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
21
Jun
2015
Qubit measurement error from coupling with a detuned neighbor in circuit QED
In modern circuit QED architectures, superconducting transmon qubits are measured via the state-dependent phase and amplitude shift of a microwave field leaking from a coupled resonator.
Determining this shift requires integrating the field quadratures for a nonzero duration, which can permit unwanted concurrent evolution. Here we investigate such dynamical degradation of the measurement fidelity caused by a detuned neighboring qubit. We find that in realistic parameter regimes, where the qubit ensemble-dephasing rate is slower than the qubit-qubit detuning, the joint qubit-qubit eigenstates are better discriminated by measurement than the bare states. Furthermore, we show that when the resonator leaks much more slowly than the qubit-qubit detuning, the measurement tracks the joint eigenstates nearly adiabatically. However, the measurement process also causes rare quantum jumps between the eigenstates. The rate of these jumps becomes significant if the resonator decay is comparable to or faster than the qubit-qubit detuning, thus significantly degrading the measurement fidelity in a manner reminiscent of energy relaxation processes.
20
Jun
2015
Controlling the nonclassical properties of a hybrid Cooper pair box system and an intensity dependent nanomechanical resonator
We employ a more realistic treatment to investigate the entropy and the excitation-inversion of a coupled system that consists of a nanomechanical resonator and a superconducting Cooper
pair box. The procedure uses the Buck-Sukumar model in the microwave domain, considers the nanoresonator with a time dependent frequency and both subsystems in the presence of losses. Interesting results were found for the temporal evolutions of the entropy of each subsystem and of the excitation-inversion in the Cooper pair box. A comparison was also performed about which of these two subsystems is more sensitive to the presence of losses. The results suggest that appropriate choices of the involved time dependent parameters allow us to monitor these two features of the subsystems and may offer potential applications, e.g., in the generation of nonclassical states, quantum communication, quantum lithography.
19
Jun
2015
Entangling superconducting qubits in a multi-cavity system
Important tasks in cavity quantum electrodynamics include the generation and control of quantum states of spatially-separated particles distributed in different cavities. An interesting
question in this context is how to prepare entanglement among particles located in different cavities, which are important for large-scale quantum information processing. We here consider a multi-cavity system where cavities are coupled to a superconducting (SC) qubit and each cavity hosts many SC qubits. We show that all intra-cavity SC qubits plus the coupler SC qubit can be prepared in an entangled Greenberger-Horne-Zeilinger (GHZ) state, by using a single operation and without the need of measurements. The GHZ state is created without exciting the cavity modes; thus greatly suppressing the decoherence caused by the cavity-photon decay and the effect of unwanted inter-cavity crosstalk on the operation. We also introduce two simple methods for entangling the intra-cavity SC qubits in a GHZ state. As an example, our numerical simulations show that it is feasible, with current circuit-QED technology, to prepare high-fidelity GHZ states, for up to nine SC qubits by using SC qubits distributed in two cavities. This proposal can in principle be used to implement a GHZ state for {\it an arbitrary number} of SC qubits distributed in multiple cavities. The proposal is quite general and can be applied to a wide range of physical systems, with the intra-cavity qubits being either atoms, NV centers, quantum dots, or various SC qubits.
18
Jun
2015
Cooling and Autonomous Feedback in a Bose-Hubbard chain
We engineer a quantum bath that enables entropy and energy exchange with a one-dimensional Bose-Hubbard lattice with attractive on-site interactions. We implement this in an array of
three superconducting transmon qubits coupled to a single cavity mode; the transmons represent lattice sites and their excitation quanta embody bosonic particles. Our cooling protocol preserves particle number–realizing a canonical ensemble– and also affords the efficient preparation of dark states which, due to symmetry, cannot be prepared via coherent drives on the cavity. Furthermore, by applying continuous microwave radiation, we also realize autonomous feedback to indefinitely stabilize particular eigenstates of the array.
17
Jun
2015
Coherent Feedback Improved Qubit Initialization in the Dispersive Regime
Readout of the state of a superconducting qubit by homodyne detection of the output signal from a dispersively coupled microwave resonator is a common technique in circuit quantum electrodynamics,
and is often claimed to be quantum non-demolition (QND) up to the same order of approximation as in the dispersive approximation. However, in this work we show that only in the limit of infinite measurement time is this protocol QND, as the formation of a dressed coherent state in the qubit-cavity system applies an effective rotation to the qubit state. We show how this rotation can be corrected by a coherent operation, leading to improved qubit initialization by measurement and coherent feedback.
