Showing papers on "Quantum published in 2014"
TL;DR: An overview is given of the state-of-the-art research into secure communication based on quantum cryptography, together with its assumptions, strengths and weaknesses.
Abstract: An overview is given of the state-of-the-art research into secure communication based on quantum cryptography. The present security model together with its assumptions, strengths and weaknesses is discussed. Recent experimental progress and remaining challenges are surveyed as are the latest developments in quantum hacking and countermeasures.
1,052 citations
Journal Article•
TL;DR: In this article, the authors report on quantum simulations using trapped ions to investigate quantum relativistic effects and spin systems and use them to make predictions on another quantum system under investigation.
Abstract: Quantum simulation makes use of a well controlled quantum system to make predictions on another quantum system under investigation. Here, we report on quantum simulations using trapped ions to investigate quantum relativistic effects and spin systems.
885 citations
TL;DR: In this article, the authors present Exploring the Quantum: Atoms, Cavities, and Photons by Serge Haroche, Jean-Michel Raimond 616 pp., Oxford UK, 2013.
Abstract: This article reviews Exploring the Quantum: Atoms, Cavities, and Photons. by Serge Haroche, Jean-Michel Raimond 616 pp. , Oxford UK, 2013. Price: $59.95 (paperback) ISBN 978-0-19968031-3.
868 citations
TL;DR: In this article, the authors analyze how driven quantum systems can lead to new topological states of matter, which can result from a material's intrinsic properties, or can be generated by external electromagnetic fields or mechanical deformations.
Abstract: Topological effects can result from a material's intrinsic properties, or can be generated by external electromagnetic fields or mechanical deformations. Researchers analyze how driven quantum systems can lead to new topological states of matter.
598 citations
TL;DR: A pragmatic introduction to the weak value in terms of measurable quantities is presented in this paper, along with an explanation for how it can be determined in the laboratory and its application to three distinct experimental techniques is reviewed.
Abstract: Since its introduction 25 years ago, the quantum weak value has gradually transitioned from a theoretical curiosity to a practical laboratory tool While its utility is apparent in the recent explosion of weak value experiments, its interpretation has historically been a subject of confusion Here a pragmatic introduction to the weak value in terms of measurable quantities is presented, along with an explanation for how it can be determined in the laboratory Further, its application to three distinct experimental techniques is reviewed First, as a large interaction parameter it can amplify small signals above technical background noise Second, as a measurable complex value it enables novel techniques for direct quantum state and geometric phase determination Third, as a conditioned average of generalized observable eigenvalues it provides a measurable window into nonclassical features of quantum mechanics In this selective review, a single experimental configuration to discuss and clarify each of these applications is used
550 citations
TL;DR: In this paper, the authors point out a connection between the emergence of bulk locality in AdS/CFT and the theory of quantum error correction and suggest a tensor network calculation that may settle the issue.
Abstract: We point out a connection between the emergence of bulk locality in AdS/CFT and the theory of quantum error correction. Bulk notions such as Bogoliubov transformations, location in the radial direction, and the holographic entropy bound all have natural CFT interpretations in the language of quantum error correction. We also show that the question of whether bulk operator reconstruction works only in the causal wedge or all the way to the extremal surface is related to the question of whether or not the quantum error correcting code realized by AdS/CFT is also a "quantum secret sharing scheme", and suggest a tensor network calculation that may settle the issue. Interestingly, the version of quantum error correction which is best suited to our analysis is the somewhat nonstandard "operator algebra quantum error correction" of Beny, Kempf, and Kribs. Our proposal gives a precise formulation of the idea of "subregion-subregion" duality in AdS/CFT, and clarifies the limits of its validity.
536 citations
TL;DR: Two entanglement properties that are promising for the study of the many-body localization transition are explored: the variance of the half-chainEntanglement entropy of exact eigenstates and the long time change in entanglements after a local quench from an specific eigenstate.
Abstract: Many-body localization occurs in isolated quantum systems when Anderson localization persists in the presence of finite interactions. Despite strong evidence for the existence of a many-body localization transition, a reliable extraction of the critical disorder strength is difficult due to a large drift with system size in the studied quantities. In this Letter, we explore two entanglement properties that are promising for the study of the many-body localization transition: the variance of the half-chain entanglement entropy of exact eigenstates and the long time change in entanglement after a local quench from an exact eigenstate. We investigate these quantities in a disordered quantum Ising chain and use them to estimate the critical disorder strength and its energy dependence. In addition, we analyze a spin-glass transition at large disorder strength and provide evidence for it being a separate transition. We, thereby, give numerical support for a recently proposed phase diagram of many-body localization with localization protected quantum order [Huse et al., Phys. Rev. B 88, 014206 (2013)].
