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
02
Apr
2013
Steady state entanglement of two superconducting qubits engineered by dissipation
We present a scheme for dissipative preparation of an entangled steady state of two superconducting qubits in a circuit QED setup. Combining resonator photon loss, a dissipative process
already present in the setup, with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art 3D, as well as with the more commonly used 2D transmons. The promising results of our study thus open a route for the demonstration of an entangled steady state in circuit QED.
Fluxon readout of a superconducting qubit
Quantum computing using superconducting circuits underwent rapid development in the last decade. This field has propelled from quantum manipulation of single two-level systems to complex
designs employing multiple coupled qubits allowing one to execute simple quantum algorithms. On the way to a practical quantum computer, a need for scalable interfaces between classical circuits and the quantum counterparts has arisen. Low-temperature superconducting single-flux quantum (SFQ) logic employs magnetic fluxons in Josephson transmission lines (JTLs) as basic bits for classical computation. Here, we report on an experiment implementing a direct link between SFQ electronics and a superconducting qubit. We demonstrate a readout of the state of a flux qubit through a frequency shift of a single fluxon oscillating in a JTL. The energy spectrum of the flux qubit is measured using this technique. The demonstrated approach may open ways to future full-scale integration of solid-state quantum computers with digital SFQ electronics.
Simulation of the Majorana equation in circuit QED
We propose a scheme to simulate the 1D Majorana equation with two Cooper pair boxes coupled to a 1D superconducting transmission line resonator, where strong coupling limit can be achieved.
With proper chosen of systematic parameters, we are able to engineer different kinds of interaction, which is indispensable in simulating the Majorana equation in an enlarged real Hilbert space. Measurement of the conserved observable of pseudo-helicity via transmission spectrum of the cavity field can verify the simulated Majorana wave function. The measurement results are experimentally resolvable based on our estimation with conservative parameters.
29
Mä
2013
Photon-mediated electron transport in hybrid circuit-QED
We explore theoretically photon-mediated transport processes in a hybrid circuit-QED structure consisting of two double quantum dots coupled to a common microwave cavity. Under suitable
resonance conditions, electron transport in one double quantum dot is facilitated by the transport in the other dot via photon-mediated processes through the cavity. We calculate the average current in the quantum dots, the mean cavity photon occupation, and the current cross-correlations using a recursive perturbation scheme that allows us to include the influence of the cavity order-by-order in the couplings between the cavity and the quantum dot systems. Within this framework we can clearly identify the photon-mediated processes in the transport.
28
Mä
2013
Local Geometric Phase and Quantum State Tomography in a Superconducting Qubit
We investigate quantum state reconstruction of a superconducting qubit threaded by an Aharonov-Bohm flux, with particular attention to the local geometric phase. A state reconstruction
scheme is introduced with a proper account of the local geometric phase generated by Faraday’s law of induction. Our scheme is based on measurement of three complementary quantities, that is, the extra charge and two local currents. Incorporating time-reversal symmetry and the Faraday’s law, we show that the full density matrix can be reconstructed without ambiguity in the choice of gauge. This procedure clearly demonstrates that the quantum Faraday effect plays an essential role in the dynamics of a quantum system that involves Aharonov-Bohm flux.
21
Mä
2013
Charge-SQUID and Tunable Phase-slip Flux Qubit
A phase-slip flux qubit, exactly dual to a charge qubit, is composed of a superconducting loop interrupted by a phase-slip junction. Here we propose a tunable phase-slip flux qubit
by replacing the phase-slip junction with a charge-related superconducting quantum interference device (SQUID) consisting of two phase-slip junctions connected in series with a superconducting island. This charge-SQUID acts as an effective phase-slip junction controlled by the applied gate voltage and can be used to tune the energy-level splitting of the qubit. Also, we show that a large inductance inserted in the loop can reduce the inductance energy and consequently suppress the dominating flux noise of the phase-slip flux qubit. This enhanced phase-slip flux qubit is exactly dual to a transmon qubit.
17
Mä
2013
Improved superconducting qubit coherence using titanium nitride
We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ~ 60 mu s by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared
to lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that previous planar transmon coherence times are limited by surface losses from two-level system (TLS) defects residing at or near interfaces. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators which is inconsistent with the predicted TLS model.
15
Mä
2013
Stabilizing a Bell state of two superconducting qubits by dissipation engineering
We propose a dissipation engineering scheme that prepares and protects a maximally entangled state of a pair of superconducting qubits. This is done by off-resonantly coupling the two
qubits to a low-Q cavity mode playing the role of a dissipative reservoir. We engineer this coupling by applying six continuous-wave microwave drives with appropriate frequencies. The two qubits need not be identical. We show that our approach does not require any fine-tuning of the parameters and requires only that certain ratios between them be large. With currently achievable coherence times, simulations indicate that a Bell state can be maintained over arbitrary long times with fidelities above 94%. Such performance leads to a significant violation of Bell’s inequality (CHSH correlation larger than 2.6) for arbitrary long times.
14
Mä
2013
Transmon-phonon coupling of plasma oscillations and lattice vibrations
In the transmon qubit we expect from conservation of momentum and energy a coupling between the plasma oscillations and the vibrations of the underlying lattice. Specifically, the electron
velocities and their kinetic energy density are boosted by the underlying lattice vibrations. We consider this effect in a representative transmon comprising two semi-circular superconducting charge islands joined by a Josephson junction. In particular, we solve the Fourier transform of a two-dimensional radial current density having inversion symmetry. The resulting spectral density is ohmic but also scales quadratically with the critical current I_c and logarithmically with the size of the transmon: J(w) ~ I_c^2 w log(kR). We make positive-definite Born-Markov approximations in a generalized Fermi’s Golden Rule and estimate the phonon-induced dephasing rate is negligible compared to current experiments.
13
Mä
2013
Tunable interfaces for realizing universal quantum computation with topological qubits
We propose to implement tunable interfaces for realizing universal quantum computation with topological qubits. One interface is between the topological and superconducting qubits,
which can realize arbitrary single-qubit gate on the topological qubit. When two qubits are involved, the interface between the topological qubits and a microwave cavity can induce a nontrivial two-qubit gate, which can not be constructed based on braiding operations. The two interfaces, being tunable via an external magnetic flux, may serve as the building blocks towards universal quantum computation with topological qubits.