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Quantum error correction is required for a practical quantum computer because of the fragile nature of quantum information. In quantum error correction, information is redundantly stored in a large quantum state space and one or more observables must be monitored to reveal the occurrence of an error, without disturbing the information encoded in an unknown quantum state. Such observables, typically multi-quantum-bit parities, must correspond to a special symmetry property inherent in the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition way (projecting without further perturbing the state) and more quickly than errors occur. Previously, quantum non-demolition measurements of quantum jumps between states of well-defined energy have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics, nitrogen-vacancy centres and superconducting quantum bits. So far, however, no fast and repeated monitoring of an error syndrome has been achieved. Here we track the quantum jumps of a possible error syndrome, namely the photon number parity of a microwave cavity, by mapping this property onto an ancilla quantum bit, whose only role is to facilitate quantum state manipulation and measurement. This quantity is just the error syndrome required in a recently proposed scheme for a hardware-efficient protected quantum memory using Schrödinger cat states (quantum superpositions of different coherent states of light) in a harmonic oscillator. We demonstrate the projective nature of this measurement onto a region of state space with well-defined parity by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared with the cavity lifetime, has a high single-shot fidelity and has a 99.8 per cent probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier, the quantum non-demolition parity tracking that we demonstrate represents an important step towards implementing an active system that extends the lifetime of a quantum bit. [ABSTRACT FROM AUTHOR]

Quantum error correction codes are designed to protect an arbitrary state of a multi-qubit register from decoherence-induced errors, but their implementation is an outstanding challenge in the development of large-scale quantum computers. The first step is to stabilize a non-equilibrium state of a simple quantum system, such as a quantum bit (qubit) or a cavity mode, in the presence of decoherence. This has recently been accomplished using measurement-based feedback schemes. The next step is to prepare and stabilize a state of a composite system. Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result is achieved using an autonomous feedback scheme that combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative reservoir. Similar autonomous feedback techniques have been used for qubit reset, single-qubit state stabilization, and the creation and stabilization of states of multipartite quantum systems. Unlike conventional, measurement-based schemes, the autonomous approach uses engineered dissipation to counteract decoherence, obviating the need for a complicated external feedback loop to correct errors. Instead, the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building block for quantum information processing. Such autonomous schemes, which are broadly applicable to a variety of physical systems, as demonstrated by the accompanying paper on trapped ion qubits, will be an essential tool for the implementation of quantum error correction. [ABSTRACT FROM AUTHOR]

A single atom is the prototypical quantum system, and a natural candidate for a quantum bit, or qubit-the elementary unit of a quantum computer. Atoms have been successfully used to store and process quantum information in electromagnetic traps, as well as in diamond through the use of the nitrogen-vacancy-centre point defect. Solid-state electrical devices possess great potential to scale up such demonstrations from few-qubit control to larger-scale quantum processors. Coherent control of spin qubits has been achieved in lithographically defined double quantum dots in both GaAs (refs 3-5) and Si (ref. 6). However, it is a formidable challenge to combine the electrical measurement capabilities of engineered nanostructures with the benefits inherent in atomic spin qubits. Here we demonstrate the coherent manipulation of an individual electron spin qubit bound to a phosphorus donor atom in natural silicon, measured electrically via single-shot read-out. We use electron spin resonance to drive Rabi oscillations, and a Hahn echo pulse sequence reveals a spin coherence time exceeding 200?µs. This time should be even longer in isotopically enriched 28Si samples. Combined with a device architecture that is compatible with modern integrated circuit technology, the electron spin of a single phosphorus atom in silicon should be an excellent platform on which to build a scalable quantum computer. [ABSTRACT FROM AUTHOR]