515 citations
TL;DR: In this paper, the authors demonstrate extremely strong couplings using a type of multipost microwave cavity that can focus a magnetic field into submillimeter-sized samples, which can be a building block in the architecture of high-fidelity hybrid quantum systems for the processors of the future.
Abstract: Magnons are quantized quasiparticles that can in principle be used in quantum computation. To implement such computations in practice, magnons must be strongly coupled with photons, which transfer information between them. In this work, the authors demonstrate extremely strong couplings using a type of multipost microwave cavity that can focus a magnetic field into submillimeter-sized samples. This ultrastrong coupling of magnons and photons can be a building block in the architecture of high-fidelity hybrid quantum systems for the processors of the future.
474 citations
TL;DR: This work couple propagating phonons to an artificial atom in the quantum regime and reproduce findings from quantum optics, with sound taking over the role of light.
Abstract: Quantum information can be stored in micromechanical resonators, encoded as quanta of vibration known as phonons. The vibrational motion is then restricted to the stationary eigenmodes of the resonator, which thus serves as local storage for phonons. In contrast, we couple propagating phonons to an artificial atom in the quantum regime and reproduce findings from quantum optics, with sound taking over the role of light. Our results highlight the similarities between phonons and photons but also point to new opportunities arising from the characteristic features of quantum mechanical sound. The low propagation speed of phonons should enable new dynamic schemes for processing quantum information, and the short wavelength allows regimes of atomic physics to be explored that cannot be reached in photonic systems.
429 citations
TL;DR: It is proved that the second law of thermodynamics holds in this framework, and a simple protocol is given to extract the optimal amount of work from the system, equal to its change in free energy.
Abstract: Thermodynamics is traditionally concerned with systems comprised of a large number of particles. Here we present a framework for extending thermodynamics to individual quantum systems, including explicitly a thermal bath and work-storage device (essentially a 'weight' that can be raised or lowered). We prove that the second law of thermodynamics holds in our framework, and gives a simple protocol to extract the optimal amount of work from the system, equal to its change in free energy. Our results apply to any quantum system in an arbitrary initial state, in particular including non-equilibrium situations. The optimal protocol is essentially reversible, similar to classical Carnot cycles, and indeed, we show that it can be used to construct a quantum Carnot engine.
414 citations
TL;DR: In this article, the authors proposed a new hardware-efficient paradigm for universal quantum computation which is based on encoding, protecting and manipulating quantum information in a quantum harmonic oscillator, and they considered two schemes.
Abstract: We present a new hardware-efficient paradigm for universal quantum computation which is based on encoding, protecting and manipulating quantum information in a quantum harmonic oscillator. This proposal exploits multi-photon driven dissipative processes to encode quantum information in logical bases composed of Schrodinger cat states. More precisely, we consider two schemes. In a first scheme, a two-photon driven dissipative process is used to stabilize a logical qubit basis of two-component Schrodinger cat states. While such a scheme ensures a protection of the logical qubit against the photon dephasing errors, the prominent error channel of single-photon loss induces bit-flip type errors that cannot be corrected. Therefore, we consider a second scheme based on a four-photon driven dissipative process which leads to the choice of four-component Schrodinger cat states as the logical qubit. Such a logical qubit can be protected against single-photon loss by continuous photon number parity measurements. Next, applying some specific Hamiltonians, we provide a set of universal quantum gates on the encoded qubits of each of the two schemes. In particular, we illustrate how these operations can be rendered fault-tolerant with respect to various decoherence channels of participating quantum systems. Finally, we also propose experimental schemes based on quantum superconducting circuits and inspired by methods used in Josephson parametric amplification, which should allow one to achieve these driven dissipative processes along with the Hamiltonians ensuring the universal operations in an efficient manner.
TL;DR: A quantum error-correcting code in which one qubit is encoded in entangled states distributed over seven trapped-ion qubits, which represents a fully functional instance of a topologically encoded qubit, or color code, and opens a route toward fault-tolerant quantum computing.
Abstract: The construction of a quantum computer remains a fundamental scientific and technological challenge because of the influence of unavoidable noise. Quantum states and operations can be protected from errors through the use of protocols for quantum computing with faulty components. We present a quantum error-correcting code in which one qubit is encoded in entangled states distributed over seven trapped-ion qubits. The code can detect one bit flip error, one phase flip error, or a combined error of both, regardless on which of the qubits they occur. We applied sequences of gate operations on the encoded qubit to explore its computational capabilities. This seven-qubit code represents a fully functional instance of a topologically encoded qubit, or color code, and opens a route toward fault-tolerant quantum computing.
