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Optical Fiber Communication Conference

Superconducting qubits meet Schrödinger’s cat

James E. Lukens

Published Jun 13, 2001• 0 citations
OpenAlex

Abstract

One of the many challenges facing those who would build a quantum computer is how to assemble a large enough group of qubits in a way that would permit useful computation. For this, solid state qubits have a clear advantage, and the natural order already present in a superconductor makes superconducting qubits a logical choice. The challenge for these “macroscopic” superconducting qubits is to achieve the required internal coherence between states in a single qubit in the face of interactions with the environment. Results of recent experiments at Stony Brook will be presented that demonstrate one can, in fact, produce a coherent superposition of flux states in SQUIDs (Superconducting Quantum Interference Devices). This coherence in SQUIDs is the strongest test to date of the predictions of quantum mechanics in a macroscopic system, e.g. Schrodinger’s cat.

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arXiv:0910.3622•Physica Scripta•Dec 1, 2009•19 citations
One of the many challenges facing those who would build a quantum computer is how to assemble a large enough group of qubits in a way that would permit useful computation. For this, solid state qubits have a clear advantage, and the natural order already present in a superconductor makes superconducting qubits a logical choice. The challenge for these “macroscopic” superconducting qubits is to achieve the required internal coherence between states in a single qubit in the face of interactions with the environment. Results of recent experiments at Stony Brook will be presented that demonstrate one can, in fact, produce a coherent superposition of flux states in SQUIDs (Superconducting Quantum Interference Devices). This coherence in SQUIDs is the strongest test to date of the predictions of quantum mechanics in a macroscopic system, e.g. Schrodinger’s cat.
Two classes of superconducting devices have been proposed as quantum bits (qubits) for realizing quantum logic operations. The flux qubits based on a superconducting quantum interference device (SQUID) appear to be particularly promising owing to the macroscopic nature of the qubit and potential integration with high-speed control circuitry in the form of rapid single-flux quantum electronics. Recent progress is discussed and near-term challenges mentioned. The radio frequency SQUID-based qubit offers a prospect for a reliably manufacturable scalable approach.
Over the last decade and a half, superconducting circuits have advanced to the point where we can generate and detect highly-entangled states, and perform universal quantum gates. Meanwhile, the coherence properties of these systems have improved more than 10,000-fold. I will describe recent experiments, such as the latest advance in coherence using a three-dimensional implementation of qubits interacting with microwave cavities, called “3D circuit QED.” The control and strong interactions possible in superconducting circuits make it possible to generate non-classical states of light, including large superpositions known as “Schrodinger cat” states. This field has many interesting prospects both for applications in quantum information processing, and fundamental investigations of the boundary between the macroscopic classical world and the microscopic world of the quantum.
In this paper I first briefly review the apparently bizarre consequences of attempting to apply quantum mechanics consistently to macroscopic bodies in certain kinds of situations (the ‘Schrödinger's Cat’ paradox). The paradox results from the assumption that the linear superpositions of macroscopically distinct states, which are unambiguously predicted by the formalism, do indeed exist in nature. Until recently, this assumption was experimentally untested. However, recent advances in techniques of microfabrication, cryogenics and noise control have opened up the possibility of indirect and possibly even direct tests, in particular in superconducting systems incorporating Josephson devices. I describe some of these tests, review the current experimental and theoretical situation and discuss the implications of various possible outcomes to the experiments.
Abstract Superconducting quantum interference devices (SQUIDs) are made by a superconducting loop interrupted by one or more Josephson junctions. They are described in terms of a macroscopic variable, the magnetic flux, which shows quantum effects such as tunnelling through a potential barrier. Besides making up the source of a quantum state, SQUIDs also provide the instruments necessary for its probing: as a fact, SQUID based magnetometers have a sensitivity approaching the quantum limit. In this paper I will review the working principle of these devices and illustrate the system of SQUIDs realized in my group to test the quantum behavior at a macroscopic level.
Abstract ‘Quantum matter’ describes how superconductors and superfluids can have properties like those found in the strange quantum world of atoms, electrons, and photons. In quantum theory, properties such as energy cannot have any possible value, but are quantized. Energy comes in discrete lumps. Particles can act like waves, be in more than one place at the same time, and interfere with themselves. The weirdness of quantum theory is captured in the paradox of Schrödinger’s cat, where quantum effects occur on the macroscopic scale. This is realized in a superconductor, where magnetic flux is quantized. Brian Josephson proposed an electrical circuit that is the basis for a Superconducting Quantum Interference Device (SQUID). These devices are of technological significance as they are used in quantum computing, and to make precise measurements of fundamental physical constants and of magnetic field strengths. SQUIDs are now used in metrology, the science of precise measurements, being the basis for the international standard for the volt, the unit of electrical voltage, and are used in quantum computing.
Flux qubits, small superconducting loops interrupted by Josephson junctions, are successful realizations of quantum coherence for macroscopic variables. Superconductivity in these loops is carried by ∼106\sim 10^6∼106 -- 101010^{10}1010 electrons, which has been interpreted as suggesting that coherent superpositions of such current states are macroscopic superpositions analogous to Schrödinger's cat. We provide a full microscopic analysis of such qubits, from which the macroscopic quantum description can be derived. This reveals that the number of microscopic constituents participating in superposition states for experimentally accessible flux qubits is surprisingly but not trivially small. The combination of this relatively small size with large differences between macroscopic observables in the two branches is seen to result from the Fermi statistics of the electrons and the large disparity between the values of superfluid and Fermi velocity in these systems.