Abstract Josephson parametric oscillators (JPOs) have potential as fundamental building blocks for large-scale quantum annealing systems. To scale up such systems, both readout and control circuits need to be implemented at cryogenic temperatures. Adiabatic quantum flux parametron (AQFP) circuits are well suited for such environments due to their ultra-low power consumption. This paper proposes a phase-state readout scheme for JPOs based on AQFP circuits, which enables real-time digitization of oscillation phases. A JPO was fabricated, and the concept was experimentally validated at 4.2 K. After confirming stable oscillations in the JPO, a digital readout method using AQFP circuits was demonstrated. Analysis of the probability of obtaining a logical ‘1’ at the output revealed a clear phase-dependent behavior. Furthermore, our simulations predicted a readout fidelity of nearly 100% at millikelvin temperatures, demonstrating the feasibility of AQFP-based phase-state readout for future large-scale quantum annealing systems.
Abstract Josephson parametric oscillators (JPOs) have potential as fundamental building blocks for large-scale quantum annealing systems. To scale up such systems, both readout and control circuits need to be implemented at cryogenic temperatures. Adiabatic quantum flux parametron (AQFP) circuits are well suited for such environments due to their ultra-low power consumption. This paper proposes a phase-state readout scheme for JPOs based on AQFP circuits, which enables real-time digitization of oscillation phases. A JPO was fabricated, and the concept was experimentally validated at 4.2 K. After confirming stable oscillations in the JPO, a digital readout method using AQFP circuits was demonstrated. Analysis of the probability of obtaining a logical ‘1’ at the output revealed a clear phase-dependent behavior. Furthermore, our simulations predicted a readout fidelity of nearly 100% at millikelvin temperatures, demonstrating the feasibility of AQFP-based phase-state readout for future large-scale quantum annealing systems.
Parametric oscillations are well studied phenomena, with applications in amplification, quantum optics, and quantum information processing. They can occur as a parameter of a system, such as the resonant frequency, is being modulated or “pumped” by an external field. A nonlinearity of the system can then transfer power from the pumping frequency to two frequencies called signal and idler. When the signal falls within a resonance, and when the pump amplitude exceeds an instability threshold, parametric oscillations occur. When signal and idler fall within the same resonance, the pumping is called degenerate.Due to somewhat different implementations, in combination with rich nonlinear physics, the details of the parametric oscillations can differ in different systems. In this work, we expand on the research done on parametric oscillations using a superconducting microwave resonator with a tunable boundary condition. Previously, a degenerate Josephson parametric oscillator (JPO) has been demonstrated by modulation of this boundary condition at twice the resonator's resonant frequency. We further use the JPO to implement a novel read-out method for a superconducting qubit. Moreover, we demonstrate a different, nondegenerate regime of the JPO, which we denote the NJPO, by modulating the frequency at the sum of two resonant frequencies of a multimode resonator.Both the JPO qubit read-out and the NJPO have applications within quantum information processing. The perhaps most promising platform for quantum information processing is represented by the circuit-QED architecture with superconducting resonators and artificial atoms, where the two lowest atomic levels represent the qubit. We make these circuits using lithographic techniques, and control and measure them at low temperature using microwaves. The qubit's state is fragile and difficult to read out with high fidelity. To this end, we have developed a method based on the JPO, in which the qubit state is mapped onto a zero-amplitude or a large-amplitude state of the JPO. We achieved a large contrast (185±15 photons), therefore eliminating the need for a following quantum-limited parametric amplifier. Our readout fidelity was 81.5 %, and by carefully modeling we can distinguish fidelity loss from the qubit and the readout, respectively. This analysis displays an actual readout fidelity of 98.7 %.An alternative model for quantum computing is based on continuous variables, and uses harmonic oscillators instead of qubits. Entangled states, which can be used as quantum resources, can then be created, for example, by two-mode squeezing of the oscillator ground state. The state of the NJPO produces correlated oscillations in its two modes, which might be useful for continuous-variable quantum computing. In this work we fully characterize the classical properties of the NJPO, and show good quantitative agreement with a theoretical model.
Abstract We believe that the adiabatic quantum flux parametron (AQFP) circuit, as a novel low-power superconducting circuit technology, has potential application in various fields, including utilizing its high precision to determine the phase of input signals. To improve the precision of AQFP circuits, it is crucial to reduce the gray zone of the system. In this paper, we investigate a digital integration method to reduce the gray zone and enhance the precision of the AQFP circuits. Theoretical analysis demonstrates that the gray zone narrows, and the precision of the system improves as more data points are integrated. Additionally, we establish the quantitative relationship between the reduction in gray zone and the number of integrated data points. To validate these findings, we fabricate AQFP integrator circuits and perform experimental verification. Furthermore, we estimate the relationship between the scale of the integration circuit and the chip area, emphasizing possibilities for various future application scenarios.
Abstract We have been developing readout interfaces for superconducting nanowire single-photon detectors (SSPDs) using adiabatic quantum-flux-parametron (AQFP) logic. AQFP circuits operate with low power consumption, low bias currents, and high sensitivity, and thus are suitable as readout interfaces for large SSPD arrays. In this study, we develop a high-sensitivity AQFP interface, consisting of a current transformer, comparator, and rising-edge detector. We systematically investigated the current sensitivity of the AQFP interface by operating an NbTiN SSPD with the interface in a 0.1 W Gifford–McMahon cryocooler. We compared the outputs from the AQFP interface with the direct outputs from the SSPD, thereby demonstrating a sensitivity of 3.5 μ A, which is much smaller than that of the single-flux-quantum interfaces that we developed before.
An adiabatic quantum flux parametron (AQFP) is a two-terminal superconducting device capable of amplifying or inverting digital input signals at near-kT energy dissipation. This ultra-low power device is desirable for myriad reasons, including high performance accelerator applications as CMOS processors become more and more limited by energy consumption. Promising performance results have been realized on AQFP processors; however, scaling these results to larger computing systems faces engineering challenges due to device density and power distribution. AQFP circuits require a multi-phase activation signal to propagate logic. If data is not properly aligned with the activation phase, it can be dropped or shifted to an incorrect phase and disrupt circuit operation. This work presents design and simulation of an activation phase synchronizer: a simple circuit that will accept data arriving on any phase and re-align it to a known phase of the subsequent cycle. The phase synchronizer can be useful for mitigating clock skew across clock domains or play an important role in asynchronous design where the arrival time of data may be unknown.
We propose and demonstrate a low-power and low-current cryogenic readout interface for a superconducting nanowire single-photon detector (SSPD) using adiabatic quantum-flux-parametron (AQFP) logic. The AQFP readout interface samples and digitizes the current signal from an SSPD, generating binary output data in accordance with the detection behavior of the SSPD. We demonstrate the correct operation of an SSPD with the interface, where the AQFP readout interface and the SSPD are placed in separate 0.1-W Gifford–McMahon (GM) cryocoolers and are interconnected via coaxial cables. It was found that the temperature of the sample stage did not change even after the AQFP readout interface was turned on, which revealed that the AQFP readout interface uses sufficiently low power and current for a compact cryocooler.
A Josephson parametric oscillator (JPO) is an interesting system from the viewpoint of quantum optics because it has two stable self-oscillating states and can deterministically generate quantum cat states. A theoretical proposal has been made to operate a network of multiple JPOs as a quantum annealer, which can solve adiabatically combinatorial optimization problems at high speed. Proof-of-concept experiments have been actively conducted for application to quantum computations. This article provides a review of the mechanism of JPOs and their application as a quantum annealer.