Microwave Link
A microwave link contains four main functions: signal generation, transmission, processing and detection. A microwave link can be a communication link between two cell phones or it can be used to transfer information within a superconducting quantum computer. The goal is to engineer the microwave link in a way that guarantees successful signal detection, be it a message transmitted from a cell phone or the state of a qubit. This section will examine a microwave link in a superconducting quantum computer, as illustrated in Figure 4, where each functional box shows the components involved in the microwave link.
Figure 4 Microwave link for a superconducting quantum computer.
Microwave signals are used to control and read out the state of superconducting qubits. As shown in Figure 4, the microwave signals are generated at room temperature and transmitted into a dilution refrigerator using microwave coaxial cables. Along the way, filters and attenuators process the signal to reduce noise. After interacting with the qubit, the signal is processed using cryogenic amplifiers and sent back to room temperature using special microwave cables. The signal is amplified, filtered and down-converted as the final room temperature processing step to prepare it for detection. Finally, a digitizer converts the analog signal into a digital signal that a computer can process.
As a signal travels down a microwave link, it is affected by cables, amplifiers, filters and various other passive and active components that amplify or attenuate it. To guarantee the successful detection of the signal at the receiving end, a comprehensive link budget analysis is conducted. This analysis calculates all gains and losses within the microwave link, ensuring the presence of adequate power and a sufficient signal-to-noise-ratio for accurate detection of quantum states.
Microwave Systems
This section explores how a microwave signal is influenced by noise, interference, distortion and nonlinear effects as it travels through a link. To ensure successful signal detection, it is crucial to comprehend and analyze these effects on microwave signals. To effectively analyze and design a microwave system, it is important to have a system-level understanding of the critical performance factors along with the effects that will degrade this performance. The analytical tools used for microwave system analysis are powerful, enabling a microwave system and its components to be treated as a black box. This allows designers to examine the inputs and outputs of the system without requiring a deep knowledge of electromagnetics or microwave engineering.
As mentioned in the previous section, the essential first step in designing and evaluating a microwave link begins with performing link budget calculations. Subsequently, the effects of distortion, interference and noise on microwave systems need to be examined because these are crucial factors that affect system performance. Furthermore, nonlinear effects need to be explored in microwave systems. Some of these effects include 1 dB compression point, harmonic levels and intermodulation distortion, which are essential for designing and optimizing system performance and efficiency.
Microwave Components
Figure 5 (a) A single qubit coupled to a coplanar waveguide resonator. (b) Two qubits coupled to a coplanar waveguide resonator.
The next step is to understand cryogenic and room temperature microwave components. These components include, but are not limited to, amplifiers, mixers, filters, power combiners, attenuators, up-converters and down-converters. This knowledge can be divided into three main areas:
- Noise engineering needs a detailed understanding of all the coupling paths to the qubit and how to block them using components such as filters, attenuators, special microwave cables and circulators.
- Microwave signal processing techniques involve filtering, amplification, down-conversion and up-conversion. These techniques play a vital role in qubit operation. They involve transmitting modulated pulses to the qubits and subsequently down-converting readout signals for digital post-processing.
- Microwave measurement techniques use instruments like network and spectrum analyzers, signal generators and high frequency oscilloscopes. These instruments are used to perform component-level, system-level and qubit measurements.
Single-qubit and two-qubit operations are the backbone of quantum algorithm implementation. Figure 5a depicts a single qubit superconducting system where the qubit is coupled to a coplanar waveguide resonator. Figure 5b shows a two-qubit system coupled to a coplanar waveguide resonator. Microwave components and techniques are necessary for the control and readout of the qubits.
CRYOGENIC ENGINEERING
As mentioned earlier, semiconductor qubit experiments are commonly conducted near absolute zero in a dilution refrigerator, at temperatures ranging from 10 mK to 50 mK. These low temperatures play a crucial role in mitigating disruptive factors, such as noise, which could lead to the collapse of the qubit’s delicate quantum states. A typical quantum processing unit is shown in Figure 6.
Figure 6 Dilution refrigerator.
Figure 7 Noise coupling paths to a qubit.
Figure 8 (a) Josephson junction. (b) Implementation of Josephson junction. (c) Parallel Josephson junctions form a SQUID. (d) SQUID implementation.
