We present an impedance engineered Josephson parametric amplifier capable of providing bandwidth beyond the traditional gain-bandwidth product. We achieve this by introducing a positive linear slope in the imaginary component of the input impedance seen by the Josephson oscillator using a λ/2 transformer. Our theoretical model predicts an extremely flat gain profile with a bandwidth enhancement proportional to the square root of amplitude gain. We experimentally demonstrate a nearly flat 20 dB gain over a 640 MHz band, along with a mean 1-dB compression point of -110 dBm and near quantum-limited noise. The results are in good agreement with our theoretical model.Josephson parametric amplifiers (JPAs) have become a crucial component of superconducting qubit 1 measurement circuitry, enabling recent studies of quantum jumps 2 , generation and detection of squeezed microwave field 3 , quantum feedback 4,5 , real-time tracking of qubit state evolution 6-8 and quantum error detection 9,10 . Although JPAs based on Josephson junctions embedded in a resonator 11-13 regularly achieve 20 dB power gain and quantum-limited noise, typical bandwidth is restricted to 10-50 MHz 11,14 , making them suitable for single qubit measurements only. The rapid progress towards multi-qubit architectures 9 for fault-tolerant quantum computing 15,16 demands an amplifier with much larger bandwidth to enable simultaneous readout of multiple qubits with minimal resources.There have been several attempts in this direction in recent years. One such attempt used a broadband impedance transformer 17 to lower the quality factor of a lumped-element Josephson oscillator which is the main component of a parametric amplifier. While the observed large bandwidth was qualitatively explained by a model consisting of a negative resistance 18 coupled to a frequency dependent impedance, no clear prescription on the design principle was provided. A different approach using Josephson non-linear transmission lines 19,20 was recently demonstrated 21,22 with nearly 4 GHz of bandwidth. However, this design requires fabrication of about 2000 nearly identical blocks of oscillator stages, demanding fairly sophisticated fabrication facilities. Multimode systems utilizing dissipative interactions have also been suggested theoretically as a route for enhancing bandwidth 23 , but have not been realized experimentally. In this Letter, we present a simple technique for enhancing the bandwidth of a JPA and beating the standard gain-bandwidth limit. It involves engineering the imaginary part of the environmental impedance: in particular, we introduce a positive linear slope in the imaginary component of the impedance shunting the JPA, while keeping the real part unchanged at the pump frequency. Our design uses a combination of a λ/4 and a λ/2 impedance transformers which are significantly easier to fabricate than a broadband impedance transformer 17 . Our theoretical model explains why the imaginary part of the impedance plays a crucial role in determining the amplifier bandw...

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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...

A twofold delayed-choice experiment was performed, where the behavior of a quantum system was a posteriori chosen twice.

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