What is a superconducting qubit? How It Works

Superconducting quantum computing is a projecting and actively researched approach towards building a functional quantum computer. It controls the principles of superconductivity to create and operate quantum bits, or qubits. These qubits can be used to perform quantum computations that have the potential to beat classical computers for certain types of problems.

Superconducting quantum computing implements quantum computers using superconducting electronic circuits. Superconductivity is a wonder where certain materials show zero electrical resistance below a critical temperature. This allows for the creation of circuits with slight energy dissipation, which is critical for maintaining the mild quantum states of qubits.

How do superconducting qubits work?

Superconducting qubits function by applying the principles of superconductivity to create and control quantum states.

Here’s a step-by-step explanation

1. Choosing Superconducting Materials: Superconducting qubits are built using materials that exhibit zero electrical resistance below a critical temperature. This tolerates the creation of circuits with minimal energy loss, which is essential for maintaining the delicate quantum states.

 Examples of organisations working with superconducting qubits include Google, IBM, and Rigetti.

2. Constructing Superconducting Circuits: These superconducting materials are used to construct complex electrical circuits on a chip. These circuits typically include components like capacitors (C) and inductors (L), which can be designed to have specific quantum properties. The ability to create printable circuits and utilise VLSI techniques is an advantage of this technology.
3. Integrating Josephson Junctions:  A decisive element in superconducting qubits is the Josephson junction. This consists of two superconducting electrodes separated by a very thin insulating barrier. The Josephson junction is a non-linear circuit element that is essential for creating usable qubit states. Its nonlinearity breaks the immorality of energy level spacings, allowing the system to behave as a two-level quantum system, i.e., a qubit.
4. Defining Qubit States: The two essential states of a qubit, |0⟩ and |1⟩, are mapped to specific quantum states within the superconducting circuit. These states are repeatedly related to different energy levels in the system, which are determined by the circuit parameters (capacitance, inductance, and Josephson junction characteristics).

For example, in some designs, the states might correspond to different charge configurations or magnetic flux states across the Josephson junction.

5. Reaching Superposition: Different from classical bits that are either 0 or 1, a superconducting qubit can exist in a superposition of both |0⟩ and |1⟩ simultaneously. This means the qubit’s state can be denoted as a linear combination of the basis states: |ψ⟩ = α|0⟩ + β|1⟩, where α and β are complex amplitudes. The superposition allows a quantum computer to process exponentially larger data spaces compared to classical computers of the same size. The exact mechanism for creating a superposition state depends on the implementation technology, frequently involving the application of carefully timed electromagnetic pulses, typically microwaves, at specific frequencies to operate the energy levels of the qubit.
6. Working Qubit States with Quantum Gates: To perform computations, the states of the superconducting qubits are operated using quantum gates. These gates are equivalent to classical logic gates but operate on quantum states. Quantum gates are implemented by applying exactly controlled electromagnetic pulses (usually microwaves) to the qubits. The frequency, duration, and shape of these pulses determine the specific quantum gate that is applied, causing a controlled evolution of the qubit’s quantum state.

Examples of quantum gates include single-qubit gates like Hadamard (H), Pauli-X, Y, and Z gates, and multi-qubit gates like the Controlled-NOT (CNOT) gate, which can create entanglement between qubits. For instance, a CNOT gate can be implemented by electromagnetically coupling two qubits so that they influence each other.

7. Creating Entanglement:  Entanglement is a vital quantum mechanical property where the quantum states of two or more qubits become linked together in such a way that they share the same fate, no matter how far apart they are. Multi-qubit gates like the CNOT are used to create entanglement between superconducting qubits. Entanglement allows for powerful quantum algorithms that offer potential speedups over classical algorithms.
8. Measuring the Qubit State: The final step in a quantum computation is measurement, which is the process of extracting information from the qubits. When a superconducting qubit is measured, its superposition state collapses into one of the basis states, either |0⟩ or |1⟩, with a probability determined by the amplitudes of the superposition (α and β). Measurement is typically done by interacting the qubit with its environment, causing a noticeable change, such as the emission of a photon, which can be detected. The measurement outcome is a classical bit (0 or 1) that represents the result of the quantum computation for that particular run. Due to the probabilistic nature of quantum mechanics, quantum algorithms are often run multiple times to obtain a statistical distribution of the results, which can then be used to infer the solution to the problem.
9. Operating at Cryogenic Temperatures: Superconductivity only happens at very low temperatures, typically a few millikelvin. Therefore, superconducting quantum computers must operate in sophisticated cryogenic systems that use liquid helium to achieve these ultra-cold temperatures. These low temperatures are essential to maintain the superconducting state and to minimise thermal noise and decoherence, which can disrupt the delicate quantum states of the qubits.

Advantages of Superconducting Qubits

• Compatibility with Traditional Integrated Circuits: These implementations accordingly match with traditional classical integrated circuits and can be easily invented utilising existing hardware technology. This is a significant advantage for scalability and manufacturing.
• Fast Gate Operations: Superconducting qubits are known for their potential for fast gate operations. Quantum gates are the basic building blocks of quantum algorithms, and faster gates can lead to quicker computation times.
• Printable Circuits and VLSI: The ability to create printable circuits and utilise Very-Large-Scale Integration (VLSI) techniques is another benefit that aids in scaling up the number of qubits.

What are the limitations of superconducting qubits?

