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
08
Aug
2014
The Interspersed Spin Boson Lattice Model
. We"]explore the full phase diagram of the model using Matrix-Product-State methods. Guided by these numerical results, we describe a modified variational ansatz to improve our analytic description of the groundstate at low boson frequencies. Additionally, we introduce an experimental protocol capable of inferring the low-energy excitations of the system by means of Fano scattering spectroscopy. Finally, we discuss the implementation and characterization of this model with current circuit-QED technology.
Tunable-Cavity QED with Phase Qubits
We describe a tunable-cavity QED architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both microwave
readout of tunneling and dispersive measurements of the qubit. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling and detuning. A tunable cavity frequency provides a way to strongly vary both the qubit-cavity detuning and coupling strength, which can reduce Purcell losses, cavity-induced dephasing of the qubit, and residual bus coupling for a system with multiple qubits. With our qubit-cavity system, we show that dynamic control over the cavity frequency enables one to avoid Purcell losses during coherent qubit evolutions and optimize state readout during qubit measurements. The maximum qubit decay time T1 = 1.5 μs is found to be limited by surface dielectric losses from a design geometry similar to planar transmon qubits.
07
Aug
2014
Robust manipulation of superconducting qubits in the presence of fluctuations
Superconducting quantum systems are promising candidates for quantum information processing due to their scalability and design flexibility. However, the existence of defects, fluctuations,
and inaccuracies is unavoidable for practical superconducting quantum circuits. In this paper, a sampling-based learning control (SLC) method is used to guide the design of control fields for manipulating superconducting quantum systems. Numerical results for one-qubit systems and coupled two-qubit systems show that the „smart“ fields learned using the SLC method can achieve robust manipulation of superconducting qubits even in the presence of large fluctuations and inaccuracies.
05
Aug
2014
Passive correction of quantum logical errors in a driven, dissipative system: a blueprint for an analog quantum code fabric
A physical realization of self correcting quantum code would be profoundly useful for constructing a quantum computer. In this theoretical work, we provide a partial solution to major
challenges preventing self correcting quantum code from being engineered in realistic devices. We consider a variant of Kitaev’s toric code coupled to propagating bosons, which induce a long-ranged interaction between anyonic defects. By coupling the primary quantum system to an engineered dissipation source through resonant energy transfer, we demonstrate a „rate barrier“ which leads to a potentially enormous increase in the system’s quantum state lifetime through purely passive quantum error correction, even when coupled to an infinite temperature bath. While our mechanism is not scalable to infinitely large systems, the maximum effective size can be very large, and it is fully compatible with active error correction schemes. Our model uses only on-site and nearest-neighbor interactions, and could be implemented in superconducting qubits. We sketch one such implementation at the end of this work.
Photon-assisted tunneling with non-classical light
Among the most exciting recent advances in the field of superconducting quantum circuits is the ability to coherently couple microwave photons in low-loss cavities to quantum electronic
conductors (e.g.~semiconductor quantum dots or carbon nanotubes). These hybrid quantum systems hold great promise for quantum information processing applications; even more strikingly, they enable exploration of completely new physical regimes. Here we study theoretically the new physics emerging when a quantum electronic conductor is exposed to non-classical microwaves (e.g.~squeezed states, Fock states). We study this interplay in the experimentally-relevant situation where a superconducting microwave cavity is coupled to a conductor in the tunneling regime. We find the quantum conductor acts as a non-trivial probe of the microwave state; in particular, the emission and absorption of photons by the conductor is characterized by a non-positive definite quasi-probability distribution. This negativity has a direct influence on the conductance of the conductor.
02
Aug
2014
Accurate Qubit Control with Single Flux Quantum Pulses
We describe the coherent manipulation of harmonic oscillator and qubit modes using resonant trains of single flux quantum pulses in place of microwaves. We show that coherent rotations
are obtained for pulse-to-pulse spacing equal to the period of the oscillator. We consider a protocol for preparing bright and dark harmonic oscillator pointer states. Next we analyze rotations of a two-state qubit system. We calculate gate errors due to timing jitter of the single flux quantum pulses and due to weak anharmonicity of the qubit. We show that gate fidelities in excess of 99.9% are achievable for sequence lengths of order 20 ns.
01
Aug
2014
Large gain quantum-limited qubit measurement using a two-mode nonlinear cavity
We provide a thorough theoretical analysis of qubit state measurement in a setup where a driven, parametrically-coupled cavity system is directly coupled to the qubit, with one of the
cavities having a weak Kerr nonlinearity. Such a system could be readily realized using circuit QED architectures. We demonstrate that this setup is capable in the standard linear-response regime of both producing a highly amplified output signal while at the same time achieving near quantum-limited performance: the measurement backaction on the qubit is near the minimal amount required by the uncertainty principle. This setup thus represents a promising route for performing efficient large-gain qubit measurement that is completely on-chip, and that does not rely on the use of circulators or complex non-reciprocal amplifiers.
27
Jul
2014
Josephson parametric phase-locked oscillator and its application to dispersive readout of superconducting qubits
The parametric phase-locked oscillator (PPLO), also known as a parametron, is a resonant circuit in which one of the reactances is periodically modulated. It can detect, amplify, and
store binary digital signals in the form of two distinct phases of self-oscillation. Indeed, digital computers using PPLOs based on a magnetic ferrite ring or a varactor diode as its fundamental logic element were successfully operated in 1950s and 1960s. More recently, basic bit operations have been demonstrated in an electromechanical resonator, and an Ising machine based on optical PPLOs has been proposed. Here, using a PPLO realized with Josephson-junction circuitry, we demonstrate the demodulation of a microwave signal digitally modulated by binary phase-shift keying. Moreover, we apply this demodulation capability to the dispersive readout of a superconducting qubit. This readout scheme enables a fast and latching-type readout, yet requires only a small number of readout photons in the resonator to which the qubit is coupled, thus featuring the combined advantages of several disparate schemes. We have achieved high-fidelity, single-shot, and non-destructive qubit readout with Rabi-oscillation contrast exceeding 90%, limited primarily by the qubit’s energy relaxation.
24
Jul
2014
Photon-mediated interactions between distant artificial atoms
Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quantum simulations and quantum information processing. The exchange of real and virtual
photons between atoms gives rise to non-trivial interactions the strength of which decreases rapidly with distance in three dimensions. Here we study much stronger photon mediated interactions using two superconducting qubits in an open onedimensional transmission line. Making use of the unique possibility to tune these qubits by more than a quarter of their transition frequency we observe both coherent exchange interactions at an effective separation of 3λ/4 and the creation of super- and sub-radiant states at a separation of one photon wavelength λ. This system is highly suitable for exploring collective atom/photon interactions and applications in quantum communication technology.
23
Jul
2014
Quantum simulation of DIII class topological superconductors using superconducting quantum circuits
An exotic DIII model of one-dimensional p-wave spin-triplet superconductors with the ℤ2 topological phase protected by the time-reversal symmetry is simulated by an array of inductively
coupled transmon qubits with tunable nearest-neighbor couplings and scalability. The anti-commutation relation between opposite spin components in the DIII model is realized by a novel dispersive dynamic modulation approach, while previous schemes consider only spinless fermions. Our detailed analysis reveals that distinctive topological phenomena can be visualized with the state-of-the-art technology in this superconducting-circuit array.