The quantum internet is predicted to be the next-generation information processing platform, promising secure communication and an exponential speed-up in distributed computation. The distribution of single qubits over large distances via quantum teleportation is a key ingredient for realizing such a global platform. By using quantum teleportation, unknown quantum states can be transferred over arbitrary distances to a party whose location is unknown. Since the first experimental demonstrations of quantum teleportation of independent external qubits, an internal qubit and squeezed states, researchers have progressively extended the communication distance. Usually this occurs without active feed-forward of the classical Bell-state measurement result, which is an essential ingredient in future applications such as communication between quantum computers. The benchmark for a global quantum internet is quantum teleportation of independent qubits over a free-space link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feed-forward in real time. The experiment uses two free-space optical links, quantum and classical, over 143?kilometres between the two Canary Islands of La Palma and Tenerife. To achieve this, we combine advanced techniques involving a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors and entanglement-assisted clock synchronization. The average teleported state fidelity is well beyond the classical limit of two-thirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feed-forward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in real-world scenarios, in particular for future satellite-based quantum teleportation. [ABSTRACT FROM AUTHOR]

Quantum computers could be used to solve certain problems exponentially faster than classical computers, but are challenging to build because of their increased susceptibility to errors. However, it is possible to detect and correct errors without destroying coherence, by using quantum error correcting codes. The simplest of these are three-quantum-bit (three-qubit) codes, which map a one-qubit state to an entangled three-qubit state; they can correct any single phase-flip or bit-flip error on one of the three qubits, depending on the code used. Here we demonstrate such phase- and bit-flip error correcting codes in a superconducting circuit. We encode a quantum state, induce errors on the qubits and decode the error syndrome-a quantum state indicating which error has occurred-by reversing the encoding process. This syndrome is then used as the input to a three-qubit gate that corrects the primary qubit if it was flipped. As the code can recover from a single error on any qubit, the fidelity of this process should decrease only quadratically with error probability. We implement the correcting three-qubit gate (known as a conditional-conditional NOT, or Toffoli, gate) in 63 nanoseconds, using an interaction with the third excited state of a single qubit. We find 85?±?1 per cent fidelity to the expected classical action of this gate, and 78?±?1 per cent fidelity to the ideal quantum process matrix. Using this gate, we perform a single pass of both quantum bit- and phase-flip error correction and demonstrate the predicted first-order insensitivity to errors. Concatenation of these two codes in a nine-qubit device would correct arbitrary single-qubit errors. In combination with recent advances in superconducting qubit coherence times, this could lead to scalable quantum technology. [ABSTRACT FROM AUTHOR]

Quantum computers hold the promise of massive performance enhancements across a range of applications, from cryptography and databases to revolutionary scientific simulation tools. Such computers would make use of the same quantum mechanical phenomena that pose limitations on the continued shrinking of conventional information processing devices. Many of the key requirements for quantum computing differ markedly from those of conventional computers. However, silicon, which plays a central part in conventional information processing, has many properties that make it a superb platform around which to build a quantum computer. [ABSTRACT FROM AUTHOR]

Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement and in the production of the highest-quality entanglement reported so far. However, current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed, single photons, and linear optics gates are inherently probabilistic. Here we introduce a deterministic process-coherent photon conversion (CPC)-that provides a new way to generate and process complex, multiquanta states for photonic quantum information applications. The technique uses classically pumped nonlinearities to induce coherent oscillations between orthogonal states of multiple quantum excitations. One example of CPC, based on a pumped four-wave-mixing interaction, is shown to yield a single, versatile process that provides a full set of photonic quantum processing tools. This set satisfies the DiVincenzo criteria for a scalable quantum computing architecture, including deterministic multiqubit entanglement gates (based on a novel form of photon-photon interaction), high-quality heralded single- and multiphoton states free from higher-order imperfections, and robust, high-efficiency detection. It can also be used to produce heralded multiphoton entanglement, create optically switchable quantum circuits and implement an improved form of down-conversion with reduced higher-order effects. Such tools are valuable building blocks for many quantum-enabled technologies. Finally, using photonic crystal fibres we experimentally demonstrate quantum correlations arising from a four-colour nonlinear process suitable for CPC and use these measurements to study the feasibility of reaching the deterministic regime with current technology. Our scheme, which is based on interacting bosonic fields, is not restricted to optical systems but could also be implemented in optomechanical, electromechanical and superconducting systems with extremely strong intrinsic nonlinearities. Furthermore, exploiting higher-order nonlinearities with multiple pump fields yields a mechanism for multiparty mediation of the complex, coherent dynamics. [ABSTRACT FROM AUTHOR]

