The cryogenic hardware needed to build a superconducting qubit based quantum computer requires a variety of microwave components including microwave couplers, filters, amplifiers, andcirculators/isolators. Traditionally, these are implemented via discrete components inserted in to the signal path. As qubit counts climb over the 100+ mark, the integration of these peripheral components, in an effort to reduce overall footprint, thermal load, and added noise in the overall system, is a key challenge to scaling. Ferrite-based microwave isolators are one of the physically largest devices that continue to remain as discrete components. They are generally employed in the readout chain to protect qubits and resonators from broadband noise and unwanted signals emanating from downstream components such as amplifiers. Here we present an alternative two-port isolating integrated circuit derived from the DC Superconducting Quantum Interference Device (DC-SQUID). The non-reciprocal transmission is achieved using the three-wave microwave mixing properties of a flux-modulated DC-SQUID. We show experimentally that, when multiple DC-SQUIDs are embedded in a multi-pole admittance inverting filter structure, RF flux pumping of the DC-SQUIDs can provide directional microwave power flow. For a three-pole filter device, we experimentally demonstrate a directionality greater than 15 dB over a 600 MHz bandwidth.
Fast, high-fidelity measurement is a key ingredient for quantum error correction. Conventional approaches to the measurement of superconducting qubits, involving linear amplificationof a microwave probe tone followed by heterodyne detection at room temperature, do not scale well to large system sizes. Here we introduce an alternative approach to measurement based on a microwave photon counter. We demonstrate raw single-shot measurement fidelity of 92%. Moreover, we exploit the intrinsic damping of the counter to extract the energy released by the measurement process, allowing repeated high-fidelity quantum non-demolition measurements. Crucially, our scheme provides access to the classical outcome of projective quantum measurement at the millikelvin stage. In a future system, counter-based measurement could form the basis for a scalable quantum-to-classical interface.
We describe the design and characterization of superconducting coplanar waveguide cavities tailored to facilitate strong coupling between superconducting quantum circuits and singletrapped Rydberg atoms. For initial superconductor-atom experiments at 4.2 K, we show that resonator quality factors above 104 can be readily achieved. Furthermore, we demonstrate that the incorporation of thick-film copper electrodes at a voltage antinode of the resonator provides a route to enhance the zero-point electric fields of the resonator in a trapping region that is 40 μm above the chip surface, thereby minimizing chip heating from scattered trap light. The combination of high resonator quality factor and strong electric dipole coupling between the resonator and the atom should make it possible to achieve the strong coupling limit of cavity quantum electrodynamics with this system.
We propose a novel hybrid quantum gate between an atom and a microwave photon in a superconducting coplanar waveguide cavity by exploiting the strong resonant microwave coupling betweenadjacent Rydberg states. Using experimentally achievable parameters gate fidelities >0.99 are possible on sub-μs timescales for waveguide temperatures below 40 mK. This provides a mechanism for generating entanglement between two disparate quantum systems and represents an important step in the creation of a hybrid quantum interface applicable for both quantum simulation and quantum information processing.