Three-qubit quantum gates are crucial for quantum error correction and quantum information processing. We generate policies for quantum control procedures to design three types of three-qubitgates, namely Toffoli, Controlled-Not-Not and Fredkin gates. The design procedures are applicable to an architecture of nearest-neighbor-coupled superconducting artificial atoms. The resultant fidelity for each gate is above 99.9%, which is an accepted threshold fidelity for fault-tolerant quantum computing. We test our policy in the presence of decoherence-induced noise as well as show its robustness against random external noise generated by the control electronics. The three-qubit gates are designed via our machine learning algorithm called Subspace-Selective Self-Adaptive Differential Evolution (SuSSADE).
We devise a scalable scheme for simulating a quantum phase transition from paramagnetism to frustrated magnetism in a superconducting flux-qubit network, and we show how to characterizethis system experimentally both macroscopically and microscopically simultaneously. Macroscopic characterization of the quantum phase transition is based on the expected sudden transition of the probability distribution for the spin-network net magnetic moment with this transition quantified by the Kullback-Leibler divergence between measured and theoretical distributions for a given quantum phase. Microscopic characterization of the quantum phase transition is performed using the standard local-entanglement-witness approach. Simultaneous macro and micro characterizations of quantum phase transitions would serve to verify in two ways a quantum phase transition and provide empirical data for revisiting the foundational emergentist-reductionist debate regarding reconciliation of macroscopic thermodynamics with microscopic statistical mechanics especially in the quantum realm for the classically intractable case of frustrated quantum magnetism.
Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quantum simulations and quantum information processing. The exchange of real and virtualphotons 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.
We study the collective effects that emerge in waveguide quantum electrodynamics where several (artificial) atoms are coupled to a one-dimensional (1D) superconducting transmissionline. Since single microwave photons can travel without loss for a long distance along the line, real and virtual photons emitted by one atom can be reabsorbed or scattered by a second atom. Depending on the distance between the atoms, this collective effect can lead to super- and subradiance or to a coherent exchange-type interaction between the atoms. Changing the artificial atoms transition frequencies, something which can be easily done with superconducting qubits (two levels artificial atoms), is equivalent to changing the atom-atom separation and thereby opens the possibility to study the characteristics of these collective effects. To study this waveguide quantum electrodynamics system, we extend previous work and present an effective master equation valid for an ensemble of inhomogeneous atoms. Using input-output theory, we compute analytically and numerically the elastic and inelastic scattering and show how these quantities reveal information about collective effects. These theoretical results are compatible with recent experimental results using transmon qubits coupled to a superconducting one-dimensional transmission line [A.F. van Loo {\it et al.} (2013)].
We propose a quantum-electrodynamics scheme for implementing the
discrete-time, coined quantum walk with the walker corresponding to the phase
degree of freedom for a quasi-magnon fieldrealized in an ensemble of
nitrogen-vacancy centres in diamond. The coin is realized as a superconducting
flux qubit. Our scheme improves on an existing proposal for implementing
quantum walks in cavity quantum electrodynamics by removing the cumbersome
requirement of varying drive-pulse durations according to mean quasiparticle
number. Our improvement is relevant to all indirect-coin-flip cavity
quantum-electrodynamics realizations of quantum walks. Our numerical analysis
shows that this scheme can realize a discrete quantum walk under realistic
conditions.
Coherent pulse control for quantum memory is viable in the optical domain but
nascent in microwave quantum circuits. We show how to realize coherent storage
and on-demand pulse retrievalentirely within a superconducting circuit by
exploiting and extending existing electromagnetically induced transparency
technology in superconducting quantum circuits. Our scheme employs a linear
array of superconducting artificial atoms coupled to a microwave transmission
line.