The original motivation to build a quantum computer came from Feynman, who imagined a machine capable of simulating generic quantum mechanical systems-a task that is believed to be intractable for classical computers. Such a machine could have far-reaching applications in the simulation of many-body quantum physics in condensed-matter, chemical and high-energy systems. Part of Feynman's challenge was met by Lloyd, who showed how to approximately decompose the time evolution operator of interacting quantum particles into a short sequence of elementary gates, suitable for operation on a quantum computer. However, this left open the problem of how to simulate the equilibrium and static properties of quantum systems. This requires the preparation of ground and Gibbs states on a quantum computer. For classical systems, this problem is solved by the ubiquitous Metropolis algorithm, a method that has basically acquired a monopoly on the simulation of interacting particles. Here we demonstrate how to implement a quantum version of the Metropolis algorithm. This algorithm permits sampling directly from the eigenstates of the Hamiltonian, and thus evades the sign problem present in classical simulations. A small-scale implementation of this algorithm should be achievable with today's technology. [ABSTRACT FROM AUTHOR]

Entanglement is one of the key resources required for quantum computation, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers. In superconducting devices, two-qubit entangled states have been demonstrated and used to show violations of Bell's inequality and to implement simple quantum algorithms. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways. These are typified by the states |GHZ? = (|000??+?|111?)/ and |W? = (|001??+?|010??+?|100?)/. Here we demonstrate the operation of three coupled superconducting phase qubits and use them to create and measure |GHZ? and |W? states. The states are fully characterized using quantum state tomography and are shown to satisfy entanglement witnesses, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement. [ABSTRACT FROM AUTHOR]

An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons. At present, entangled-light sources are optically driven with lasers, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates. However, these sources are Poissonian and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays, in core components of quantum computing such as teleportation, and in entanglement swapping. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications. [ABSTRACT FROM AUTHOR]

Improved control of the motional and internal quantum states of ultracold neutral atoms and ions has opened intriguing possibilities for quantum simulation and quantum computation. Many-body effects have been explored with hundreds of thousands of quantum-degenerate neutral atoms, and coherent light–matter interfaces have been built. Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms and to search for variations of fundamental constants in precision atomic clocks. Until now, atomic quantum gases and single trapped ions have been treated separately in experiments. Here we investigate whether they can be advantageously combined into one hybrid system, by exploring the immersion of a single trapped ion into a Bose–Einstein condensate of neutral atoms. We demonstrate independent control over the two components of the hybrid system, study the fundamental interaction processes and observe sympathetic cooling of the single ion by the condensate. Our experiment calls for further research into the possibility of using this technique for the continuous cooling of quantum computers. We also anticipate that it will lead to explorations of entanglement in hybrid quantum systems and to fundamental studies of the decoherence of a single, locally controlled impurity particle coupled to a quantum environment. [ABSTRACT FROM AUTHOR]

Over the past several decades, quantum information science has emerged to seek answers to the question: can we gain some advantage by storing, transmitting and processing information encoded in systems that exhibit unique quantum properties? Today it is understood that the answer is yes, and many research groups around the world are working towards the highly ambitious technological goal of building a quantum computer, which would dramatically improve computational power for particular tasks. A number of physical systems, spanning much of modern physics, are being developed for quantum computation. However, it remains unclear which technology, if any, will ultimately prove successful. Here we describe the latest developments for each of the leading approaches and explain the major challenges for the future. [ABSTRACT FROM AUTHOR]

