[Paper Review] Flux-driven Josephson parametric amplifier

Tsuyoshi Yamamoto, K. Inomata+6 more|ArXiv.org|Aug 10, 2008

Physics of Superconductivity and Magnetism25 citations

TL;DR

This paper presents a flux-driven Josephson parametric amplifier based on a superconducting coplanar waveguide resonator terminated by a dc SQUID, where microwave pump signals at ~20 GHz modulate the magnetic flux through the SQUID to parametrically amplify signals at ~10 GHz. The amplifier achieves up to 17 dB gain with a noise temperature below 0.87 K, demonstrating tunable band center and phase-dependent amplification/deamplification.

ABSTRACT

We have developed a Josephson parametric amplifier, comprising a superconducting coplanar waveguide resonator terminated by a dc SQUID (superconducting quantum interference device). An external field (the pump, $\sim 20$ GHz) modulates the flux threading the dc SQUID, and, thereby, the resonant frequency of the cavity field (the signal, $\sim 10$ GHz), which leads to parametric signal amplification. We operated the amplifier at different band centers, and observed amplification (17 dB at maximum) and deamplification depending on the relative phase between the pump and the signal. The noise temperature is estimated to be less than 0.87 K.

Motivation & Objective

• To develop a Josephson parametric amplifier with a widely tunable band center for flexible operation in quantum information systems.
• To demonstrate parametric amplification and deamplification via flux modulation of a dc SQUID without requiring direct current bias.
• To achieve quantum-limited performance with low noise temperature suitable for sensitive quantum measurements.
• To enable separation of pump and signal frequencies through inductive coupling, minimizing pump leakage into the signal path.

Proposed method

• The amplifier uses a superconducting coplanar waveguide resonator terminated by a dc SQUID, where the resonant frequency is controlled by an external dc flux bias.
• A microwave pump at approximately twice the signal frequency (20 GHz) is applied to the SQUID loop to modulate the magnetic flux, inducing parametric amplification of the signal at ~10 GHz.
• The resonant frequency of the cavity is dynamically modulated at the pump frequency, enabling parametric gain through time-varying nonlinearity in the Josephson inductance.
• The pump and signal are coupled through different ports, and the absence of a second resonance near 20 GHz suppresses pump leakage into the signal line.
• The system is operated at 30 mK in a dilution refrigerator, with signal and gain measured using a directional coupler and cryogenic HEMT amplifier.
• The noise temperature is estimated by comparing signal gain with background noise levels, accounting for system losses and HEMT amplifier noise.

Experimental results

Research questions

• RQ1 Can a flux-driven Josephson parametric amplifier achieve high gain with low noise temperature while enabling wide tunability of the band center?
• RQ2 How does the relative phase between the pump and signal carriers affect amplification and deamplification in the flux-pumped scheme?
• RQ3 To what extent does the amplifier's performance degrade at high input signal powers due to nonlinearities in the Josephson junctions?
• RQ4 What is the achievable bandwidth and dynamic range of the amplifier at fixed band center, and how does it scale with device parameters?

Key findings

• The amplifier achieved a maximum gain of 17 dB when operated at a band center of 10.00 GHz, with a signal frequency of 10.00 GHz and a pump frequency of 20.00 GHz.
• A gain of 14 dB was measured at a band center of 10.78 GHz, with the gain showing a periodic dependence on the signal carrier phase with a period of π.
• Deamplification (negative gain) was observed at specific phase values, confirming the potential for noise squeezing in the amplifier.
• The noise temperature was estimated to be less than 0.87 K, indicating near-quantum-limited performance.
• The bandwidth of the amplifier at a fixed band center was measured to be approximately 20 MHz, consistent with the cavity linewidth.
• At high input signal powers (>−120 dBm), the gain response became distorted and hysteretic, indicating nonlinear saturation effects due to the Josephson junctions.