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
06
Dez
2018
Mode Structure in Superconducting Metamaterial Transmission Line Resonators
Superconducting metamaterials are a promising resource for quantum information science. In the context of circuit QED, they provide a means to engineer on-chip, novel dispersion relations
and a band structure that could ultimately be utilized for generating complex entangled states of quantum circuitry, for quantum reservoir engineering, and as an element for quantum simulation architectures. Here we report on the development and measurement at millikelvin temperatures of a particular type of circuit metamaterial resonator composed of planar superconducting lumped-element reactances in the form of a discrete left-handed transmission line (LHTL). We discuss the details of the design, fabrication, and circuit properties of this system. As well, we provide an extensive characterization of the dense mode spectrum in these metamaterial resonators, which we conducted using both microwave transmission measurements and laser scanning microscopy (LSM). Results are observed to be in good quantitative agreement with numerical simulations and also an analytical model based upon current-voltage relationships for a discrete transmission line. In particular, we demonstrate that the metamaterial mode frequencies, spatial profiles of current and charge densities, and damping due to external loading can be readily modeled and understood, making this system a promising tool for future use in quantum circuit applications and for studies of complex quantum systems.
04
Dez
2018
Nonexponential decay of a giant artificial atom
In quantum optics, light-matter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger
than the atomic dimensions. In contrast, here we experimentally demonstrate the vastly different giant atom regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal time-delay for the field to propagate across the giant atom, giving rise to non-Markovian dynamics. We demonstrate the non-Markovian character of the giant atom in the frequency spectrum as well as nonexponential relaxation in the time domain.
02
Dez
2018
Hamiltonian quantum computing with superconducting qubits
can be implemented using a 2D array of superconducting transmon qubits. We show how the scheme requires the"]engineering of strong attractive cross-Kerr and weak flip-flop or hopping interactions and we detail how this can be achieved. Our proposal uses a new electric circuit for obtaining the attractive cross-Kerr coupling between transmons via a dipole-like element. We discuss and numerically analyze the forward motion and execution of the computation and its dependence on coupling strengths and their variability. We extend [1] by explicitly showing how to construct a direct Toffoli gate, thus establishing computational universality via the Hadamard and Toffoli gate or via controlled- Hadamard, Hadamard and CNOT.
30
Nov
2018
Tuning the energy gap of a flux qubit by AC-Zeeman shift
We propose to tune the minimal energy level splitting of a superconducting qubit by a microwave induced ac Zeeman shift. We experimentally investigate the usability of this approach
to overcome parameter spread induced by the micro fabrication of superconducting artificial quantum circuits. To do so, we dress the qubit by a strong tone, effectively shifting its energy levels. By a two-tone spectroscopy of this dressed system the shift of the qubit’s energy levels can be probed. A theoretical treatment allowed us to completely explain the observed experimental dependencies and reconstruct the influence of the strong driving to the dissipative dynamics of the qubit.
29
Nov
2018
Using a Recurrent Neural Network to Reconstruct Quantum Dynamics of a Superconducting Qubit from Physical Observations
At it’s core, Quantum Mechanics is a theory developed to describe fundamental observations in the spectroscopy of solids and gases. Despite these practical roots, however, quantum
theory is infamous for being highly counterintuitive, largely due to its intrinsically probabilistic nature. Neural networks have recently emerged as a powerful tool that can extract non-trivial correlations in vast datasets. They routinely outperform state-of-the-art techniques in language translation, medical diagnosis and image recognition. It remains to be seen if neural networks can be trained to predict stochastic quantum evolution without a priori specifying the rules of quantum theory. Here, we demonstrate that a recurrent neural network can be trained in real time to infer the individual quantum trajectories associated with the evolution of a superconducting qubit under unitary evolution, decoherence and continuous measurement from raw observations only. The network extracts the system Hamiltonian, measurement operators and physical parameters. It is also able to perform tomography of an unknown initial state without any prior calibration. This method has potential to greatly simplify and enhance tasks in quantum systems such as noise characterization, parameter estimation, feedback and optimization of quantum control.
