We review recent work and some new results on the role of quantum superpositions in providing new modes of computation, not available to classical computers. For certain problems, these new modes can provide an exponential reduction in complexity over any classical computer demonstrating the importance of quantum processes for issues in complexity theory.
We review recent work and some new results on the role of quantum superpositions in providing new modes of computation, not available to classical computers. For certain problems, these new modes can provide an exponential reduction in complexity over any classical computer demonstrating the importance of quantum processes for issues in complexity theory.
Recent theoretical results confirm that quantum theory provides the possibility of new ways of performing efficient calculations. The most striking example is the factoring problem. It has recently been shown that computers that exploit quantum features could factor large composite integers This task is believed to be out of reach of classical computers as soon as the number of digits in the number to factor exceeds a certain limit. The additional power of quantum computers comes from the possibility of employing a superposition of states, of following many distinct computation paths and of producing a final output that depends on the interference of all of them. This "quantum parallelism" outstrips by far any parallelism that can be thought of in classical computation and is responsible for the "exponential" speed-up of computation. Experimentally, however, it will be extremely difficult to "decouple" a quantum computer from its environment. Noise fluctuations due to the outside world...
Recent theoretical results confirm that quantum theory provides the possibility of new ways of performing efficient calculations. The most striking example is the factoring problem. It has recently been shown that computers that exploit quantum features could factor large composite integers. This task is believed to be out of reach of classical computers as soon as the number of digits in the number to factor exceeds a certain limit. The additional power of quantum computers comes from the possibility of employing a superposition of states, of following many distinct computation paths and of producing a final output that depends on the interference of all of them. This ‘quantum parallelism’ outstrips by far any parallelism that can be thought of in classical computation and is responsible for the ‘exponential’ speed-up of computation. Experimentally, however, it will be extremely difficult to ‘decouple’ a quantum computer from its environment. Noise fluctuations due to the outside world, no matter how little, are sufficient to drastically reduce the performance of these new computing devices. To control the nefarious effects of this environmental noise, one needs to implement efficient error-correcting techniques.
In this chapter we shall deal with the most recent and challenging development of quantum theory, and also one of the most important ones for interpretational, foundational, and even technological issues. This field finds its roots in the observation that quantum states can be viewed as bricks of information in a way that is intrinsically different from classical information. As a consequence, the ability to manipulate quantum states translates immediately into a new form of information processing and exchange. What has been discovered during this conceptual passage is that this type of information processing is in many respects much richer than its classical analogue. This has contributed to the understanding of quantum states as an extension of the classical concept of state and not as a defective reality (see also Subsec. 2.3.4 and Sec. 15.5). The impossibility of knowing the value of all observables at the same time that had been seen as a strong limitation in the early days of quantum mechanics, now turns out to be a manifestation of a different – but not necessarily poorer – resource. On the contrary, we have increasingly discovered that superposition and entanglement are additional informational resources. These resources, for instance, allow for certain particular computations that are much faster on a quantum device than on its classical counterpart, and therefore the former is able to solve problems that cannot be practically solved using classical means.
Quantum computing operates by manipulating the quantum state of a system to solve problems intractable for classical computing. These problems, previously considered limitations of classical algorithms, have solutions through quantum approaches. Therefore, quantum computing suggests that the complexity of such problems lies in the current epistemic state rather than the objective state of the problem. Computational complexity can be classified into objective and epistemic complexity. Some classical problems exhibiting exponential complexity fall into the epistemic category, as their solutions depend on the current state of human knowledge and available tools. Quantum computing suggests that even mathematics isn’t entirely objective, and mathematics, in one sense, is human-made and a kind of technology, which reflects our intention and planning. Computational complexity, therefore, bridges the objective and subjective. Overcoming classical complexity necessitates advancements in scientific theories, algorithms, and quantum technology, all of which rely on the development of human cognitive abilities. Quantum computing is not just a mathematical construct, but also has physical meaning. If it were purely mathematical, classical physics and systems could potentially achieve the same results. What makes the quantum world special is its empirical nature—the physical reality of quantum chips, quantum algorithms, and quantum evolution. Quantum supremacy, or quantum advantage, refers to the potential superiority of quantum computers over classical ones. Controllable quantum systems offer an overwhelming advantage in terms of manipulation and dominance compared to classical systems. Essentially, quantum computing has the potential to significantly outperform classical computing. Many experts, including Shor, have proposed that quantum computing’s power stems from its ability to perform multiple computations simultaneously—a concept often referred to as the Quantum parallelism thesis. Rather, some argue that the true source of quantum computing acceleration lies in the fundamental differences between classical and quantum logic. However, it’s important to note that quantum logic, while providing a new perspective, does not directly address the core principle of quantum parallelism. This dominance stems from quantum physics’ unique properties: Exponential storage capacity, parallel computing inherent to its nature (ontological), and the ability to perform calculations with exponential speedup through quantum algorithms.
Finding exponential separation between quantum and classical information tasks is like striking gold in quantum information research. Such an advantage is believed to hold for quantum computing but is proven for quantum communication complexity. Recently, a novel quantum resource called the quantum switch-which creates a coherent superposition of the causal order of events, known as quantum causality-has been harnessed theoretically in a new protocol providing provable exponential separation. We experimentally demonstrate such an advantage by realizing a superposition of communication directions for a two-party distributed computation. Our photonic demonstration employs d-dimensional quantum systems, qudits, up to d=2^{13} dimensions and demonstrates a communication complexity advantage, requiring less than (0.696±0.006) times the communication of any causally ordered protocol. These results elucidate the crucial role of the coherence of communication direction in achieving the exponential separation for the one-way processing task, and open a new path for experimentally exploring the fundamentals and applications of advanced features of indefinite causal structures.