Quantum Computing

A paradigm shift for information technology? 

The aim of this brief application note is to introduce the reader to some of the fundamental concepts of quantum computing – distinguishing it from a classical machine. In the first part, we will introduce some of the key properties of quantum systems, as well as how these concepts are being used for quantum information processing. The second part consists of an overview of the most common physical platforms used for building quantum processors, as well as comparing their relative performance to date.

Table of contents

• Classic vs. Quantum Bits
• Superposition States and Quantum Measurements
• Decoherence Characterization
• Entanglement and Quantum Interference
• Different Qubit Types
• Conclusion

Classical vs. Quantum Bits 

To explore the underlying mechanisms of a quantum processor, it is natural to first consider how information is represented and processed in today’s “classical” computers. Information in classical computers is stored and processed in binary, meaning that single bits can only represent two values – either “0” or “1”. Therefore, in order to build the powerful computers that are used today, billions of bits are required. It turns out, however, that there are certain classes of problems where classical computers fall short in terms of architecture and processing power. Examples of such problems are simulations of systems with many degrees of freedom, e.g. quantum systems such as atoms and molecules, logistics modeling, and other problems where the number of potential solutions is large. 

About forty years ago, Richard Feynman postulated that quantum systems could be simulated much more efficiently by using computer hardware that operates quantum mechanics. So, what does it mean to operate quantum mechanically? In short, it means that information is stored and processed in a system that is obeying the laws of quantum mechanics and that these laws form the basis for the information processing. To do this, we introduce the concept of a quantum bit (qubit) – a quantum two-level system with some remarkable properties. 

The information in a quantum processor is encoded into the states of its qubits. A qubit is a quantum two-level system that can be physically realized in many different quantum-mechanical platforms, such as trapped ions, superconducting circuits, and silicon spins, to just mention a few.