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
14
Jan
2014
A hybrid quantum system: Coupling spin ensembles via superconducting flux qubits
We study a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the NV centers in a diamond crystal and the
nearestneighbor spin ensembles are effectively coupled via a flux qubit. We show that the coupling strengths between flux qubits and spin ensembles can reach the strong and even ultrastrong coupling regimes by either engineering the hybrid structure in advance or tuning the excitation frequencies of spin ensembles via external magnetic fields. When extending the hybrid structure to an array with equal coupling strengths, we find that in the strong coupling regime, the hybrid array is reduced to a tight-binding model of a 1D bosonic lattice. In the ultrastrong coupling regime, it exhibits quasi-particle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition which is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.
13
Jan
2014
Defining and detecting quantum speedup
The development of small-scale digital and analog quantum devices raises the question of how to fairly assess and compare the computational power of classical and quantum devices, and
of how to detect quantum speedup. Here we show how to define and measure quantum speedup in various scenarios, and how to avoid pitfalls that might mask or fake quantum speedup. We illustrate our discussion with data from a randomized benchmark test on a D-Wave Two device with up to 503 qubits. Comparing the performance of the device on random spin glass instances with limited precision to simulated classical and quantum annealers, we find no evidence of quantum speedup when the entire data set is considered, and obtain inconclusive results when comparing subsets of instances on an instance-by-instance basis. Our results for one particular benchmark do not rule out the possibility of speedup for other classes of problems and illustrate that quantum speedup is elusive and can depend on the question posed.
01
Jan
2014
Fast Scalable State Measurement with Superconducting Qubits
Progress in superconducting qubit experiments with greater numbers of qubits or advanced techniques such as feedback requires faster and more accurate state measurement. We have designed
a multiplexed measurement system with a bandpass filter that allows fast measurement without increasing environmental damping of the qubits. We use this to demonstrate simultaneous measurement of four qubits on a single superconducting integrated circuit, the fastest of which can be measured to 99.8% accuracy in 140ns. This accuracy and speed is suitable for advanced multi-qubit experiments including surface code error correction.
27
Dez
2013
Parametric amplification by coupled flux qubits
We report the parametric amplification of a microwave signal in a Kerr medium formed from superconducting qubits. Two mutually coupled flux qubits, embedded in the current antinode
of a superconducting coplanar waveguide resonator, are used as a nonlinear element. Shared Josephson junctions provide the qubit-resonator coupling, resulting in a device with a measured gain of about 20 dB. We argue, that this arrangement represents a unit cell which can be straightforwardly extended to a quasi one-dimensional quantum metamaterial with a large tunable Kerr nonlinearity.
20
Dez
2013
Study of a quantized resonator field coupled to a current-biased Josephson junction in circuit QED
A Lagrangian formalism is used to derive the Hamiltonian for a λ/4-resonator shunted by a current-biased Josephson junction. The eigenstates and the quantum dynamics of the system
are analyzed numerically, and we show that the system can function as an efficient detector of weak incident microwave fields.
18
Dez
2013
Spin-Orbit Qubit on a Multiferroic Insulator in a Superconducting Resonator
We propose a spin-orbit qubit in a nanowire quantum dot on the surface of a multiferroic insulator with a cycloidal spiral magnetic order. The spiral exchange field from the multiferroic
insulator causes inhomogeneous Zeeman-like interaction on the electron spin in the quantum dot, assisting the realization of a spin-orbit qubit. The absence of an external magnetic field benefits the integration of such spin-orbit qubit into high-quality superconducting resonators for manipulation. By exploiting the Rashba spin-orbit coupling in the quantum dot via a gate voltage, one can obtain an effective spin-photon coupling with an efficient on/off switching. This makes the proposed device promising for hybrid quantum communications.
11
Dez
2013
Direct spectrum analysis using a threshold detector with application to a superconducting circuit
We introduce a new and quantitative theoretical framework for noise spectral analysis using a threshold detector, which is then applied to a superconducting device: the Cavity BifurcationAmplifier (CBA). We show that this new framework provides direct access to the environmental noise spectrum with a sensitivity approaching the standard quantum limit of weak continuous measurements. In addition, the accessible frequency range of the spectrum is, in principle, limited only by the ring down time of the resonator. This on-chip noise detector is non-dissipative and works with low probing powers, allowing it to be operated at low temperatures (T<10mK). We exploit this technique for measuring the frequency fluctuations of the CBA and find a low frequency noise with an amplitude and spectrum compatible with a dielectric origin.[/expand]
10
Dez
2013
Observation of a Dissipation-Induced Classical to Quantum Transition
The emergence of non-trivial structure in many-body physics has been a central topic of research bearing on many branches of science. Important recent work has explored the non-equilibrium
quantum dynamics of closed many-body systems. Photonic systems offer a unique platform for the study of open quantum systems. We report here the experimental observation of a novel dissipation driven dynamical localization transition of strongly correlated photons in an extended superconducting circuit. Monitoring the homodyne signal reveals this transition to be from a regime of classical oscillations into a macroscopically self-trapped state manifesting revivals, a fundamentally quantum phenomenon. This experiment also demonstrates a new class of scalable quantum simulators with well controlled coherent and dissipative dynamics suited to the study of quantum many-body phenomena out of equilibrium.
06
Dez
2013
Dynamically protected cat-qubits: a new paradigm for universal quantum computation
We present a new hardware-efficient paradigm for universal quantum computation which is based on encoding, protecting and manipulating quantum information in a quantum harmonic oscillator.
This proposal exploits multi-photon driven dissipative processes to encode quantum information in logical bases composed of Schr\“odinger cat states. More precisely, we consider two schemes. In a first scheme, a two-photon driven dissipative process is used to stabilize a logical qubit basis of two-component Schr\“odinger cat states. While such a scheme ensures a protection of the logical qubit against the photon dephasing errors, the prominent error channel of single-photon loss induces bit-flip type errors that cannot be corrected. Therefore, we consider a second scheme based on a four-photon driven dissipative process which leads to the choice of four-component Schr\“odinger cat states as the logical qubit. Such a logical qubit can be protected against single-photon loss by continuous photon number parity measurements. Next, applying some specific Hamiltonians, we provide a set of universal quantum gates on the encoded qubits of each of the two schemes. In particular, we illustrate how these operations can be rendered fault-tolerant with respect to various decoherence channels of participating quantum systems. Finally, we also propose experimental schemes based on quantum superconducting circuits and inspired by methods used in Josephson parametric amplification, which should allow to achieve these driven dissipative processes along with the Hamiltonians ensuring the universal operations in an efficient manner.
04
Dez
2013
Notes on implementing generalized measurements with superconducting qubits
We describe a method to perform any generalized purity-preserving measurement of a qubit with techniques tailored to superconducting systems. We start with considering two methods for
realizing a two-outcome partial projection: using a thresholded continuous measurement in the circuit QED setup and using an indirect ancilla qubit measurement. Then we decompose an arbitrary purity-preserving two-outcome measurement into single qubit unitary rotations and a partial projection. Finally, we systematically reduce any multiple-outcome measurement to a sequence of two-outcome measurements and unitary operations.