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
01
Feb
2016
Superconducting qubit-oscillator circuit beyond the ultrastrong-coupling regime
To control light-matter interaction at the single-quantum level in cavity quantum electrodynamics (cavity-QED) or circuit-QED, strong coupling between the light and matter components
is indispensable. Specifically, the coupling rate g must be larger than the decay rates. If g is increased further and becomes as large as the frequencies of light and matter excitations, the energy eigenstates including the ground state are predicted to be highly entangled. This qualitatively new coupling regime can be called the deep strong-coupling regime. One approach toward the deep strong-coupling regime is to use huge numbers of identical systems to take advantage of ensemble enhancement. With the emergence of so-called macroscopic artificial atoms, superconducting qubits for example, it has become possible for a single artificial atom to realize ultrastrong coupling, where ℏg exceeds ~10% of the energies of the qubit ℏωq and the harmonic oscillator ℏωo. By making use of the macroscopic magnetic dipole moment of a flux qubit, large zero-point-fluctuation current of an LC oscillator, and large Josephson inductance of a coupler junction, we have realized circuits in the deep strong-coupling regime, where g/ωo ranges from 0.72 to 1.34 and g/ωq >> 1. Using energy spectroscopy measurements, we have observed unconventional transition spectra between Schrodinger cat-like energy eigenstates. These states involve quantum superpositions of Fock states with phase-space displacements of ±g/ωo and remarkably survive with environmental noise. Our results provide a basis for ground-state-based entangled-pair generation and open a new direction in circuit-QED.
Ultrastrong coupling of a single artificial atom to an electromagnetic continuum
The study of the interaction of light and matter has led to many fundamental discoveries as well as numerous important technologies. Over the last decades, great strides have been made
in increasing the strength of this interaction at the single-photon level, leading to a continual exploration of new physics and applications. In recent years, a major achievement has been the demonstration of the so-called strong coupling regime, a key advancement enabling great progress in quantum information science. In this work, we demonstrate light-matter interaction over an order of magnitude stronger than previously reported, reaching a new regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest values of the coupling, the spontaneous emission rate of the atom is comparable to its transition frequency. In this USC regime, the conventional quantum description of the atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required. Our results open the door to a wealth of new physics and applications. Beyond light-matter interaction itself, the tunability of our system makes it promising as a tool to study a number of important physical systems such as the well-known spin-boson and Kondo models.
31
Jan
2016
Protected ground states in short chains of coupled spins in circuit quantum electrodynamics
The quasi-degenerate ground state manifold of the anisotropic Ising spin model can encode quantum information, but its degree of protection against local perturbations is known to be
only partial. We explain how the coupling between the two ground states can be used to observe signatures of Majorana zero modes in a small controlled chain of qubits. We argue that the protection against certain local perturbations persists across a range of parameters even away from the ideal point. Remarkably, when additional non-local interactions are considered the system enters a phase where the ground states are fully protected against all local field perturbations.
30
Jan
2016
Towards Quantum Simulation of Chemical Dynamics with Prethreshold Superconducting Qubits
The single excitation subspace (SES) method for universal quantum simulation is investigated for a number of diatomic molecular collision complexes. Assuming a system of n tunably-coupled,
and fully-connected superconducting qubits, computations are performed in the n-dimensional SES which maps directly to an n-channel collision problem within a diabatic molecular wave function representation. Here we outline the approach on a classical computer to solve the time-dependent Schr\“odinger equation in an n-dimensional molecular basis – the so-called semiclassical molecular-orbital close-coupling (SCMOCC) method – and extend the treatment beyond the straight-line, constant-velocity approximation which is restricted to large kinetic energies (≳0.1 keV/u). We explore various multichannel potential averaging schemes and an Ehrenfest symmetrization approach to allow for the application of the SCMOCC method to much lower collision energies (approaching 1 eV/u). In addition, a computational efficiency study for various propagators is performed to speed-up the calculations on classical computers. These computations are repeated for the simulation of the SES approach assuming typical parameters for realistic pretheshold superconducting quantum computing hardware. The feasibility of applying future SES processors to the quantum dynamics of large molecular collision systems is briefly discussed.
Four-junction superconducting circuit in both flux and phase regimes
We develop a theory for the quantum circuit consisting of a superconducting loop interrupted by four Josephson junctions and pierced by a magnetic flux (either static or time-dependent).
In addition to the similarity with the typical three-junction flux qubit, we demonstrate the difference of the four-junction circuit from its three-junction analogue, especially its distinct advantages over the latter. Moreover, the four-junction circuit in the phase regime is also investigated. Our theory provides a tool to explore the physical properties of this four-junction superconducting circuit.
