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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
Abstract: "Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist

1,878 citations

Journal ArticleDOI
TL;DR: In this paper, the theory underpinning the measurement of density matrices of a pair of quantum two-level systems is described, and a detailed error analysis is presented, allowing errors in quantities derived from the density matrix, such as the entropy or entanglement of formation.
Abstract: We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems ~‘‘qubits’’ !. Our particular emphasis is on qubits realized by the two polarization degrees of freedom of a pair of entangled photons generated in a down-conversion experiment; however, the discussion applies in general, regardless of the actual physical realization. Two techniques are discussed, namely, a tomographic reconstruction ~in which the density matrix is linearly related to a set of measured quantities ! and a maximum likelihood technique which requires numerical optimization ~but has the advantage of producing density matrices that are always non-negative definite!. In addition, a detailed error analysis is presented, allowing errors in quantities derived from the density matrix, such as the entropy or entanglement of formation, to be estimated. Examples based on down-conversion experiments are used to illustrate our results.

1,838 citations

Journal ArticleDOI
02 Oct 2008-Nature
TL;DR: An approach to nanoscale magnetic sensing is experimentally demonstrated, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature to achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging.
Abstract: Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 muT, and the corresponding field from a single proton is a few nanoteslas. A sensor able to detect such magnetic fields with nanometre spatial resolution would enable powerful applications, ranging from the detection of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. Here we experimentally demonstrate an approach to such nanoscale magnetic sensing, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature. Using an ultra-pure diamond sample, we achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging. In addition, we demonstrate a sensitivity of 0.5 muT Hz(-1/2) for a diamond nanocrystal with a diameter of 30 nm.

1,817 citations

Journal ArticleDOI
01 Apr 2010-Nature
TL;DR: This work shows that conventional cryogenic refrigeration can be used to cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator.
Abstract: Quantum mechanics provides a highly accurate description of a wide variety of physical systems. However, a demonstration that quantum mechanics applies equally to macroscopic mechanical systems has been a long-standing challenge, hindered by the difficulty of cooling a mechanical mode to its quantum ground state. The temperatures required are typically far below those attainable with standard cryogenic methods, so significant effort has been devoted to developing alternative cooling techniques. Once in the ground state, quantum-limited measurements must then be demonstrated. Here, using conventional cryogenic refrigeration, we show that we can cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator. We further show that we can controllably create single quantum excitations (phonons) in the resonator, thus taking the first steps to complete quantum control of a mechanical system. The bizarre, often counter-intuitive predictions of quantum mechanics have been observed in atomic-scale optical and electrical systems, but efforts to demonstrate that quantum mechanics applies equally to a mechanical system, especially one large enough to be seen with the naked eye, have proved challenging. The difficulty is cooling a mechanical system to its quantum ground state, where all classical noise is eliminated. A team at the Department of Physics at the University of California, Santa Barbara, has overcome this obstacle. Using conventional cryogenic refrigeration, they cool a mechanical resonator with a very high oscillation frequency to one-fortieth of a degree above absolute zero. This resonator, called a 'quantum drum', is coupled to a superconducting quantum bit that acts as a quantum thermometer to detect whether there are any excitations left in the resonator. When it is confirmed there are none, it is further shown that a single quantum of excitation, a phonon, can be introduced in this system and exchanged between resonator and qubit many times, thereby taking the first steps towards complete quantum control of a mechanical system. Quantum mechanics provides an accurate description of a wide variety of physical systems but it is very challenging to prove that it also applies to macroscopic (classical) mechanical systems. This is because it has been impossible to cool a mechanical mode to its quantum ground state, in which all classical noise is eliminated. Recently, various mechanical devices have been cooled to a near-ground state, but this paper demonstrates the milestone result of a piezoelectric resonator with a mechanical mode cooled to its quantum ground state.

1,800 citations

Journal ArticleDOI
TL;DR: In this paper, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed, where the effective long-range interaction is mediated by the vacuum field of a high finesse microcavity.
Abstract: The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant time scales. A solid-state quantum computer based on localized electron spins as qubits is therefore of potential interest. Here, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed. The effective long-range interaction is mediated by the vacuum field of a high finesse microcavity. By using conduction-band-hole Raman transitions induced by classical laser fields and the cavity-mode, parallel controlled-not operations, and arbitrary single qubit rotations can be realized.

1,702 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