A pivotal role of a dilution refrigerator lies in protecting qubits from coupling to the environment. A quantum engineer must understand that there are conducted and radiated pathways that allow noise to affect the qubit. This is shown, generically, in Figure 7. This understanding enables the application of techniques, such as filtering and shielding, to block these pathways and protect the qubit from decoherence. For this purpose, a combination of filters, circulators, attenuators, shielding and various cable types, like lossy microwave cables, are used to minimize the noise level coupled to the qubit. For a quantum engineer, a comprehensive understanding of a dilution refrigerator and its components, along with noise pathways, is essential before undertaking qubit experiments.
NANOFABRICATION
Nanofabrication techniques are crucial for the development of nanocircuits used in semiconductor qubits, with electron beam lithography machines being a common tool in this process. Proficiency in diverse fabrication methods is advantageous for quantum engineers. These engineers can benefit from familiarizing themselves with key processes and tools like electron beam lithography, as well as metal deposition techniques such as sputtering and thermal evaporation. In academic settings, a single individual may handle all aspects of qubit fabrication and operation, but in the industrial landscape, dedicated teams specialize in this field. Quantum hardware engineers in the industry may not need extensive knowledge of nanofabrication techniques since requirements will be based on specific roles within the company. To illustrate typical nanofabrication techniques used on a superconducting quantum computer, Figure 8a shows a Josephson junction that is comprised of two superconducting electrodes separated by an extremely thin insulating layer. Cooper pairs can coherently tunnel across the insulating barrier. The Josephson junction equivalent circuit is also shown in the diagram.
Figure 8b shows the implementation of a Josephson junction using aluminum electrodes and an Al/AlOx/Al oxide layer.
Figure 8c shows two parallel Josephson junctions forming a superconducting quantum interference device (SQUID). The Josephson energy or the total inductance can be controlled using an external flux.
Finally, Figure 8d shows the implementation of a SQUID with a Josephson junction with a charging energy of EJ1 and EC. Cg is the gate capacitance to control the charge. Josephson junctions and SQUIDs are essential components for a superconducting quantum computer, so it becomes very important to master these nanofabrication techniques.
DATA ACQUISITION AND QUANTUM MEASUREMENTS
After fabricating the qubits and housing them in a dilution refrigerator, along with addressing all microwave engineering aspects, the quantum system is ready for operation. Operating a quantum processor involves the precise control and readout of qubits to successfully implement single-qubit and two-qubit gates, which are fundamental for implementing quantum algorithms. This phase of the process demands a thorough understanding of quantum mechanics and quantum measurements, along with expertise in instrument automation and data acquisition.
These processes are coordinated with control software that interfaces with various instruments and microwave subsystems to generate the necessary microwave pulses for qubit control and readout. Quantum engineers need to be adept at utilizing diverse communication protocols such as serial communication, GPIB, LAN and USB. Software that is commonly employed to ensure efficient data acquisition includes Matlab, Python and Labber. Quantum hardware engineers should be proficient in at least one of these software tools to ensure that data acquisition tasks are completed seamlessly. Additionally, coding skills are invaluable, particularly for those wishing to develop customized measurement codes within these software environments.
CONCLUSION
This article has discussed four of the most important skill sets for semiconductor quantum hardware development. It is important to realize that the next generation of quantum engineers does not need to be experts in all four areas. Individuals will specialize and focus on specific areas to gain expertise and companies may choose to specialize in different niches within the larger quantum market.
The upcoming generation of quantum engineers will play a crucial role in advancing revolutionary technologies and their success hinges on effective skill-based training. Quantum hardware engineers need proficiency in diverse areas like microwave engineering, cryogenic techniques, nanofabrication, data acquisition and quantum measurements to successfully contribute to the development of quantum processors. Closing the talent gap requires a strategic emphasis on well-designed courses and hands-on training with real-world applications. Utilizing resources like books and online courses can significantly accelerate the learning process for aspiring quantum engineers.
References
- C. Hughes, D. Finke, D. -A. German, C. Merzbacher, P. M. Vora and H. J. Lewandowski, “Assessing the Needs of the Quantum Industry,” IEEE Transactions on Education, doi: 10.1109/TE.2022.3153841.
- A. Salari, “Microwave Techniques in Superconducting Quantum Computers,” Artech House, 2024.