To process information using quantum-mechanical principles, the states of individual particles need to be entangled and manipulated. One way to do this is to use trapped, laser-cooled atomic ions. Attaining a general-purpose quantum computer is, however, a distant goal, but recent experiments show that just a few entangled trapped ions can be used to improve the precision of measurements. If the entanglement in such systems can be scaled up to larger numbers of ions, simulations that are intractable on a classical computer might become possible. [ABSTRACT FROM AUTHOR]

The fractional quantum Hall effect, where plateaus in the Hall resistance at values of h/νe2 coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. (Here h is Planck’s constant, ν the filling factor and e the electron charge.) Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of ν, the observation of the even denominator ν = 5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are ‘non-abelian’—in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the ν = 5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the non-abelian nature of the ν = 5/2 state, it is the first step towards a full understanding of these new fractional charges. [ABSTRACT FROM AUTHOR]

As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states, and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits. Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor. Although these systems have successfully passed tests of coherent coupling for up to four qubits, communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended superconducting transmission line of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved later by the second qubit connected to the opposite end of the cavity. Beyond simple state transfer, these results suggest that a high-quality-factor superconducting cavity could also function as a useful short-term memory element. The basic architecture presented here can be expanded, offering the possibility for the coherent interaction of a large number of superconducting qubits. [ABSTRACT FROM AUTHOR]

Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine, and several examples of two-qubit interactions and gates have been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gate operations between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a ‘quantum bus’, which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip. [ABSTRACT FROM AUTHOR]

As information carriers in quantum computing, photonic qubits have the advantage of undergoing negligible decoherence. However, the absence of any significant photon–photon interaction is problematic for the realization of non-trivial two-qubit gates. One solution is to introduce an effective nonlinearity by measurements resulting in probabilistic gate operations. In one-way quantum computation, the random quantum measurement error can be overcome by applying a feed-forward technique, such that the future measurement basis depends on earlier measurement results. This technique is crucial for achieving deterministic quantum computation once a cluster state (the highly entangled multiparticle state on which one-way quantum computation is based) is prepared. Here we realize a concatenated scheme of measurement and active feed-forward in a one-way quantum computing experiment. We demonstrate that, for a perfect cluster state and no photon loss, our quantum computation scheme would operate with good fidelity and that our feed-forward components function with very high speed and low error for detected photons. With present technology, the individual computational step (in our case the individual feed-forward cycle) can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states. [ABSTRACT FROM AUTHOR]

A quantum point contact (QPC) is a narrow constriction between two wider electron reservoirs, and is the standard building block of sub-micrometre devices such as quantum dots and qubits (the proposed basic elements of quantum computers). The conductance through a QPC changes as a function of its width in integer steps of G0 = 2e2/h (where e is the charge on an electron, and h is Planck's constant), signalling the quantization of its transverse modes. But measurements of these conductance steps also reveal an additional shoulder at a value around 0.7G0 (refs 1–4), an observation that has remained a puzzle for more than a decade. It has recently been suggested that this phenomenon can be explained by the existence of a magnetic ‘impurity’ in the QPC at low electron densities. Here we present extensive numerical density-functional calculations that reveal the formation of an electronic state with a spin-1/2 magnetic moment in the channel under very general conditions. In addition, we show that such an impurity will also form at large magnetic fields, for a specific value of the field, and sometimes even at the opening of the second transverse mode in the QPC. Beyond explaining the source of the ‘0.7 anomaly’, these results may have far-reaching implications for spin-filling of electronic states in quantum dots and for the dephasing of quantum information stored in semiconductor qubits. [ABSTRACT FROM AUTHOR]

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits. [ABSTRACT FROM AUTHOR]

The article provides information on the would-be practical quantum computers intended to be used for quantum computing. Despite some hurdles, Andrew Steane of the quantum-computing group at the University of Oxford in Great Britain declared that useful computer is realistic by 2020. Likewise, David Deutsch agreed the declaration of Steane stressing that he is optimistic that a practical quantum computer will be achieved within the next decade in relation to the recent theoretical advances.