16
Jun
2015
Encoding a Qubit into a Cavity Mode in Circuit-QED using Phase Estimation
Gottesman, Kitaev and Preskill have formulated a way of encoding a qubit into an oscillator such that the encoded qubit is protected against small shifts in phase space. We propose
and analyze the approximate creation of these encoded states with ancilla qubits. The preparation of the code states uses the idea of phase estimation where the phase of the stabilizer checks is approximately determined. We propose a physical implementation of this protocol using the dispersive coupling between a transmon ancilla qubit and a cavity mode in circuit-QED. We provide an estimate that in a current experimental set-up one can prepare a good code state from a squeezed vacuum state using 8 rounds of adapative phase estimation, lasting in total less than 5 {\mu} sec., with 80 % (heralded) chance of success.
13
Jun
2015
Circuit electromechanics with single photon strong coupling
In circuit electromechanics, the coupling strength is usually very small. Here, replacing the capacitor in circuit electromechanics by a superconducting flux qubit, we show that the
coupling among the qubit and the two resonators can induce effective electromechanical coupling which can attain the strong coupling regime at the single photon level with feasible experimental parameters. We use dispersive couplings among two resonators and the qubit while the qubit is also driven by an external classical field. These couplings form a three-wave mixing configuration among the three elements where the qubit degree of freedom can be adiabatically eliminated, and thus results in the enhanced coupling between the two resonators. Therefore, our work opens up the possibility of studying quantum nonlinear effect in circuit electromechanics.
09
Jun
2015
Origin and Implications of A2-like Contribution in the Quantization of Circuit-QED Systems
By placing an atom into a cavity, the electromagnetic mode structure of the cavity is modified. In Cavity QED, one manifestation of this phenomenon is the appearance of a gauge-dependent
diamagnetic term, known as the A2 contribution. Although in atomic Cavity QED, the resulting modification in the eigenmodes is negligible, in recent superconducting circuit realizations, such corrections can be observable and may have qualitative implications. We revisit the canonical quantization procedure of a circuit QED system consisting of a single superconducting transmon qubit coupled to a multimode superconducting microwave resonator. A complete derivation of the quantum Hamiltonian of an open circuit QED system consisting of a transmon qubit coupled to a leaky transmission line cavity is presented. We introduce a complete set of modes that properly conserves the current in the entire structure and present a sum rule for the dipole transition matrix elements of a multi-level transmon qubit coupled to a multi-mode cavity. Finally, an effective multi-mode Rabi model is derived with coefficients that are given in terms of circuit parameters.
07
Jun
2015
Quantum-enabled temporal and spectral mode conversion of microwave signals
Electromagnetic waves are ideal candidates for transmitting information in a quantum network as they can be routed rapidly and efficiently between locations using optical fibers or
microwave cables. Yet linking quantum-enabled devices with cables has proved difficult because most cavity or circuit quantum electrodynamics (cQED) systems used in quantum information processing can only absorb and emit signals with a specific frequency and temporal envelope. Here we show that the temporal and spectral content of microwave-frequency electromagnetic signals can be arbitrarily manipulated with a flexible aluminum drumhead embedded in a microwave circuit. The aluminum drumhead simultaneously forms a mechanical oscillator and a tunable capacitor. This device offers a way to build quantum microwave networks using separate and otherwise mismatched components. Furthermore, it will enable the preparation of non-classical states of motion by capturing non-classical microwave signals prepared by the most coherent circuit QED systems.
05
Jun
2015
Controllable coupling between a nanomechanical resonator and a coplanar-waveguide resonator via a superconducting flux qubit
We study a tripartite quantum system consisting of a coplanar-waveguide (CPW) resonator and a nanomechanical resonator (NAMR) connected by a flux qubit, where the flux qubit has a large
detuning from both resonators. By a unitray transformation and a second-order approximation, we obtain a strong and controllable (i.e., magnetic-field-dependent) effective coupling between the NAMR and the CPW resonator. Due to the strong coupling, vacuum Rabi splitting can be observed from the voltage-fluctuation spectrum of the CPW resonator. We further study the properties of single photon transport as inferred from the reflectance or equivalently the transmittance. We show that the reflectance and the corresponding phase shift spectra both exhibit doublet of narrow spectral features due to vacuum Rabi splitting. By tuning the external magnetic field, the reflectance and the phase shift can be varied from 0 to 1 and −π to π, respectively. The results indicate that this hybrid quantum system can act as a quantum router.