With optical communication technology today, it is practical to communicate with polarization division multiplexed (PDM) quadrature amplitude modulated (QAM) symbols, which are quantum superposition of horizontally and vertically polarized photons, which are, so called, qubits. As the number of bits encoded by a PDM QAM symbol is limited, according to Shannon-Hartley theorem, by signal to noise ratio, the degree of parallelism of quantum computers is limited. The result is consistent with quantum threshold theorem. Quantum entanglement between qubits only makes the number of bits encoded by the qubits smaller, because entanglement means correlation between the qubits. Thus, quantum computers are not more powerful than classical ones. Finally, it is shown that purely classical computers can be arbitrarily fast and ideal, that is, noiseless, quantum computers are classical.
With optical communication technology today, it is practical to communicate with polarization division multiplexed (PDM) quadrature amplitude modulated (QAM) symbols, which are quantum superposition of horizontally and vertically polarized photons, which are, so called, qubits. As the number of bits encoded by a PDM QAM symbol is limited, according to Shannon-Hartley theorem, by signal to noise ratio, the degree of parallelism of quantum computers is limited. The result is consistent with quantum threshold theorem. Quantum entanglement between qubits only makes the number of bits encoded by the qubits smaller, because entanglement means correlation between the qubits. Thus, quantum computers are not more powerful than classical ones. Finally, it is shown that purely classical computers can be arbitrarily fast and ideal, that is, noiseless, quantum computers are classical.
Today's computers work on bits that exist as either 0 or 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits, which can exist in superposition. Qubits represent atoms, ions, photons or electrons and their respective control devices that are working together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers. A processor that can use registers of qubits will be able to perform calculations using all the possible values of the input registers simultaneously. This superposition causes a phenomenon called quantum parallelism, and is the motivating force behind the research being carried out in quantum computing. Due to technical obstacles, till date, a quantum computer has not yet been realized. But the concepts and ideas of quantum computing has been demonstrated using various methods like NMR, Ion Trap, Quantum Dot, Optical Methods, etc. A quantum computer manipulates qubits by executing a series of quantum gates, each a unitary transformation acting on a single qubit or pair of qubits. In applying these gates in succession, a quantum computer can perform a complicated unitary transformation to a set of qubits in some initial state. The qubits can then be measured, with this measurement serving as the final computational result. Research must devise a way to maintain decoherence and other potential sources of error at an acceptable level. Probably the most important idea in this field is the application of error correction in phase coherence as a means to extract information and reduce error in a quantum system without actually measuring that system. Thereby, quantum computers will emerge as the superior computational devices and perhaps one day make today's modern computer obsolete.
Now there is a great interest in the field of information technologies cause development a quantum computer that make direct use of quantum-mechanical phenomena, such as superposition and entanglement, to perform operations on data. Qubit - is a unit of quantum information—the quantum analogue of the classical bit. As a bit, a qubit admits two of its own state, but it can be in superposition. Following the development of a quantum computer will need to establish a standard unit of quantum information. The search for a candidate for the role of the qubit (and, stable) can give perspectives to the development of quantum metrology.
Dirac’s misconception that classical and quantum superpositions are different because absolute phase is quantum physically meaningless is, even today, seemingly blindly believed by many physicists. However, there is no such difference because, time translation symmetry means absolute time and, thus, absolute phase are classical physically meaningless, too. As a qubit can be a polarization division multiplexed (PDM) quadrature amplitude modulated (QAM) symbol of light, its state has direct correspondence with polarization state of classical light. Even more intuitively, the state may be represented by two-dimensional mechanical oscillations of a pendulum, which enables intuitive understanding of not-really-quantum superpositioned states of a qubit even by elementary school pupils.
The natural choice of physical qubit for quantum communication is the photon: a photon can be transmitted quickly from the sender to a distant receiver and the technology for creating, manipulating, distributing, and measuring light pulses is well established. Many of these classical techniques can also be employed for quantum communication protocols. We will therefore focus our attention on optical setups for quantum communication. We will study a number of optical setups in the next few sections and use the conventions introduced in Chapter 4 to analyze them. In particular, we will assume that the qubits are encoded in the degrees of freedom of a single photon. Some of the main technical challenges in quantum communication arise from this need to work with single photon pulses. For instance, photons do not interact with each other in vacuum or linear media, and interactions between two photons in non-linear optical media are relatively small, so that realizing entangling two photon gates via coherent interactions is a challenging task. The quantum communication schemes discussed here circumvent this technical problem to a large extent. They only use non-linear optical materials for parametric down-conversion to create Bell-pairs of photons and then exploit standard linear optical devices and photo-detectors to manipulate them. No further non-linear entangling gates are required. Parametric down-conversion In non-linear optical media a single photon can be down-converted into a pair of photons. In this coherent process the incoming photon is destroyed and two photons of lower energy are created.
Reliable communication under additive noise, multipath fading, and mobility-induced channel variation remains a central requirement in contemporary wireless systems. This study develops a unified quantum-classical framework for examining bit error rate (BER) performance across ideal and impaired channel conditions. The classical component models orthogonal frequency-division multiplexing with BPSK, QPSK, 8-PSK, and 4-, 16-, 64-, and 128-QAM using MATLAB R2026a. The quantum-proxy component is implemented in Qiskit 2.3.0 using computational-basis qubits and Kraus-operator models for bit-flip, amplitude-damping, and phase damping noise. BER is evaluated against signal-to-noise ratio (SNR) for additive white Gaussian noise and combined fading and Doppler conditions. The classical results show a consistent reduction in BER as SNR increases, with BPSK exhibiting the greatest robustness and higher-order PSK and QAM schemes showing greater sensitivity to channel impairment. In the quantum-proxy simulation, BER remains largely independent of the number of bits represented per symbol because each qubit is encoded and measured as an independent binary state. Phase damping has negligible influence on computational-basis measurement outcomes, whereas bit-flip and amplitude-damping processes principally determine the observed errors. The numerical BER values from the two frameworks are not directly comparable because the quantum SNR axis represents a proxy mapping to noise probabilities rather than a calibrated physical Eb/N0 measure. The framework therefore supports a qualitative comparison of error mechanisms while clarifying the present limitations of basis-state quantum encoding for modelling higher-order modulation.
All computers are quantum computers but some are more quantum than others. Universal classical computers rely on the transistor, a quantum semiconductor, for their operation but use simple binary digital logic based on bits, 1 and 0. Quantum computers, those we are interested in here, utilize the qbits https://www.w3.org/1998/Math/MathML" display="inline"> | 1 〉 and https://www.w3.org/1998/Math/MathML" display="inline"> | 0 〉 , which are quantum states, to realize their quantum logic. These qbits are used in conjunction with superposition probability amplitudes, such as https://www.w3.org/1998/Math/MathML" display="inline"> | ψ 〉 + , https://www.w3.org/1998/Math/MathML" display="inline"> | ψ 〉 − , https://www.w3.org/1998/Math/MathML" display="inline"> | ψ 〉 + , and https://www.w3.org/1998/Math/MathML" display="inline"> | ψ 〉 − , to interact with optical components such as beam splitters, interferometers, and polarization rotators to perform quantum logical operations. In this chapter, a basic description of quantum logic, and quantum optical configurations, utilized in workable quantum computers is given.