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Showing papers on "Qubit published in 2007"


Journal ArticleDOI
22 Feb 2007-Nature
TL;DR: Observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum.
Abstract: Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot-cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.

1,679 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed spin qubits in graphene quantum dots and showed that in an array of many qubits it is possible to couple any two of them via Heisenberg exchange with the others being decoupled by detuning.
Abstract: The main characteristics of good qubits are long coherence times in combination with fast operating times. It is well known that carbon-based materials could increase the coherence times of spin qubits, which are among the most developed solid-state qubits. Here, we propose how to form spin qubits in graphene quantum dots. A crucial requirement to achieve this goal is to find quantum-dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with armchair boundaries. The most remarkable new feature of the proposed spin qubits is that, in an array of many qubits, it is possible to couple any two of them via Heisenberg exchange with the others being decoupled by detuning. This unique feature is a direct consequence of the quasi-relativistic spectrum of graphene.

962 citations



Journal ArticleDOI
27 Apr 2007-Science
TL;DR: It is shown that, even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear, this “sudden death” constitutes yet another distinct and counterintuitive trait of entangled systems.
Abstract: We demonstrate the difference between local, single-particle dynamics and global dynamics of entangled quantum systems coupled to independent environments. Using an all-optical experimental setup, we showed that, even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear. This "sudden death" constitutes yet another distinct and counterintuitive trait of entanglement.

792 citations


Journal ArticleDOI
01 Feb 2007-Nature
TL;DR: A circuit QED experiment is reported in the strong dispersive limit, a new regime where a single photon has a large effect on the qubit without ever being absorbed, the basis of a logic bus for a quantum computer.
Abstract: Electromagnetic signals are always composed of photons, although in the circuit domain those signals are carried as voltages and currents on wires, and the discreteness of the photon's energy is usually not evident. However, by coupling a superconducting quantum bit (qubit) to signals on a microwave transmission line, it is possible to construct an integrated circuit in which the presence or absence of even a single photon can have a dramatic effect. Such a system can be described by circuit quantum electrodynamics (QED)-the circuit equivalent of cavity QED, where photons interact with atoms or quantum dots. Previously, circuit QED devices were shown to reach the resonant strong coupling regime, where a single qubit could absorb and re-emit a single photon many times. Here we report a circuit QED experiment in the strong dispersive limit, a new regime where a single photon has a large effect on the qubit without ever being absorbed. The hallmark of this strong dispersive regime is that the qubit transition energy can be resolved into a separate spectral line for each photon number state of the microwave field. The strength of each line is a measure of the probability of finding the corresponding photon number in the cavity. This effect is used to distinguish between coherent and thermal fields, and could be used to create a photon statistics analyser. As no photons are absorbed by this process, it should be possible to generate non-classical states of light by measurement and perform qubit-photon conditional logic, the basis of a logic bus for a quantum computer.

782 citations


Journal ArticleDOI
TL;DR: Wallraff et al. as mentioned in this paper theoretically studied single and two-qubit dynamics in the circuit QED architecture, in which superconducting charge qubits are capacitively coupled to a single high-Q$ super-conducting coplanar resonator.
Abstract: We theoretically study single and two-qubit dynamics in the circuit QED architecture. We focus on the current experimental design [Wallraff et al., Nature (London) 431, 162 (2004); Schuster et al., Nature (London) 445, 515 (2007)] in which superconducting charge qubits are capacitively coupled to a single high-$Q$ superconducting coplanar resonator. In this system, logical gates are realized by driving the resonator with microwave fields. Advantages of this architecture are that it allows for multiqubit gates between non-nearest qubits and for the realization of gates in parallel, opening the possibility of fault-tolerant quantum computation with superconduting circuits. In this paper, we focus on one- and two-qubit gates that do not require moving away from the charge-degeneracy ``sweet spot.'' This is advantageous as it helps to increase the qubit dephasing time and does not require modification of the original circuit QED. However, these gates can, in some cases, be slower than those that do not use this constraint. Five types of two-qubit gates are discussed, these include gates based on virtual photons, real excitation of the resonator, and a gate based on the geometric phase. We also point out the importance of selection rules when working at the charge degeneracy point.

