Quantum Computer Temperature: Do They Need to Be Cold?

Introduction

Welcome to world of quantum computing, where the intuitive laws of classical physics give way to the complex phenomena of quantum mechanics. At kiutra, we are addressing some of the key challenges around operating and testing quantum technologies at low temperatures.

This article has been written for a layman audience and therefore some explanations have been truncated or oversimplified in favor of accessibility.

Why do we need quantum devices in extreme cold

Let’s start with quantum computers. The answer lies in the quirky behaviour of quantum bits or qubits, the fundamental units of information in a quantum computer. Unlike classical bits, which can exist in one of two states, 0 or 1, qubits can exist in a superposition of probabilistic states. This remarkable property allows quantum computers to process information in ways that classical computers simply cannot match.

Qubits are incredibly delicate and prone to what is called ‘decoherence’. This means that their quantum states can be easily disrupted by any interaction with their surroundings, such as heat and electromagnetic radiation. To mitigate this, we must create an environment where external influences are minimized, and that’s where the extreme cold comes into play.

Do quantum computers need to be cold?

Yes – some quantum computers, such as those leveraging superconducting qubits, typically need to be kept at very low temperatures, close to absolute zero, for several important reasons:

Reducing thermal noise:

At higher temperatures, particles within the quantum computer’s components exhibit greater thermal vibrations and can introduce errors into the delicate quantum states of qubits. Cooling the system helps minimize these vibrations, reducing the impact of thermal noise and enhancing the stability of qubits.

Preserving quantum coherence:

Quantum states, such as superposition and entanglement, are exceptionally sensitive to their surroundings. Lower temperatures help prolong the coherence time of qubits, allowing them to maintain their quantum properties for longer durations, which is essential for performing complex quantum computations.

Minimizing interaction with the environment:

High temperatures are associated with increased interactions between quantum systems and their external environment, causing decoherence and information loss. Cooling down the system helps isolate the quantum components from external influences.

Quantum computer modalities

This section covers the main modalities being pursued to develop quantum computers. There are several approaches to developing quantum computers that do not always require ultra-low temperature cooling. For the avoidance of doubt, most of these approaches do require some form of cooling (though further work is being done on developing room-temperature quantum computing).

Superconducting qubits:

The most well-known approach for quantum computing involving superconducting qubits in quantum computing utilizes tiny circuits made of superconducting materials that can carry electrical current without resistance when cooled to extremely low temperatures, typically close to absolute zero. These superconducting qubits are manipulated and entangled using microwave pulses and controlled through the application of external magnetic fields. They are highly scalable and have become a leading platform for building quantum processors. Superconducting qubits are the basis for quantum devices developed by large companies like IBM, Google, AWS and a broad set of start-ups.

Silicon spin qubits:

Silicon spin qubits encode quantum information in the spin states of individual electrons or nuclei in silicon. This approach leverages advanced semiconductor manufacturing techniques, allowing for the integration of spin qubits into silicon chips, similar to conventional electronics. The quantum states are manipulated using microwave and magnetic fields, with the potential for high-density qubit arrays due to the small size of the qubits. A key advantage of silicon spin qubits is their compatibility with existing semiconductor technology, which may facilitate easier scaling and integration into current electronic infrastructure. Furthermore, silicon-based qubits can operate at slightly higher temperatures than superconducting or ion trap qubits, potentially reducing cooling requirements. This approach is being actively researched and developed by several organizations, including tech giants like Intel and smaller specialized companies. mK cooling is currently in use, and future mature chips will still necessitate low single-digit Kelvin temperatures.

Trapped ion qubits:

Trapped ion qubits represent another prominent technique in quantum computing, wherein individual ions (charged atoms) are confined and isolated in an electromagnetic field. These ions are cooled to near absolute zero, similarly to superconducting qubits, but the information is stored in the quantum state of the ions themselves. Laser pulses are used to manipulate and entangle these qubits, enabling quantum computations. Trapped ion qubits are known for their long coherence times, which is crucial for maintaining quantum information over extended periods. This approach has been instrumental in advancing quantum computing research and is adopted by leading organizations and startups in the field, including IonQ and Honeywell (now part of Quantinuum), signifying its significance alongside superconducting qubit-based quantum computers.

kiutra might be known best for its magnetic sub-Kelvin coolers, but we also craft advanced cryostats tailored for trapped ion chips, addressing the unique demands of the expanding trapped ion (TI) community and their emerging applications. Operating at ‘moderate’ cryogenic temperatures between 2-10 K is crucial for scaling trapped ion quantum processing units (QPUs) to a level where quantum advantage is attainable. This is because cryogenic cooling is essential for maintaining extremely high vacuum (XHV) conditions, cooling the control and readout electronics, and managing the expanding RF wiring trees necessary for trapped ion quantum computers. Our commitment to continual enhancement of our solutions is evident through active collaborations with various partners within the TI community, such as our involvement in the Millenion project.

