Superconducting quantum computing

[1] Research in superconducting quantum computing is conducted by companies such as Google,[2] IBM,[3] IMEC,[4] BBN Technologies,[5] Rigetti,[6] and Intel.

[3] In October 2019, the Martinis group, partnered with Google, published an article demonstrating novel quantum supremacy, using a chip composed of 53 superconducting qubits.

[citation needed] Superconducting capacitors and inductors are used to produce a resonant circuit that dissipates almost no energy, as heat can disrupt quantum information.

Physical implementation of qubits and gates is challenging for the same reason that quantum phenomena are difficult to observe in everyday life given the minute scale on which they occur.

Unlike typical conductors, superconductors possess a critical temperature at which resistivity plummets to nearly zero and conductivity is drastically increased.

The prospect of actualizing superconducting quantum computers becomes all the more promising considering NASA's recent development of the Cold Atom Lab in outer space where Bose-Einstein Condensates are more readily achieved and sustained (without rapid dissipation) for longer periods of time without the constraints of gravity.

[15] At each point of a superconducting electronic circuit (a network of electrical elements), the condensate wave function describing charge flow is well-defined by some complex probability amplitude.

In typical conductor electrical circuits, this same description is true for individual charge carriers except that the various wave functions are averaged in macroscopic analysis, making it impossible to observe quantum effects.

Differing from microscopic implementations of quantum computers (such as atoms or photons), parameters of superconducting circuits are designed by setting (classical) values to the electrical elements composing them such as by adjusting capacitance or inductance.

Firstly, all electrical elements must be described by the condensate wave function amplitude and phase rather than by closely related macroscopic current and voltage descriptions used for classical circuits.

For instance, the square of the wave function amplitude at any arbitrary point in space corresponds to the probability of finding a charge carrier there.

Finally, these equations of motion must be reformulated to Lagrangian mechanics such that a quantum Hamiltonian is derived describing the total energy of the system.

Typical dimensions fall on the range of micrometers, with sub-micrometer resolution, allowing for the convenient design of a Hamiltonian system with well-established integrated circuit technology.

Manufacturing superconducting qubits follows a process involving lithography, depositing of metal, etching, and controlled oxidation as described in.

The condensate wave function on the two sides of the junction are weakly correlated, meaning that they are allowed to have different superconducting phases.

are mapped to different states of the physical system (typically to discrete energy levels or their quantum superpositions).

[24][25] Another crucial advantage of the fluxonium qubit biased at the sweet spot is the large anharmonicity, which allows fast local microwave control and mitigates spectral crowding problems, leading to better scalability.

[36] A relatively simple qubit, the Unimon consists of a single Josephson junction shunted by a linear inductor (possessing an inductance not depending on current) inside a (superconducting) resonator.

[37] Unimons have increased anharmocity and display faster operation time resulting in lower susceptibility to noise errors.

Note that particle mass corresponds to an inverse function of the circuit capacitance and that the shape of the potential is governed by regular inductors and Josephson junctions.

Temperatures of tens of millikelvins are achieved in dilution refrigerators and allow qubit operation at a ~5 GHz energy level separation.

Notably, D-Wave Systems' nearest-neighbor coupling achieves a highly connected unit cell of 8 qubits in Chimera graph configuration.

[42] Following the dark state manifold, the Khazali-Mølmer scheme[42] performs complex multi-qubit operations in a single step, providing a substantial shortcut to the conventional circuit model.

Rotation direction depends on the state of the first qubit, allowing a controlled phase gate construction.

[52] DiVincenzo's criteria is a list describing the requirements for a physical system to be capable of implementing a logical qubit.

The final two criteria have been experimentally proven by research performed by ETH with two superconducting qubits connected by a coaxial cable.

[57] One of the primary challenges of superconducting quantum computing is the extremely low temperatures at which superconductors like Bose-Einstein Condensates exist.

[16] Moreover, superconducting quantum computing devices must be reliably reproducible at increasingly large scales such that they are compatible with these improvements.

[58] The journey has been long, arduous and full of breakthroughs but has seen significant advancements in the recent history and has massive potential for revolutionizing computing.

In August 2022, Baidu released its plans to build a fully integrated top to bottom quantum computer which incorporated superconducting qubits.