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Barreiro, Julio T. |
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Quantum physics: Environmental effects controlled |
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Journal Article |
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2011 |
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Nature Physics |
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Nat. Phys. |
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7 |
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927–928 |
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fromIPMRAS |
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An open quantum system loses its 'quantumness' when information about the state leaks into its surroundings. Researchers now show how this decoherence can be controlled between two incompatible regimes in the case of a single photon. |
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RPLAB @ gujma @ |
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817 |
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Bialczak, R. C.; Ansmann, M.; Hofheinz, M.; Lucero, E.; Neeley, M.; O'Connell, A. D.; Sank, D.; Wang, H.; Wenner, J.; Steffen, M.; Cleland, A. N.; Martinis, J. M. |
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Quantum process tomography of a universal entangling gate implemented with Josephson phase qubits |
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Journal Article |
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2010 |
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Nature Physics |
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Nat. Phys. |
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6 |
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6 |
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409-413 |
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fromIPMRAS |
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Quantum gates must perform reliably when operating on standard input basis states and on complex superpositions thereof. Experiments using superconducting qubits have validated truth tables for particular implementations of, for example, the controlled-NOT gate, but have not fully characterized gate operation for arbitrary superpositions of input states. Here we demonstrate the use of quantum process tomography (QPT) to fully characterize the performance of a universal entangling gate between two superconducting qubits. Process tomography permits complete gate analysis, but requires precise preparation of arbitrary input states, control over the subsequent qubit interaction and ideally simultaneous single-shot measurement of output states. In recent work, it has been proposed to use QPT to probe noise properties and time dynamics of qubit systems and to apply techniques from control theory to create scalable qubit benchmarking protocols. We use QPT to measure the fidelity and noise properties of an entangling gate. In addition to demonstrating a promising fidelity, our entangling gate has an on-to-off ratio of 300, a level of adjustable coupling that will become a requirement for future high-fidelity devices. This is the first solid-state demonstration of QPT in a two-qubit system, as QPT has previously been demonstrated only with single solid-state qubits. |
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RPLAB @ gujma @ |
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803 |
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Perseguers, S.; Lewenstein, M.; Acín, A.; Cirac, J. I. |
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Quantum random networks |
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Journal Article |
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2010 |
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Nature Physics |
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Nat. Phys. |
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6 |
Issue |
7 |
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539-543 |
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Quantum mechanics offers new possibilities to process and transmit information. In recent years, algorithms and cryptographic protocols exploiting the superposition principle and the existence of entangled states have been designed. They should allow us to realize communication and computational tasks that outperform any classical strategy. Here we show that quantum mechanics also provides fresh perspectives in the field of random networks. Already the simplest model of a classical random graph changes markedly when extended to the quantum case, where we obtain a distinct behaviour of the critical probabilities at which different subgraphs appear. In particular, in a network of N nodes, any quantum subgraph can be generated by local operations and classical communication if the entanglement between pairs of nodes scales as N-2. This result also opens up new vistas in the domain of quantum networks and their applications. |
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804 |
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Ma, Xiao-Song; Dakic, Borivoje; Naylor, William; Zeilinger, Anton; Walther, Philip |
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Quantum simulation of the wavefunction to probe frustrated Heisenberg spin systems |
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Journal Article |
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2011 |
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Nature Physics |
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Nat. Phys. |
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7 |
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5 |
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399-405 |
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Quantum simulators are controllable quantum systems that can reproduce the dynamics of the system of interest in situations that are not amenable to classical computers. Recent developments in quantum technology enable the precise control of individual quantum particles as required for studying complex quantum systems. In particular, quantum simulators capable of simulating frustrated Heisenberg spin systems provide platforms for understanding exotic matter such as high-temperature superconductors. Here we report the analogue quantum simulation of the ground-state wavefunction to probe arbitrary Heisenberg-type interactions among four spin-1/2 particles. Depending on the interaction strength, frustration within the system emerges such that the ground state evolves from a localized to a resonating-valence-bond state. This spin-1/2 tetramer is created using the polarization states of four photons. The single-particle addressability and tunable measurement-induced interactions provide us with insights into entanglement dynamics among individual particles. We directly extract ground-state energies and pairwise quantum correlations to observe the monogamy of entanglement. |
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RPLAB @ gujma @ |
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842 |
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Zakka-Bajjani, Eva; Nguyen, François; Lee, Minhyea; Vale, Leila R.; Simmonds, Raymond W.; Aumentado, José |
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Title ![sorted by Title field, ascending order (up)](img/sort_asc.gif) |
Quantum superposition of a single microwave photon in two different 'colour' states |
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Journal Article |
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Year |
2011 |
Publication |
Nature Physics |
Abbreviated Journal |
Nat. Phys. |
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Volume |
7 |
Issue |
8 |
Pages |
599-603 |
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fromIPMRAS |
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Fully controlled coherent coupling of arbitrary harmonic oscillators is an important tool for processing quantum information. Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems using a two-level system as a mediating element. Direct interaction at the quantum level has only recently been realized by means of resonant coupling between trapped ions. Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by means of parametric frequency conversion. We accomplish this by coupling the mode currents of two harmonics through a superconducting quantum interference device (SQUID) and modulating its flux at the difference (~7GHz) of the harmonic frequencies. We deterministically prepare a single-photon Fock state and coherently manipulate it between multiple modes, effectively controlling it in a superposition of two different 'colours'. This parametric interaction can be described as a beamsplitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols on-chip. |
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RPLAB @ gujma @ |
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822 |
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