Neutral atom qubits:

Neutral atom qubits offer a distinct method in quantum computing, where neutral atoms, typically alkali atoms like rubidium or cesium, are used as qubits. These atoms are held in place by highly focused laser beams in an ultra-high vacuum environment. Information is stored in the quantum states of these atoms, and quantum operations are performed using precisely controlled laser pulses. This method is noted for its potential scalability, as large arrays of neutral atoms can be manipulated simultaneously. The neutral atoms’ approach allows for long-range interactions between qubits, enabling unique quantum computing architectures. Companies such as QuEra, Pasqal and Infleqtion (p.k.a ColdQuanta) are exploring this technology.

At kiutra, we recognize the potential need for cooling solutions in scaling up, for instance, to mitigate black-body radiation background and improving vacuum conditions. We understand that the requirements will be different from, e.g., trapped ion cooling architecture due to, higher laser input power, a need for more optical access, and perhaps even tougher vibration requirements.

Photonic qubits:

Photonic qubits encode quantum information in the properties of photons, such as their polarization or phase. This method utilizes optical systems – like lasers, beam splitters, and photon detectors – to generate, manipulate, and measure photonic qubits. Photonic qubits demonstrate inherent robustness against environmental noise. Companies and research institutions such as Xanadu, PsiQuantum, ORCA Computing and various academic labs are actively pursuing this technology.

These modalities offer the potential for quantum computing and quantum communication without the need for the same level of cryogenic cooling as superconducting qubits.

Given their faint interaction, which is advantageous for ‘flying qubits,’ photons are complex to detect, typically requiring cryogenic single-photon detectors. Additionally, deterministic quantum light sources function at cryogenic temperatures, necessitating cooling in the 4–10 K range.

See also: What Is Magnetic Refrigeration

Components and cryogenic testing

Though keeping quantum computers at ultra-low temperatures is an important, and well-known image, in quantum computing, the importance of cryogenic testing is not always recognized.

All the components that go into quantum systems need to be tested in relevant conditions:

Coherence enhancement:

Certain critical components, such as microwave sources or control electronics are often required to be kept at cryogenic temperatures. This is because lower temperatures can help reduce electronic noise and maintain the stability and precision required for qubit manipulation.

Thermal management:

Some heat is still generated during quantum computations. Cooling specific components helps manage this heat and prevents it from affecting the qubits’ performance.

The role of cryogenic testing in quantum technology

Cryogenic testing in quantum computing involves subjecting quantum-based systems and components to low temperatures to evaluate their performance and reliability. It serves two main purposes:

Characterization:

Cryogenic testing helps researchers understand how qubits and related hardware behave in a challenging low-temperature environment. It provides valuable data on factors like coherence times, error rates, and fidelity, crucial for designing more robust and efficient quantum systems.

Quality control:

Cryogenic testing serves as a quality control measure during the manufacturing and development of quantum devices. It helps identify and rectify issues that may arise due to temperature-related effects, ensuring that quantum computers operate reliably when deployed in real-world applications.

In essence, cryogenic testing is essential for assessing and optimizing the performance of quantum hardware, ultimately advancing the development and practical use of quantum computing technology.

How are components such as amplifiers and interconnects tested at cryogenic temperatures?

Components like amplifiers and interconnects are tested at cryogenic temperatures by placing them in specialized cryostats or refrigeration systems. These systems can maintain extremely low temperatures, typically just above absolute zero. The components are carefully installed within the cryostat, and electrical connections are established.

During testing, the cryogenic environment helps evaluate the component’s performance under conditions similar to those it would experience in a quantum computing system. Researchers can measure properties such as signal amplification, signal loss, and electrical conductivity to assess how these components behave at low temperatures.

Cryogenic testing ensures that these components can operate reliably in the demanding conditions of quantum computers, where maintaining the stability of qubits and minimizing noise is crucial for achieving accurate quantum computations.

Conclusion

The world of quantum technology is a fascinating frontier that often relies on very low temperatures to harness the incredible power of quantum mechanics. kiutra and other pioneering organizations are pushing the boundaries of technology to address the challenges associated with operating and testing quantum systems at low temperatures.

While the most mature quantum computers, like those using superconducting qubits, require extreme cold to reduce thermal noise, preserve quantum coherence, and minimize environmental interactions, there are promising alternatives like trapped ion, neutral atom, silicon spin, and photonic qubits hat promise operation at more moderate (still cryogenic) temperature.

Cryogenic testing plays a pivotal role in characterizing and ensuring the quality of quantum hardware, advancing the development and practical application of quantum computing technology.

One final word from kiutra

At kiutra, we immerse ourselves in the realm of cryogenics, a field deeply rooted in the world of low-temperature physics.

Our goal revolves around harnessing cryogenic cooling technologies. We recognize that one of the most critical challenges in quantum computing is the preservation of qubit coherence. Thanks to innovations in cryogenic engineering and magnetic cooling technology, we are proud to provide scalable and efficient solutions that drive quantum forward.

You can learn more about our quantum cooling solutions here.