As the size of semiconductor devices continues to shrink, the normally random distribution of the individual dopant atoms within the semiconductor becomes a critical factor in determining device performance—homogeneity can no longer be assumed. Here we report the fabrication of semiconductor devices in which both the number and position of the dopant atoms are precisely controlled. To achieve this, we make use of a recently developed single-ion implantation technique, which enables us to implant dopant ions one-by-one into a fine semiconductor region until the desired number is reached. Electrical measurements of the resulting transistors reveal that device-to-device fluctuations in the threshold voltage (Vth; the turn-on voltage of the device) are less for those structures with ordered dopant arrays than for those with conventional random doping. We also find that the devices with ordered dopant arrays exhibit a shift in Vth, relative to the undoped semiconductor, that is twice that for a random dopant distribution (- 0.4 V versus -0.2 V); we attribute this to the uniformity of electrostatic potential in the conducting channel region due to the ordered distribution of dopant atoms. Our results therefore serve to highlight the improvements in device performance that can be achieved through atomic-scale control of the doping process. Furthermore, ordered dopant arrays of this type may enhance the prospects for realizing silicon-based solid-state quantum computers. [ABSTRACT FROM AUTHOR]

Standard quantum computation is based on sequences of unitary quantum logic gates that process qubits. The one-way quantum computer proposed by Raussendorf and Briegel is entirely different. It has changed our understanding of the requirements for quantum computation and more generally how we think about quantum physics. This new model requires qubits to be initialized in a highly entangled cluster state. From this point, the quantum computation proceeds by a sequence of single-qubit measurements with classical feedforward of their outcomes. Because of the essential role of measurement, a one-way quantum computer is irreversible. In the one-way quantum computer, the order and choices of measurements determine the algorithm computed. We have experimentally realized four-qubit cluster states encoded into the polarization state of four photons. We characterize the quantum state fully by implementing experimental four-qubit quantum state tomography. Using this cluster state, we demonstrate the feasibility of one-way quantum computing through a universal set of one- and two-qubit operations. Finally, our implementation of Grover's search algorithm demonstrates that one-way quantum computation is ideally suited for such tasks. [ABSTRACT FROM AUTHOR]

In theory, quantum computers offer a means of solving problems that would be intractable on conventional computers. Assuming that a quantum computer could be constructed, it would in practice be required to function with noisy devices called‘gates’. These gates cause decoherence of the fragile quantum states that are central to the computer's operation. The goal of so-called‘fault-tolerant quantum computing’is therefore to compute accurately even when the error probability per gate (EPG) is high. Here we report a simple architecture for fault-tolerant quantum computing, providing evidence that accurate quantum computing is possible for EPGs as high as three per cent. Such EPGs have been experimentally demonstrated, but to avoid excessive resource overheads required by the necessary architecture, lower EPGs are needed. Assuming the availability of quantum resources comparable to the digital resources available in today's computers, we show that non-trivial quantum computations at EPGs of as high as one per cent could be implemented. [ABSTRACT FROM AUTHOR]

One hundred years ago Albert Einstein introduced the concept of the photon. Although in the early years after 1905 the evidence for the quantum nature of light was not compelling, modern experiments-especially those using photon pairs-have beautifully confirmed its corpuscular character. Research on the quantum properties of light (quantum optics) triggered the evolution of the whole field of quantum information processing, which now promises new technology, such as quantum cryptography and even quantum computers. [ABSTRACT FROM AUTHOR]

The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation. [ABSTRACT FROM AUTHOR]

Proposes an implementation of Grover's algorithm for quantum computing that uses molecular magnets, which are solid-state systems with a large spin. How Shor and Grover demonstrated the superiority of quantum computers for searching databases; Theoretical evidence that molecular magnets can be used to build dense and efficient memory devices based on the Grover algorithm; How one single crystal can serve as a storage unit of a dynamic random access memory device; Demonstration of feasability.