632 citations


Journal ArticleDOI
TL;DR: An efficient quantum algorithm for simulating the evolution of a quantum state for a sparse Hamiltonian H over a given time t is presented in terms of a procedure for computing the matrix entries of H.
Abstract: We present an efficient quantum algorithm for simulating the evolution of a quantum state for a sparse Hamiltonian H over a given time t in terms of a procedure for computing the matrix entries of H. In particular, when H acts on n qubits, has at most a constant number of nonzero entries in each row/column, and ||H|| is bounded by a constant, we may select any positive integer k such that the simulation requires O((log*n)t1+1/2k) accesses to matrix entries of H. We also show that the temporal scaling cannot be significantly improved beyond this, because sublinear time scaling is not possible.

626 citations


Journal ArticleDOI
16 Aug 2007-Nature
TL;DR: A protocol is demonstrated that allows the generation of arbitrarily large squeezed Schrödinger cat states, using homodyne detection and photon number states as resources, and clearly exhibits several quantum phase-space interference fringes between the ‘dead’ and ‘alive’ components.
Abstract: Schrodinger's cat is a Gedankenexperiment in quantum physics, in which an atomic decay triggers the death of the cat. Because quantum physics allow atoms to remain in superpositions of states, the classical cat would then be simultaneously dead and alive. By analogy, a 'cat' state of freely propagating light can be defined as a quantum superposition of well separated quasi-classical states-it is a classical light wave that simultaneously possesses two opposite phases. Such states play an important role in fundamental tests of quantum theory and in many quantum information processing tasks, including quantum computation, quantum teleportation and precision measurements. Recently, optical Schrodinger 'kittens' were prepared; however, they are too small for most of the aforementioned applications and increasing their size is experimentally challenging. Here we demonstrate, theoretically and experimentally, a protocol that allows the generation of arbitrarily large squeezed Schrodinger cat states, using homodyne detection and photon number states as resources. We implemented this protocol with light pulses containing two photons, producing a squeezed Schrodinger cat state with a negative Wigner function. This state clearly exhibits several quantum phase-space interference fringes between the 'dead' and 'alive' components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory.

608 citations


Journal ArticleDOI
TL;DR: The Nextnano simulator as discussed by the authors is a simulation tool for semiconductor nanodevice simulation that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nano-structures.
Abstract: nextnano is a semiconductor nanodevice simulation tool that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nanostructures. The underlying idea is to provide a robust and generic framework for modeling device applications in the field of nanosized semiconductor heterostructures. The simulator deals with realistic geometries and almost any relevant combination of materials in one, two, and three spatial dimensions. It focuses on an accurate and reliable treatment of quantum mechanical effects and provides a self-consistent solution of the Schrodinger, Poisson, and current equations. Exchange-correlation effects are taken into account in terms of the local density scheme. The electronic structure is represented within the single-band or multiband kldrp envelope function approximation, including strain. The code is not intended to be a ldquoblack boxrdquo tool. It requires a good understanding of quantum mechanics. The input language provides a number of tools that simplify setting up device geometry or running repetitive tasks. In this paper, we present a brief overview of nextnano and present four examples that demonstrate the wide range of possible applications for this software in the fields of solid-state quantum computation, nanoelectronics, and optoelectronics, namely, 1) a realization of a qubit based on coupled quantum wires in a magnetic field, 2) and 3) carrier transport in two different nano-MOSFET devices, and 4) a quantum cascade laser.

571 citations


Journal ArticleDOI
TL;DR: A general strategy to maintain the coherence of a quantum bit is proposed based on an optimized pi-pulse sequence for dynamic decoupling extending the Carr-Purcell-Meiboom-Gill cycle.
Abstract: A general strategy to maintain the coherence of a quantum bit is proposed. The analytical result is derived rigorously including all memory and backaction effects. It is based on an optimized $\ensuremath{\pi}$-pulse sequence for dynamic decoupling extending the Carr-Purcell-Meiboom-Gill cycle. The optimized sequence is very efficient, in particular, for strong couplings to the environment.

