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Review of Scientific Instruments

Low-noise computer-controlled current source for quantum coherence experiments

S. Linzen, Timothy L. Robertson, T. Hime, B. L. T. Plourde, P. A. Reichardt, John L. Clarke

Published Jul 23, 2004• 13 citations
Publisher

Abstract

We describe a dual current source designed to provide static flux biases for a superconducting qubit and for the Superconducting QUantum Interference Device (SQUID) which measures the qubit state. The source combines digitally programmable potentiometers with a stabilized voltage source. Each channel has a maximum output of ±1 mA, and can be adjusted with an accuracy of about ±1 nA. Both current supplies are fully computer controlled and designed not to inject digital noise into the quantum bit and SQUID during manipulation and measurement of the flux. For a 275 μA setting, the measured noise current is 2.6 parts per million (ppm) rms, in a bandwidth of 0.0017–10 Hz, from which we estimate dephasing times of hundreds of nanoseconds in the particular case of our own qubit design. By resetting the current every 10 min, we are able to reduce the drift to no more than 5 ppm at a current of 750 μA over a period of 3 days. The current source has been implemented without thermal regulation inside a radiofrequency-shielding room, and is used routinely in our quantum coherence experiments.

DOI
10.1063/1.1771499

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Review of Scientific Instruments•Nov 1, 1997•43 citations
We describe a dual current source designed to provide static flux biases for a superconducting qubit and for the Superconducting QUantum Interference Device (SQUID) which measures the qubit state. The source combines digitally programmable potentiometers with a stabilized voltage source. Each channel has a maximum output of ±1 mA, and can be adjusted with an accuracy of about ±1 nA. Both current supplies are fully computer controlled and designed not to inject digital noise into the quantum bit and SQUID during manipulation and measurement of the flux. For a 275 μA setting, the measured noise current is 2.6 parts per million (ppm) rms, in a bandwidth of 0.0017–10 Hz, from which we estimate dephasing times of hundreds of nanoseconds in the particular case of our own qubit design. By resetting the current every 10 min, we are able to reduce the drift to no more than 5 ppm at a current of 750 μA over a period of 3 days. The current source has been implemented without thermal regulation inside a radiofrequency-shielding room, and is used routinely in our quantum coherence experiments.
Abstract With the rapid scaling of superconducting quantum processors, electronic control systems relying on commercial off-the-shelf instruments face critical bottlenecks in signal density, power consumption, and crosstalk mitigation. Here we present a custom dual-channel direct current (DC) source module (QPower) dedicated to large-scale superconducting quantum processors. The module delivers a voltage range of ±7 V with 200 mA maximum current per channel, while achieving the following key performance benchmarks: noise spectral density of 20 nV/ Hz at 10 kHz, output ripple <500 μVpp within 20 MHz bandwidth, and long-term voltage drift <5 μVpp over 12 hours. Integrated into the control electronics of a 66-qubit quantum processor, QPower enables qubit coherence time of T 1 = 87.6 μs and Ramsey dephasing time of T 2 = 5.1 μs, with qubit resonance frequency drift constrained to ±40 kHz during 12-hour operation. This modular design is compact in size and efficient in energy consumption, providing a scalable DC source solution for intermediate-scale quantum processors with stringent noise and stability requirements, with potential extensions to other quantum hardware platforms and precision measurement systems.
