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Quantum 101 - What is a Qubit?

QNu Labs   |   Jan 30, 2024

Just as bits are the fundamental object of information in classical computing, qubits (quantum bits) are the fundamental object of information in quantum computing. 

To comprehend the concept of a qubit, one must first grasp the essence of quantum computing. Unlike classical computers, which operate based on binary bits—units of information—quantum computers harness the peculiar phenomena of superposition and entanglement to process data in a fundamentally different manner.

While a bit, or binary digit, can have a value of either 0 or 1, a qubit can have a value that is either 0, 1, or a quantum superposition of 0 and 1.

At its core, a qubit (quantum bit) encapsulates the essence of superposition, existing in a state that is both 0 and 1 simultaneously, until measured. This inherent duality enables quantum computers to perform parallel computations, exponentially increasing their processing power compared to classical counterparts.

Qubits may be pictured in 3D using the Bloch sphere representation. The Bloch sphere gives a way of describing a single-qubit quantum state (which is a two-dimensional complex vector) as a three-dimensional real-valued vector. This is important because it allows us to visualise single-qubit states and thereby develop reasoning that can be invaluable in understanding multi-qubit states.


Types of Qubits: The Quantum Diversity

Qubits manifest in various physical forms, each harnessing distinct properties and mechanisms to encode and manipulate quantum information. Some of the most prominent types of qubits are:

Superconducting Qubits

When cooled to a low temperature, some materials allow an electrical current to flow with no resistance. These are called superconductors. Electrical circuits designed based on superconductors can behave like qubits.

Superconducting qubits comprise tiny circuits fabricated from superconducting materials, such as niobium. These qubits exploit the quantized energy levels of superconducting circuits to encode and process quantum information, making them a leading candidate for building scalable quantum processors.

Trapped Ion Qubits 

One can use the energy levels of electrons in neutral atoms or ions as qubits. In their natural state, these electrons occupy the lowest possible energy levels. Using lasers, they can be “excited” to a higher energy level. Qubit values can be assigned based on their energy state.

Individual ions are typically of elements like ytterbium or calcium. Manipulating the internal states of these ions (through laser-induced transitions) enables precise control over quantum information, offering exceptional coherence times and fidelity crucial for quantum computations.

Spin Qubits

Most quantum particles behave like little magnets. This property is called spin. The spin orientation is always pointing either fully up or fully down but never in between. Using the spin states of up and down, a spin qubit is built.

Spin qubits capitalise on the intrinsic angular momentum (spin) of particles, such as electrons or atomic nuclei, as the basis for encoding quantum information. In semiconductor quantum dots or defect centres within crystals, the spin states of these particles serve as qubit carriers, governed by external magnetic and electric fields.

Photon Qubits

Photonic qubits exploit the quantum properties of photons, the fundamental particles of light, to encode and process information. Through techniques like linear optics and nonlinear processes, photons can be entangled and manipulated, laying the groundwork for applications in quantum communication and quantum cryptography.

Photon qubits can be Polarisation Qubit, Path Qubit, or Time Qubit.

Topological Qubits 

Theoretical in nature, topological qubits rely on exotic states of matter, such as topological superconductors or Majorana fermions, to encode and protect quantum information from decoherence—a pervasive challenge in quantum computing. While still in the nascent stages of exploration, topological qubits hold promise for realising fault-tolerant quantum computation.

Underlying Potential of Qubits

As the pursuit of quantum supremacy accelerates, fuelled by breakthroughs in qubit technology and quantum algorithms, the transformative impact of quantum computing on diverse fields looms on the horizon. From optimising complex problems and simulating quantum systems to revolutionising cryptography and drug discovery, the capabilities of quantum computers herald a new era of innovation and discovery.

However, amidst the promise lies the task of surmounting challenges, including decoherence, error correction, and scalability, which stand as barriers to realising the full potential of quantum computing. Addressing these hurdles demands interdisciplinary collaboration and perseverance to unlock the transformative power of qubits and pave the way towards a quantum future.

To Summarise….

Qubits stand as the building blocks of a new computational paradigm. As researchers, engineers, and visionaries continue to push the boundaries of possibility, the journey into the quantum frontier promises to redefine the limits of human knowledge and reshape the trajectory of technological progress.



University of Waterloo