We propose and demonstrate a frequency-multiplexed readout scheme in 3D cQED architecture. We use four transmon qubits coupled to individual rectangular cavities which are aperture-coupled to a common rectangular waveguide feedline. A coaxial to waveguide transformer at the other end of the feedline allows one to launch and collect the multiplexed signal. The reflected readout signal is amplified by an impedance engineered broadband parametric amplifier with 380 MHz of bandwidth. This provides us high fidelity single-shot readout of multiple qubits using compact microwave circuitry, an efficient way for scaling up to more qubits in 3D cQED.

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Existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to reduced circuit depth, requiring large series of gates to perform an arbitrary unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross resonance interaction in such systems and characterize the entangling operation as well as the unitary imperfections and cross-talk as a function of device parameters. Finally, we tune up the cross resonance drive to implement multi-qubit gates in this architecture.Superconducting qubits have become one of the most promising platforms for quantum computation and quantum information processing 1 in the near term. Over the past decade small quantum processors with superconducting qubits have shown tremendous improvement in terms of coherence times reaching milliseconds 2,3 and scalability up to 10-70 qubits 4,5 . However almost all the existing architectures 6-9 in superconducting qubits have only nearest-neighbor coupling. With limited connectivity this often imposes strong constraints on available multi-qubit operations in such architectures and leads to inefficient implementation of quantum algorithms and quantum simulations 10 . On the other hand, always on all-to-all interaction in longitudinally coupled multimodal devices 11,12 leads to implementation of fast high fidelity N-qubit gates in the circuit. Previous experiments have demonstrated such devices as an effective three qubit processor with efficient implementation of small quantum algorithms 13 . Using multi-modal devices as building blocks for a larger quantum processor could enable greater interqubit connectivity and increased circuit depth for quantum information processing. This is a useful approach to enhance the performance of near-term imperfect quantum processors 14,15 without fault tolerance.In this letter, we demonstrate a circuit QED architecture consisting of a multi-modal superconducting circuit 12 and a transmon 16 qubit coupled via an exchange (σ x σ x ) coupling mediated via a bus cavity. We numerically analyze the effect of a cross resonance 17,18 drive in such systems and estimate the elements of the effective Hamiltonian 19 for experimentally realizable parameters. Then we use a frequency tunable transmon to experimentally study the cross resonance effect as a function of detuning between the two qubits. We identify the optimum detuning range and tune up cross resonance interaction for a multi-qubit entangling gate. We characterize the performance of ...

Josephson junctions (JJ) and their tunable properties, including their nonlinearities, form the core of superconducting circuit quantum electrodynamics (cQED) [1]. In quantum circuits, lownoise amplification of feeble microwave signals is essential and the Josephson parametric amplifiers (JPA) [2] are the widely used devices. The existing JPAs are based on Al-AlO x -Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of 2D van der Waals JJs [3,4] provide an opportunity to implement various cQED devices [5][6][7] with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals cQED architecture have been demonstrated -quantum noise limited amplifier, an essential component, has not been realized. Here we implement a quantum noise limited JPA, using a graphene JJ, that has linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and −130 dBm saturation power; performance on par with the best single-junction JPAs [2,8]. Importantly, our gate tunable JPA works in the quantumlimited noise regime which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers -the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum noise-limited amplifier. In general, our work will open up exploration of scalable device architecture of 2D van der Waals materials by integrating a sensor with the quantum amplifier.