Showing papers on "Quantum computer published in 2012"
TL;DR: The concept of the stabilizer, using two qubits, is introduced, and the single-qubit Hadamard, S and T operators are described, completing the set of required gates for a universal quantum computer.
Abstract: This article provides an introduction to surface code quantum computing. We first estimate the size and speed of a surface code quantum computer. We then introduce the concept of the stabilizer, using two qubits, and extend this concept to stabilizers acting on a two-dimensional array of physical qubits, on which we implement the surface code. We next describe how logical qubits are formed in the surface code array and give numerical estimates of their fault tolerance. We outline how logical qubits are physically moved on the array, how qubit braid transformations are constructed, and how a braid between two logical qubits is equivalent to a controlled-not. We then describe the single-qubit Hadamard, Ŝ and T operators, completing the set of required gates for a universal quantum computer. We conclude by briefly discussing physical implementations of the surface code. We include a number of Appendices in which we provide supplementary information to the main text. © 2012 American Physical Society.
2,205 citations
TL;DR: An object-oriented open-source framework for solving the dynamics of open quantum systems written in Python that is particularly well suited to the fields of quantum optics, superconducting circuit devices, nanomechanics, and trapped ions, while also being ideal for use in classroom instruction.
1,430 citations
01 May 2012
TL;DR: In this article, a review of the state of the art in continuous-variable quantum information processing can be found, ranging from the basic theoretical tools and landmark experimental realizations to the most recent successful developments.
Abstract: The science of quantum information has arisen over the last two decades centered on the manipulation of individual quanta of information, known as quantum bits or qubits. Quantum computers, quantum cryptography and quantum teleportation are among the most celebrated ideas that have emerged from this new field. It was realized later on that using continuous-variable quantum information carriers, instead of qubits, constitutes an extremely powerful alternative approach to quantum information processing. This review focuses on continuous-variable quantum information processes that rely on any combination of Gaussian states, Gaussian operations, and Gaussian measurements. Interestingly, such a restriction to the Gaussian realm comes with various benefits, since on the theoretical side, simple analytical tools are available and, on the experimental side, optical components effecting Gaussian processes are readily available in the laboratory. Yet, Gaussian quantum information processing opens the way to a wide variety of tasks and applications, including quantum communication, quantum cryptography, quantum computation, quantum teleportation, and quantum state and channel discrimination. This review reports on the state of the art in this field, ranging from the basic theoretical tools and landmark experimental realizations to the most recent successful developments.
1,374 citations
TL;DR: In this article, the authors present a survey of photonic quantum simulation and discuss the prospects for tackling outstanding problems in physics, chemistry, and biology, as well as their potential for solving problems that are intractable on conventional computers.
Abstract: Quantum simulators are controllable quantum systems that can be used to mimic other quantum systems. They have the potential to enable the tackling of problems that are intractable on conventional computers. The photonic quantum technology available today is reaching the stage where significant advantages arise for the simulation of interesting problems in quantum chemistry, quantum biology and solid-state physics. In addition, photonic quantum systems also offer the unique benefit of being mobile over free space and in waveguide structures, which opens new perspectives to the field by enabling the natural investigation of quantum transport phenomena. Here, we review recent progress in the field of photonic quantum simulation, which should break the ground towards the realization of versatile quantum simulators. Quantum optics has played an important role in the exploration of foundational issues in quantum mechanics, and in using quantum effects for information processing and communications purposes. Photonic quantum systems now also provide a valuable test bed for quantum simulations. This article surveys the first generation of such experiments, and discusses the prospects for tackling outstanding problems in physics, chemistry and biology.
930 citations
TL;DR: This paper has reviewed several algorithms based on both discrete- and continuous-time quantum walks as well as a most important result: the computational universality of both continuous- and discrete- time quantum walks.
Abstract: Quantum walks, the quantum mechanical counterpart of classical random walks, is an advanced tool for building quantum algorithms that has been recently shown to constitute a universal model of quantum computation. Quantum walks is now a solid field of research of quantum computation full of exciting open problems for physicists, computer scientists and engineers. In this paper we review theoretical advances on the foundations of both discrete- and continuous-time quantum walks, together with the role that randomness plays in quantum walks, the connections between the mathematical models of coined discrete quantum walks and continuous quantum walks, the quantumness of quantum walks, a summary of papers published on discrete quantum walks and entanglement as well as a succinct review of experimental proposals and realizations of discrete-time quantum walks. Furthermore, we have reviewed several algorithms based on both discrete- and continuous-time quantum walks as well as a most important result: the computational universality of both continuous- and discrete-time quantum walks.
