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What Is The Physics OF Quantum Computing

What Is The Physics OF Quantum Computing

April 17 , 2024 0

It is well known that computers perform a wide range of tasks in science, education, economy and daily life. Hungarian physicist and mathematician John von Neumann, inspired by British mathematician Alan Turing, proposed an architecture of the same name based on program storage, which laid the logical mathematical foundation of computing and modern computer science.
A traditional digital computer system has four basic components: input/output ports, main memory, control unit, and arithmetic logic unit (ALU). For current supercomputers, limited by traditional computing structures, there are many limitations in handling the most difficult tasks, so researchers have turned their eyes to quantum computers. Quantum computing, together with artificial intelligence, represents a major near-term technical and scientific challenge.
In fact, the first computers were mechanical, invented in the 17th century by Blaise PASCAL in France and Gottfried Wilhelm Leibniz in Germany. The first electronic digital computer is believed to have been built by American scientist John Vincent Atanasoff, who built it between 1939 and 1942 with the help of a graduate student. In 1946, based on Atanasoff's machine, J. Presper Eckert and John W. Mauchly of the University of Pennsylvania built the ENIAC(Electronic Numerical Integration Computer). Both computers used vacuum tubes instead of relays as active logic blocks, a feature that significantly increased processing speed. Indeed, the digital computer road map was dotted with other important innovations, from transistors to integrated circuits, and finally to microprocessors and very large scale integrated circuits in the 1980s.
Although advances in microelectronics have led to amazing advances in processing technology, in reality, computer architecture has remained largely unchanged.

New Concepts OF Quantum Computing

In 1959, a new formula describing the interaction of light and matter, quantum electrodynamics (QED), was proposed by the American physicist and Nobel Prize winner Richard Feynman. He believes that as electronic devices approach microscopic size, exotic effects predicted by quantum physics emerge, and these effects can be used to design more powerful computers. This mysterious phenomenon, which occurs only at the atomic or particle scale, is the foundation of quantum computing hardware.
Quantum computing takes advantage of three key quantum principles: superposition, entanglement, and interference. These concepts play a fundamental role in quantum computers, thus making quantum computers very different from traditional computers.

Superposition

In the world of quantum mechanics, objects such as particles do not necessarily have well-defined states, as demonstrated by the famous double-slit experiment. In the experiment, when a single photon passes through a light-sensitive screen with two small slits, an interference pattern, similar to that produced by light waves, is seen as the result of all available paths being superimposed. If a detector is used to try to determine which of the two slits the photon passes through, the interference pattern disappears. The explanation for this strange result is that before the measurement, the quantum system "exists" in all possible states, and when fatal perturbations are introduced, the system crashes into one state, which is called decoherence. If this phenomenon is reproduced in a computer, it is expected to expand its computing power exponentially.
Traditional digital computers take binary numbers or bits that can be in one of two states, represented as 0 and 1. This way, for a 4-bit computer register, any one of 16(24) possible numbers can be held. Qubits (qubits) exist in wavy superpositions of 0 and 1 values, so a 4-qubit computer register can process 16 different numbers simultaneously. In theory, quantum computers can process so many values in parallel that a quantum computer with just 30 qubits could be comparable to a digital computer that performs 10 trillion floating-point operations per second (TFLOPS), which is comparable to a superdigital computer.
Today's fastest digital computer is Frontier, installed at the U.S. Department of Energy's Oak Ridge National Laboratory in Tennessee, which reaches 1,018 FLOPS. Supercomputers built on classical architectures are very complex and bulky machines that require a large number of parallel devices and processors. Frontier, for example, contains 9,408 cpus, 37,632 Gpus, and 8730,112 cores, all of which are connected by a 145km long cable. The computer covers an area of 372m2 and has a power consumption of 21MW, up to 40MW at peak times.

Entanglement

Superposition is the ability of qubits to exist in more than one state (0, 1, or any combination of the two) at the same time, while entanglement is the quantum phenomenon in which two or more qubits are related to each other. In other words, the state description of a qubit cannot be independent of the state of its partner. This interdependence allows information to be shared instantly between entangled qubits, no matter how far apart they are. Einstein referred to this phenomenon as "spooky action at a distance" to highlight his annoyance with the uncertainty and non-locality of quantum mechanics. Entanglement, however, is the backbone of many quantum algorithms that can solve problems faster and more efficiently.