562 citations


01 Mar 2007
TL;DR: In this article, the authors describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement, since it involves long sequences of randomly chosen gates, and also verifies that error behavior is stable when used in long computations.
Abstract: A key requirement for scalable quantum computing is that elementary quantum gates can be implemented with sufficiently low error. One method for determining the error behavior of a gate implementation is to perform process tomography. However, standard process tomography is limited by errors in state preparation, measurement and one-qubit gates. It suffers from inefficient scaling with number of qubits and does not detect adverse error-compounding when gates are composed in long sequences. An additional problem is due to the fact that desirable error probabilities for scalable quantum computing are of the order of 0.0001 or lower. Experimentally proving such low errors is challenging. We describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement. Since it involves long sequences of randomly chosen gates, it also verifies that error behavior is stable when used in long computations. We implemented randomized benchmarking on trapped atomic ion qubits, establishing a one-qubit error probability per randomized $\ensuremath{\pi}/2$ pulse of 0.00482(17) in a particular experiment. We expect this error probability to be readily improved with straightforward technical modifications.

Journal ArticleDOI
TL;DR: In this article, the use of trapped ytterbium ions as quantum bits for quantum information processing was demonstrated and the high efficiency and high fidelity of these operations was achieved through the stabilization and frequency modulation of relevant laser sources.
Abstract: We demonstrate the use of trapped ytterbium ions as quantum bits for quantum information processing. We implement fast, efficient state preparation and state detection of the first-order magnetic field-insensitive hyperfine levels of $^{171}\mathrm{Yb}^{+}$, with a measured coherence time of $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. The high efficiency and high fidelity of these operations is accomplished through the stabilization and frequency modulation of relevant laser sources.

Journal ArticleDOI
27 Jul 2007-Science
TL;DR: This work reports on the realization of an atom-photon quantum interface based on an optical cavity, using it to entangle a single atom with a single photon and then to map the quantum state of the atom onto a second single photon.
Abstract: A major challenge for a scalable quantum computing architecture is the faithful transfer of information from one node to another. We report on the realization of an atom-photon quantum interface based on an optical cavity, using it to entangle a single atom with a single photon and then to map the quantum state of the atom onto a second single photon. The latter step disentangles the atom from the light and produces an entangled photon pair. Our scheme is intrinsically deterministic and establishes the basic element required to realize a distributed quantum network with individual atoms at rest as quantum memories and single flying photons as quantum messengers.

Journal ArticleDOI
TL;DR: A spin chain is a permanently coupled 1D system of spins as discussed by the authors, which can be used to connect quantum registers without resorting to optics, and it has been shown that it is possible to achieve perfect quantum state transfer through spin chains.
Abstract: We present an introductory overview of the use of spin chains as quantum wires, which has recently developed into a topic of lively interest. The principal motivation is in connecting quantum registers without resorting to optics. A spin chain is a permanently coupled 1D system of spins. When one places a quantum state on one end of it, the state will be dynamically transmitted to the other end with some efficiency if the spins are coupled by an exchange interaction. No external modulations or measurements on the body of the chain, except perhaps at the very ends, is required for this purpose. For the simplest (uniformly coupled) chain and the simplest encoding (single qubit encoding), however, dispersion reduces the quality of transfer. We present a variety of alternatives proposed by various groups to achieve perfect quantum state transfer through spin chains. We conclude with a brief discussion of the various directions in which the topic is developing.