883 citations
TL;DR: In this article, the authors present a prototype of a quantum network based on single atoms embedded in optical cavities and demonstrate the faithful transfer of an atomic quantum state and the creation of entanglement between two identical nodes in separate laboratories.
Abstract: Quantum networks are distributed quantum many-body systems with tailored topology and controlled information exchange. They are the backbone of distributed quantum computing architectures and quantum communication. Here we present a prototype of such a quantum network based on single atoms embedded in optical cavities. We show that atom–cavity systems form universal nodes capable of sending, receiving, storing and releasing photonic quantum information. Quantum connectivity between nodes is achieved in the conceptually most fundamental way—by the coherent exchange of a single photon. We demonstrate the faithful transfer of an atomic quantum state and the creation of entanglement between two identical nodes in separate laboratories. The non-local state that is created is manipulated by local quantum bit (qubit) rotation. This efficient cavity-based approach to quantum networking is particularly promising because it offers a clear perspective for scalability, thus paving the way towards large-scale quantum networks and their applications. Single atoms in optical cavities in two separate laboratories are the nodes of an elementary quantum network, in which quantum information is distributed via the controlled emission and absorption of single photons. Quantum networks, following the principles of quantum teleportation, form the backbone of distributed quantum-computing architectures and quantum communication. This paper reports the first realization of an elementary quantum network with two quantum nodes based on single atoms trapped in optical cavities in separate laboratories. The approach is particularly promising in that it demonstrates all the necessary ingredients of a full-scale quantum network.
742 citations
TL;DR: A controlled two-qubit operation between singlet-triplet qubits is performed using a dynamically decoupled sequence that maintains the two- qubit coupling while decoupling each qubit from its fluctuating environment, providing proof of entanglement.
Abstract: Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.
542 citations
TL;DR: The study of topological phases does not have to remain limited to static or quasi-static/adiabatic situations, and can be extended to periodically driven systems, which have recently been proposed to also exhibit topological behaviors.
Abstract: Topological phases exhibit some of the most striking phenomena in modern physics. Much of the rich behaviour of quantum Hall systems, topological insulators, and topological superconductors can be traced to the existence of robust bound states at interfaces between different topological phases. This robustness has applications in metrology and holds promise for future uses in quantum computing. Engineered quantum systems--notably in photonics, where wavefunctions can be observed directly--provide versatile platforms for creating and probing a variety of topological phases. Here we use photonic quantum walks to observe bound states between systems with different bulk topological properties and demonstrate their robustness to perturbations--a signature of topological protection. Although such bound states are usually discussed for static (time-independent) systems, here we demonstrate their existence in an explicitly time-dependent situation. Moreover, we discover a new phenomenon: a topologically protected pair of bound states unique to periodically driven systems.
512 citations
TL;DR: The present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres.
Abstract: Future quantum networks will combine ideally stationary quantum bits (qubits), such as single electron spins, with 'flying' qubits, which are photons that transfer quantum states between distant qubits. It has therefore been a long-standing challenge in the field of quantum computation and communication to couple a single electron spin to a single photon in a solid-state platform. Two groups working independently have now achieved that goal, by demonstrating entanglement between a photon and a single electron spin trapped in a semiconductor 'quantum dot' structure. The quantum dot acts as the stationary node. This achievement is a small step towards eventual implementation of quantum networks that can support long-distance quantum communication.
503 citations
TL;DR: In this paper, the authors developed a quantum algorithm to compute relativistic scattering probabilities in a massive quantum field theory with quartic self-interactions (φ^4 theory) in spacetime of four and fewer dimensions.
Abstract: Quantum field theory reconciles quantum mechanics and special relativity,
and plays a central role in many areas of physics. We develop a quantum
algorithm to compute relativistic scattering probabilities in a massive quantum
field theory with quartic self-interactions (φ^4 theory) in spacetime of four
and fewer dimensions. Its run time is polynomial in the number of particles,
their energy, and the desired precision, and applies at both weak and strong
coupling. In the strong-coupling and high-precision regimes, our quantum
algorithm achieves exponential speedup over the fastest known classical algorithm.
