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
09
Okt
2016
Efficient transfer of an arbitrary qutrit state in circuit QED
Compared with a qubit, a qutrit (i.e., three-level quantum system) has a larger Hilbert space and thus can be used to encode more information in quantum information processing and communication.
Here, we propose a scheme to transfer an arbitrary quantum state between two flux qutrits coupled to two resonators. This scheme is simple because it only requires two basic operations. The state-transfer operation can be performed fast because of using resonant interactions only. Numerical simulations show that high-fidelity transfer of quantum states between the two qutrits is feasible with current circuit-QED technology. This scheme is quite general and can be applied to accomplish the same task for other solid-state qutrits coupled to resonators.
08
Okt
2016
High Quality Stepped-impedance Resonators suitable for Circuit-QED Measurement of Superconducting Artificial Atoms
High quality factor coplanar resonators are critical elements in superconducting quantum circuits. We describe the design, fabrication and measurement of stepped impedance resonators
(SIRs), which have more compact size than commonly used uniform impedance resonators (UIRs). With properly chosen impedance ratio, SIRs can be much shorter in total length than that of UIRs. Two kinds of designs containing both SIRs and UIRs are fabricated and measured. The power dependence of the extracted internal quality factors (Qi) for all the resonators indicated that SIRs and UIRs had comparable performance at high incident power. However, as the incident power decreased, the internal quality factor of SIRs decreased much slower than that of UIRs. All the SIRs in design I kept near half-million Qi at single-photon level, while the two UIRs on the same chip decreased heavily to less than 2×105. These results indicate potential advantages of SIRs in quantum computer architectures: they consume less space than UIRs, while perform excellent under single-photon level. The resonators in design II were measured under a large residual magnetic field. The measured results showed that the internal quality factor of all the SIRs and UIRs were more or less suppressed. Such behavior confirmed that trapped vortices in the coplanar resonators provide another loss channel.
07
Okt
2016
Tunable quantum gate between a superconducting atom and a propagating microwave photon
We propose a two-qubit quantum logic gate between a superconducting atom and a propagating microwave photon. The atomic qubit is encoded on its lowest two levels and the photonic qubit
is encoded on its carrier frequencies. The gate operation completes deterministically upon reflection of a photon, and various two-qubit gates (SWAP, SWAP‾‾‾‾‾‾‾√, and Identity) are realized through {\it in situ} control of the drive field. The proposed gate is applicable to construction of a network of superconducting atoms, which enables gate operations between non-neighboring atoms.
Quantum information processing with superconducting circuits: a review
During the last ten years, superconducting circuits and systems have passed from interesting physical devices to contenders for useful information processing in the near future. There
are now advanced simulation experiments with nine qubits, and commitments to demonstrate quantum supremacy with fifty qubits within just a few years. The time is therefore ripe for providing an overview of superconducting devices and systems: to discuss the state of the art of applications to quantum information processing (QIP), and to describe recent and upcoming applications of superconducting systems to digital and analogue computing and simulation in Physics and Chemistry. On top of that, the review will try to address general questions like „What can a quantum computer do that a classical computer can’t?“.
04
Okt
2016
A fluxonium-based artificial molecule with a tunable magnetic moment
Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for
building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for protected qubits and quantum simulation.
Resolving magnon number states in quantum magnonics
Collective excitation modes in solid state systems play a central role in circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics. In the latter, quanta of collective
excitation modes in a ferromagnet, called magnons, interact with qubits to provide the nonlinearity necessary to access quantum phenomena in magnonics. A key ingredient for future quantum magnonics systems is the ability to probe magnon states. Here we observe individual magnons in a millimeter-sized ferromagnet coherently coupled to a superconducting qubit. Specifically, we resolve magnon number states in spectroscopic measurements of a transmon qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic dipole of the ferromagnet equivalent to a single spin flipped among more than 1019 spins. The strong dispersive regime of quantum magnonics opens up the possibility of encoding superconducting qubits into non-classical magnon states, potentially providing a coherent interface between a superconducting quantum processor and optical photons.