Journal ArticleDOI
20 Sep 2007-Nature
TL;DR: An on-chip, on-demand single-photon source, where the microwave photons are injected into a wire with high efficiency and spectral purity is demonstrated, accomplished in a circuit quantum electrodynamics architecture that enhances the spontaneous emission of a single superconducting qubit.
Abstract: Microwaves have widespread use in classical communication technologies, from long-distance broadcasts to short-distance signals within a computer chip. Like all forms of light, microwaves, even those guided by the wires of an integrated circuit, consist of discrete photons. To enable quantum communication between distant parts of a quantum computer, the signals must also be quantum, consisting of single photons, for example. However, conventional sources can generate only classical light, not single photons. One way to realize a single-photon source is to collect the fluorescence of a single atom. Early experiments measured the quantum nature of continuous radiation, and further advances allowed triggered sources of photons on demand. To allow efficient photon collection, emitters are typically placed inside optical or microwave cavities, but these sources are difficult to employ for quantum communication on wires within an integrated circuit. Here we demonstrate an on-chip, on-demand single-photon source, where the microwave photons are injected into a wire with high efficiency and spectral purity. This is accomplished in a circuit quantum electrodynamics architecture, with a microwave transmission line cavity that enhances the spontaneous emission of a single superconducting qubit. When the qubit spontaneously emits, the generated photon acts as a flying qubit, transmitting the quantum information across a chip. We perform tomography of both the qubit and the emitted photons, clearly showing that both the quantum phase and amplitude are transferred during the emission. Both the average power and voltage of the photon source are characterized to verify performance of the system. This single-photon source is an important addition to a rapidly growing toolbox for quantum optics on a chip.

Journal ArticleDOI
26 Jul 2007-Nature
TL;DR: This experiment uses an optical lattice of double-well potentials to isolate and manipulate arrays of paired 87Rb atoms, inducing controlled entangling interactions within each pair, and demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits), which forms a set of universal gates for quantum computation.
Abstract: Controlled two-particle interaction is a fundamental requirement for quantum computing, and achieving it has long been a goal for research on neutral atom systems. Anderlini et al. have used a system, consisting of arrays of paired ultracold rubidium-87 atoms in an optical lattice of double-well potentials, to induce controlled entangling interactions within each atom pair. Repeated interchange of spin between atoms occupying different vibrational levels occurs with a coherence time of more than ten milliseconds. This demonstrates an essential component of a quantum gate. An optical lattice of double-well potentials is used to isolate and manipulate arrays of paired 87Rb atoms, inducing controlled entangling interactions within each pair. Repeated interchange of spin between atoms occupying different vibrational levels occurs with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a quantum gate important for quantum computation. Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms1 confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials2,3 to isolate and manipulate arrays of paired 87Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling4 in a system of neutral atoms5. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its ‘half-implementation’, the gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation4.

Journal ArticleDOI
04 May 2007-Science
TL;DR: This work reports on the time-domain tunable coupling of optimally biased superconducting flux qubits by modulating the nonlinear inductance of an additional coupling element and parametrically induced a two-qubit transition that was otherwise forbidden.
Abstract: To do large-scale quantum information processing, it is necessary to control the interactions between individual qubits while retaining quantum coherence. To this end, superconducting circuits allow for a high degree of flexibility. We report on the time-domain tunable coupling of optimally biased superconducting flux qubits. By modulating the nonlinear inductance of an additional coupling element, we parametrically induced a two-qubit transition that was otherwise forbidden. We observed an on/off coupling ratio of 19 and were able to demonstrate a simple quantum protocol.

Journal ArticleDOI
TL;DR: This work proposes to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core, and two-qubit gates and qubit readout can be implemented.
Abstract: Spin qubits offer one of the most promising routes to the implementation of quantum computers. Very recent results in semiconductor quantum dots show that electrically-controlled gating schemes are particularly well-suited for the realization of a universal set of quantum logical gates. Scalability to a larger number of qubits, however, remains an issue for such semiconductor quantum dots. In contrast, a chemical bottom-up approach allows one to produce identical units in which localized spins represent the qubits. Molecular magnetism has produced a wide range of systems with properties that can be tailored, but so far, there have been no molecules in which the spin state can be controlled by an electrical gate. Here we propose to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core. Through electrical manipulation of the molecular redox potential, the charge of the core can be changed. With this setup, two-qubit gates and qubit readout can be implemented.