474 citations
TL;DR: This paper proposes synthetic methods aimed at accessing such type of functional molecules, based on ligand design and inorganic synthesis, and consists in targeting molecules containing a pair of well-defined and weakly coupled paramagnetic metal aggregates.
Abstract: A very exciting prospect in coordination chemistry is to manipulate spins within magnetic complexes for the realization of quantum logic operations. An introduction to the requirements for a paramagnetic molecule to act as a 2-qubit quantum gate is provided in this tutorial review. We propose synthetic methods aimed at accessing such type of functional molecules, based on ligand design and inorganic synthesis. Two strategies are presented: (i) the first consists in targeting molecules containing a pair of well-defined and weakly coupled paramagnetic metal aggregates, each acting as a carrier of one potential qubit, (ii) the second is the design of dinuclear complexes of anisotropic metal ions, exhibiting dissimilar environments and feeble magnetic coupling. The first systems obtained from this synthetic program are presented here and their properties are discussed.
TL;DR: An alternative method of simulating fermions with qubits, first proposed by Bravyi and Kitaev, is developed that reduces the simulation cost to O(log n) qubit operations for one fermionic operation and demonstrates the superior efficiency of the Bravyi-Kitaev method for all quantum computations of electronic structure.
Abstract: Quantum simulation is an important application of future quantum computers with applications in quantum chemistry, condensed matter, and beyond. Quantum simulation of fermionic systems presents a specific challenge. The Jordan-Wigner transformation allows for representation of a fermionic operator by O(n) qubit operations. Here, we develop an alternative method of simulating fermions with qubits, first proposed by Bravyi and Kitaev [Ann. Phys. 298, 210 (2002); e-print arXiv:quant-ph/0003137v2], that reduces the simulation cost to O(log n) qubit operations for one fermionic operation. We apply this new Bravyi-Kitaev transformation to the task of simulating quantum chemical Hamiltonians, and give a detailed example for the simplest possible case of molecular hydrogen in a minimal basis. We show that the quantum circuit for simulating a single Trotter time step of the Bravyi-Kitaev derived Hamiltonian for H(2) requires fewer gate applications than the equivalent circuit derived from the Jordan-Wigner transformation. Since the scaling of the Bravyi-Kitaev method is asymptotically better than the Jordan-Wigner method, this result for molecular hydrogen in a minimal basis demonstrates the superior efficiency of the Bravyi-Kitaev method for all quantum computations of electronic structure.
TL;DR: An experimental demonstration of blind quantum computing in which the input, computation, and output all remain unknown to the computer is presented and the conceptual framework of measurement-based quantum computation that enables a client to delegate a computation to a quantum server is exploited.
Abstract: Quantum computers, besides offering substantial computational speedups, are also expected to preserve the privacy of a computation. We present an experimental demonstration of blind quantum computing in which the input, computation, and output all remain unknown to the computer. We exploit the conceptual framework of measurement-based quantum computation that enables a client to delegate a computation to a quantum server. Various blind delegated computations, including one- and two-qubit gates and the Deutsch and Grover quantum algorithms, are demonstrated. The client only needs to be able to prepare and transmit individual photonic qubits. Our demonstration is crucial for unconditionally secure quantum cloud computing and might become a key ingredient for real-life applications, especially when considering the challenges of making powerful quantum computers widely available.
TL;DR: This work describes and expands upon the scalable randomized benchmarking protocol proposed in Phys.
Abstract: We describe and expand upon the scalable randomized benchmarking protocol proposed in Phys. Rev. Lett. 106, 180504 (2011) which provides a method for benchmarking quantum gates and estimating the gate dependence of the noise. The protocol allows the noise to have weak time and gate dependence, and we provide a sufficient condition for the applicability of the protocol in terms of the average variation of the noise. We discuss how state-preparation and measurement errors are taken into account and provide a complete proof of the scalability of the protocol. We establish a connection in special cases between the error rate provided by this protocol and the error strength measured using the diamond norm distance.
Journal Article•
TL;DR: A scalable experimental protocol is described that consists of interleaving random Clifford gates between the gate of interest and provides an estimate as well as theoretical bounds for the average error of the gate under test, so long as the average noise variation over all Clifford gates is small.