TL;DR: In this paper, a new formalism, alternative to the old thermodynamic-Bethe- ansatz-like approach, for solution of the spectral problem of planar N=4 super Yang-Mills theory is presented.
Abstract: We present a new formalism, alternative to the old thermodynamic-Bethe- ansatz-like approach, for solution of the spectral problem of planar N=4 super Yang-Mills theory. It takes a concise form of ...
Journal Article•
TL;DR: In this article, the motion of a macroscopic mechanical oscillator was entangled with a propagating electrical signal and one half of the entangled state was stored in the oscillator.
Abstract: When two physical systems share the quantum property of entanglement, measurements of one system appear to determine the state of the other. This peculiar property is used in optical, atomic, and electrical systems in an effort to exceed classical bounds when processing information. We extended the domain of this quantum resource by entangling the motion of a macroscopic mechanical oscillator with a propagating electrical signal and by storing one half of the entangled state in the mechanical oscillator. This result demonstrates an essential requirement for using compact and low-loss micromechanical oscillators in a quantum processor, can be extended to sense forces beyond the standard quantum limit, and may enable tests of quantum theory.
TL;DR: A general method is developed to extract the Fisher information, which reveals that the quantum dynamics of a classically unstable system creates quantum states that are not spin squeezed but nevertheless entangled, which quantifies metrologically useful entanglement.
Abstract: Entanglement is the key quantum resource for improving measurement sensitivity beyond classical limits. However, the production of entanglement in mesoscopic atomic systems has been limited to squeezed states, described by Gaussian statistics. Here, we report on the creation and characterization of non-Gaussian many-body entangled states. We develop a general method to extract the Fisher information, which reveals that the quantum dynamics of a classically unstable system creates quantum states that are not spin squeezed but nevertheless entangled. The extracted Fisher information quantifies metrologically useful entanglement, which we confirm by Bayesian phase estimation with sub–shot-noise sensitivity. These methods are scalable to large particle numbers and applicable directly to other quantum systems.
TL;DR: The question of whether the linearity of quantum mechanics extends into the macroscopic domain has been studied for decades as discussed by the authors, and it is an open question whether this debate may be settled by table-top experiments.
Abstract: Quantum physics has intrigued scientists and philosophers alike, because it challenges our notions of reality and locality — concepts that we have grown to rely on in our macroscopic world. It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain. Scientific progress over the past decades inspires hope that this debate may be settled by table-top experiments. Testing the limits of the quantum mechanical description of nature has become a subject of intense experimental interest. Recent advances in investigating macroscopic quantum superpositions are pushing these limits.
TL;DR: In this article, a spin-based spin-strain interaction with a single-crystal diamond resonator has been studied and quantitatively characterized with axial and transverse strain sensitivities of the ground state spin.
Abstract: The development of hybrid quantum systems is central to the advancement of emerging quantum technologies, including quantum information science and quantum-assisted sensing. The recent demonstration of high-quality single-crystal diamond resonators has led to significant interest in a hybrid system consisting of nitrogen-vacancy centre spins that interact with the resonant phonon modes of a macroscopic mechanical resonator through crystal strain. However, the nitrogen-vacancy spin-strain interaction has not been well characterized. Here, we demonstrate dynamic, strain-mediated coupling of the mechanical motion of a diamond cantilever to the spin of an embedded nitrogen-vacancy centre. Via quantum control of the spin, we quantitatively characterize the axial and transverse strain sensitivities of the nitrogen-vacancy ground-state spin. The nitrogen-vacancy centre is an atomic scale sensor and we demonstrate spin-based strain imaging with a strain sensitivity of 3 × 10(-6) strain Hz(-1/2). Finally, we show how this spin-resonator system could enable coherent spin-phonon interactions in the quantum regime.
TL;DR: In this article, two types of spin-orbit coupling are discussed: the Raman process induced coupling between spin and motion along one of the spatial directions, and the Rashba SO coupling.
Abstract: This review focuses on recent developments on studying synthetic spin-orbit (SO) coupling in ultracold atomic gases. Two types of SO coupling are discussed. One is Raman process induced coupling between spin and motion along one of the spatial directions, and the other is Rashba SO coupling. We emphasize their common features in both single-particle and two-body physics and their consequences in many-body physics. For instance, single particle ground state degeneracy leads to novel features of superfluidity and richer phase diagram; increased low-energy density-of-state enhances interaction effects; the absence of Galilean invariance and spin-momentum locking give rise to intriguing behaviors of superfluid critical velocity and novel quantum dynamics; and mixing of two-body singlet and triplet states yields novel fermion pairing structure and topological superfluids. With these examples, we show that investigating SO coupling in cold atom systems can enrich our understanding of basic phenomena such as superfluidity, provide a good platform for simulating condensed matter states such as topological superfluids, and more importantly, result in novel quantum systems such as SO coupled unitary Fermi gas or high spin quantum gases. Finally we also point out major challenges and possible future directions.
