Phase-sensitive cascade quantum amplifier with nearly noiseless operation

AI summary

Overview

Research area: Superconducting quantum electronics, specifically low-noise microwave amplification for quantum computing and quantum communication—Josephson parametric amplifiers (JPAs), traveling-wave parametric amplifiers (TWPAs), and their cascading.

Technical level: Intermediate. The experimental results are accessible to anyone familiar with amplifier noise concepts, while the theoretical background sections (input–output operators, quadrature variances, the standard quantum limit) require comfort with quantum optics formalism.

Scope: The paper compares the phase-preserving and phase-sensitive operation of a single SQUID-based JPA, using a TWPA as a second-stage pre-amplifier so that the JPA's noise can be measured directly even at low gain.

What This Paper Is About

Quantum amplifiers must add noise to preserve the laws of quantum mechanics, and this added noise normally prevents them from reaching the ideal (quantum-limited) performance. This paper asks whether a phase-sensitive (degenerate) amplifier mode, in which only one quadrature is amplified, can be shown experimentally to operate below the standard quantum limit. The goal is a measurement chain with so little noise that the state of a quantum device can be read out directly, without subtracting background noise afterwards.

Key Contributions

1. Demonstrates a JPA–TWPA cascaded amplification scheme in which the TWPA acts as a low-noise pre-amplifier for the JPA output, which makes it possible to probe the JPA's noise performance directly at low gains.
2. Reports phase-sensitive operation of the SQUID-based JPA with a system noise temperature of 94 ± 12 mK at 6 GHz, well below the standard quantum limit of 288 mK.
3. Quantifies the added noise in both regimes: the phase-preserving mode saturates at 200 ± 54 mK (0.71 added photons), while the phase-sensitive mode saturates at 17 ± 12 mK (0.06 added photons).
4. Places the result against prior phase-sensitive amplifier measurements (Yurke et al., 0.31 added photons; Zhong et al., 0.12; Parker et al., 0.1), attributing the improvement mainly to the low-noise pre-amplification from the TWPA.

Main Findings

• Phase-preserving system noise: The JPA in phase-preserving mode gives a system noise temperature T_S,JPA = 351 ± 53 mK at 6 GHz, close to the quantum limit.
• Phase-sensitive system noise: In phase-sensitive (degenerate) mode the minimum system noise temperature falls to 94 ± 12 mK, below the standard quantum limit of 288 mK stated in the abstract.
• Added noise saturation: The JPA's added noise in phase-preserving operation saturates at 200 ± 54 mK, about 56 mK above the phase-preserving quantum limit T_q = ℏω_s/(2k_B) = 144 mK.
• Near-noiseless amplification: In phase-sensitive operation the added noise saturates at only 17 ± 12 mK, equivalent to 0.06 added input-referred photons versus 0.71 photons in the phase-preserving case.
• Noise accounting: Of the 94 ± 12 mK system noise temperature in phase-sensitive mode, approximately 89 mK comes from quantum fluctuations and the JPA's intrinsic added noise, while about 5 mK comes from the rest of the amplifier chain.
• Gain enhancement: Phase-locking the local oscillator to the amplified quadrature makes signal and idler amplitudes add coherently, producing a 6 dB increase in measured phase-sensitive gain (approximately 6 dB improvement in signal visibility).
• Cascade benefit: Without the TWPA the low system noise temperatures could not be reached, because the TWPA suppresses the noise contribution of the amplifiers that follow it.
• Device parameters: The JPA has half-pump frequency ω_p/2π = 6.045 GHz, gain bandwidth product 400 MHz, saturation power −120 dBm at about 20 dB gain, and pump power −90 dBm; the TWPA has 5.5 GHz, 400 GHz, −101 dBm, and −66 dBm respectively; both operate at T_0 = 15 mK. The JPA is flux-tunable from 5.4 to 6.1 GHz.
• Internal loss: The JPA's intrinsic cavity loss rate, from a standard S_21 complex-circle fit, is γ = 0.22κ.

Methodology in Plain English

The team built a SQUID-based Josephson parametric amplifier in which a quarter-wave transmission-line resonator is terminated to ground by a SQUID, with an on-chip flux line to drive the SQUID with a strong pump tone. The pump sits at roughly twice the cavity resonance, so it drives the device in the three-wave mixing regime where one pump photon splits into a signal and an idler; this also keeps the pump far outside the measurement band so it does not overlap the signal. A DC bias field, applied through a bias-T outside the sample box, is needed for this mode. The device is measured in reflection, with a circulator separating incoming and outgoing fields, inside a millikelvin cryostat.

