The Bluefors Dilution Refrigerator as an Integrated Quantum Measurement System

Context

Measurements of quantum devices at millikelvin temperatures take on new importance today because of immediate applications in quantum simulations (Georgescu, Ashhab, & Nori, 2014), quantum sensing (Degen, Reinhard, & Cappellaro, 2017), and the world-wide race to build a fault tolerant quantum computer (Kjaergaard, et al., 2020). Scientists and engineers require integrated measurement systems to perform fast, and accurate characterization of quantum devices. The main measurement challenges are that devices require detection of very weak microwave signals and are exquisitely sensitive to environmental factors. Bluefors systems are often used as a platform for quantum devices as evidenced by more than 200 references (as of March 2021) to the Bluefors system in technical literature that focuses on quantum measurements or qubits.

This application note reports a decisive step towards demonstrating a "turn-key" integrated quantum measurement system that requires a vastly reduced number of steps required by the end-user to begin a quantum measurement. This advance is enabled by the integration of a PXI-based quantum measurement system from Keysight Quantum Engineering Solutions (QES) and the Labber instrument control software.

Application

The purpose of this application note is to demonstrate a working example of a superconducting qubit measurement in a Bluefors cryostat using the Keysight quantum control hardware. Our motivation is twofold. First, we provide pre-qualification data that the Bluefors cryostat, including filtering and wiring, can support long-lived qubits. Second, we demonstrate that the Keysight system (controlled using Labber) provides a straightforward solution to perform these characterization measurements. This document is intended as a brief guide for starting an experimental platform for testing superconducting qubits. The setup described here is an immediate jumping off point for a suite of applications including testing quantum logical gates, quantum optics with microwaves, or even using the qubit itself as a sensitive probe of local electromagnetic fields. Qubit measurements rely on high performance of both the physical sample environment and the measurement electronics.

Systems integration

Keysight qubit control solution

The control and readout of the qubit state has been outlined in a previous Keysight application note "Characterizing Superconducting Qubits." The signal generation and data acquisition sub-components of the measurement system are contained within a PXI-based quantum engineering tool-kit from Keysight. All modules are controlled using an embedded controller (M9037A) that runs the Labber software. 

The PXIe chassis includes an arbitrary waveform generator (M3202A), a local oscillator (M9347A), and a digitizer (M3102A). The PXI offers advanced backplane triggering features known as a hard virtual instrument (HVI). In the present example, upconversion for qubit control and downconversion for qubit readout are implemented with discrete IQ mixers outside of the PXI chassis.

Measurements and data acquisition are performed using ”Labber” — a powerful, yet user-friendly software package for instrument control and lab automation — with a focus on quantum applications.