TL;DR: This work shows that, for any graph, there is always a correlation experiment such that the set of quantum probabilities is exactly the Grötschel-Lovász-Schrijver theta body, and provides a method for singling out experiments with quantum correlations on demand.
Abstract: Correlations in Bell and noncontextuality inequalities can be expressed as a positive linear combination of probabilities of events. Exclusive events can be represented as adjacent vertices of a graph, so correlations can be associated to a subgraph. We show that the maximum value of the correlations for classical, quantum, and more general theories is the independence number, the Lovasz number, and the fractional packing number of this subgraph, respectively. We also show that, for any graph, there is always a correlation experiment such that the set of quantum probabilities is exactly the Grotschel-Lovasz-Schrijver theta body. This identifies these combinatorial notions as fundamental physical objects and provides a method for singling out experiments with quantum correlations on demand.
TL;DR: Optimal control of a prototype spin qubit system consisting of two proximal nitrogen-vacancy centres in diamond is experimentally demonstrated, and nuclear spin entanglement over a length scale of 25 nm is demonstrated.
Abstract: Precise control of quantum systems is of fundamental importance in quantum information processing, quantum metrology and high-resolution spectroscopy When scaling up quantum registers, several challenges arise: individual addressing of qubits while suppressing cross-talk, entangling distant nodes and decoupling unwanted interactions Here we experimentally demonstrate optimal control of a prototype spin qubit system consisting of two proximal nitrogen-vacancy centres in diamond Using engineered microwave pulses, we demonstrate single electron spin operations with a fidelity F≈099 With additional dynamical decoupling techniques, we further realize high-quality, on-demand entangled states between two electron spins with F>082, mostly limited by the coherence time and imperfect initialization Crosstalk in a crowded spectrum and unwanted dipolar couplings are simultaneously eliminated to a high extent Finally, by high-fidelity entanglement swapping to nuclear spin quantum memory, we demonstrate nuclear spin entanglement over a length scale of 25 nm This experiment underlines the importance of optimal control for scalable room temperature spin-based quantum information devices
TL;DR: In this article, the authors provide a brief introduction to quantum thermalization, paying particular attention to the ''Eigenstate Thermalization Hypothesis' (ETH), and the resulting ''single-eigenstate statistical mechanics''.
Abstract: We review some recent developments in the statistical mechanics of isolated quantum systems. We provide a brief introduction to quantum thermalization, paying particular attention to the `Eigenstate Thermalization Hypothesis' (ETH), and the resulting `single-eigenstate statistical mechanics'. We then focus on a class of systems which fail to quantum thermalize and whose eigenstates violate the ETH: These are the many-body Anderson localized systems; their long-time properties are not captured by the conventional ensembles of quantum statistical mechanics. These systems can locally remember forever information about their local initial conditions, and are thus of interest for possibilities of storing quantum information. We discuss key features of many-body localization (MBL), and review a phenomenology of the MBL phase. Single-eigenstate statistical mechanics within the MBL phase reveals dynamically-stable ordered phases, and phase transitions among them, that are invisible to equilibrium statistical mechanics and can occur at high energy and low spatial dimensionality where equilibrium ordering is forbidden.
TL;DR: In this paper, a new experiment using matter-wave interferometry confirms that different atoms free fall in gravity at the same rate in the same way as other atoms in the universe.
Abstract: A new experiment using matter-wave interferometry confirms that different atoms free fall in gravity at the same rate.
Book•
14 Mar 2014
TL;DR: In this article, the authors discuss the real world and quantum mechanics as a stochastic theory, including the zeropoint radiation field and the harmonic oscillator, and the wave properties of matter.
Abstract: Preface. Part I: Prelude. 1. Quantum mechanics and the real world. 2. Quantum mechanics as a stochastic theory. 3. Elements of electrodynamics. Part II: Theme. 4. The zeropoint radiation field. 5. The equilibrium radiation field. 6. Environmental effects through the zero-point field. 7. The harmonic oscillator. 8. Quantum properties of other simple systems. 9. Breakdown of detailed energy balance. Part III: Coda. 10. Linear stochastic electrodynamics. 11. Radiative corrections in linear SED. 12. The wave properties of matter. 13. Stochastic optics. 14. Outlook and some corollaries. Bibliography. Index.