Journal ArticleDOI
21 Dec 2007-Science
TL;DR: This work demonstrated the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit; it manipulated the qubit geometrically by means of microwave radiation and observed the accumulated phase in an interference experiment, finding excellent agreement with Berry's predictions.
Abstract: In quantum information science, the phase of a wave function plays an important role in encoding information. Although most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance. We demonstrated the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit; we manipulated the qubit geometrically by means of microwave radiation and observed the accumulated phase in an interference experiment. We found excellent agreement with Berry's predictions and also observed a geometry-dependent contribution to dephasing.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate trapping and imaging of 250 single atoms in a three-dimensional optical lattice and show that imaging is highly unlikely to change the pattern of site occupancy, which in combination with reproducible imaging should allow verifiable filling of vacancies, execution of site-specific quantum gates and measurement of each atom's final quantum state.
Abstract: Asingle neutral atom trapped by light is a promising qubit. It has weak, well-understood interactions with the environment, its internal state can be precisely manipulated1, interactions that entangle atoms can be varied from negligible to strong2,3,4 and many single atoms can be trapped near each other in an optical lattice5. This collection of features would allow for a relatively large quantum computer6 if each neutral atom qubit could be independently detected and addressed7,8,9,10. A quantum computer with even 50 qubits would allow quantum simulations that are out of the reach of classical computers11,12. So far, fewer than ten single atoms have been simultaneously imaged13. Here we demonstrate trapping and imaging of 250 single atoms in a three-dimensional optical lattice and show that imaging is highly unlikely to change the pattern of site occupancy. Our lattice spacing is large enough that, in principle, individual atoms can be addressed, which in combination with reproducible imaging should allow for verifiable filling of vacancies, execution of site-specific quantum gates and measurement of each atom’s final quantum state14,15. The lattice we use can readily be scaled to include thousands of trapped atoms.

Journal ArticleDOI
TL;DR: A double quantum dot based on Ge/Si nanowires in which one can completely control the coupling between the dots and to the leads and it is demonstrated that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot.
Abstract: One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.

Journal ArticleDOI
04 Oct 2007-Nature
TL;DR: This work demonstrates a lasing effect with a single artificial atom—a Josephson-junction charge qubit—embedded in a superconducting resonator.
Abstract: A lasing effect with a single artificial atom (a Josephson-junction charge qubit) that is embedded in a superconducting resonator is demonstrated, making use of the property that such artificial atoms are strongly and controllably coupled to resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons. Solid-state superconducting circuits1,2,3 are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments4,5,6,7,8 it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom—a Josephson-junction charge qubit9—embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons.

Journal ArticleDOI
TL;DR: In this paper, an additional capacitor shunted in parallel to the smaller Josephson junction (JJ) in the loop was proposed to improve the dephasing performance of flux qubits.
Abstract: A flux qubit can have a relatively long decoherence time at the degeneracy point, but away from this point the decoherence time is greatly reduced by dephasing. This limits the practical applications of flux qubits. Here we propose a qubit design modified from the commonly used flux qubit by introducing an additional capacitor shunted in parallel to the smaller Josephson junction (JJ) in the loop. Our results show that the effects of noise can be considerably suppressed, particularly away from the degeneracy point, by both reducing the coupling energy of the JJ and increasing the shunt capacitance. This shunt capacitance provides a novel way to improve the qubit.

Journal ArticleDOI
TL;DR: In spite of the presence of gapless propagating Majorana fermion excitations, dynamical two spin correlation functions are identically zero beyond nearest neighbor separation, which shows existence of a gapless but short range spin liquid.
Abstract: We present certain exact analytical results for dynamical spin correlation functions in the Kitaev Model. It is the first result of its kind in nontrivial quantum spin models. The result is also novel: in spite of the presence of gapless propagating Majorana fermion excitations, dynamical two spin correlation functions are identically zero beyond nearest neighbor separation. This shows existence of a gapless but short range spin liquid. An unusual, all energy scale fractionalization of a spin-flip quanta, into two infinitely massive $\ensuremath{\pi}$ fluxes and a dynamical Majorana fermion, is shown to occur. As the Kitaev Model exemplifies topological quantum computation, our result presents new insights into qubit dynamics and generation of topological excitations.

