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Qubit

About: Qubit is a research topic. Over the lifetime, 29978 publications have been published within this topic receiving 723084 citations. The topic is also known as: quantum bit & qbit.


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Journal ArticleDOI
TL;DR: This work encoding in a single cavity mode, together with a protection protocol, significantly reduces the error rate due to photon loss and describes in detail how to implement these operations in a circuit quantum electrodynamics system.
Abstract: We propose to encode a quantum bit of information in a superposition of coherent states of an oscillator, with four different phases. Our encoding in a single cavity mode, together with a protection protocol, significantly reduces the error rate due to photon loss. This protection is ensured by an efficient quantum error correction scheme employing the nonlinearity provided by a single physical qubit coupled to the cavity. We describe in detail how to implement these operations in a circuit quantum electrodynamics system. This proposal directly addresses the task of building a hardware-efficient quantum memory and can lead to important shortcuts in quantum computing architectures.

266 citations

Journal ArticleDOI
17 Apr 2014-Nature
TL;DR: A striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducted quantum bits.
Abstract: The long-predicted suppression of quasiparticle dissipation in a Josephson junction when the phase difference across the junction is π is inferred from a sharp maximum in the energy relaxation time of a superconducting artificial atom. Josephson junctions, which consist of two superconductors connected by a weak link, have a central role in quantum electronic applications, such as in sensitive magnetic field detectors, high-speed processing and quantum information networks. However, a fundamental prediction concerning the Josephson effect has not yet been confirmed. It is known that the current flowing through a Josephson junction is made up from superconducting Cooper pairs as well as excitations called quasiparticles, which contribute in a few different ways. One contribution causes dissipation but can in theory be suppressed by tuning the phase difference between the superconductors. This has been achieved experimentally. Ioan Pop et al. have made a qubit comprising a Josephson junction. The energy relaxation time of this qubit increases by almost two orders of magnitude owing to the suppression of quasiparticle dissipation. This finding confirms the existence of a fundamental quantum phenomenon predicted over 50 years ago. Owing to the low-loss propagation of electromagnetic signals in superconductors, Josephson junctions constitute ideal building blocks for quantum memories, amplifiers, detectors and high-speed processing units, operating over a wide band of microwave frequencies. Nevertheless, although transport in superconducting wires is perfectly lossless for direct current, transport of radio-frequency signals can be dissipative in the presence of quasiparticle excitations above the superconducting gap1. Moreover, the exact mechanism of this dissipation in Josephson junctions has never been fully resolved experimentally. In particular, Josephson’s key theoretical prediction that quasiparticle dissipation should vanish in transport through a junction when the phase difference across the junction is π (ref. 2) has never been observed3. This subtle effect can be understood as resulting from the destructive interference of two separate dissipative channels involving electron-like and hole-like quasiparticles. Here we report the experimental observation of this quantum coherent suppression of quasiparticle dissipation across a Josephson junction. As the average phase bias across the junction is swept through π, we measure an increase of more than one order of magnitude in the energy relaxation time of a superconducting artificial atom. This striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducting quantum bits.

266 citations

Journal ArticleDOI
13 May 2019
TL;DR: This work proposes a variational hybrid quantum-classical algorithm called quantum-assisted quantum compiling (QAQC), and presents both gradient-free and gradient-based approaches to minimizing the cost of this algorithm's cost.
Abstract: Compiling quantum algorithms for near-term quantum computers (accounting for connectivity and native gate alphabets) is a major challenge that has received significant attention both by industry and academia. Avoiding the exponential overhead of classical simulation of quantum dynamics will allow compilation of larger algorithms, and a strategy for this is to evaluate an algorithm's cost on a quantum computer. To this end, we propose a variational hybrid quantum-classical algorithm called quantum-assisted quantum compiling (QAQC). In QAQC, we use the overlap between a target unitary $U$ and a trainable unitary $V$ as the cost function to be evaluated on the quantum computer. More precisely, to ensure that QAQC scales well with problem size, our cost involves not only the global overlap ${\rm Tr} (V^\dagger U)$ but also the local overlaps with respect to individual qubits. We introduce novel short-depth quantum circuits to quantify the terms in our cost function, and we prove that our cost cannot be efficiently approximated with a classical algorithm under reasonable complexity assumptions. We present both gradient-free and gradient-based approaches to minimizing this cost. As a demonstration of QAQC, we compile various one-qubit gates on IBM's and Rigetti's quantum computers into their respective native gate alphabets. Furthermore, we successfully simulate QAQC up to a problem size of 9 qubits, and these simulations highlight both the scalability of our cost function as well as the noise resilience of QAQC. Future applications of QAQC include algorithm depth compression, black-box compiling, noise mitigation, and benchmarking.

266 citations

Journal ArticleDOI
TL;DR: This work demonstrates how to realize nonadiabatic holonomic quantum computation in decoherence-free subspaces by using only three neighboring physical qubits undergoing collective dephasing to encode one logical qubit, and realizes a universal set of quantum gates.
Abstract: Quantum computation that combines the coherence stabilization virtues of decoherence-free subspaces and the fault tolerance of geometric holonomic control is of great practical importance Some schemes of adiabatic holonomic quantum computation in decoherence-free subspaces have been proposed in the past few years However, nonadiabatic holonomic quantum computation in decoherence-free subspaces, which avoids a long run-time requirement but with all the robust advantages, remains an open problem Here, we demonstrate how to realize nonadiabatic holonomic quantum computation in decoherence-free subspaces By using only three neighboring physical qubits undergoing collective dephasing to encode one logical qubit, we realize a universal set of quantum gates

266 citations

Journal ArticleDOI
TL;DR: The quadrupole S(1/2)-D(5/2) optical transition of a single trapped Ca+ ion is coherently coupled to the standing wave field of a high finesse cavity and deterministic coupling of the cavity mode to the ion's vibrational state is achieved.
Abstract: The quadrupole S(1/2)-D(5/2) optical transition of a single trapped Ca+ ion, well suited for encoding a quantum bit of information, is coherently coupled to the standing wave field of a high finesse cavity. The coupling is verified by observing the ion's response to both spatial and temporal variations of the intracavity field. We also achieve deterministic coupling of the cavity mode to the ion's vibrational state by selectively exciting vibrational state-changing transitions and by controlling the position of the ion in the standing wave field with nanometer precision.

265 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,977
20224,380
20213,014
20203,119
20192,594
20182,228