Towards phase-coherent caloritronics in superconducting quantum circuits
The emerging field of coherent caloritronics (from the Latin word „calor“, i.e., heat) is based on the possibility to manipulate the phase-coherent heat currents flowing
in mesoscopic superconducting structures. The goal is to design and implement quantum technologies able to master energy transfer with the same degree of accuracy reached for charge transport in contemporary electronic devices. This can be obtained by exploiting the macroscopic quantum coherence intrinsic to superconducting condensates, which manifests itself through the Josephson and the proximity effect. Here, we review recent experimental results obtained in the realization of heat interferometers and thermal rectifiers, and discuss a few proposals for exotic non-linear phase-coherent caloritronic devices, such as thermal transistors, solid-state memories, coherent heat splitters, microwave refrigerators, thermal engines and heat valves. Besides being extremely attractive from the fundamental physics point of view, these systems are expected to have a vast impact on all cryogenic microcircuits requiring energy management, and (possibly) lay the first stone for the foundation of electronic thermal logic.
30
Sep
2016
A model study of present-day Hall-effect circulators
Stimulated by the recent implementation of a three-port Hall-effect microwave circulator of Mahoney et al. (MEA), we present model studies of the performance of this device. Our calculations
are based on the capacitive-coupling model of Viola and DiVincenzo (VD). Based on conductance data from a typical Hall-bar device obtained from a two-dimensional electron gas (2DEG) in a magnetic field, we numerically solve the coupled field-circuit equations to calculate the expected performance of the circulator, as determined by the S parameters of the device when coupled to 50Ω ports, as a function of frequency and magnetic field. Above magnetic fields of 1.5T, for which a typical 2DEG enters the quantum Hall regime (corresponding to a Landau-level filling fraction ν of 20), the Hall angle θH=tan−1σxy/σxx always remains close to 90∘, and the S parameters are close to the analytic predictions of VD for θH=π/2. As anticipated by VD, MEA find the device to have rather high (kΩ) impedance, and thus to be extremely mismatched to 50Ω, requiring the use of impedance matching. We incorporate the lumped matching circuits of MEA in our modeling and confirm that they can produce excellent circulation, although confined to a very small bandwidth. We predict that this bandwidth is significantly improved by working at lower magnetic field when the Landau index is high, e.g. ν=20, and the impedance mismatch is correspondingly less extreme. Our modeling also confirms the observation of MEA that parasitic port-to-port capacitance can produce very interesting countercirculation effects.
Staggered quantum walks with superconducting microwave resonators
The staggered quantum walk model on a graph is defined by an evolution operator that is the product of local operators related to two or more independent graph tessellations. A graph
tessellation is a partition of the set of nodes that respects the neighborhood relation. Flip-flop coined quantum walks with the Hadamard or Grover coins can be expressed as staggered quantum walks by converting the coin degree of freedom into extra nodes in the graph. We propose an implementation of the staggered model with superconducting microwave resonators, where the required local operations are provided by the nearest neighbor interaction of the resonators coupled through superconducting quantum interference devices. The tunability of the interactions makes this system an excellent toolbox for this class of quantum walks. We focus on the one-dimensional case and discuss its generalization to a more general class known as triangle-free graphs.
29
Sep
2016
Direct Probe of Topological Invariants Using Bloch Oscillating Quantum Walks
The topology of a single-particle band structure plays a fundamental role in understanding a multitude of physical phenomena. Motivated by the connection between quantum walks and such
topological band structures, we demonstrate that a simple time-dependent, Bloch-oscillating quantum walk enables the direct measurement of topological invariants. We consider two classes of one-dimensional quantum walks and connect the global phase imprinted on the walker with its refocusing behavior. By disentangling the dynamical and geometric contributions to this phase we describe a general strategy to measure the topological invariant in these quantum walks. As an example, we propose an experimental protocol in a circuit QED architecture where a superconducting transmon qubit plays the role of the coin, while the quantum walk takes place in the phase space of a cavity.