July 3, 2020

L’esperimento di cancellazione quantistica a scelta ritardata, eseguito per la prima volta da . Le altre versioni del cancellatore quantistico usando fotoni entangled, tuttavia, sono intrinsecamente non-classici. A causa di ciò, al fine di evitare. Il misticismo quantico è un termine che si usa per riferirsi a un insieme di credenze metafisiche In modo particolare, il teorema di non-località ed il fenomeno sperimentale entanglement quantistico, furono motivo di discussioni e interrogativi. Quantum computing is computing using quantum-mechanical phenomena, such as superposition and entanglement. A quantum computer is a device that.

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Quantum computing is computing using quantum-mechanical phenomenasuch as superposition and entanglement. Such a computer is completely different from binary digital electronic computers based on transistors and capacitors. Whereas common digital computing requires that the data be encoded into binary digits bitseach of which is always in one of two definite states 0 or 1quantum computation uses quantum bits or qubits, which can be in superpositions of states.

A quantum Turing entangleent is a theoretical model of such a computer and is also known as the universal quantum entannglement.

entanglement quantistico – Wiktionary

The field of quantum computing was initiated by the work of Paul Benioff [2] and Yuri Manin in[3] Richard Feynman in[4] and David Deutsch in As of[update] the development of actual quantum computers is still in its infancy, but experiments have been carried out in which quantum computational operations were executed on a very small number of quantum bits.

D-Wave Systems has been developing their own version of a quantum computer that uses annealing. Large-scale quantum computers would theoretically be able to solve certain problems much more quickly than any classical computers that use even the best currently known algorithmslike integer factorization using Shor’s algorithm which is a quantum algorithm and the simulation of quantum many-body systems.

There exist quantum algorithmssuch as Simon’s algorithmthat run faster than any possible probabilistic classical algorithm. A classical computer has a memory made up of bitswhere each bit is represented by either a one or a zero. A quantum computer, on the other hand, maintains a sequence of qubitswhich can represent a one, a zero, or any quantum superposition of those two qubit states ; [12]: A quantum computer operates on its qubits using quantum gates and measurement which also alters the observed state.

An algorithm is composed of a fixed sequence of quantum logic gates and a problem is encoded by setting the initial values of the qubits, similar to how a classical computer works. If the algorithm did not end with a measurement, the result is an unobserved quantum state.

Such unobserved states may be sent to other computers as part of distributed quantum algorithms. Quantum algorithms are often probabilistic, in that they provide the correct solution only with a certain known probability.

An example of an implementation of qubits of a quantum computer could start with the use of particles with two spin states: A quantum computer with a given number of qubits is fundamentally different from a classical computer composed of the same number of classical bits.

For example, representing the state of an n -qubit system on a classical computer requires the storage of 2 n complex coefficients, while to characterize the state of a classical n -bit system it is sufficient to provide the values of the n bits, that is, only n numbers. Although this fact may seem to indicate that qubits can hold exponentially more information than their classical counterparts, care must be taken not to overlook the fact that the qubits are only in a probabilistic quantisticoo of all of their states.

This means that when the final state of the qubits is entajglement, they will only be found in one of the possible configurations they were in before the measurement.

It is generally incorrect to think of a system of qubits as being in one particular state before the measurement. Since the fact that they were in a superposition of states before the measurement was made directly affects the possible outcomes of the computation. To better understand this point, consider a classical computer that operates on a three-bit register.

If there is no uncertainty over its state, then it is in exactly one of these states with probability 1. However, quantistiico it is a probabilistic computerthen there is a possibility of it being in any one of a number of different states.

However, because a complex number encodes not just a magnitude but also a direction in the complex planethe phase difference between any two coefficients states represents a meaningful parameter.

This is a fundamental difference between quantum computing and probabilistic classical computing. If you measure the three qubits, you will observe a three-bit string. The probability of measuring a given string is the squared magnitude of that string’s coefficient i.

An eight-dimensional vector can be specified in many different ways depending on what basis is chosen for the space. The basis of bit strings e. Other possible bases are unit-lengthorthogonal vectors and the eigenvectors of the Pauli-x operator. Ket notation is often used to make the choice of basis explicit. While a classical 3-bit state and a quantum 3-qubit state are each eight-dimensional vectorsthey are manipulated quite differently for classical or quantum computation.


In classical randomized computation, the system evolves according to the application of stochastic matriceswhich preserve that the probabilities wntanglement up to one i. In quantum computation, on the other hand, allowed operations are unitary matriceswhich are effectively rotations they preserve that the sum of the squares add up to one, the Euclidean or L2 norm. Exactly what unitaries can be applied depend on the physics of the quantum device.

Consequently, since rotations can enatnglement undone by rotating backward, quantum computations are reversible. Technically, quantum operations can be probabilistic combinations of unitaries, so quantum computation entanglemenh does generalize classical computation. Etnanglement quantum circuit for a more precise formulation. Finally, upon termination of the algorithm, the result needs to be read off. In the case of a classical computer, we sample from the probability distribution on the three-bit register to obtain one definite three-bit string, say Quantum mechanically, one measures the three-qubit state, which is equivalent to collapsing the quantum state down to a classical distribution with the coefficients in the classical state being the squared magnitudes of the coefficients for the quantum state, as described abovefollowed by sampling from that distribution.

This destroys the original quantum state. Many algorithms will only give the correct answer with a certain probability.

