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V Vedral

Publications and source records attributed to V Vedral.

18 recordsLinked to original sources
PubMed2005

Experimental one-way quantum computing.

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.

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PubMed2004

Spin-1/2 geometric phase driven by decohering quantum fields.

We calculate the geometric phase of a spin-1/2 system driven by one and two mode quantum fields subject to decoherence. Using the quantum jump approach, we show that the corrections to the phase in the no-jump trajectory are different when considering adiabatic and nonadiabatic evolutions. We discuss the implications of our results from both fundamental as well as quantum computational perspectives.

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PubMed2003

Geometric phase in open systems.

We calculate the geometric phase associated with the evolution of a system subjected to decoherence through a quantum-jump approach. The method is general and can be applied to many different physical systems. As examples, two main sources of decoherence are considered: dephasing and spontaneous decay. We show that the geometric phase is completely insensitive to the former, i.e., it is independent of the number of jumps determined by the dephasing operator.

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PubMed2003

Classical correlations and entanglement in quantum measurements.

We analyze a quantum measurement where the apparatus is initially in a mixed state. We show that the amount of information gained in a measurement is not equal to the amount of entanglement between the system and the apparatus, but is instead equal to the degree of classical correlations between the two. As a consequence, we derive an uncertainty-like expression relating the information gain in the measurement and the initial mixedness of the apparatus. Final entanglement between the environment and the apparatus is also shown to be relevant for the efficiency of the measurement.

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PubMed2002

Vacuum induced spin-1/2 Berry's phase.

We calculate the Berry phase of a spin-1/2 particle in a magnetic field considering the quantum nature of the field. The phase reduces to the standard Berry phase in the semiclassical limit and the eigenstate of the particle acquires a phase in the vacuum. We also show how to generate a vacuum induced Berry phase considering two quantized modes of the field which has an interesting physical interpretation.

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PubMed2002

Entanglement concentration using quantum statistics.

We propose an entanglement concentration scheme which uses only the effects of quantum statistics of indistinguishable particles. This establishes the fact that useful quantum information processing can be accomplished by quantum statistics alone. Because of the basis independence of statistical effects, our protocol requires less knowledge of the initial state than most entanglement concentration schemes. Moreover, no explicit controlled operation is required at any stage.

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PubMed2001

Subsystem purity as an enforcer of entanglement.

We show that entanglement can always arise in the interaction of an arbitrarily large system in any mixed state with a single qubit in a pure state. This small initial purity is enough to enforce entanglement even when the total entropy is close to maximum. We demonstrate this feature using the Jaynes-Cummings interaction of a two-level atom in a pure state with a field in a thermal state at an arbitrarily high temperature. We find the time and temperature variation of a lower bound on the amount of entanglement produced and study the classical correlations quantified by the mutual information.

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PubMed2001

Natural thermal and magnetic entanglement in the 1D Heisenberg model.

We investigate the entanglement between any two spins in a one dimensional Heisenberg chain as a function of temperature and the external magnetic field. We find that the entanglement in an antiferromagnetic chain can be increased by increasing the temperature or the external field. Increasing the field can also create entanglement between otherwise disentangled spins. This entanglement can be confirmed by testing Bell's inequalities involving any two spins in the solid.

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PubMed2001

Remote information concentration using a bound entangled state.

Remote information concentration, the reverse process of quantum telecloning, is presented. In this scheme, quantum information originally from a single qubit, but now distributed into three spatially separated qubits, is remotely concentrated back to a single qubit via an initially shared entangled state without performing any global operations. This entangled state is a single unlockable bound entangled state and we analyze its properties.

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PubMed2000

Communication capacity of quantum computation.

By considering quantum computation as a communication process, we relate its efficiency to its classical communication capacity. This formalism allows us to derive lower bounds on the complexity of search algorithms in the most general context. It enables us to link the mixedness of a quantum computer to its efficiency and also allows us to derive the critical level of mixedness beyond which there is no quantum advantage in computation.

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PubMed2000

Proposal for measurement of harmonic oscillator berry phase in ion traps

We propose a scheme for measuring the Berry phase in the vibrational degree of freedom of a trapped ion. Starting from the ion in a vibrational coherent state we show how to reverse the sign of the coherent state amplitude by using a purely geometric phase. This can then be detected through the internal degrees of freedom of the ion. Our method can be applied to preparation of entangled states of the ion and the vibrational mode.

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PubMed2000

Local distinguishability of multipartite orthogonal quantum states

We consider one copy of a quantum system prepared in one of two orthogonal pure states, entangled or otherwise, and distributed between any number of parties. We demonstrate that it is possible to identify which of these two states the system is in by means of local operations and classical communication alone. The protocol we outline is both completely reliable and completely general; it will correctly distinguish any two orthogonal states 100% of the time.