27
Nov
2018
Fast flux control of 3D transmon qubits using a magnetic hose
Fast magnetic flux control is a crucial ingredient for circuit quantum electrodynamics (cQED) systems. So far it has been a challenge to implement this technology with the high coherence3D cQED architecture. In this paper we control the magnetic field inside a superconducting waveguide cavity using a magnetic hose, which allows fast flux control of 3D transmon qubits on time scales < 100 ns. The hose is designed as an effective microwave filter to not compromise the energy relaxation time of the qubit. The magnetic hose is a promising tool for fast magnetic flux control in various platforms intended for quantum information processing and quantum optics. [/expand]
25
Nov
2018
Mechanically Generating Entangled Photons from the Vacuum: A Microwave Circuit-Acoustic Resonator Analogue of the Unruh Effect
We consider a model for an oscillatory, relativistic accelerating photodetector inside a cavity and show that the entangled photon pair production from the vacuum (Unruh effect) can
be accurately described in the steady state by a non-degenerate parametric amplifier (NDPA), with the detector’s accelerating center of mass serving as the parametric drive (pump). We propose an Unruh effect analogue NDPA microwave superconducting circuit scheme, where the breathing mode of the coupling capacitance between the cavity and detector provides the mechanical pump. For realizable circuit parameters, the resulting photon production from the vacuum should be detectable.
23
Nov
2018
Cavity-free vacuum-Rabi splitting in circuit quantum acoustodynamics
Artificial atoms coupled to surface acoustic waves (SAWs) have played a crucial role in the recent development of circuit quantum acoustodynamics (cQAD). In this paper, we have investigated
the interaction of an artificial atom and SAWs beyond the weak coupling regime, focusing on the role of the interdigital transducer (IDT) that enables the coupling. We find a parameter regime in which the IDT acts as a cavity for the atom, rather than an antenna. In other words, the atom forms its own cavity. Similar to an atom coupled to an explicit cavity, this regime is characterized by vacuum-Rabi splitting, as the atom hybridizes with the phononic vacuum inside the IDT. This hybridization is possible because of the interdigitated coupling, which has a large spatial extension, and the slow propagation speed of SAWs. We work out a criterion for entering this regime from a model based on standard circuit-quantization techniques, taking only material parameters as inputs. Most notably, we find this regime hard to avoid for an atom on top of a strong piezoelectric material, such as LiNbO3. The SAW-coupled atom on top of LiNbO3 can thus be regarded as an atom-cavity-bath system. On weaker piezoelectric materials, the number of IDT electrodes need to be large in order to reach this regime.
22
Nov
2018
Scalable Realization of Surface Code Quantum Memory by Applying Multi-Qubit Parity Detector Gates
We analytically designed the control bias pulses to realize new multi-qubit parity detector gates for 2-Dimensional (2D) array of superconducting flux qubits with non-tunable couplings.
We designed two 5-qubit gates such that the middle qubit is the target qubit and all four coupled neighbors are the control qubits. These new gates detect the parity between two vertically/horizontally coupled neighbor qubits while cancelling out the coupling effect of horizontally/vertically coupled neighbor qubits. For a 3 by 3 array of 9 qubits with non-tunable couplings, we simulated the effect of our new 5-qubit horizontal and vertical parity detector gates. We achieved the intrinsic fidelity of 99.9% for horizontal and vertical parity detector gates. In this paper we realize Surface Code memories based on the multi-qubit parity detector gates for nearest neighbor superconducting flux qubits with and without tunable couplings. However, our scheme is applicable to other superconducting qubits as well. In our proposed memory realization, error correction cycles can be performed in parallel on several logical qubits or even on the entire 2D array of qubits, this makes it a desirable candidate for large scale and longtime quantum computation. In addition to extensive reduction of the number of control parameters in our method, the error correction cycle time is reduced and does not grow by increasing the number of qubits in the logical qubit layout. Another advantage of this approach is that there will not be any dephasing from idle qubits since all the qubits are used in the error correction cycles.
Ideal Quantum Nondemolition Readout of a Flux Qubit Without Purcell Limitations
Dispersive coupling based on the Rabi model with large detuning is widely used for quantum nondemolition (QND) qubit readout in quantum computation. However, the measurement speed and
fidelity are usually significantly limited by the Purcell effects, i.e.: Purcell decay, critical photon numbers, and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its direct dispersive coupling to a boundary-tunable measurement resonator. We show that this novel readout mechanism is free of dipole-field interactions, and that the qubit-QND measurement is not deteriorated by intracavity photons. Both qubit-readout speed and fidelity can avoid the Purcell limitations. Moreover, this direct dispersive coupling can be conveniently turned on and off via an external control flux. We show how to extend this proposal to a multi-qubit architecture for a joint qubit readout.