However, by repeatedly initializing, running and measuring the quantum computer’s results, the probability of getting the correct answer can be increased. In contrast, counterfactual quantum computation allows the correct answer to be inferred when the quantum computer is not actually running in a technical sense, though earlier initialization and frequent measurements are part of the counterfactual computation protocol.

For more details on the sequences of operations used for various quantum algorithmssee universal quantum computerShor’s algorithmGrover’s algorithmDeutsch—Jozsa algorithmamplitude amplificationquantum Fourier transformquantum gatequantum adiabatic algorithm and quantum error correction.

Integer factorizationwhich underpins the security of public key cryptographic systems, is believed to be computationally infeasible with an ordinary computer for large integers if they are the product of few prime numbers e.

This ability would allow a quantum computer to break many of the cryptographic systems in use today, in the sense that there would be a polynomial time in the number of digits of the integer algorithm for solving the problem.

In particular, most of the popular public key ciphers are based on the difficulty of factoring integers or the discrete logarithm problem, both of which can be solved by Shor’s algorithm. These are used to protect secure Web pages, encrypted email, and many other types of data.

Breaking these would have significant ramifications for electronic privacy and security. However, other cryptographic algorithms do not appear to be broken by those algorithms. AES would have the same security against an attack using Grover’s algorithm that AES has against classical brute-force search see Key size.

Quantum cryptography could potentially fulfill some of the functions of public key cryptography. Quantum-based cryptographic systems could, therefore, be more secure than traditional systems against quantum hacking.

Besides factorization and discrete logarithms, quantum algorithms offering a more than polynomial speedup over the best known classical algorithm have been found for several problems, [23] including the simulation of quantum physical processes from chemistry and solid state physics, the approximation of Jones polynomialsand solving Pell’s equation.

No mathematical proof has been found that shows that an equally fast classical algorithm cannot be discovered, although this is considered unlikely. The most well-known example of this is quantum database searchwhich can be solved by Grover’s algorithm using quadratically fewer queries to the database than that are required by classical algorithms. In this case, the advantage is not only provable but also optimal, it has been shown that Grover’s algorithm gives the maximal possible probability of finding the desired element for any number of oracle lookups.

Several other examples of provable quantum speedups for query problems have subsequently been discovered, such as for finding collisions in two-to-one functions and evaluating NAND trees. Problems that can be addressed with Grover’s algorithm have the following properties:. For problems with all these properties, the running time of Grover’s algorithm on a quantum computer will scale as the square root of the number of inputs or elements in the databaseas opposed to the linear scaling of classical algorithms.

A general class of problems to which Grover’s algorithm can be applied [25] is Boolean satisfiability problem. In this instance, the database through which the algorithm is iterating is that of all possible answers.


Quantum computing

An example and possible application of this is a password cracker that attempts to guess the password or secret key for an encrypted file or system. Since chemistry and nanotechnology rely on understanding quantum systems, and such systems are impossible to simulate in an efficient manner classically, many believe quantum simulation will be one of the most important applications of quantum computing.

Adiabatic quantum computation relies on the adiabatic theorem to undertake calculations. A system is placed in the ground state for a simple Hamiltonian, which is slowly evolved to a more complicated Hamiltonian whose ground state represents the solution to the problem in question.

The adiabatic theorem states that if the evolution is slow enough the system will stay in its ground state at all times through the process. The Quantum algorithm for linear systems of equations or “HHL Algorithm”, named after its discoverers Harrow, Hassidim, and Lloyd, is expected to provide speedup over classical counterparts.

John Preskill has introduced the term quantum supremacy to refer to the hypothetical speedup advantage that a quantum computer would have over a classical computer in a certain field. Schlafly maintains that the Born rule is just “metaphysical fluff” and that quantum mechanics does not rely on probability any more than other branches of science but simply calculates the expected values of observables.

He also points out that arguments about Turing complexity cannot be run backwards. There are a number of technical challenges in building a large-scale quantum computer, and thus far quantum computers have yet to solve a problem faster than a classical computer.

Esperimento di cancellazione quantistica a scelta ritardata

One of the greatest challenges is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere.

However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T 2 for NMR and MRI technology, also called the dephasing timetypically range between nanoseconds and seconds at low temperature.

As a result, time-consuming tasks may render some quantum algorithms inoperable, as maintaining the state of qubits for a long enough duration will eventually corrupt the superpositions. These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

As described in the Quantum threshold theoremif the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them.

Meeting this scalability condition is possible for a wide range of systems. However, the use of error correction brings with it the cost of a greatly increased number of required qubits. The number required to factor integers using Shor’s algorithm is still polynomial, and thought to be between L and L 2where L is the number of qubits in the number to be factored; error correction algorithms would inflate this figure by an additional factor of L.

For a bit number, this implies a need for about 10 4 bits without error correction. A very different approach to the stability-decoherence problem is to create a topological quantum computer with anyonsquasi-particles used as threads and relying on braid theory to form stable logic gates.

There are a number of quantum computing models, distinguished by the basic elements in which the computation is decomposed. The four main models of practical importance are:. The quantum Turing machine is theoretically important but the direct implementation of this model is not pursued.

quantum entanglement – Wikidata

All four models of computation have been shown to be equivalent; each can simulate the other with no more than polynomial overhead. For physically implementing a quantum computer, many different candidates are being pursued, among them distinguished by the physical system used to realize the qubits:.

A large number of candidates demonstrates that the topic, in spite of rapid progress, is still in its infancy.