TL;DR: In this article, a resource theory analogous to the theory of entanglement has been developed for fault-tolerant stabilizer computation and two quantitative measures for the amount of non-stabilizer resources are introduced.
Abstract: Recent results on the non-universality of fault-tolerant gate sets underline the critical role of resource states, such as magic states, to power scalable, universal quantum computation. Here we develop a resource theory, analogous to the theory of entanglement, that is relevant for fault-tolerant stabilizer computation. We introduce two quantitative measures?monotones?for the amount of non-stabilizer resource. As an application we give absolute bounds on the efficiency of magic state distillation. One of these monotones is the sum of the negative entries of the discrete Wigner representation of a quantum state, thereby resolving a long-standing open question of whether the degree of negativity in a quasi-probability representation is an operationally meaningful indicator of quantum behavior.
TL;DR: A review of quantum mechanical and optical pseudo-Hermitian systems with an emphasis on PT-symmetric systems important for optics and electrodynamics is given in this article.
Abstract: We review quantum mechanical and optical pseudo- Hermitian systems with an emphasis on PT-symmetric systems important for optics and electrodynamics. One of the most
TL;DR: In this paper, an effective Hamiltonian was derived for the conduction band of monolayer transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields.
Abstract: We derive an effective Hamiltonian that describes the dynamics of electrons in the conduction band of monolayer transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields. We discuss in detail both the intrinsic and the Bychkov-Rashba spin-orbit coupling induced by an external electric field. We point out interesting differences in the spin-split conduction band between different TMDC compounds. An important consequence of the strong intrinsic spin-orbit coupling is an effective out-of-plane g factor for the electrons that differs from the free-electron g factor g~=2. We identify a new term in the Hamiltonian of the Bychkov-Rashba spin-orbit coupling that does not exist in III-V semiconductors. Using first-principles calculations, we give estimates of the various parameters appearing in the theory. Finally, we consider quantum dots formed in TMDC materials and derive an effective Hamiltonian that allows us to calculate the magnetic field dependence of the bound states in the quantum dots. We find that all states are both valley and spin split, which suggests that these quantum dots could be used as valley-spin filters. We explore the possibility of using spin and valley states in TMDCs as quantum bits, and conclude that, due to the relatively strong intrinsic spin-orbit splitting in the conduction band, the most realistic option appears to be a combined spin-valley (Kramers) qubit at low magnetic fields.
TL;DR: Very long quantum coherence times for a transition metal complex of 68 μs at low temperature (qubit figure of merit QM=3,400) and 1 μs at room temperature are reported, much higher than previously reported values for such systems.
Abstract: The successful development of a quantum computer would change the world, and current internet encryption methods would cease to function. However, no working quantum computer that even begins to rival conventional computers has been developed yet, which is due to the lack of suitable quantum bits. A key characteristic of a quantum bit is the coherence time. Transition metal complexes are very promising quantum bits, owing to their facile surface deposition and their chemical tunability. However, reported quantum coherence times have been unimpressive. Here we report very long quantum coherence times for a transition metal complex of 68 μs at low temperature (qubit figure of merit QM=3,400) and 1 μs at room temperature, much higher than previously reported values for such systems. We show that this achievement is because of the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion.
TL;DR: In this article, a series of architecturally scalable quantum annealing (QA) processors consisting of networks of manufactured interacting spins (qubits) were built and the energy eigen spectrum of two-and eight-qubit systems within one such processor was measured.
Abstract: : Entanglement lies at the core of quantum algorithms designed to solve problems that are intractable by classical approaches. One such algorithm, quantum annealing (QA), provides a promising path to a practical quantum processor. We have built a series of architecturally scalable QA processors consisting of networks of manufactured interacting spins (qubits). Here, we use qubit tunneling spectroscopy to measure the energy eigen spectrum of two- and eight-qubit systems within one such processor, demonstrating quantum coherence in these systems. We present experimental evidence that, during a critical portion of QA, the qubits become entangled and entanglement persists even as these systems reach equilibrium with a thermal environment. Our results provide an encouraging sign that QA is a viable technology for large scale quantum computing.
TL;DR: In this article, the same authors describe quantum criticality in insulators in which the electric moments are fluctuating over a wider temperature range than in quantum critical metals, and show that these too can be described by the same framework.
Abstract: Quantum criticality is often found in metallic compounds that are close to being magnetic. What about insulators in which the electric moments are fluctuating? These too can be described by the same framework—over a wider temperature range than in quantum critical metals.
TL;DR: This review discusses how quantum coherence manifests in photosynthetic light harvesting and its implications, and examines the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of Quantum coherence in light harvesting.
Abstract: Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.