Cryogenic Measurement Infrastructure for Quantum Computing

Quantum computers require ultra-low temperatures near absolute zero to operate, which is why dilution refrigerator measurement systems are used to cool them down. But while qubits love operating at millikelvin temperatures, the measurement infrastructure needed to control the quantum computer must also function at ultra-low temperatures. In this post, we look at the fundamentals of cryogenic wiring and signal transfer for qubit control and readout.

Operating in a Quantum World of Cold

Quantum devices are placed inside a cryogenic environment at the lowest temperature stage of a dilution refrigerator, where temperatures remain close to absolute zero.

While this is great for qubits, the rest of the measurement infrastructure must also be designed to operate at ultra-low temperatures so the system can control and receive readout information from the quantum device without introducing unwanted signal interference or heat.

In a dilution refrigerator measurement system, control and readout is achieved using microwave signals. Control signals are attenuated to a very low level and transmitted to the dilution refrigerator’s base temperature stage to operate the quantum device. On the return journey, readout signals are amplified to a signal level detectable at room temperature.

The measurement infrastructure needed to achieve both attenuation and amplification must therefore be capable of operating in an ultra-low temperature, cryogenic environment.

Measurement Infrastructure for Quantum Computing

Quantum computing measurement infrastructure is built using radio frequency components. The inputs and outputs are connected to electronic devices and components using wiring. Attenuators, amplifiers, and filters are all needed to condition the signals, with some of them needing to operate in the low temperature stages of the dilution refrigerator.

The wiring lines conduct signals from room temperature into the cryogenic environment. They connect the control electronics operating at room temperature to the quantum device operating close to absolute zero in the dilution refrigerator. It is important that the thermal conductivity of the wiring is low, so heat is not transmitted to the ultra-low temperature stages.

To control the qubits and receive readouts, the infrastructure uses control lines and readout lines:

• The control line carries the signal to drive the qubit.
• The readout line transmits the output of the measurements.
• Flux control lines are also used to tune qubits to certain frequencies.

Components on the Control Lines

On the control lines, attenuators are used to reduce thermal radiation and signal power (amplitude) to a suitable level needed to control the quantum device. When used as input lines, attenuators are situated on the dilution refrigerator’s flanges. In this way, the signals are attenuated further at each of the different cooling stages.

Filters are also needed to limit the noise around the signals that are being sent to the sample space. They are necessary to achieve high performance from superconducting qubits. For example, low-pass filters and infrared filters (IR Filter) are used to absorb and dissipate high frequency noise.

Diagnostic tools can also be placed on the input lines for measurement or calibration. Cryogenic noise sources, for example, might be placed inside the cryogenic environment to add a controlled amount of noise in the input line.

Components on the Readout Lines

Readout lines also need components to enable the optimized readout of signals coming from the qubits. Since the signals used to drive qubits are attenuated to a low power, amplifiers are needed on the readout lines to make the signal detectable back at room temperature, so they can be accurately measured. Again, the signals are amplified at different temperature stages to optimize the signal-to-noise ratio of received output signals at room temperature.

Traveling Wave Parametric Amplifiers (TWPA), for example, are placed in the coldest stage of the dilution refrigerator. TWPAs are close to quantum-limited amplifiers, providing high gain for the signals, while creating a minimal amount of noise.

High-Electron Mobility Transistor (HEMT) amplifiers are placed in the 4K stage, giving a final amplification in the cryogenic environment. They add a very high gain to the signal.  The type and placement of these amplifiers enables the accurate measurement of signals without compromising the qubits with noise.

Isolators and circulators are also added to the readout line, to isolate the quantum device. These components protect the qubit from interference created on the readout line, for example coming from the amplifiers. A filter can also be added on the output line, to reduce noise outside the measurement band.

Cooling the Measurement Infrastructure

Finally, besides the components that attenuate, amplify, or filter signals, it is important to consider temperature and heat. As mentioned earlier, many of the components in a dilution refrigerator are used to reduce thermal radiation — but even those components generate heat themselves, and must therefore be cooled by appropriate thermal anchoring at the different stages, and thoughtful thermal engineering throughout the dilution refrigerator, to prevent them contributing thermal radiation while in use.

Similarly, the more wiring lines that are added to the system, the higher the potential heat transfer due to conduction. To solve this dilemma, in addition to cooling the quantum processor, the dilution refrigerator is also used to cool all the components that control and measure the system and the state of the qubits.

Cool Developments

Maintaining an ultra-low temperature environment for quantum devices necessitates a complex infrastructure. Not only does the quantum device itself need to be cooled, but the entire infrastructure that both operates and measures the system needs to both function in cold environments, and modify and adapt the signals used to transmit information.

This is why constant development is needed for the components used to create the efficient measurement infrastructure required for quantum computing. Want to dive deeper into the cryogenic environment of dilution refrigerators? Learn more about the different components of the measurement system in the application note The Bluefors Dilution Refrigerator as an Integrated Quantum Measurement System.