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Better Understanding of Quantum Theory |
| Paper ID: |
432 |
Last updated: 31/01/2012 09:08:31
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Where: Global |
When: 11-20yrs |
How Fast: Years |
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| Keywords: |
Theoretical physics - quantum, entanglement, uncertainty, quantum computing, encryption |
Summary  |
| Quantum theory has counterintuitive features, which physicists are only now beginning to understand. They can be exploited to achieve things that would not be possible in a classical universe, such as more powerful computers and unbreakable encryption. These developments will also have important philosophical implications and give us a better understanding of nature at the most fundamental level. |
Discussion |
Quantum mechanics is an extremely successful theory with many important applications, and along with relativity, it is the foundation of modern physics. However, it has features that still appear mysterious and counterintuitive.
The main source of the mystery is the so-called measurement problem. [1], [2] In quantum theory an object can be in two states at the same time. This is called a superposition of states and is described by a mathematical object called the wavefunction. For example, a particle can be in two positions at the same time. The time evolution of the wavefunction is deterministic, meaning that its future is completely determined by its initial state.
However, when we measure the position of the particle, we always find it in one particular position, and we cannot predict which position it is. The wavefunction only tells us the probability of finding the particle in a given position. It therefore appears that measurements are truly random processes. The problem is how the probabilities given by the wavefunction are converted into a well-defined outcome. Is this process truly random, or does it simply appear random because we do not fully understand it?
One concrete consequence of this feature of quantum physics is ‘entanglement’. [3] If two quantum objects are entangled, a measurement of a property of one object can be used to predict the corresponding property of the other object. However, according to quantum theory these properties are not determined until they are measured, so it seems that measurement of one object affects the other instantaneously.
To find out if this is true, physicists have developed and carried out increasingly sophisticated experiments. The results confirm the predictions of the quantum theory and rule out almost all alternatives. [4], [5] Physicists have also been able to create quantum superposition states of molecules consisting of more than a hundred atoms, demonstrating that quantum principles are not restricted to elementary particles. [4]
These successes still leave many fundamental questions unanswered. Do wavefunctions really exist, or are they simply a mathematical representation of our ignorance? What really happens when we carry out a measurement? According to the original “Copenhagen” interpretation, the wavefunction collapses from a superposition into a definite state. In the more recent many-world interpretation, the whole universe splits into braches that correspond to the different possible outcomes of the measurement. [6] But how can we then interpret the probabilities of the different outcomes?
Recent theoretical developments indicate that we may soon be able to answer some of these questions. Interaction with the rest of the universe leads to decoherence, which appears as a collapse of the wavefunction of the measured system. [2], [4], [6], [7] It also seems to explain the probabilities of the different outcomes. [2], [7] Decoherence does not really address the deep philosophical questions, [8] but it is often claimed to support the many-worlds interpretation. [9]
Other deep questions arise when one tries to combine quantum theory with gravitation. For instance, black holes seem to be able to erase information completely, which would violate the principles of quantum mechanics. [10]
Even if we do not fully understand these counterintuitive features, we can exploit them. Using quantum superpositions, one can design a quantum computer that can carry out multiple calculations simultaneously. [11] Quantum computers consisting of a few atoms have already been built. If this can be repeated on macroscopic scale, they would speed up some calculations dramatically. For instance, a quantum computer would be able to break some widely-used encryption methods. The main problem in building large-scale quantum computers is avoiding decoherence which would destroy the quantum state.
The counterintuitive features of quantum theory can also be used for quantum cryptography, a completely secure way of transmitting information. [12] Quantum-encrypted communication has been achieved over distances of more than 100 kilometres, and some commercial quantum cryptography products are already available.
Quantum theory also tells us that some operations that could in principle be done in a classical universe are impossible. For instance, one cannot make an exact copy of a quantum state, but one can move the exact quantum state to another system, which is known as quantum teleportation. [13]
These developments are clearly only the first steps in exploitation of quantum mechanics. Our pragmatic understanding of quantum theory will continue to improve. [5], [8], [14] This, together with developments in related fields such as high energy physics and cosmology, will also help us understand quantum theory and reality at a deeper level. [6] |
Implications |
Better understanding of quantum theory will give us a deeper understanding of the nature of reality. It will have significant philosophical implications and transform our worldview. It is closely tied to progress in cosmology and high energy physics, and it will give a framework to answer deep questions about the beginning and future of the universe. Understanding of fundamental questions will also help us tackle practical problems in applications of quantum theory and may lead to completely new applications in the long term.
More pragmatic understanding and exploitation of quantum theory will give us a new generation of computers which will be able to do calculations that would be almost impossible with current technology. They will be able to break some of today’s encryption methods, but quantum cryptography will also provide completely secure new ones. There are likely to be many other useful applications which have not been thought of yet. Quantum technology may become as important as semiconductor technology is today.
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Early indicators |
• Demonstration of quantum superposition of states of a virus
• Practical calculations with quantum computers
• Quantum encryption in routine use |
Drivers & Inhibitors |
Drivers:
o Human curiosity
o Discoveries in cosmology and high energy physics
o Need for secure communications
o Limitations of classical computers
o Institutions and Organisations:
- Academic institutions
- Los Alamos National Laboratory
- Computer industry (Toshiba, IBM, HP)
- Engineering and Physical Sciences Research Council
- Science and Technology Facilities Council
• Inhibitors:
o Decline in physics student numbers
o Short term view on research funding, leading to an emphasis on immediate applications at the expense of fundamental research |
Parallels & Precedents |
Development of statistical physics and understanding of thermodynamics
• Einstein’s special and general relativity
• Godel’s incompleteness theorem in logic
• Invention of digital computers[1][2][3][4][5][6][7][8][9][10][11][12][13][14] |
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Sources |
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| The contents of this paper were provided by the Outsights-Ipsos MORI Partnership. Any views expressed are independent of government and do not constitute government policy. |
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