Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:0805.0164

Superconducting qubits

Alexandre M. Zagoskin, Alexandre Blais

Published May 1, 2008• 10 citations
arXivHTMLPDF

Abstract

From a physicist's standpoint, the most interesting part of quantum computing research may well be the possibility to probe the boundary between the quantum and the classical worlds. The more macroscopic are the structures involved, the better. So far, the most "macroscopic" qubit prototypes that have been studied in the laboratory are certain kinds of superconducting qubits. To get a feeling for how macroscopic these systems can be, the states of flux qubits which are brought in a quantum superposition corresponds to currents composed of as much as 10^5-10^6 electrons flowing in opposite directions in a superconducting loop. Non-superconducting qubits realized so far are all essentially microscopic.

DOI
10.48550/arxiv.0805.0164

Related papers

Physical Realizations of Qubits

Eduardo R. Mucciolo

In the mid 1990s, with the advent of quantum algorithms that could solve hard problems of practical interest, a race to develop qubits began. A myriad of approaches were proposed and tried, from nuclear spins in molecules to trapped ions and electrons to superconducting circuits, not to mention photons. Some of these qubit realizations were far more advanced than others from a technical standpoint (such as nuclear spins and superconducting circuits), thanks to decades of fundamental studies and applications in areas other than quantum computing. Other proposals were untested but very promising such as defect states in insulating materials and Rydberg atomic systems.

CRC Press eBooks•Aug 6, 2025•0 citations

Quantum in the Microscopic and Macroscopic Worlds

Mahn-Soo CHOI

It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.

Physics and High Technology•Dec 23, 2025•0 citations

Superconducting Qubits

Ray LaPierre

Thus far, we have examined quantum computing based on electron states in atoms, ions and semiconductor structures. In this chapter, we will examine quantum states in superconductors and their application as qubits. This chapter is particularly extensive due to the large variety of possible superconducting quantum circuits. We introduce superconductivity and examine the three main types of superconducting qubit: the flux qubit, the charge qubit and the phase qubit. We will also examine the transmon qubit and circuit quantum electrodynamics.

The Materials Research Society series•Oct 24, 2025•0 citations

Proposed Experiments to Test the Foundations of Quantum Computing

Alan M. Kadin, Steven B. Kaplan

Quantum computing promises computational performance that is exponentially faster than any conceivable classical computer. This is due to the theoretically expected scaling of N entangled qubits, with parallel evolution of 2^N quantum states. This is in sharp contrast to classical computing, where N bits may have 2^N classical states, but only one at a time. It is widely believed that quantum superposition and entanglement have been demonstrated in several experimental systems, and that practical quantum computing can be achieved once sufficiently long quantum relaxation times are obtained. On the contrary, we suggest that there may be serious problems with quantum computing on both the macroscopic and microscopic levels, and that the experiments thus far have not proven the existence of non-classical superposition states, which are necessary for the proper functioning of qubits. In order to investigate this further, we propose new experiments in three physical systems: electron spins, single photons, and superconducting loops. We further suggest that certain more limited classes of quantum computing, such as quantum annealing, do not require quantum entanglement, and can achieve significant performance enhancements even if universal quantum computing proves to be impossible.

•Jul 1, 2016•1 citation

Toward a superconducting quantum computer Harnessing macroscopic quantum coherence

Jaw-Shen Tsai

Intensive research on the construction of superconducting quantum computers has produced numerous important achievements. The quantum bit (qubit), based on the Josephson junction, is at the heart of this research. This macroscopic system has the ability to control quantum coherence. This article reviews the current state of quantum computing as well as its history, and discusses its future. Although progress has been rapid, the field remains beset with unsolved issues, and there are still many new research opportunities open to physicists and engineers.

