The Toffoli gate is a three-qubit operation that inverts the state of a target qubit conditioned on the state of two control qubits. It enables universal reversible classical computation, forms a universal set of gates in quantum computation together with a Hadamard gate and is also a key element in quantum error correction schemes.
Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for quantum computation. In this publication, the group of Andreas Wallraff at ETH Zurich demonstrated the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator.
We have realized a novel device in which a semiconductor double quantum dot is dipole coupled to a GHz-frequency high-quality transmission line resonator. This approach allows us to characterize the properties of the double dot by measuring both its dispersive and dissipative interaction with the resonator. In addition to providing a new readout mechanism, this architecture has the potential to isolate the dots from the environment and to provide long distance coupling between spatially separated dots.
When a quantum mechanical system evolves under a time-dependent Hamiltonian, it acquires not only the wellknown dynamic phase, which is the time-integral of the energy of the system, but also a geometric phase. As indicated by its name, it is of a purely geometric nature in that it solely depends on the trajectory of the quantum system in state space.
Tomography is the main method used for measuring the fidelity of an experimentally implemented quantum process. However, it is a very inefficient method since the number of measurements as well as the time needed for the data post-processing scale exponentially with the number of qubits. With the ongoing experimental progress and growth in system size, quantum process tomography will soon become infeasible in state-of-the art experiments.
Microwave circuits based on superconducting transmission lines can also be used to investigate electron transport through nanometer scale electronic devices based on single molecules or carbon nanotubes. By measuring the radiofrequency properties of such a circuit, the properties of the integrated device can be determined with a larger bandwidth and better signal-to-noise compared to DC measurements, allowing, for example, for measurements with microsecond or better time resolution.
We have exploited the resonant interaction between three superconducting transmon-type qubits and a microwave transmission line resonator to show that a W-state can be generated with high efficiency in this system by harnessing its collective dynamics. Interestingly, our method also benefits from the √N-nonlinearity of the coupling strength between N qubits and a single field mode.
Propagating photons are ideal carriers for distributing entanglement between distant matter systems in a quantum network. Entanglement between photons and stationary qubits has thus far been exclusively studied at optical frequencies with single atoms or electron spins.
Imagine, you plan for a Sundays hike along a splendid trail laid out in a figure-eight and you have to decide which of the two lobes you walk about first. Usually, it makes no difference and any choice leaves you afterwards in the same weary but contented state of mind. We have now demonstrated that in a quantum world you have to be more careful in your choice. Because of the non-abelian geometric effect the final state of a superconducting quantum device depends on which loop is traversed first.
We present the very first measurement of the Hong-Ou-Mandel effect outside the domain of optical frequency photons. This genuinely quantum effect allows us to demonstrate the fundamental indistinguishability of single particles of light (photons) at microwave frequencies by measuring their coalescence into a photon pair at a beam splitter. Our experiments open new possibilities for using this effect in quantum communication and information, e.g. for creating remote entanglement.
