Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain.However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials.Here we report a fast (750 ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit (T 1 = 52 μs, T 2E = 35 μs) without any nonreciprocal magnetic devices.We use in our readout chain a microwave-controlled qubit readout multichip module (QRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator or amplifier device, and an off-chip low-pass filter.Using the QRMCM, we demonstrate isolation up to 45 dB within 13 MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10 dB within a dynamical bandwidth of 10 MHz, when the reconfigurable device is operated as an amplifier.We also investigate the dependence of the qubit coherence times T ϕ and T 2E on the isolation response of the QRMCM, which we control in situ using the microwave drives feeding the isolators.Furthermore, by directly comparing the QRMCM performance with that of a state-of-art configuration (with T 2E ≈ 2T 1 ) that uses a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the QRMCM (for which T 2E ≈ T 1 ) is likely limited by the residual thermal photon population in the readout resonator.Improved versions of the QRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.
Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain.However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials.Here we report a fast (750 ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit (T 1 = 52 μs, T 2E = 35 μs) without any nonreciprocal magnetic devices.We use in our readout chain a microwave-controlled qubit readout multichip module (QRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator or amplifier device, and an off-chip low-pass filter.Using the QRMCM, we demonstrate isolation up to 45 dB within 13 MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10 dB within a dynamical bandwidth of 10 MHz, when the reconfigurable device is operated as an amplifier.We also investigate the dependence of the qubit coherence times T ϕ and T 2E on the isolation response of the QRMCM, which we control in situ using the microwave drives feeding the isolators.Furthermore, by directly comparing the QRMCM performance with that of a state-of-art configuration (with T 2E ≈ 2T 1 ) that uses a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the QRMCM (for which T 2E ≈ T 1 ) is likely limited by the residual thermal photon population in the readout resonator.Improved versions of the QRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.
Nonreciprocal microwave devices, such as circulators and isolators, are critical in high-fidelity qubit readout schemes. They unidirectionally route the readout signals and protect the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magneto-optical effects. Here, we realize an on-chip, single-microwave-pump Josephson ISolator (JIS), formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. We unravel the interplay between the orientation parity of the magnetic fluxes, biasing the mixers, and the JIS directionality. Furthermore, we build a motherboard, which integrates the JIS and other superconducting components, including a Josephson directional amplifier, into a printed circuit and use it to read out a qubit with 92% fidelity, while maintaining 75% of its T2E. Improved versions of this motherboard could replace magnetic circulators and isolators in large superconducting quantum processors.
Abstract Nonreciprocal microwave devices play critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They impose unidirectional routing of readout signals and protect the quantum systems from unwanted noise originated by the output chain. However, cryogenic circulators and isolators are disadvantageous in scalable superconducting architectures because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme and driving them with phase-shifted, same-frequency pumps. By incorporating our Josephson-based isolator into a superconducting qubit setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing 20 dB of protection within a bandwidth of 10 MHz against amplified noise reflected off the Josephson amplifier in the output chain. A moderate reduction of 35% is observed in T2E when the Josephson-based isolator is turned on. Such a moderate degradation can be mitigated by minimizing heat dissipation in the pump lines.
Abstract Nonreciprocal microwave devices play critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They impose unidirectional routing of readout signals and protect the quantum systems from unwanted noise originated by the output chain. However, cryogenic circulators and isolators are disadvantageous in scalable superconducting architectures because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme and driving them with phase-shifted, same-frequency pumps. By incorporating our Josephson-based isolator into a superconducting qubit setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing 20 dB of protection within a bandwidth of 10 MHz against amplified noise reflected off the Josephson amplifier in the output chain. A moderate reduction of 35% is observed in T2E when the Josephson-based isolator is turned on. Such a moderate degradation can be mitigated by minimizing heat dissipation in the pump lines.
The cryogenic hardware required to build a superconducting-qubit-based quantum computer demands a variety of microwave components. These elements include microwave couplers, filters, amplifiers, and circulators and/or isolators. Traditionally implemented as discrete components, integration of this peripheral hardware, in an effort to reduce overall footprint, thermal load, and added noise, is a key challenge to scaling modern quantum processors with qubit counts climbing over the one hundred plus mark. Ferrite-based microwave isolators, generally employed in the readout chain to decouple qubits and resonators from readout electronics, persist as one of the volumetrically largest devices still utilized as discrete components. Here, we present an alternative two-port isolating integrated circuit derived from the dc superconducting quantum interference device (dc SQUID). Nonreciprocal transmission is achieved using the three-wave microwave-mixing properties of a flux-modulated dc SQUID. We show that when multiple dc SQUIDs are embedded in a multipole admittance-inverting filter structure, the three-wave mixing derived from the 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.
Abstract We realize and characterize a quantum-limited, directional Josephson amplifier suitable for qubit readout. The device consists of two nondegenerate, three-wave-mixing amplifiers that are coupled together in an interferometric scheme, embedded in a printed circuit board. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the device, which plays the role of the magnetic field in a Faraday medium. Directional amplification and reflection-gain elimination are induced via wave interference between multiple paths in the system. We measure and discuss the main figures of merit of the device and show that the experimental results are in good agreement with theory. An improved version of this directional amplifier is expected to eliminate the need for bulky, off-chip isolation stages that generally separate quantum systems and preamplifiers in high-fidelity, quantum-nondemolition measurement setups.
Nonreciprocal microwave routing plays a crucial role in measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between noninteracting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and controllable directional elements. Here, we report the design and experimental realization of a minimal controllable directional interface that can be directly coupled to superconducting qubits. In the device presented, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30 dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the 1 % level or below. Our work offers a promising route for realizing high-fidelity signal routing and entanglement generation in all-to-all connected microwave quantum networks, and provides a path for isolator-free qubit readout schemes. Published by the American Physical Society 2024