Journal ArticleDOI
TL;DR: In this article, the Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction and entanglement teleportation when using two independent Heisenberg chains as the quantum channel are investigated.
Abstract: Thermal entanglement of a two-qubit Heisenberg chain in the presence of the Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction and entanglement teleportation when using two independent Heisenberg chains as the quantum channel are investigated. It is found that the DM interaction can excite entanglement and teleportation fidelity. The output entanglement increases linearly with increasing value of the input; its dependences on the temperature, DM interaction, and spin coupling constant are given in detail. Entanglement teleportation will be better realized via an antiferromagnetic spin chain when the DM interaction is turned off and the temperature is low. However, the introduction of the DM interaction can cause the ferromagnetic spin chain to be a better quantum channel for teleportation. A minimal entanglement of the thermal state in the model is needed to realize the entanglement teleportation regardless of whether the spin chains are antiferromagnetic or ferromagnetic.

Journal ArticleDOI
TL;DR: This work demonstrates fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin by exploiting the spontaneous decay rate of the excited state.
Abstract: Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order ${10}^{9}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, which is set by the spontaneous decay rate of the excited state.

Journal ArticleDOI
TL;DR: In this paper, it was shown that correlations are present in large amounts in the DQC1 circuit, as measured through the operator Schmidt rank, and this provides evidence for the preclusion of efficient classical simulation by means of a whole class of classical simulation algorithms.
Abstract: In a quantum computation with pure states, the generation of large amounts of entanglement is known to be necessary for a speedup with respect to classical computations. However, examples of quantum computations with mixed states are known, such as the deterministic computation with one quantum qubit (DQC1) model [Knill and Laflamme, Phys. Rev. Lett. 81, 5672 (1998)], in which entanglement is at most marginally present, and yet a computational speedup is believed to occur. Correlations, and not entanglement, have been identified as a necessary ingredient for mixed-state quantum computation speedups. Here we show that correlations, as measured through the operator Schmidt rank, are indeed present in large amounts in the DQC1 circuit. This provides evidence for the preclusion of efficient classical simulation of DQC1 by means of a whole class of classical simulation algorithms, thereby reinforcing the conjecture that DQC1 leads to a genuine quantum computational speedup.

Journal ArticleDOI
TL;DR: In this article, the hysteretic behavior of a coupled nonlinear resonator has been investigated in the context of quantum non-demolition (QND) measurements on a superconducting flux qubit.
Abstract: In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A quantum non-demolition (QND) measurement minimizes this disturbance by using a particular system—detector interaction that preserves the eigenstates of a suitable operator of the quantum system. This leads to an ideal projective measurement. We present experiments in which we carry out two consecutive measurements on a quantum two-level system, a superconducting flux qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator. The large correlation between the results of the two measurements demonstrates the QND nature of the readout method. The fact that a QND measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum-information processing for protocols such as state preparation and error correction.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the small valley splittings observed in previous experiments on Si-SiGe heterostructures result from atomic steps at the quantum-well interface.
Abstract: Silicon has many attractive properties for quantum computing, and the quantum-dot architecture is appealing because of its controllability and scalability. However, the multiple valleys in the silicon conduction band are potentially a serious source of decoherence for spin-based quantum-dot qubits. Only when a large energy splits these valleys do we obtain well-defined and long-lived spin states appropriate for quantum computing. Here, we show that the small valley splittings observed in previous experiments on Si–SiGe heterostructures result from atomic steps at the quantum-well interface. Lateral confinement in a quantum point contact limits the electron wavefunctions to several steps, and enhances the valley splitting substantially, up to 1.5 meV. The combination of electrostatic and magnetic confinement produces a valley splitting larger than the spin splitting, which is controllable over a wide range. These results improve the outlook for realizing spin qubits with long coherence times in silicon-based devices.

Journal ArticleDOI
TL;DR: The recently predicted two-dimensional "weak-pairing" px + ipy superfluid state of fermionic cold atoms is used as a platform for topological quantum computation and realistic schemes suitable for atomic superfluids are proposed.
Abstract: We propose to use the recently predicted two-dimensional "weak-pairing" px + ipy superfluid state of fermionic cold atoms as a platform for topological quantum computation. In the core of a vortex, this state supports a zero-energy Majorana mode, which moves to finite energy in the corresponding topologically trivial "strong-pairing" state. By braiding vortices in the "weak-pairing" state, unitary quantum gates can be applied to the Hilbert space of Majorana zero modes. For readout of the topological qubits, we propose realistic schemes suitable for atomic superfluids.