29
Jan
2016
New class of quantum error-correcting codes for a bosonic mode
We construct a new class of quantum error-correcting codes for a bosonic mode which are advantageous for applications in quantum memories, communication, and scalable computation. These
`binomial quantum codes‘ are formed from a finite superposition of Fock states weighted with binomial coefficients. The binomial codes can exactly correct errors that are polynomial up to a specific degree in bosonic creation and annihilation operators, including amplitude damping and displacement noise as well as boson addition and dephasing errors. For realistic continuous-time dissipative evolution, the codes can perform approximate quantum error correction to any given order in the timestep between error detection measurements. We present an explicit approximate quantum error recovery operation based on projective measurements and unitary operations. The binomial codes are tailored for detecting boson loss and gain errors by means of measurements of the generalized number parity. We discuss optimization of the binomial codes and demonstrate that by relaxing the parity structure, codes with even lower unrecoverable error rates can be achieved. The binomial codes are related to existing two-mode bosonic codes but offer the advantage of requiring only a single bosonic mode to correct amplitude damping as well as the ability to correct other errors. Our codes are similar in spirit to `cat codes‘ based on superpositions of the coherent states, but offer several advantages such as smaller mean number, exact rather than approximate orthonormality of the code words, and an explicit unitary operation for repumping energy into the bosonic mode. The binomial quantum codes are realizable with current superconducting circuit technology and they should prove useful in other quantum technologies, including bosonic quantum memories, photonic quantum communication, and optical-to-microwave up- and down-conversion.
28
Jan
2016
Coherent dynamics and decoherence in a superconducting weak link
We demonstrate coherent dynamics of quantized magnetic fluxes in a superconducting loop with a weak link – a nanobridge patterned from the same thin NbN film as the loop. The
bridge is a short rounded shape constriction, close to 10 nm long and 20 – 30 nm wide, having minimal width at its center. Quantum state control and coherent oscillations in the driven time evolution of the tunnel-junctionless system are achieved. Decoherence and energy relaxation in the system are studied using a combination of microwave spectroscopy and direct time-domain techniques. The effective flux noise behavior suggests inductance fluctuations as a possible cause of the decoherence.
27
Jan
2016
Efficient single sideband microwave to optical conversion using an electro-optical whispering gallery mode resonator
Linking classical microwave electrical circuits to the optical telecommunication band is at the core of modern communication. Future quantum information networks will require coherent
microwave-to-optical conversion to link electronic quantum processors and memories via low-loss optical telecommunication networks. Efficient conversion can be achieved with electro-optical modulators operating at the single microwave photon level. In the standard electro-optic modulation scheme this is impossible because both, up- and downconverted, sidebands are necessarily present. Here we demonstrate true single sideband up- or downconversion in a triply resonant whispering gallery mode resonator by explicitly addressing modes with asymmetric free spectral range. Compared to previous experiments, we show a three orders of magnitude improvement of the electro-optical conversion efficiency reaching 0.1% photon number conversion for a 10GHz microwave tone at 0.42mW of optical pump power. The presented scheme is fully compatible with existing superconducting 3D circuit quantum electrodynamics technology and can be used for non-classical state conversion and communication. Our conversion bandwidth is larger than 1MHz and not fundamentally limited.
26
Jan
2016
Holonomic quantum computation with all resonant control in circuit QED
The implementation of holonomic quantum computation generally requires controllable and complicated interaction among addressable multi-level systems, which is challenging on superconducting
circuit. Here, we propose a scalable architecture for non-adiabatic holonomic quantum computation on a circuit QED lattice with hybrid transmon and photon encoding of the logical qubits in a decoherence-free subspace. With proper driven on the transmon, we can obtain tunable resonate interaction between the transmon and each of the resonators, which leads to arbitrary single-qubit operation on the encoded logical qubit. Meanwhile, for a nontrivial two-qubit gate, we only need resonate interactions among the three resonators from the two logical qubits, which can be induced by commonly coupled to a grounding SQUID with ac magnetic driven. More importantly, our scheme is achieved with all resonate interactions among the involved elements, and thus leads to quantum gates with very high fidelity. Therefore, our scheme opens up the possibility of realizing high fidelity universal holonomic quantum computation in solid-state system.
25
Jan
2016
Displacement of propagating squeezed microwave states
Displacement of propagating quantum states of light is a fundamental operation for quantum communication. It enables fundamental studies on macroscopic quantum coherence and plays an
important role in quantum teleportation protocols with continuous variables. In our experiments we have successfully implemented this operation for propagating squeezed microwave states. We demonstrate that, even for strong displacement amplitudes, there is no degradation of the squeezing level in the reconstructed quantum states. Furthermore, we confirm that path entanglement generated by using displaced squeezed states stays constant over a wide range of the displacement power.