The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is therefore a critical goal in quantum information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing read-out time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of 7.6 µs. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

Systems of interacting quantum spins show a rich spectrum of quantum phases and display interesting many-body dynamics. Computing characteristics of even small systems on conventional computers poses significant challenges. A quantum simulator has the potential to outperform standard computers in calculating the evolution of complex quantum systems. Here, we perform a digital quantum simulation of the paradigmatic Heisenberg and Ising interacting spin models using a two transmon-qubit circuit quantum electrodynamics setup. We make use of the exchange interaction naturally present in the simulator to construct a digital decomposition of the model-specific evolution and extract its full dynamics. This approach is universal and efficient, employing only resources which are polynomial in the number of spins and indicates a path towards the controlled simulation of general spin dynamics in superconducting qubit platforms.

Sharing information coherently between nodes of a quantum network is fundamental to distributed quantum information processing. In this scheme, the computation is divided into subroutines and performed on several smaller quantum registers that are connected by classical and quantum channels . A direct quantum channel, which connects nodes deterministically rather than probabilistically, achieves larger entanglement rates between nodes and is advantageous for distributed fault-tolerant quantum computation . Here we implement deterministic state-transfer and entanglement protocols between two superconducting qubits fabricated on separate chips. Superconducting circuits constitute a universal quantum node that is capable of sending, receiving, storing and processing quantum information. Our implementation is based on an all-microwave cavity-assisted Raman process , which entangles or transfers the qubit state of a transmon-type artificial atom with a time-symmetric itinerant single photon. We transfer qubit states by absorbing these itinerant photons at the receiving node, with a probability of 98.1 ± 0.1 per cent, achieving a transfer-process fidelity of 80.02 ± 0.07 per cent for a protocol duration of only 180 nanoseconds. We also prepare remote entanglement on demand with a fidelity as high as 78.9 ± 0.1 per cent at a rate of 50 kilohertz. Our results are in excellent agreement with numerical simulations based on a master-equation description of the system. This deterministic protocol has the potential to be used for quantum computing distributed across different nodes of a cryogenic network.

Engineered macroscopic quantum systems based on superconducting electronic circuits are attractive for experimentally exploring diverse questions in quantum information science. At the current state of the art, quantum bits (qubits) are fabricated, initialized, controlled, read out and coupled to each other in simple circuits. This enables the realization of basic logic gates, the creation of complex entangled states and the demonstration of algorithms or error correction. Using different variants of low-noise parametric amplifiers, dispersive quantum non-demolition single-shot readout of single-qubit states with high fidelity has enabled continuous and discrete feedback control of single qubits. Here we realize full deterministic quantum teleportation with feed-forward in a chip-based superconducting circuit architecture. We use a set of two parametric amplifiers for both joint two-qubit and individual qubit single-shot readout, combined with flexible real-time digital electronics. Our device uses a crossed quantum bus technology that allows us to create complex networks with arbitrary connecting topology in a planar architecture. The deterministic teleportation process succeeds with order unit probability for any input state, as we prepare maximally entangled two-qubit states as a resource and distinguish all Bell states in a single two-qubit measurement with high efficiency and high fidelity. We teleport quantum states between two macroscopic systems separated by 6 mm at a rate of 10(4) s(-1), exceeding other reported implementations. The low transmission loss of superconducting waveguides is likely to enable the range of this and other schemes to be extended to significantly larger distances, enabling tests of non-locality and the realization of elements for quantum communication at microwave frequencies. The demonstrated feed-forward may also find application in error correction schemes.

We demonstrate a coupled cavity realization of a Bose Hubbard dimer to achieve quantum limited amplification and to generate frequency entangled microwave fields with squeezing parameters well below -12 dB. In contrast to previous implementations of parametric amplifiers our dimer can be operated both as a degenerate and as a nondegenerate amplifier. The large measured gain-bandwidth product of more than 250 MHz for nondegenerate operation and the saturation at input photon numbers as high as 2000 per µs are both expected to be improvable even further, while maintaining wide frequency tunability of about 2 GHz. Featuring flexible control over all relevant system parameters, the presented Bose-Hubbard dimer based on lumped element circuits has significant potential as an elementary cell in nonlinear cavity arrays for quantum simulation.The high level of control achievable over collections of massive or massless particles, such as atoms, spins or photons, enables the detailed study of intricate many-body phenomena in man-made quantum systems [1]. In this context coupled nonlinear resonators both provide a viable avenue for studying light-matter interactions and constitute a generic building block for photonic quantum simulators of strongly interacting systems [2][3][4]. Therefore, their theoretical and experimental investigation is pursued in a wide variety of physical settings such as photonic structures [5,6], optomechanical systems [7][8][9] and superconducting circuits [10][11][12].The remarkable progress in quantum science using microwave radiation has stimulated broad interest in low noise amplification [13,14] and has lead to the development of novel versatile amplifiers in the recent years [15][16][17][18][19][20][21][22]. Many of these implementations rely on parametric processes in which the noise temperature of the amplifier is solely limited by the radiation temperature of the input fields, ultimately by the vacuum fluctuations [23]. In parametric amplification the presence of a signal stimulates conversion processes from a pump field into the signal field, while creating an additional idler field. When signal and idler field occupy the same mode, this is referred to as degenerate parametric amplification, whereas in nondegenerate amplifiers the signal and idler modes are separated either spatially or in frequency [23]. While degenerate parametric amplifiers [15] are often preferable for the fast dispersive readout of qubits in quantum feedback protocols, nondegenerate amplification [19] can be more practical for multiplexed readout, the measurement of photon correlation functions and more general applications in which amplification is to be independent of the phase of the signal relative to the pump.Here, we consider a system described by two bosonic modes a L and a R , which are coupled with hopping rate J and have an on-site interaction strength U = U L = U R , see generic representation in Fig. 1(a). In a frame rotating at the bare cavity frequency ω 0 = ω L = ω R the system is described by the Bos...