Abstract: We describe a scalable experimental protocol for estimating the average error of individual quantum computational gates. This protocol consists of interleaving random Clifford gates between the gate of interest and provides an estimate as well as theoretical bounds for the average error of the gate under test, so long as the average noise variation over all Clifford gates is small. This technique takes into account both state preparation and measurement errors and is scalable in the number of qubits. We apply this protocol to a superconducting qubit system and find a bounded average error of 0.003 [0,0.016] for the single-qubit gates ${X}_{\ensuremath{\pi}/2}$ and ${Y}_{\ensuremath{\pi}/2}$. These bounded values provide better estimates of the average error than those extracted via quantum process tomography.
TL;DR: This paper introduces a new technique enabling the coupling of two planar codes without transversal operations, maintaining the 2DNN of the encoded computer, and shows how lattice surgery allows us to distribute encoded GHZ states in a more direct manner, and how a demonstration of an encoded CNOT between two distance-3 logical states is possible with 53 physical qubits.
Abstract: In recent years, surface codes have become a leading method for quantum error correction in theoretical large-scale computational and communications architecture designs. Their comparatively high fault-tolerant thresholds and their natural two-dimensional nearest-neighbour (2DNN) structure make them an obvious choice for large scale designs in experimentally realistic systems. While fundamentally based on the toric code of Kitaev, there are many variants, two of which are the planar- and defect-based codes. Planar codes require fewer qubits to implement (for the same strength of error correction), but are restricted to encoding a single qubit of information. Interactions between encoded qubits are achieved via transversal operations, thus destroying the inherent 2DNN nature of the code. In this paper we introduce a new technique enabling the coupling of two planar codes without transversal operations, maintaining the 2DNN of the encoded computer. Our lattice surgery technique comprises splitting and merging planar code surfaces, and enables us to perform universal quantum computation (including magic state injection) while removing the need for braided logic in a strictly 2DNN design, and hence reduces the overall qubit resources for logic operations. Those resources are further reduced by the use of a rotated lattice for the planar encoding. We show how lattice surgery allows us to distribute encoded GHZ states in a more direct (and overhead friendly) manner, and how a demonstration of an encoded CNOT between two distance-3 logical states is possible with 53 physical qubits, half of that required in any other known construction in 2D.
TL;DR: The algorithm can be used to provide an efficient parametric estimation of the quantum state and therefore can be applied as an alternative to full quantum-state tomography given a fault tolerant quantum computer.
Abstract: We provide a new quantum algorithm that efficiently determines the quality of a least-squares fit over an exponentially large data set by building upon an algorithm for solving systems of linear equations efficiently [Harrow et al., Phys. Rev. Lett. 103, 150502 (2009)]. In many cases, our algorithm can also efficiently find a concise function that approximates the data to be fitted and bound the approximation error. In cases where the input data are pure quantum states, the algorithm can be used to provide an efficient parametric estimation of the quantum state and therefore can be applied as an alternative to full quantum-state tomography given a fault tolerant quantum computer.
TL;DR: The result generalizes the Gottesman-Knill theorem and provides a way of sampling from the output distribution of a computation or a simulation, including the efficient sampling from an approximate output distribution in the case of sampling imperfections for initial states, gates, or measurements.
Abstract: We show that quantum circuits where the initial state and all the following quantum operations can be represented by positive Wigner functions can be classically efficiently simulated. This is true both for continuous-variable as well as discrete variable systems in odd prime dimensions, two cases which will be treated on entirely the same footing. Noting the fact that Clifford and Gaussian operations preserve the positivity of the Wigner function, our result generalizes the Gottesman-Knill theorem. Our algorithm provides a way of sampling from the output distribution of a computation or a simulation, including the efficient sampling from an approximate output distribution in the case of sampling imperfections for initial states, gates, or measurements. In this sense, this work highlights the role of the positive Wigner function as separating classically efficiently simulable systems from those that are potentially universal for quantum computing and simulation, and it emphasizes the role of negativity of the Wigner function as a computational resource.
IBM1
TL;DR: A full universal set of all-microwave gates on two superconducting single-frequency single-junction transmon qubits are characterized and a process map representation in the Pauli basis is introduced which is visually efficient and informative.
