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Quantum Computing Explained


Quantum computers are quite thoroughgoing in how they approach the problem solving dynamic. Quantum computers shy away from the traditional transistor methodology and opt for spins. The hype around quantum computers is unmatched, they have been dubbed as'The Cancer, aging and death solvers', some go as fare as calling them the'ultimate hackers' and will break RSA encryption. Well I am currently playing with these babies, specifically the IBM quantum computers and will share what I have learned so far. Traditional computers process tasks using binary 101100, lets imagine this as a heads and tales system with a coin.

Quantum Mechanics on the Classical Computer


In recent years, there has been a surge of interest in quantum computing, with many major companies and organizations investing significant resources into developing this technology. Hybrid classical-quantum computing (HCQC) is a promising approach that could potentially enable the construction of powerful quantum computers. In this article, we will firstly review the basics of quantum computing and classical computing. We will then describe how hybrid classical-quantum computing works and discuss some of the advantages and disadvantages of this approach. Finally, we will outline some potential applications of hybrid classical-quantum computing.

Quantum logic at a distance


Quantum computers could revolutionize how specific computational problems are solved that remain untractable even for the world's best supercomputers. However, although the basic elements of a quantum computer—realizing a register of qubits that preserve superposition states, controlling and reading out qubits individually, and performing quantum gates between them—have been scaled to a few dozen qubits, millions are needed to attack problems such as integer factorization. One approach for scaling up quantum computers is to “divide and conquer”—keep individual processing units smaller and connect many of them together. This approach leaves local processing nodes tractable but requires generation of entanglement and performance of quantum gates on qubits located at distant nodes to keep the advantages of quantum processing. On page 614 of this issue, Daiss et al. ([ 1 ][1]) made substantial progress toward this goal by performing quantum-logic operations on two distant qubits in an elementary network. Entangling distant qubits in quantum networks can enable distributed computing and secure data transmission . Basic quantum networks have been demonstrated with a few different systems such as ultracold atoms ([ 2 ][2]), trapped ions ([ 3 ][3]), color centers in diamond ([ 4 ][4]), and superconducting qubits ([ 5 ][5]). By using schemes to improve the quality of initially imperfect entanglement, e.g., by so-called entanglement distillation ([ 6 ][6]), it should be possible to build noise-resilient, error-corrected quantum networks that perform better than their individual components ([ 7 ][7]). The additional benefits of improved qubit addressability, reduced cross-talk, and improved connectivity between arbitrary qubits controlled through the network connections suggest that a distributed quantum computer could outperform one large computing core. Daiss et al. realized a quantum gate between two separated qubits in independent setups connected by a 60-m-long optical fiber. The qubits are implemented by internal spin states of two atoms that are trapped inside optical cavities. The qubits become connected when a single photon sent through the two setups is successively reflected from the two cavities and then detected ([ 8 ][8]). The presence of an atom strongly coupled to a cavity changes the reflection phase of the photon. A photon reflected off an empty resonant cavity undergoes a π phase shift. However, an atom strongly coupled to a cavity causes a frequency shift of the cavity resonance. This shift prevents photon entry upon reflection , and the reflected-photon phase remains unchanged. One qubit state of the atom, but not the other, couples strongly to the cavity, so one state does not change the photon reflection phase, and the other adds a π phase shift. An atom in a superposition state of its qubit levels will produce a corresponding superposition of the photon phase and create an atom-photon entangled state. When such an entangled photon is bounced off a second cavity and undergoes another atom-photon entangling operation with the second atom, a final state can be produced that corresponds to a NOT-gate operation. ![Figure][9] Large-scale quantum circuits Daiss et al. created quantum processors with single photons guided by optical fibers that were reflected successively by two atom–cavity devices. Scaling to multiple qubits could be achieved with large-scale photonic networks connecting optical cavities containing multiple atomic spin qubits. The full scheme is actually more complicated and requires encoding through polarization states, a measurement of the resulting combined atom-atom state based on the photon polarization, and a conditional change of the first atom's state that depends on the measurement result. Daiss et al. realized a controlled-NOT gate with <15% deviation from the ideal gate performance. Together with the simpler single-qubit gates, this result represents a complete toolbox to implement any kind of quantum logic operation. The scheme is heralded, meaning that it produces a measurable signal when the gate operation is successful and becomes immune against photon loss as an error source. At a first glance, the scheme requires surprisingly few resources—only a single photon that is reflected successively by two atomcavity devices. Indeed, strong atom-cavity coupling makes the scheme efficient as it increased the probability that the photon is reflected rather than lost through processes such as spontaneous emission. However, realizing such strongly coupled atom-cavity systems requires overcoming hurdles that include atom control, cavity performance, atomic coherence, and minimized photon loss. Also, the experiment was not yet operated with single photons but with attenuated laser pulses, which is easier to do but introduces errors caused by the presence of two-photon contributions and enforces low average photon numbers. Although all of these issues limit the efficiency and fidelity of the gate demonstrated by Daiss et al. , none of them introduce fundamental limits and could be improved in the future. Thus, it should be possible to scale up the system (see the figure). For example, multiple atoms coupled to a single cavity could allow the reflection of a single photon to immediately produce an N -qubit Toffoli gate ([ 8 ][8]), an element of the Grover's search algorithm. Also, elegant ways for scaling multiple cavities in interferometer-type configurations have been proposed ([ 9 ][10]) in which a single photon can generate an entangling quantum gate between any selected qubits in a network. Although formidable challenges remain, it is intriguing to imagine the possibilities when distributed quantum computers form a quantum internet ([ 10 ][11], [ 11 ][12]). 1. [↵][13]1. S. Daiss et al ., Science 371, 614 (2021). [OpenUrl][14][Abstract/FREE Full Text][15] 2. [↵][16]1. S. Ritter et al ., Nature Nature, 195 (2012). 3. [↵][17]1. D. L. Moehring et al ., Nature 449, 68 (2007). [OpenUrl][18][CrossRef][19][PubMed][20][Web of Science][21] 4. [↵][22]1. P. C. Humphreys et al ., Nature 558, 268 (2018). [OpenUrl][23] 5. [↵][24]1. N. Roch et al ., Phys. Rev. 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Quantum supremacy: What can we do with a quantum computer?

New Scientist

It could still be decades before we have a quantum computer that can do anything useful, but once we get there, what will we use them for? The first use could be in chemistry. As the physicist Richard Feynman once said, "Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical."

DOE pushes for useful quantum computing


The U.S. Department of Energy (DOE) is joining the quest to develop quantum computers, devices that would exploit quantum mechanics to crack problems that overwhelm conventional computers. The initiative comes as Google and other companies race to build a quantum computer that can demonstrate "quantum supremacy" by beating classical computers on a test problem. But reaching that milestone will not mean practical uses are at hand, and the new $40 million DOE effort is intended to spur the development of useful quantum computing algorithms for its work in chemistry, materials science, nuclear physics, and particle physics. With the resources at its 17 national laboratories, DOE could play a key role in developing the machines, researchers say, although finding problems with which quantum computers can help isn't so easy.