Discusses the observation of coherent optical information storage in an atomic medium using halted light pulses. Use of electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms; Presentation of a theoretical model which reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations; Applications for quantum information processing.

Presents a study on quantum computation. Impact of quantum computers on the efficiency of solving problems, including factoring large integers; Proposals for quantum computation, which were based on implementing a quantum bit with two optical modes containing one photon; Method of computation that uses beam splitters, phase shifters, single photon sources, and photon detectors.

Presents a study in which an explicit scheme was introduced in which the Heisenberg interaction alone suffices to implement exactly any quantum computer circuit. Cost of a factor of three in additional qubits, and a factor of about ten in additional two-qubit operations; The ability to eliminate the complexity of one-bit operations, which this method offers; Progress towards solid-state implementations of quantum computation which should be accelerated.

Discusses the possible use of quantum effects like interference and entanglement to break codes, create unbreakable codes and speed up otherwise intractable computations. Difference between quantum information and conventional digital data and data-processing operations; Quantum analogues of central techniques of classical information theory; Use of quantum gates and wires.

The article discusses the potential use of Majorana modes that were invented by Ettore Majorana in 1937 in providing qubits, the basic element for a quantum computer. It says that Majorana modes enable the availability of forms of excitation to a few specific and special kinds of quasiparticles. It adds that Majorana modes can generate a kind of emergent spin for quasiparticles supporting them. It also discusses the constraints relating to the attachment of Majorana states into mode-icules.

The article presents a study which shows a technique that uses topological impacts to lessen the sensitivity of a quantum computer to errors. The study reveals that the probability of discovering the system in the 0 or 1 state is the same to the square of a complex number recognized as the probability amplitude. As a result, the information stocked in a qubit matches to a continuous range of probability amplitudes.

The article presents questions and answers related to quantum computing including what is a quantum computer, what are qubits made of and how are quantum information manipulated.

The article discusses the presentation of the experiment which is the single photons transfer quantum information between distant quantum bits (qubits) in a nanofabricated circuit. The controllable qubits, despite their macroscopic sizes, provide an alternative basis for a quantum computer. Moreover, the said quantum computer has an essential ingredient that controls the interactions between individual qubits for a universal entangling operation to intertwine the quantum states. It is noted that qubits should be able to couple strongly and should also efficiently decouple.

Quantum computers offer a new kind of processing power. Silicon chips are easy to manufacture. Scientists are trying to take advantages of the two approaches through their combination. A materials scientist at University College in London, England, Marshall Stoneham is using his experience in semiconductors to marry the worlds of silicon chips and quantum computing. Like all quantum computers, Stoneham's design shares a common feature with conventional processors, both carry out calculations by manipulating bits of information, the ones and zeros of binary code. The simplest operation involves flipping the value of a bit, so that 1 becomes 0 or vice versa. More complex operations involve two bits. One bit may be flipped if the value of a second bit is 0, for example, but not if it is 1. In principle, quantum devices work in the same way. But they also take advantage of the rules of quantum mechanics, which say that particles can exist in two states simultaneously.

Presents a study that developed a quantum charge-coupled device consisting of a large number of interconnected ion traps. Features of the quantum computer; Sources of decoherence during ion transport; Discussion on how to reduce the sources of decoherence by several levels of magnitude; Role of decoherence-free subspace in logic gate synchronization.

Profiles the emerging field of spintronics, one that exploits the spins of electrons, rather than their charges. Departure from conventional electronics; Potential powerful impact on information technology; Obstacles for spintronics; Spin being an ideal candidate for the quantum bits of quantum computing; Goal to provide a host of devices for very-high-speed, low-power electronics and optoelectronics.

Proposes a model for an ion trap quantum computer that combines scalability with the advantages of quantum optical systems. Application of an external field to a particular ion; Displacement of motional wave packet; Induction of differential shift due to the Coulomb interaction; Quantum computations for two-qubit phase gates.

Shows how the generalization of quantum teleportation reduces resource requirements for quantum computers and unifies known protocols for fault-tolerant quantum computation. Systematic constructions for an infinite class of reliable quantum gates; Construction of a universal quantum computer.

