the qubits directly with classical bitstreams derived from optimal control theory. Qubit measurement is performed by a Josephson photon counter, which provides access to the classical result of projective quantum measurement at the millikelvin stage. We analyze the power budget and physical footprint of the SFQ coprocessor and discuss challenges and opportunities associated with this approach.