Steady state entanglement of two superconducting qubits engineered by dissipation

We present a scheme for the dissipative preparation of an entangled steady state of two superconducting qubits in a circuit QED setup. Combining resonator photon loss, a dissipative process already present in the setup, with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art 3D, as well as with the more commonly used 2D transmons. The promising results of our study thus open a route for the demonstration of an entangled steady state in circuit QED.Comment: 12 pages, 5 figures; close to published versio

Engineering dissipative channels for realizing Schrödinger cats in SQUIDs

We show that by engineering the interaction with the environment, there exists a large class of systems that can evolve irreversibly to a cat state. To be precise, we show that it is possible to engineer an irreversible process so that the steady state is close to a pure Schrödinger’s cat state by using double well systems and an environment comprising two-photon (or phonon) absorbers.We also show that it should be possible to prolong the lifetime of a Schrödinger’s cat state exposed to the destructive effects of a conventional single-photon decohering environment. In addition to our general analysis, we present a concrete circuit realization of both system and environment that should be fabricatable with current technologies. Our protocol should make it easier to prepare and maintain Schrödinger cat states, which would be useful in applications of quantum metrology and information processing as well as being of interest to those probing the quantum to classical transition

Stabilizing two-qubit entanglement with engineered synthetic squeezing

It is well known that qubits immersed in a squeezed vacuum environment exhibit many exotic phenomena, including dissipative entanglement stabilization. Here, we show that these effects only require interference between excitation and decay processes, and can be faithfully mimicked without non-classical light using simple classical temporal modulation. We present schemes that harnesses this idea to stabilize entanglement between two remote qubits coupled via a transmission line or waveguide, where either the qubit-waveguide coupling is modulated, or the qubits are directly driven. We analyze the resilience of these approaches against various imperfections, and also characterize the trade-off between the speed and quality of entanglement stabilization. Our protocols are compatible with state of the art cavity QED systems.Comment: 16 pages, 6 figure

Backaction-driven, robust, steady-state long-distance qubit entanglement over lossy channels

We present a scheme for generating robust and persistent entanglement between qubits that do not interact and that are separated by a long and lossy transmission channel, using Markovian reservoir engineering. The proposal uses only the correlated decay into the common channel of remotely separated, driven single-photon qubit transitions. This simple scheme is generic and applicable to various experimental implementations, including circuit and cavity QED, with little experimental overhead compared with methods requiring dynamic control, initialization, measurement, or feedback. In addition to avoiding these inefficiencies, the simple protocol is highly robust against noise, miscalibration, and loss in the channel. We find high quality solutions over a wide range of parameters and show that the optimal strategy reflects a transition from ballistic to diffusive photon transmission, going from symmetrically and coherently driving a common steady state to asymmetrically absorbing photons that are emitted from one qubit by the second. Detailed analysis of the role of the transmission channel shows that allowing bi-directional decay drastically increases indistinguishability and thereby quadratically suppresses infidelity.Comment: 11 pages, 6 figure