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
15
Dez
2016
Schrodinger’s catapult: Launching multiphoton quantum states from a microwave cavity memory
Encoding quantum states in complex multiphoton fields can overcome loss during signal transmission in a quantum network. Transmitting quantum information encoded in this way requires
that locally stored states can be converted to propagating fields. Here we experimentally show the controlled conversion of multiphoton quantum states, like „Schr\“odinger cat“ states, from a microwave cavity quantum memory into propagating modes. By parametric conversion using the nonlinearity of a single Josephson junction, we can release the cavity state in ~500 ns, about 3 orders of magnitude faster than its intrinsic lifetime. This `catapult‘ faithfully converts arbitrary cavity fields to traveling signals with an estimated efficiency of > 90%, enabling on-demand generation of complex itinerant quantum states. Importantly, the release process can be controlled precisely on fast time scales, allowing us to generate entanglement between the cavity and the traveling mode by partial conversion. Our system can serve as the backbone of a microwave quantum network, paving the way towards error-correctable distribution of quantum information and the transfer of highly non-classical states to hybrid quantum systems.
13
Dez
2016
Generating higher order quantum dissipation from lower order parametric processes
Stabilization of quantum manifolds is at the heart of error-protected quantum information storage and manipulation. Nonlinear driven-dissipative processes achieve such stabilization
in a hardware efficient manner. Josephson circuits with parametric pump drives implement these nonlinear interactions. In this article, we propose a scheme to engineer a four-photon drive and dissipation on a harmonic oscillator by cascading experimentally demonstrated two-photon processes. This would stabilize a four-dimensional degenerate manifold in a superconducting resonator. We analyze the performance of the scheme using numerical simulations of a realizable system with experimentally achievable parameters.
Efficient protocol for qubit initialization with a tunable environment
We propose an efficient qubit initialization protocol based on a dissipative environment that can be dynamically adjusted. Here the qubit is coupled to a thermal bath through a tunable
harmonic oscillator. On-demand initialization is achieved by sweeping the oscillator rapidly into resonance with the qubit. This resonant coupling with the engineered environment induces fast relaxation to the ground state of the system, and a consecutive rapid sweep back to off resonance guarantees weak excess dissipation during quantum computations. We solve the corresponding quantum dynamics using a Markovian master equation for the reduced density operator of the qubit-bath system. This allows us to optimize the parameters and the initialization protocol for the qubit. Our analytical calculations show that the ground-state occupation of our system is well protected during the fast sweeps of the environmental coupling and, consequently, we obtain an estimate for the duration of our protocol by solving the transition rates between the low-energy eigenstates with the Jacobian diagonalization method. Our results suggest that the current experimental state of the art for the initialization speed of superconducting qubits at a given fidelity can be considerably improved.
12
Dez
2016
Nonadiabatic Holonomic Quantum Computation with Dressed-state Qubits
Implementing holonomic quantum computation is a challenging task as it requires complicated interaction among multilevel systems. Here, we propose to implement nonadiabatic holonomic
quantum computation based on dressed-state qubits in circuit QED. An arbitrary holonomic single-qubit gate can be conveniently achieved using external microwave fields and tuning their amplitudes and phases. Meanwhile, nontrivial two-qubit gates can be implemented in a coupled cavities scenario assisted by a grounding SQUID with tunable interaction, where the tuning is achieved by modulating the ac flux threaded through the SQUID. In addition, our proposal is directly scalable, up to a two-dimensional lattice configuration. In our scheme, the dressed states only involve the lowest two levels of each transmon qubits and the effective interactions exploited are all of resonant nature. Therefore, we release the main difficulties for physical implementation of holonomic quantum computation on superconducting circuits.
10
Dez
2016
Electronic zero-point fluctuation forces inside circuit components
One of the most intriguing manifestations of quantum zero-point fluctuations are the van der Waals and Casimir forces, often associated with vacuum fluctuations of the electromagnetic
field. Here we study generalized fluctuation potentials acting on internal degrees of freedom of components in electrical circuits. These electronic Casimir-like potentials are induced by the zero-point current fluctuations of any general conductive circuit. For realistic examples of an electromechanical capacitor and a superconducting qubit, our results reveal the possibility of tunable repulsive and attractive forces between the capacitor plates, or the level shifts of the qubit, respectively. Our analysis suggests an alternative route towards the exploration of Casimir-like fluctuation potentials, namely, by characterizing and measuring them as a function of parameters of the environment. Such tunable potentials may be useful for future nano-electromechanical technologies.
Fisher information of two superconducting charge qubits under dephasing noisy
The dynamics of a charged two-qubit system prepared initially in a maximum entangled state is discussed, where each qubit interacts independently with a dephasing channel. The Fisher
information is used to estimate the channel and the energy parameters. Moreover, the contribution from different parts of Fisher information is discussed. We show that, the degree of estimation of any parameter depends on the initial value of this parameter. It turns out that the upper bounds of Fisher information with respect to the dephasing parameter increase and that corresponding to coupling parameter decreases as the initial energy of each qubit increases.