Superconducting qubits face several limitations:

• Short Coherence Times: A significant problem is the tremendously limited coherence time of early superconducting qubits. Coherence time refers to how long a qubit can maintain its quantum state before it is disturbed by the environment, leading to errors. Superconducting qubits may lose their information in tens of microseconds. To overcome the effect of phonon-assisted excitons and thermal energy disturbances, superconducting qubits typically need to operate at millikelvin temperatures. This requires the use of large cryostats filled with liquid helium for refrigeration.
• Qubit Variability: Initially, a challenge was that all qubits were different. However, developments in creation are working to improve the uniformity of superconducting qubits.
• Control Signal Heat: The control signals can carry in troublemaking heat, posing a challenge for maintaining the extremely low operating temperatures required. Proposals exist to run some of the control within the cryostat to minimize heat production.
• Error Rates: Quantum operations on superconducting qubits are unreliable, with error rates around 0.1% in earlier stages. This requires the execution of Quantum Error Correction (QEC) techniques, which use multiple physical defective qubits to compose more reliable logical qubits.

Types of Superconducting Qubits

There are various types of superconducting qubits, including:

• Charge Qubits: These qubits are based on the charging energy of a superconducting island.
• Flux Qubits: These qubits utilize the magnetic flux through a superconducting loop.
• Phase Qubits: These qubits are based on the phase difference across a Josephson junction.

Current Status of Superconducting Qubits

Significant progress has been made in the field of superconducting quantum computing:

• Early Demonstrations: The first experimental demonstration of a superconductor qubit was attributed to NEC Corporation in 1999, although with a very short coherence time. This experiment spurred further development worldwide.
• Quantum Advantage Claims: In 2019, Google’s Sycamore processor claimed quantum advantage using a programmable superconducting processor. This was a landmark achievement suggesting that quantum computers could perform certain calculations practically impossible for classical machines.
• Increasing Qubit Counts: IBM has also made significant strides, building its Eagle quantum computer with 127 qubits in 2021. IBM has further announced plans to increase the number of available qubits, aiming for a 1000-qubit quantum chip. Other companies like Rigetti and Intel have also developed superconducting chips with increasing qubit numbers. As of 2018, D-Wave’s technology had up to 2000 superconducting qubits in their quantum annealers.
• Improved Coherence and Fidelity: Ongoing research focuses on improving the coherence times and the fidelity of quantum gates in superconducting qubits. Fidelity refers to the accuracy of the quantum operations.

Examples of Superconducting Qubit Companies

Some important organizations are deeply involved in researching and developing superconducting quantum computing technologies:

• Google: Demonstrated quantum advantage with their Sycamore processor.
• IBM: Has developed several quantum processors, including the 127-qubit Eagle. They also offer cloud access to their quantum computers through IBM Quantum.
• Rigetti: Another company developing superconducting quantum computers and providing cloud access.
• Intel: Has invested in developing superconducting qubit technology.
• IMEC: A research and innovation hub involved in superconducting quantum computing research.
• BBN Technologies: Has also contributed to the development of superconducting qubits.
• D-Wave Systems: Builds superconducting quantum annealers designed for optimization problems.
• Fujitsu: Has invested in a digital annealer, a quantum-inspired computer that handles similar optimization problems as D-Wave.

Applications

Superconducting quantum computers have the potential to revolutionize various fields, including:

• Cryptography: Shor’s algorithm, which can be run on a quantum computer, poses a threat to many modern cryptographic systems used for secure communication. This has spurred research into post-quantum cryptography.
• Drug Discovery and Materials Science: Quantum computers can be used to simulate molecules and materials, potentially leading to the discovery of new drugs and materials with novel properties.
• Optimization Problems: Quantum annealing using superconducting qubits, as pioneered by D-Wave, is being explored for solving complex optimization problems in areas like logistics, finance, and scheduling.
• Machine Learning: Quantum machine learning aims to leverage quantum algorithms to enhance the computational efficiency of machine learning models.

Superconducting qubits Vs trapped ion qubits

Superconducting qubits and trapped ion qubits are two of the important technologies in quantum computing, but they work in fundamentally different ways using different physical systems.

Superconducting qubits are like small, specifically designed electrical circuits made from superconducting materials that lose all electrical resistance at extremely low temperatures. The quantum information, or the qubit state (representing 0, 1, or a mix of both), is stored in the energy levels of these circuits, regularly involving a component called a Josephson junction. These qubits are typically controlled and manipulated using microwave pulses sent to the circuits. One of the main strengths of superconducting qubits is their fast operation speeds, meaning they can perform quantum calculations quickly. However, they tend to have shorter coherence times, meaning they lose their quantum state relatively quickly due to interactions with their environment. They also require very low operating temperatures, very close to absolute zero.

Trapped ion qubits, on the other hand, use individual atoms (ions) that are held in place using electromagnetic fields, like a small cage. The qubit state is stored in the internal energy levels of these ions, for example, the ground and an excited state. These qubits are controlled and operated using exactly focused laser beams that interact with the ions. Trapped ion qubits are known for their high stability and long coherence times, meaning they can hold quantum information for a longer duration. However, their quantum operations are generally slower compared to superconducting qubits. A unique advantage of trapped ions is that they can potentially achieve high connectivity, as ions can be moved around or their states can be linked using photons. While they also operate at cryogenic temperatures, the requirements might be less extreme than for superconductors. Moreover, qubits can even be physically transported between different locations in some trapped ion architectures.

In simple terms, superconducting qubits are fast but delicate electrical systems, while trapped ion qubits are more stable individual atoms that are manipulated with light. Both have their own strengths and weaknesses, and researchers are actively working to improve both technologies.