Abstract: We use quantum process tomography to characterize a full universal set of all-microwave gates on two superconducting single-frequency single-junction transmon qubits. All extracted gate fidelities, including those for Clifford group generators, single-qubit $\ensuremath{\pi}/4$ and $\ensuremath{\pi}/8$ rotations, and a two-qubit controlled-not, exceed $95%$ ($98%$), without (with) subtracting state preparation and measurement errors. Furthermore, we introduce a process map representation in the Pauli basis which is visually efficient and informative. This high-fidelity gate set serves as a critical building block towards scalable architectures of superconducting qubits for error correction schemes and pushes up on the known limits of quantum gate characterization.
TL;DR: In this article, the Toffoli gate was implemented with three transmon qubits coupled to a microwave resonator, achieving a phase fidelity of 68.5 ± 0.5 per cent.
Abstract: Use of a three-level system allows the Toffoli gate, an important primitive for quantum error correction schemes, to be implemented with many fewer elementary gates than was previously thought possible. Quantum processors based on fragile quantum coherence are especially prone to errors due to sources of disturbance inevitable in any real system. The Toffoli gate, a logic gate that makes universal reversible classical computation possible, is a key element of universal quantum computation and error-correction schemes. Its realization is challenging, requiring many single and two-qubit operations executed with high fidelity. Fedorov et al. have developed a Toffoli gate in a superconducting circuit, exploiting three-level qubits to simplify implementation. Its performance confirms the potential of macroscopic superconducting qubits in complex quantum operations. The Toffoli gate is a three-quantum-bit (three-qubit) operation that inverts the state of a target qubit conditioned on the state of two control qubits. It makes universal reversible classical computation1 possible and, together with a Hadamard gate2, forms a universal set of gates in quantum computation. It is also a key element in quantum error correction schemes3,4,5,6,7. The Toffoli gate has been implemented in nuclear magnetic resonance3, linear optics8 and ion trap systems9. Experiments with superconducting qubits have also shown significant progress recently: two-qubit algorithms10 and two-qubit process tomography have been implemented11, three-qubit entangled states have been prepared12,13, first steps towards quantum teleportation have been taken14 and work on quantum computing architectures has been done15. Implementation of the Toffoli gate with only single- and two-qubit gates requires six controlled-NOT gates and ten single-qubit operations16, and has not been realized in any system owing to current limits on coherence. Here we implement a Toffoli gate with three superconducting transmon qubits coupled to a microwave resonator. By exploiting the third energy level of the transmon qubits, we have significantly reduced the number of elementary gates needed for the implementation of the Toffoli gate, relative to that required in theoretical proposals using only two-level systems. Using full process tomography and Monte Carlo process certification, we completely characterize the Toffoli gate acting on three independent qubits, measuring a fidelity of 68.5 ± 0.5 per cent. A similar approach15 to realizing characteristic features of a Toffoli-class gate has been demonstrated with two qubits and a resonator and achieved a limited characterization considering only the phase fidelity. Our results reinforce the potential of macroscopic superconducting qubits for the implementation of complex quantum operations with the possibility of quantum error correction17.
TL;DR: In this paper, a non-adiabatic generalization of holonomic quantum computation in which high-speed universal quantum gates can be realized using non-Abelian geometric phases is presented.
Abstract: We develop a non-adiabatic generalization of holonomic quantum computation in which high-speed universal quantum gates can be realized using non-Abelian geometric phases. We show how a set of non-a ...
TL;DR: Shor's quantum algorithm factorizes integers as discussed by the authors, and implementing this is a benchmark test in the early development of quantum processors, and it has been used to demonstrate this important test in a solid-state system.
Abstract: Shor’s quantum algorithm factorizes integers, and implementing this is a benchmark test in the early development of quantum processors. Researchers now demonstrate this important test in a solid-state system: a circuit made up of four superconducting qubits factorizes the number 15.
TL;DR: In this article, a connection between the potential for quantum speed-up and the onset of negative values in a distinguished quasi-probability representation, a discrete analogue of the Wigner function for quantum systems of odd dimension, was established.