09
Dez
2016
Quantum caustics in resonance fluorescence trajectories
We employ phase-sensitive amplification to perform homodyne detection of the resonance fluorescence from a driven superconducting artificial atom. Entanglement between the emitter and
its fluorescence allows us to track the individual quantum state trajectories of the emitter conditioned on the outcomes of the field measurements. We analyze the ensemble properties of these trajectories by considering trajectories that connect specific initial and final states. By applying the stochastic path integral formalism, we calculate equations-of-motion for the most likely path between two quantum states and compare these predicted paths to experimental data. Drawing on the mathematical similarity between the action formalism of the most likely quantum paths and ray optics we study the emergence of caustics in quantum trajectories – places where multiple extrema in the stochastic action occur. We observe such multiple most likely paths in experimental data and find these paths to be in reasonable quantitative agreement with theoretical calculations.
Is the quantum Rabi model adequate in circuit QED for any atom-resonator coupling?
In circuit quantum electrodynamics, an artificial „circuit atom“ can couple to a quantized microwave radiation much stronger than its real atomic counterpart. The celebrated
quantum Rabi model describes the simplest interaction of a two-level system with a single-mode boson field. When the coupling is arbitrary large, the bare multilevel structure of a realistic circuit atom cannot be ignored even if the circuit is strongly anharmonic. We explored this situation theoretically for flux (fluxonium) and charge (Cooper pair box) type multi-level circuit atoms at maximal frustration and identified which spectral features of the quantum Rabi model survive and which are renormalized for arbitrary large coupling. We provide a quantitative comparison with the ideal quantum Rabi model by inspecting not only the circuit energy level spectrum, but also the entanglement spectrum. Despite significant renormalization of the low-energy energy spectrum in the fluxonium case, the key quantum Rabi feature — nearly-degenerate vacuum consisting of an atomic state entangled with a multi-photon field — appears in both circuits when the coupling is sufficiently large. Like in the quantum Rabi model, for very large couplings the entanglement spectrum is dominated by only two, nearly equal eigenvalues, in spite of the fact that a large number of bare atomic states are actually involved in the ground state. We interpret the emergence of the vacuum degeneracy in both circuits as an environmental suppression of flux/charge tunneling due to their dressing by virtual low-/high-impedance photons in the resonator. For flux tunneling, the dressing is nothing else than the shunting of a Josephson atom with a large capacitance of the resonator. Suppression of charge tunneling appears to have the same origin as the dynamical Coulomb blockade of transport in tunnel junctions connected to resistive leads.
05
Dez
2016
Nonreciprocal microwave signal processing with a Field-Programmable Josephson Amplifier
We report on the design and implementation of a Field Programmable Josephson Amplifier (FPJA) – a compact and lossless superconducting circuit that can be programmed extit{in
situ} by a set of microwave drives to perform reciprocal and nonreciprocal frequency conversion and amplification. In this work we demonstrate four modes of operation: frequency conversion (−0.5 dB transmission, −30 dB reflection), circulation (−0.5 dB transmission, −30 dB reflection, 30 dB isolation), phase-preserving amplification (gain >20 dB, 1 photon of added noise) and directional phase-preserving amplification (−10 dB reflection, 18 dB forward gain, 8 dB reverse isolation, 1 photon of added noise). The system exhibits quantitative agreement with theoretical prediction. Based on a gradiometric Superconducting Quantum Interference Device (SQUID) with Nb/Al-AlOx/Nb Josephson junctions, the FPJA is first-order insensitive to flux noise and can be operated without magnetic shielding at low temperature. Due to its flexible design and compatibility with existing superconducting fabrication techniques, the FPJA offers a straightforward route toward on-chip integration with superconducting quantum circuits such as qubits or microwave optomechanical systems.
03
Dez
2016
Hybrid quantum device with a carbon nanotube and a flux qubit for dissipative quantum engineering
We describe a hybrid quantum system composed of a micrometer-size carbon nanotube (CNT) longitudinally coupled to a flux qubit. We demonstrate the usefulness of this device for generating
high-fidelity nonclassical states of the CNT via dissipative quantum engineering. Sideband cooling of the CNT to its ground state and generating a squeezed ground state, as a mechanical analogue of the optical squeezed vacuum, are two additional examples of the dissipative quantum engineering studied here. Moreover, we show how to generate a long-lived macroscopically-distinct superposition (i.e., a Schr\“odinger cat-like) state. This cat state can be trapped via dark-state methods assuming that the CNT dissipation is negligible compared to the qubit dissipation, and can be verified by detecting the optical response of control fields.