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Register a new userEfficient qubit measurement with a nonreciprocal microwave amplifier
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The act of observing a quantum object fundamentally perturbs its state, resulting in a random walk toward an eigenstate of the measurement operator. Ideally, the measurement is responsible for all dephasing of the quantum state. In practice, imperfections in the measurement apparatus limit or corrupt the flow of information required for quantum feedback protocols, an effect quantified by the measurement efficiency. Here we demonstrate the efficient measurement of a superconducting qubit using a nonreciprocal parametric amplifier to directly monitor the microwave field of a readout cavity. By mitigating the losses between the cavity and the amplifier we achieve a measurement efficiency of $72%$. The directionality of the amplifier protects the readout cavity and qubit from excess backaction caused by amplified vacuum fluctuations. In addition to providing tools for further improving the fidelity of strong projective measurement, this work creates a testbed for the experimental study of ideal weak measurements, and it opens the way towards quantum feedback protocols based on weak measurement such as state stabilization or error correction.
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The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by loss or excess noise on the way to the detector. Even a textit{quantum-limited} detector that simultaneously measures a second non-commuting observable would double the output noise, therefore limiting the efficiency to $50%$. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field, but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of $62_{-9}^{+17} %$ that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.
We report on the design and implementation of a Field Programmable Josephson Amplifier (FPJA) - a compact and lossless superconducting circuit that can be programmed textit{in situ} by a set of microwave drives to perform reciprocal and nonreciprocal frequency conversion and amplification. In this work we demonstrate four modes of operation: frequency conversion ($-0.5~mathrm{dB}$ transmission, $-30~mathrm{dB}$ reflection), circulation ($-0.5~mathrm{dB}$ transmission, $-30~mathrm{dB}$ reflection, $30~mathrm{dB}$ isolation), phase-preserving amplification (gain $>20~mathrm{dB}$, $1~mathrm{photon}$ of added noise) and directional phase-preserving amplification ($-10~mathrm{dB}$ reflection, $18~mathrm{dB}$ forward gain, $8~mathrm{dB}$ reverse isolation, $1~mathrm{photon}$ of added noise). The system exhibits quantitative agreement with theoretical prediction. Based on a gradiometric Superconducting Quantum Interference Device (SQUID) with Nb/Al-AlO$_x$/Nb Josephson junctions, the FPJA is first-order insensitive to flux noise and can be operated without magnetic shielding at low temperature. Due to its flexible design and compatibility with existing superconducting fabrication techniques, the FPJA offers a straightforward route toward on-chip integration with superconducting quantum circuits such as qubits or microwave optomechanical systems.
High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable, universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.
The interaction of photons and coherent quantum systems can be employed to detect electromagnetic radiation with remarkable sensitivity. We introduce a quantum radiometer based on the photon-induced-dephasing process of a superconducting qubit for sensing microwave radiation at the sub-unit-photon level. Using this radiometer, we demonstrated the radiative cooling of a 1-K microwave resonator and measured its mode temperature with an uncertainty ~0.01 K. We have thus developed a precise tool for studying the thermodynamics of quantum microwave circuits, which provides new solutions for calibrating hybrid quantum systems and detecting candidate particles for dark matter.
We propose a design to realize integrated broadband nonreciprocal microwave isolators and circulators using superconducting circuit elements without any magnetic materials. To obtain a broadband response, we develop a waveguide-based design by temporal modulations. The corresponding compact traveling-wave structure is implemented with integrated superconducting composite right-/left-handed transmission lines. The calculations show that the bandwidth of 580 MHz can be realized over a nonreciprocal isolation of 20 dB in reflections. Such on-chip isolators and circulators are useful for cryogenic integrated microwave connections and measurements, such as protecting qubits from the amplified reflected signal in multiplexed readout.
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