To measure noise directly rather than inferring it, they added a VTT-fabricated TWPA after the JPA (separated by a circulator and a directional coupler/diplexer so the TWPA pump does not disturb the JPA). The TWPA is biased in phase-preserving mode to provide about 20 dB of gain with added noise of roughly 700 mK, giving a system noise temperature of roughly 900 mK before the JPA is considered.

Calibration uses the standard Y-factor method: heating the mixing chamber, using a 500 mK cold load and a 1 K hot load with 1 MHz measurement bandwidth and 100 averages, yielding about 10 K for the LNF HEMT cooled preamplifier referred to the JPA input, with an estimated uncertainty of ±1 dB. The added noise of each amplifier is then extracted from the measured gain and the improvement in signal-to-noise ratio when the amplifier is switched on, using the appropriate relation (which includes a correction term for quantum fluctuations already present at the input), and the cascade noise is recomputed with the Friis formula. Both tones are generated by an Anapico APMS12G-4 signal generator, with relative phase stability of 3 mrad over 5 hours.

Why This Matters

Impact on research: The result gives an experimentally demonstrated route below the standard quantum limit for a practical, flux-tunable cavity-based amplifier, and shows that a JPA–TWPA cascade can be used to probe quantum devices with high fidelity without background-noise subtraction. It also provides a clean, directly measured comparison between phase-preserving and phase-sensitive operation of the same device.

Real-world applications:

• Readout of superconducting qubits and other quantum devices placed before the amplifier in the signal chain.
• Quantum communication protocols, specifically quantum secure direct communication and continuous-variable quantum key distribution, both named in the paper as beneficiaries of an ultimate-sensitivity amplifier chain.
• Measurement preparation using squeezed vacuum states, which the authors note can push JPA-based experiments below the quantum limit.
• Low-noise microwave measurement more generally across the JPA's flux-tunable 5.4 to 6.1 GHz range.

Industry relevance: Amplifier chains of this kind are a core component of superconducting quantum computing hardware and of quantum communication links, and the devices were fabricated with VTT's SWAPS technology, indicating a foundry-compatible fabrication route.

Future Directions

• Extending degenerate three-wave-mixing operation to short traveling-wave parametric amplifier sections, which the authors note would offer flexible frequency tunability beyond the roughly half-linewidth range of transmission-line-cavity JPAs.
• Combining the cascade with squeezed-vacuum input states to operate further below the quantum limit.
• Applying the chain to direct, high-fidelity probing of actual quantum devices in the component chain, replacing noise-subtraction techniques.
• Deploying such ultra-low-noise chains in quantum communication protocols, where amplifier sensitivity is a limiting factor.

Target Audience

Researchers and engineers working on superconducting quantum devices, quantum-limited microwave amplification, qubit readout, and quantum communication. It is also useful for readers who want a concrete experimental comparison of phase-preserving versus phase-sensitive amplifier operation, and for those designing measurement setups in which amplifier-chain noise rather than the device under test sets the sensitivity floor.

Authors’ abstract

Phase-sensitive parametric devices enable quadrature-selective amplification with the potential for sub-quantum-limited noise performance. In this work, we investigate the operation of a SQUID-based Josephson Parametric Amplifier (JPA), comparing its performance in the phase-preserving and phase-sensitive regimes. The device, fabricated using VTT SWAPS technology, is driven in a three-wave mixing configuration and characterized in a reflection-based measurement setup at millikelvin temperatures. To directly probe the noise performance at low JPA gains, we employ a cascaded amplification scheme in which a Traveling-Wave Parametric Amplifier (TWPA) provides low-noise pre-amplification of the JPA output. In a phase-preserving operation, the JPA exhibits near-quantum-limited performance with a system noise temperature of $351\pm53$ mK at 6 GHz. In contrast, phase-sensitive operation yields a minimum system noise temperature of $94\pm12$ mK, well below the standard quantum limit of 288 mK. Our results demonstrate that a JPA-TWPA amplifier cascade opens the door to direct, high-fidelity probing of quantum devices without the need for background noise subtraction.