Intervene

Interference occurs when two or more quantum states combine to form a new state, resulting in constructive or destructive interference. Constructive interference amplifies the probability of obtaining a correct output, while destructive interference reduces the probability of an incorrect output. By manipulating interference patterns, quantum computers can quickly parse potential solutions and arrive at the right answer faster than classical computers.
But how are qubits built?

Consider a single electron and its angular momentum, or spin. The quantified spin can be either up or down. By defining 0 as the spin up state and 1 as the spin down state, electrons can be used as qubits. Here, it is useful to use the bra-ket notation of theoretical mathematician and physicist Paul Dirac. Quantum states can be represented by "ket" (basically many column vectors), so the two states can be written as |0> and |1>. Thus, spin acts in the same way that transistors implement a bit/bit in standard Boolean logic.
The superposition principle states that unlike traditional digital bits, qubits can be represented by a superposition of 0 and 1 at the same time. In mathematical notation, if |ψ> is treated as the state of a qubit, it can be expressed as:
| bits of > = W0 | | 1, 0 > + W1 >

Where, W0 and W1 are two digits, representing the relative weight of |0> and |1> in superposition, respectively. More formally, these numbers are the complex probability amplitude of the qubit, determining the probability of getting 0 or 1 when measuring the qubit state. Of course, they must be subject to the following normalization conditions, namely:
|W0|2+|W1|2=1
When W0 = 1 and W1 = 0, the qubit is in its |0> state, corresponding to the cutoff state of the transistor; Conversely, if W0 = 0 and W1 = 1, the qubit state corresponds to the on-state of the transistor. For any other value of W0 and W1, it is as if the transistor, in classical terms, is neither "off" nor "on," but both "on" and "off," as in the famous thought experiment conceived by Edwin Schrodinger, one of the founders of quantum mechanics, the cat is both dead and alive.
Superposition allows a qubit to perform two calculations at the same time, and if two qubits are connected by entanglement, 22 or 4 calculations can be performed simultaneously:
| bits of > = W00 | 00 > + W01 | > + W10 | > 10 + 01 W11 | 11 >
Three qubits can process 23 or 8 calculations in parallel, and so on.

In principle, a quantum computer with 300 qubits could perform more calculations in an instant than there are visible atoms in the universe. Today, a quantum computer with that many qubits has emerged: IBM's Osprey Quantum Computer (433 bits), with the most powerful quantum processor to date, has been made available for exploratory technology demonstrations on the IBM cloud.

After encoding the data in the qubit, it is necessary to modify and manipulate the state of the qubit. In a digital computer, this is done through basic operations performed by logic gates such as AND, NAND, and NOR. The corresponding operations in quantum computers are carried out by quantum gates, which can be classified according to the number of qubits involved. In contrast to traditional logic gates, quantum gates can create and manipulate entanglement and superposition, which is crucial to improving the computational power of quantum computers.

Some quantum gates that perform operations on qubits through a set of quantum logic operations are Pauli-X, Pauli-Y, Pauli-Z, Hadamard, and CNOT(Controlled NOT). For example, Pauli-X is a quantum simulation of a traditional NOT logic gate. Hadamard converts individual qubits into a perfect superposition of |0> and |1> states. Thus, the measure of a single qubit "transformed" by the gate will yield |0> or |1> with the same probability, i.e. : W1=W2=1/√2. Actually, (1/√2)2+(1/√2)2=1.

Quantum Processing Unit

The core component of quantum computing hardware, the quantum processing unit (QPU), processes qubits through a series of quantum gates to perform quantum algorithms. Traditional processors such as cpus, Gpus and Dpus, which are widely used in data center data processing units, utilize the principles of classical physics, while Qpus process qubits to enable quantum computers to perform complex calculations faster than traditional computers. The underlying technologies for Qpus can vary, such as nuclear magnetic resonance, trapped ions, superconducting qubits, and photonic chips, and each approach has unique advantages and challenges. Due to differences in implementation and architecture, Qpus cannot be compared simply by looking at the number of qubits being processed.

Quantum computing is becoming one of the most transformative technologies in the world, but the constraints are strict. Quantum computers must maintain coherence (or quantum entanglement) between qubits for a long enough period of time to run a full algorithm. However, decoherence can occur due to almost inevitable interaction with the environment. Therefore, it is necessary to develop robust error detection and correction methods. Finally, since the behavior of measuring a quantum system can also interfere with its state, reliable methods of information extraction must also be devised.

In any case, it is certain that we will enjoy another revolution in computational science. Many of today's intractable problems can be solved with new machines. Indeed, the impressive capabilities of quantum computing have led to intense competition among related companies to achieve "quantum supremacy."