We present the "trimon", a multi-mode superconducting circuit implementing three qubits with all-to-all longitudinal coupling. This always-on interaction enables simple implementation of generalized controlled-NOT gates which form a universal set. Further, two of the three qubits are protected against Purcell decay while retaining measurability. We demonstrate high-fidelity state swapping operations between two qubits and characterize the coupling of all three qubits to a neighbouring transmon qubit. Our results offer a new paradigm for multi-qubit architecture with applications in quantum error correction, quantum simulations and quantum annealing.Controlling and manipulating the interactions between multiple qubits is at the heart of quantum information processing, and the superconducting circuit architecture 1 has emerged as a leading candidate. Previous demonstrations of multi-qubit devices 2-8 have used transmon qubits 9 along with separate coupling elements to implement transverse inter-qubit coupling. Typically, this transverse coupling is weak and restricted to nearest neighbours which limits the kind of multi-qubit operations that can be performed. Recently, longitudinal inter-qubit coupling has been proposed as an alternative for building a universal multi-qubit architecture [10][11][12] and for quantum annealing architectures with all-to-all coupling 13 . While the transmon design uses a single anharmonic oscillator mode to implement a qubit, this idea can be extended to a circuit that can support several oscillator modes to implement a multi-qubit system with strong longitudinal coupling 14,15 . However, previous experiments 16,17 have not demonstrated multi-qubit operations and their coherence times have not matched that of typical transmon qubits.In this Letter, we present a new quantum device, the "Trimon", implementing a three-qubit system that arises from a single superconducting circuit. Our device ( Fig. 1(a)) is based on the Josephson ring modulator (JRM) consisting of four nominally identical Josephson junctions in a superconducting loop to implement three orthogonal electrical modes 18 . This three-mode structure has been previously exploited to couple different harmonic oscillators for parametric amplification 19 , while more recently, it has been proposed as a coupling element between two qubits 13 . Here, we capacitively shunt the JRM by connecting superconducting pads to each node (Fig. 1(b)) to create three coupled anharmonic oscillator modes: two dipolar and one quadrupolar ( Fig. 1(c)). Each mode has properties similar to 3D-transmon qubits 20 with the resonant frequency and anharmonicity controllable by design. The longitudinal inter-qubit coupling 13 of the cross-Kerr type originates due to the sharing of the four junctions amongst all three modes. One of the two dipolar modes couples directly to the host 3D electromagnetic cavity ( Fig. 1(c)); we call this the "A" qubit. The other dipolar mode (qubit B) and the quadrupolar mode (qubit C) ideally stay uncoupled from the cavity an...

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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 ...