Noisy voltage sources can be a limiting factor for both fundamental physics experiments and device applications in quantum information, mesoscopic circuits, magnetometry, and other fields. The best commercial dc voltage sources can be programmed to approximately six digits and have intrinsic noise in the microvolt range. On the other hand, the noise levels in metrological Josephson-junction-based voltage standards is subfemtovolt. Although such voltage standards can be considered “noiseless,” they are generally not designed for continuous tuning of the output voltage nor for supplying current to a load at cryogenic temperatures. We propose a Josephson-effect-based voltage , as opposed to a voltage standard, operating in the 30 − 160 − μ V range, which can supply over 100 nA of current to loads at mK temperatures. We describe the operating principle, the sample design, and the calibration procedure to obtain continuous tunability. We show the current-voltage characteristics of the device, demonstrate how the voltage can be adjusted without dc control connections to room-temperature electronics, and showcase an experiment coupling the source to a mesoscopic load: a small Josephson junction. Finally, we characterize the performance of our source by measuring the voltage noise at the load, 50 pV rms, which is attributed to parasitic resistances in the cabling. This work establishes the use of the Josephson effect for voltage biasing of extremely sensitive quantum devices.
User-friendly ultra-high-speed readout electronics for dc superconducting quantum interference devices (SQUIDs) are presented. To maximize the system bandwidth, the SQUID is directly read out without flux modulation. A composite preamplifier is used consisting of a slow dc amplifier in parallel with a fast ac amplifier. In this way, excellent dc precision and a high amplifier bandwidth of 50 MHz are achieved, simultaneously. A virtual 50 Ω amplifier input resistance with negligible excess noise is realized by active shunting, i.e., by applying feedback from preamplifier output to input via a high resistance. The white voltage and current noise levels are 0.33 nV Hz −1 and 2.6 pA Hz −1/2 , respectively. The electronics is fully computer controlled via a microcontroller integrated into the flux-locked loop (FLL) board. Easy-to-use software makes the various electronic settings accessible. A wide bias voltage range of 1.3 mV enables the readout of series SQUID arrays. Furthermore, additional current sources allow the operation of two-stage SQUIDs or transition edge sensors. The electronics was tested using various SQUIDs with input inductances between 30 nH and 1.5 µH. Typically, the maximum FLL bandwidth was 20 MHz, which is close to the theoretical limit given by transmission line delay within the FLL. Slew rates of up to 4.6 Φ 0 µs −1 were achieved with series SQUID arrays. Current noise levels as low as 0.47 pA Hz −1/2 and coupled energy sensitivities between 90 h and 500 h were measured at 4.2 K, where h is the Planck constant. The noise did not degrade when the system bandwidth was increased to the maximum value of about 20 MHz. With a two-stage set-up, intrinsic white energy sensitivities of 30 h and 2.3 h were measured at 4.2 and 0.3 K, respectively.
A current biased dc superconducting quantum interference device behaves as an anharmonic quantum oscillator controlled by a bias current and an applied magnetic flux. We consider here its two level limit consisting of the two lower energy states \ensuremath{\mid}0⟩ and \ensuremath{\mid}1⟩. We have measured energy relaxation times and microwave absorption for different bias currents and fluxes in the low microwave power limit. Decoherence times are extracted. The low frequency flux and current noise have been measured independently by analyzing the probability of current switching from the superconducting to the finite voltage state, as a function of applied flux. The high frequency part of the current noise is derived from the electromagnetic environment of the circuit. The decoherence of this quantum circuit can be fully accounted for by these current and flux noise sources.
An improved read-out electronics without flux modulation for dc superconducting quantum interference devices (dc SQUIDs) is presented where the preamplifier is directly coupled to the SQUID. Compared to our previous designs, the 1/f voltage and current noise levels of the preamplifier were reduced by about a factor of 4 (rms) by using bipolar matched transistors instead of operational amplifiers at the input. Three types of flux-locked-loop electronics were developed and successfully tested: a low-power (120 mW) version for large multichannel arrays, a high-speed version with a 15 MHz bandwidth and a very fast (300 ns) reset mode, and a low-noise (0.44 nV/Hz) version with a fast and precise bias reversal circuit. The flux-locked-loop electronics are compatible with each other, so that only one type of low-power (180 mW) control unit is required to operate them. All essential specifications of practical SQUID systems (noise, dynamic behavior, and temperature stability) were determined by using liquid-helium and liquid-nitrogen cooled SQUIDs, respectively.