Superconducting Qubits

[Royal]

Photons dance along a line of superconducting qubits
Qubit chain spreads out; a third quantum computing architecture is on the horizon.
Chris Lee - 5/14/2019, 3:45 AM

When I think about computing, I usually think about it in terms of individual logic gates performing specific operations. These can be strung together to create more sophisticated and useful operations and can be ultimately built into a disaster like EndNote. Even when I make a conceptual switch and think about quantum computing, I still get stuck thinking about quantum logic gates.

A quantum gate or quantum logic gate is a rudimentary quantum circuit operating on a small number of qubits. They are the analogues for quantum computers to classical logic gates for conventional digital computers. Quantum logic gates are reversible, unlike many classical logic gates. Some universal classical logic gates, such as the Toffoli gate, provide reversibility and can be directly mapped onto quantum logic gates. Quantum logic gates are represented by unitary matrices.

The most common quantum gates operate on spaces of one or two qubits. This means that as matrices, quantum gates can be described by 2 x 2 or 4 x 4 matrices with orthonormal rows.

https://www.quantiki.org/wiki/quantum-gates

But there is a better-than-even chance that quantum computing will not make direct use of logic gates. If logic gates aren't going to be a thing in quantum computing, how will we compute? One way is through annealing, which I've written about a lot.

Quantum annealing (QA) is a metaheuristic for finding the global minimum of a given objective function over a given set of candidate solutions (candidate states), by a process using quantum fluctuations. Quantum annealing is used mainly for problems where the search space is discrete (combinatorial optimization problems) with many local minima; such as finding the ground state of a spin glass.

https://en.wikipedia.org/wiki/Quantum_annealing

But the neglected stepchild of quantum computing is something called a "quantum random walk." In a minor miracle, researchers have shown a quantum random walk through a string of 12 quantum bits. This is the sort of step that may herald the beginning of actually demonstrating a quantum computer based on a random walk.

Jump to search
Quantum walks are quantum analogues of classical random walks. In contrast to the classical random walk, where the walker occupies definite states and the randomness arises due to stochastic transitions between states, in quantum walks randomness arises through:
(1) quantum superposition of states,
(2) non-random, reversible unitary evolution and
(3) collapse of the wave function due to state measurements.

As with classical random walks, quantum walks admit formulations in both discrete time and continuous time.

Quantum walks are motivated by the widespread use of classical random walks in the design of randomized algorithms, and are part of several quantum algorithms. For some oracular problems, quantum walks provide an exponential speedup over any classical algorithm.[1][2] Quantum walks also give polynomial speedups over classical algorithms for many practical problems, such as the element distinctness problem,[3] the triangle finding problem,[4] and evaluating NAND trees.[5] The well-known Grover search algorithm can also be viewed as a quantum walk algorithm.

Quantum walks exhibit very different features from classical random walks. In particular, they do not converge to limiting distributions and due to the power of quantum interference they may spread significantly faster or slower than their classical equivalents.

The quantum random walk takes advantage of the idea that quantum objects do not have to be localized in space or time: a particle is also a wave, and waves are spread out. So, if I have a quantum object and offer it two paths to travel, it will spread out and travel down both at the same time. Only after I make a measurement, or there is something along the path that destroys the quantum state, will the path the object took be certain.

A computation can be performed by encoding the problem in the network of paths available to a quantum state. The qubits will spread out through the network and interfere with themselves and each other at any locations where the paths cross. Upon exit, the most likely value for the qubits corresponds to the answer to the problem.





https://arstechnica.com/science/2019/05 ... ng-qubits/