Proceedings of the Japan Academy Series B•Jan 1, 2010•11 citations

Macroscopic many-qubit interactions in superconducting flux qubits

Sam Young Cho, Mun Dae Kim

Superconducting flux qubits are considered to investigate macroscopic many-qubit interactions. Many-qubit states based on current states can be manipulated through the current-phase relation in each superconducting loop. For flux qubit systems comprised of $N$-qubit loops, a general expression of low-energy Hamiltonian is presented in terms of low-energy levels of qubits and macroscopic quantum tunnelings between the many-qubit states. Many-qubit interactions classified by Ising-type or tunnel exchange interactions can be observable experimentally. Flux qubit systems can provide various artificial-spin systems to study many-body systems that cannot be found naturally.

arXiv:cond-mat/0703505•Physical Review B•Jun 19, 2008•4 citations
From a physicist's standpoint, the most interesting part of quantum computing research may well be the possibility to probe the boundary between the quantum and the classical worlds. The more macroscopic are the structures involved, the better. So far, the most "macroscopic" qubit prototypes that have been studied in the laboratory are certain kinds of superconducting qubits. To get a feeling for how macroscopic these systems can be, the states of flux qubits which are brought in a quantum superposition corresponds to currents composed of as much as 10^5-10^6 electrons flowing in opposite directions in a superconducting loop. Non-superconducting qubits realized so far are all essentially microscopic.
In the mid 1990s, with the advent of quantum algorithms that could solve hard problems of practical interest, a race to develop qubits began. A myriad of approaches were proposed and tried, from nuclear spins in molecules to trapped ions and electrons to superconducting circuits, not to mention photons. Some of these qubit realizations were far more advanced than others from a technical standpoint (such as nuclear spins and superconducting circuits), thanks to decades of fundamental studies and applications in areas other than quantum computing. Other proposals were untested but very promising such as defect states in insulating materials and Rydberg atomic systems.
It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.
Thus far, we have examined quantum computing based on electron states in atoms, ions and semiconductor structures. In this chapter, we will examine quantum states in superconductors and their application as qubits. This chapter is particularly extensive due to the large variety of possible superconducting quantum circuits. We introduce superconductivity and examine the three main types of superconducting qubit: the flux qubit, the charge qubit and the phase qubit. We will also examine the transmon qubit and circuit quantum electrodynamics.
Quantum computing promises computational performance that is exponentially faster than any conceivable classical computer. This is due to the theoretically expected scaling of N entangled qubits, with parallel evolution of 2^N quantum states. This is in sharp contrast to classical computing, where N bits may have 2^N classical states, but only one at a time. It is widely believed that quantum superposition and entanglement have been demonstrated in several experimental systems, and that practical quantum computing can be achieved once sufficiently long quantum relaxation times are obtained. On the contrary, we suggest that there may be serious problems with quantum computing on both the macroscopic and microscopic levels, and that the experiments thus far have not proven the existence of non-classical superposition states, which are necessary for the proper functioning of qubits. In order to investigate this further, we propose new experiments in three physical systems: electron spins, single photons, and superconducting loops. We further suggest that certain more limited classes of quantum computing, such as quantum annealing, do not require quantum entanglement, and can achieve significant performance enhancements even if universal quantum computing proves to be impossible.
Intensive research on the construction of superconducting quantum computers has produced numerous important achievements. The quantum bit (qubit), based on the Josephson junction, is at the heart of this research. This macroscopic system has the ability to control quantum coherence. This article reviews the current state of quantum computing as well as its history, and discusses its future. Although progress has been rapid, the field remains beset with unsolved issues, and there are still many new research opportunities open to physicists and engineers.
Superconducting flux qubits are considered to investigate macroscopic many-qubit interactions. Many-qubit states based on current states can be manipulated through the current-phase relation in each superconducting loop. For flux qubit systems comprised of NNN-qubit loops, a general expression of low-energy Hamiltonian is presented in terms of low-energy levels of qubits and macroscopic quantum tunnelings between the many-qubit states. Many-qubit interactions classified by Ising-type or tunnel exchange interactions can be observable experimentally. Flux qubit systems can provide various artificial-spin systems to study many-body systems that cannot be found naturally.