Characterization of Tunnel Diode Oscillator for Qubit Readout Applications

We developed a tunnel diode oscillator and characterized its performance, highlighting its potential applications in the quantum state readout of electrons in semiconductors and electrons on liquid helium. This cryogenic microwave source demonstrates significant scalability potential for large-scale qubit readout systems due to its compact design and low power consumption of only 1 uW, making it suitable for integration on the 10 mK stage of a dilution refrigerator. The tunnel diode oscillator exhibits superior amplitude stability compared to commercial microwave sources. The output frequency is centered around 140 MHz, commonly used for qubit readout of electrons in semiconductors, with a frequency tunability of 10 MHz achieved using a varactor diode. Furthermore, the phase noise was significantly improved by replacing the commercially available voltage source with a lead-acid battery, achieving a measured phase noise of -115 dBc/Hz at a 1 MHz offset.

💡 Research Summary

This paper presents the design, fabrication, and comprehensive characterization of a tunnel‑diode oscillator (TDO) intended for qubit readout applications in large‑scale quantum processors. The authors address a critical bottleneck in current quantum hardware: the need for room‑temperature microwave sources and bulky coaxial cabling that become impractical as the number of qubits grows. By moving the microwave source to the 10 mK mixing‑chamber stage of a dilution refrigerator, the proposed TDO dramatically reduces both power consumption and wiring complexity.

The core active element is a commercially available BD‑6 backward‑biased tunnel diode (American Micro Semiconductor). When biased near 0.1 V and 10 µA, the diode exhibits a negative resistance of roughly –5 kΩ, which cancels the losses of a high‑Q superconducting Nb spiral inductor (15 turns, 95 nH, measured at 4 K). The LC resonator formed by the inductor and the diode’s intrinsic capacitance (~7 pF) oscillates at a frequency centered at 140 MHz. Frequency stability is enhanced by a varactor diode (MA46H201) placed in parallel, providing a voltage‑controlled capacitance that enables a total tunability of about 10 MHz without affecting output power for varactor bias above –1.3 V.

Power consumption is exceptionally low: the operating point of the tunnel diode corresponds to roughly 1 µW, orders of magnitude lower than cryogenic CMOS sources (≈10 mW) or Josephson‑junction based microwave generators (≈100 µW). The output microwave power delivered to a hypothetical resonator is –90 dBm, matching the optimal power range for dispersive readout of semiconductor‑based electron qubits and electrons on liquid helium.

A notable engineering innovation is the inductive extraction of the RF signal. The authors couple the oscillating current to a pickup coil with a 15:1.5 ratio, isolating the DC bias line from the RF path. This prevents the bias line’s filters and impedance from perturbing the oscillation, resulting in a more stable bias voltage and reduced amplitude fluctuations.

Phase noise, a key metric for both qubit manipulation and readout fidelity, was measured under two bias conditions. Using a conventional laboratory power supply yields a phase‑noise floor of about –95 dBc/Hz at a 1 MHz offset. Replacing the supply with a low‑noise lead‑acid battery improves the floor to –115 dBc/Hz, demonstrating that supply‑induced jitter dominates the oscillator’s phase stability.

The authors also characterize the dependence of oscillation frequency on the tunnel‑diode bias voltage (V_TD). Because the diode’s depletion width changes with bias, its capacitance follows C_TD = C₀(1 – V_TD/V_d)⁻¹ᐟ², where V_d ≈ 0.5 V. Fits to measured data at 11 mK and 3.4 K yield C₀ ≈ 5.8 pF and confirm that temperature has only a minor effect on C_TD (≈0.1 pF shift). Frequency varies by roughly 0.5 MHz over the usable V_TD range, providing fine‑tuning capability.

The experimental setup includes a cryogenic low‑noise amplifier (CMT‑BA1) at the 4 K stage, a directional coupler (ZEDC‑15) for signal routing, and room‑temperature spectrum analyzers or oscilloscopes for measurement. The resonator shown in the schematic is deliberately detuned from the TDO’s operating band, acting as a high‑impedance load to isolate the oscillator during characterization.

In comparison with existing technologies, the TDO offers three decisive advantages: (1) ultra‑low power (1 µW) enabling placement at the mixing‑chamber stage without overwhelming the refrigerator’s cooling budget; (2) compact footprint (sub‑centimeter board) that eliminates the need for individual coaxial lines per qubit; and (3) superior phase‑noise performance when powered by a low‑noise battery. These attributes make the TDO a promising candidate for scalable, cryogenic microwave generation in future quantum processors.

The paper concludes with a discussion of future integration pathways. By coupling the TDO directly to a resonator designed for electron‑spin qubits, one could realize a fully cryogenic readout chain. Moreover, the low‑power, low‑phase‑noise source could serve as a clock for cryogenic analog‑to‑digital converters (Cryo‑ADCs), enabling on‑chip digitization and rapid feedback for quantum error correction. Remaining challenges include long‑term stability of the tunnel diode and varactor at millikelvin temperatures, mass‑production of the superconducting spiral inductors, and the development of multiplexed frequency‑division schemes to address many qubits simultaneously. Overall, the work demonstrates a practical, scalable solution to a key engineering hurdle in the path toward large‑scale quantum computing.