Abstract: A central problem in quantum information is to determine the minimal physical resources that are required for quantum computational speed-up and, in particular, for fault-tolerant quantum computation. We establish a remarkable connection between the potential for quantum speed-up and the onset of negative values in a distinguished quasi-probability representation, a discrete analogue of the Wigner function for quantum systems of odd dimension. This connection allows us to resolve an open question on the existence of bound states for magic state distillation: we prove that there exist mixed states outside the convex hull of stabilizer states that cannot be distilled to non-stabilizer target states using stabilizer operations. We also provide an efficient simulation protocol for Clifford circuits that extends to a large class of mixed states, including bound universal states.
Posted Content•
TL;DR: In this paper, it was shown that classical quantum computers cannot simulate highly entangled quantum systems efficiently, and the authors hope to hasten the day when well controlled quantum systems can perform tasks exceeding what can be done in the classical world.
Abstract: Quantum information science explores the frontier of highly complex quantum states, the "entanglement frontier." This study is motivated by the observation (widely believed but unproven) that classical systems cannot simulate highly entangled quantum systems efficiently, and we hope to hasten the day when well controlled quantum systems can perform tasks surpassing what can be done in the classical world. One way to achieve such "quantum supremacy" would be to run an algorithm on a quantum computer which solves a problem with a super-polynomial speedup relative to classical computers, but there may be other ways that can be achieved sooner, such as simulating exotic quantum states of strongly correlated matter. To operate a large scale quantum computer reliably we will need to overcome the debilitating effects of decoherence, which might be done using "standard" quantum hardware protected by quantum error-correcting codes, or by exploiting the nonabelian quantum statistics of anyons realized in solid state systems, or by combining both methods. Only by challenging the entanglement frontier will we learn whether Nature provides extravagant resources far beyond what the classical world would allow.
TL;DR: This work demonstrates a scalable version of Shor's quantum factoring algorithm in which then qubit control register is replaced by a single qubit that is recycled n times: the total number of qubits is one third of that required in the standard protocol.
Abstract: By using qubit recycling, researchers demonstrate a scalable version of Shor's algorithm in which the total number of qubits is one third of that required in the standard protocol. They experimentally implemented a two-photon compiled algorithm to factor N = 21, pointing to larger-scale implementations of Shor's algorithm.
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
TL;DR: In this article, the use of hyperfine-resolved optical transitions was used to detect nuclear magnetic resonance from dilute 31P in the purest available sample of 28Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds.
Abstract: A quantum computer requires systems that are isolated from their environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spins in solids promise long coherence lifetimes, but they are difficult to initialize into known states and to detect with high sensitivity. We show how the distinctive optical properties of enriched 28Si enable the use of hyperfine-resolved optical transitions, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these resolved hyperfine transitions permit rapid nuclear hyperpolarization and electrical spin-readout. We combine these techniques to detect nuclear magnetic resonance from dilute 31P in the purest available sample of 28Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds.
01 Jun 2012
TL;DR: Memory effects are ubiquitous in nature and the class of memory circuit elements - which includes memristive, memcapacitive, and meminductive systems - shows great potential to understand and simulate the associated physical processes.
Abstract: Memory effects are ubiquitous in nature and the class of memory circuit elements - which includes memristive, memcapacitive, and meminductive systems - shows great potential to understand and simulate the associated physical processes. Here, we show that such elements can also be used in electronic schemes mimicking biologically inspired computer architectures, performing digital logic and arithmetic operations, and can expand the capabilities of certain quantum computation schemes. In particular, we will discuss some examples where the concept of memory elements is relevant to the realization of associative memory in neuronal circuits, spike-timing-dependent plasticity (STDP) of synapses, and digital and field-programmable quantum computing.
01 Aug 2012
TL;DR: In this article, the quality of a least-squares fit over an exponentially large data set was determined by building upon an algorithm for solving systems of linear equations efficiently, which can be used to provide an efficient parametric estimation of the quantum state and therefore can be applied as an alternative to full quantum-state tomography given a fault tolerant quantum computer.
Abstract: We provide a new quantum algorithm that efficiently determines the quality of a least-squares fit over an exponentially large data set by building upon an algorithm for solving systems of linear equations efficiently [Harrow et al., Phys. Rev. Lett. 103, 150502 (2009)]. In many cases, our algorithm can also efficiently find a concise function that approximates the data to be fitted and bound the approximation error. In cases where the input data are pure quantum states, the algorithm can be used to provide an efficient parametric estimation of the quantum state and therefore can be applied as an alternative to full quantum-state tomography given a fault tolerant quantum computer.