An Introduction to Quantum Computing
An Introduction to Quantum Computing ____________________________________________________________________________________ Quantum entanglement produces extremely strong statistical correlations that no digital hidden variable model can replicate, and at the same time, it cannot carry any usable active information to achieve faster-than-light transmission. From the specific perspective of applied quantum computing, the role of quantum entanglement is critically important: it allows all information encoded in a large multi-qubit register to dynamically exist in complex non-local dependencies between the states of correlated qubits that are far apart, rather than being confined only to isolated individual qubits. This collective information processing capability is the core foundation required to implement computing algorithms, quantum simulation, and quantum error correction protocols.
How Entanglement Can Be Created.
_______________________________________________________________________________________________ SCTE Technical Committee : Dave Hodges and Dr Anthony Basham. © 2026 Society for Broadband Professionals. All rights reserved Page 11 A qubit is a state, not a specific component. There is no single thing that can be unambiguously referred to as "the qubit". A qubit is never a single piece of hardware; it is fundamentally defined as a two-level quantum information system, which can only be physically implemented and realised by relying on a controllable physical degree of freedom within a larger system. Building on this foundation, competing technical architectures can have completely different physical systems as their basic building blocks. The primary reason for this is that each technical approach has a unique, precise trade-off space across five key properties. These are the level of control precision, the length of quantum coherence time, the level of interconnectivity between different qubits, the complexity of large-scale manufacturing, and the environmental requirements for physical operation. This concept is the core idea, and it can be demonstrated by a simple, quantum circuit equipped with only two basic logic gates. To start the preparation process, several qubits need to be prepared, only two are needed as a starting point. Carefully initialising them to the most basic unentangled reference state, mathematically denoted as the |00 ⟩ state. After completing this first phase of the preparation process, the corresponding quantum operation should be performed on the target qubit, which is the first physical working qubit to apply to a Hadamard gate. Once this operation is finished, the first qubit immediately enters an equal-weight superposition of all possible states. At the same time, the second qubit remains completely unaffected and stays in its original |0 ⟩ state. As a result, the joint quantum state of these two systems evolves accordingly and can be precisely described by a clear mathematical expression. (|00 ⟩ + |10 ⟩ ) / √2 Next, the conditional two-qubit operation should be introduced and that must be implemented in the next step of the quantum computing workflow the controlled-NOT gate (CNOT). The first qubit specified in the previous text can be designated as the control qubit, and the following second qubit as the target qubit. The operating logic of this operation changes based on the value of the control qubit. If the control qubit’s value is 0, no operation is executed on the target qubit, and it maintains its original state at all times; if the control qubit’s value is 1, the quantum state of the target qubit undergoes a fundamental flip. Additionally, because the control qubit can simultaneously hold two potential values and exists in a fragile superposition state, executing this operation for one possible state directly affects the other. Therefore, the mathematical methods must be used to describe the combined state of all possible scenarios. (|00 ⟩ + |11 ⟩ ) / √2 It is at this precise juncture during the quantum operation process that it can be said the two qubits are entangled and must be described by a joint quantum state. This entanglement is not necessarily permanent; subsequent quantum operations can modify or remove it. The qubits remain physically distinct systems, even while sharing an entangled state. It must be recognised that what exhibits this deep level of entanglement is entirely the abstract quantum state itself. The physical devices that carry these qubits never merge at the physical level, nor are they permanently connected. In fact, in many practical implementation scenarios, almost immediately after this highly unusual quantum entanglement is established, these physical devices can be separated by billions of kilometres in principle.
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