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PubMed2000

Geometric phases for mixed states in interferometry.

We provide a physical prescription based on interferometry for introducing the total phase of a mixed state undergoing unitary evolution, which has been an elusive concept in the past. We define the parallel transport condition that provides a connection form for obtaining the geometric phase for mixed states. The expression for the geometric phase for mixed state reduces to well known formulas in the pure state case when a system undergoes noncyclic and unitary quantum evolution.

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PubMed2000

Detection of geometric phases in superconducting nanocircuits

When a quantum-mechanical system undergoes an adiabatic cyclic evolution, it acquires a geometrical phase factor' in addition to the dynamical one; this effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnology should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may be applied to the design of gates for quantum computation.

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PubMed2000

Information, relative entropy of entanglement, and irreversibility.

Previously proposed measures of entanglement, such as entanglement of formation and assistance, are shown to be special cases of the relative entropy of entanglement. The difference between these measures for an ensemble of mixed states is shown to depend on the availability of classical information about particular members of the ensemble. Based on this, relations between relative entropy of entanglement and mutual information are derived.

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PubMed2000

Geometric quantum computation using nuclear magnetic resonance

A significant development in computing has been the discovery that the computational power of quantum computers exceeds that of Turing machines. Central to the experimental realization of quantum information processing is the construction of fault-tolerant quantum logic gates. Their operation requires conditional quantum dynamics, in which one sub-system undergoes a coherent evolution that depends on the quantum state of another sub-system; in particular, the evolving sub-system may acquire a conditional phase shift. Although conventionally dynamic in origin, phase shifts can also be geometric. Conditional geometric (or 'Berry') phases depend only on the geometry of the path executed, and are therefore resilient to certain types of errors; this suggests the possibility of an intrinsically fault-tolerant way of performing quantum gate operations. Nuclear magnetic resonance techniques have already been used to demonstrate both simple quantum information processing and geometric phase shifts. Here we combine these ideas by performing a nuclear magnetic resonance experiment in which a conditional Berry phase is implemented, demonstrating a controlled phase shift gate.

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PubMed1983

[Metabolic profile of newborn calves and levels of immunoglobulins in the first days of life].

After examination of the clinical state of 128 new-born calves, blood was collected from their vena jugularis for the determination of blood actual pH value, concentration of lactic acid, pCO2, base excess, buffer base and standard acid bicarbonate. The course and difficulty of parturition exerted a significant influence on the vitality of the calves and on the studied parameters of acid-base state. In the normally born calves, compared with those after dystocia, the following values were obtained: pH 7.20 +/- 0.03 : 7.11 +/- 0.07, pCO2 = 8.4 +/- 0.9 : 10.0 +/- 1.1 kPa, base excess -2.30 +/- 2.10 : 5.80 +/- 4.60 mmol/l, buffer base 43.0 +/- 2.4 : 39.5 +/- 6.5 mmol/l, standard acid bicarbonate 22.3 +/- 1.8 : 19.6 +/- 4.1 mmol/l and lactic acid concentration 5.6 +/- 2.0 : 10.7 +/- 5.1 mmol/l. The differences were statistically significant (P less than 0.05) and statistically highly significant (P less than 0.01). The continual study of the blood actual pH value and lactic acid concentration in the calves in the first 24 hours of life showed that with the same trend of changes in calves after dystocia the initial values were less favourable and that their normalization lasted longer. Attention is drawn to the importance of dystocia for the rise of respiratory metabolic acidosis and its effect on the vitality of newborn calves, and/or on their survival. The discussion deals with the importance of immunoglobulin levels in calves in the first days after birth for their further development. The determination of antibody content in colostral serum from the first milking in 33 and 29 cows on two farms showed great drawbacks in quality. A satisfactory level of IgG was found only in 36.36% and 58.62% of the cows, and a satisfactory level of IgM only in 12.12% and 24.13% of the studied cows. The determination of immunoglobulin content in their calves two to three days from birth (33 + 33 animals) showed normoglobulinemia only in 24.24% and 15.15% of cases. In 33 and 29 cows on two farms the colostrum serum from the first milking had an average content of immunoglobulins of class G amounting to 27.99 +/- 20.25 mg/ml and 36.95 +/- 21.62 mg/ml, and class M amounting to 3.64 +/- 1.25 and 2.04 +/- 1.42 mg/ml. Three days from birth, their calves had an IgG content of 4.25 +/- 2.57 mg/ml and 3.99 +/- 1.86 mg/ml and an IgM content of 0.30 +/- 0.20 and 6.38 +/- 0.25 mg/ml.

Acid-Base Equilibrium↗