Presents research which developed a technique for quantum error-correction in quantum computers. Basis for quantum computing; Problems with coding in quantum error-correction theory; Size of a quantum computer compared with the logic device contained within it.

Presents research which developed a concept for a solid-state quiet qubit that can be environmentally decoupled for use in quantum computers. Potential for quantum computers; Difficulties in the physical realization of qubits; Concept based on macroscopic quantum coherent states; Naturally bistable system requiring no external bias.

Presents a proposal for solid-state qubits that utilizes controllable, low-capacitance Josephson junctions. Hypothetical composition of a quantum computer of coupled, two-state quantum systems or qubits; Exploitation of coherent tunnelling of Cooper pairs in the superconducting state by the design; Employment of control mechanisms of single-charge devices; Control of single- and two-bit operations by gate voltages.

The article reports on the study by Astafiev and colleagues which shows that quantum theory can explain the tunneling of magnetic flux across a nanowire in a superconducting system. It states that Astafiev and his colleagues developed a superconducting quantum bit (qubit) that was proposed by Maooj and Harmans. It says that the study paves the way for applications of nanowires supporting CQPS in quantum computing and metrology.

The article discusses the use of silicon carbide in the construction of diamond-based quantum computers. Researchers describe qubits in silicon carbide to have remarkably similar properties to diamonbased qubits. Researchers found out that the qubit states can have a long coherence times even at room temperature based on the reduced thermal vibrations in such material. They have also explored the ability of silicon carbide to emit light at wavelengths.

The article focuses on the trapped-ion quantum technology developed by C. Ospelkaus and colleagues and N. Timoney and colleagues that uses microwave radiation to execute computations in a quantum processor. It says that the quantum gates performed in a quantum processor are based on entanglement which is a non-intuitive phenomena of quantum physics. Furthermore, it mentions that the achievements of Ospelkaus and Timoney are significant innovations for quantum computing with trapped ions.

The article offers news briefs on topics related to policy, research and business in 2010. The budget of South Africa for public research and development has recorded a seven percent annual increase to 4.4 billion rand in 2011-2012. The U.S. National Aeronautics & Space Administration (NASA) plans to launch its first spacecraft aimed at returning material from an asteroid by 2016. The first commercial quantum computer from D-Wave Systems Inc. was sold to Lockheed Martin on May 25.

The article focuses on the hybrid quantum random access memory (RAM) systems, which have been believed as potential elements for the development of quantum computers. It relates on the studies made by four research groups on proof-of-principle quantum RAM, which were published in the periodicals "Physical Review Letters" and "Physical Review B." Moreover, it mentions that the hybrid quantum-computer architecture proposed in such research combines spin and superconducting qubits.

The article describes non-Abelian anyons which are hypothesized particles. It mentions that if their existence can be proven, they may form the basis of a fault-tolerant quantum computer. It explores the proposal of L. Fu and C. L. Kane in which a thin superconducting film is grown on top of a three-dimensional topological insulator. It notes that Fu and Kane showed that non-Abelian anyons are not intrinsically limited to high magnetic fields, ultra-pure materials and ultra-low temperatures but anywhere in nature.

The article presents the authors views on the study conducted by Du and colleagues which minimizes decoherence in electron spin's (quantum bits) solid-state mechanism. It mentions that decoherence happens when the mechanism interacts with itself or with its environment in unsteady manner. It notes that the quantum control system developed by Du and colleagues is an important resource in understanding and setting on the decoherence in quantum information, and in practical quantum computing.

The article discusses tailor-made quantum states. A prerequisite for creating a functional quantum computer depends upon the ability to produce superposed quantum states arbitrarily. Topics include a report by Hofheinz and colleagues published within the issue regarding tailor-made quantum states, using photons that they piled up in a resonator. Also presented is the use of superconducting (zero-resistance) macroscopic circuits in the synthesis of quantum states.