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Mariantoni, M., Wang, H., Bialczak, R. C., Lenander, M., Lucero, E., Neeley, M., et al. (2011). Photon shell game in three-resonator circuit quantum electrodynamics. Nat. Phys., 7(4), 287–293.
Abstract: The generation and control of quantum states of light constitute fundamental tasks in cavity quantum electrodynamics (QED). The superconducting realization of cavity QED, circuit QED (refs 11, 12, 13, 14), enables on-chip microwave photonics, where superconducting qubits control and measure individual photon states. A long-standing issue in cavity QED is the coherent transfer of photons between two or more resonators. Here, we use circuit QED to implement a three-resonator architecture on a single chip, where the resonators are interconnected by two superconducting phase qubits. We use this circuit to shuffle one- and two-photon Fock states between the three resonators, and demonstrate qubit-mediated vacuum Rabi swaps between two resonators. By shuffling superposition states we are also able to demonstrate the high-fidelity phase coherence of the transfer. Our results illustrate the potential for using multi-resonator circuits as photon quantum registers and for creating multipartite entanglement between delocalized bosonic modes.
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Kim, Y. - S., Lee, J. - C., Kwon, O., & Kim, Y. - H. (2012). Protecting entanglement from decoherence using weak measurement and quantum measurement reversal. Nat. Phys., 8(2), 117–120.
Abstract: Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence. For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death. Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing, quantum cryptography, quantum teleportation and quantum metrology. Here, we propose and demonstrate a scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death.
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Saffman, M. (2010). Quantum computing: A quantum telecom link. Nat. Phys., 6(11), 838–839.
Abstract: Converting data-carrying photons to telecommunication wavelengths enables distribution of quantum information over long distances.
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Raussendorf, R. (2010). Quantum computing: Shaking up ground states. Nat. Phys., 6(11), 840–841.
Abstract: Measurement-based quantum computation with an Affleck-Kennedy-Lieb-Tasaki state is experimentally realized for the first time.
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Grinolds, M. S., Maletinsky, P., Hong, S., Lukin, M. D., Walsworth, R. L., & Yacoby, A. (2011). Quantum control of proximal spins using nanoscale magnetic resonance imaging. Nat. Phys., 7(9), 687–692.
Abstract: Quantum control of individual spins in condensed-matter systems is an emerging field with wide-ranging applications in spintronics, quantum computation and sensitive magnetometry. Recent experiments have demonstrated the ability to address and manipulate single electron spins through either optical or electrical techniques. However, it is a challenge to extend individual-spin control to nanometre-scale multi-electron systems, as individual spins are often irresolvable with existing methods. Here we demonstrate that coherent individual-spin control can be achieved with few- nanometre resolution for proximal electron spins by carrying out single-spin magnetic resonance imaging (MRI), which is realized using a scanning-magnetic-field gradient that is both strong enough to achieve nanometre spatial resolution and sufficiently stable for coherent spin manipulations. We apply this scanning-field-gradient MRI technique to electronic spins in nitrogen-vacancy (NV) centres in diamond and achieve nanometre resolution in imaging, characterization and manipulation of individual spins. For NV centres, our results in individual-spin control demonstrate an improvement of nearly two orders of magnitude in spatial resolution when compared with conventional optical diffraction-limited techniques. This scanning-field-gradient microscope enables a wide range of applications including materials characterization, spin entanglement and nanoscale magnetometry.
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Hollenberg, L. C. L. (2012). Quantum control: Through the quantum chicane. Nat. Phys., 8(2), 113–114.
Abstract: In quantum control there is an inherent tension between high fidelity requirements and the need for speed to avoid decoherence. A direct comparison of quantum control protocols at these two extremes indicates where the sweet spot may lie.
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Zurek, W. H. (2009). Quantum Darwinism. Nat. Phys., 5(3), 181–188.
Abstract: Quantum Darwinism describes the proliferation, in the environment, of multiple records of selected states of a quantum system. It explains how the quantum fragility of a state of a single quantum system can lead to the classical robustness of states in their correlated multitude; shows how effective `wave-packet collapse' arises as a result of the proliferation throughout the environment of imprints of the state of the system; and provides a framework for the derivation of Born's rule, which relates the probabilities of detecting states to their amplitudes. Taken together, these three advances mark considerable progress towards settling the quantum measurement problem.
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Home, J. (2010). Quantum entanglement: Watching correlations disappear. Nat. Phys., 6(12), 938–939.
Abstract: Engineered decoherence enables tracking of multipartite entanglement as a quantum state decays.
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Shor, P. W. (2009). Quantum information theory: The bits don't add up. Nat. Phys., 5, 247–248.
Abstract: A counterexample to the 'additivity question', the most celebrated open problem in the mathematical theory of quantum information, casts doubt on the possibility of finding a simple expression for the information capacity of a quantum channel.
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Trabesinger, A. (2009). Quantum mechanics: Shaken foundations. Nat. Phys., 5(12), 863.
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Santori, C., & Beausoleil, R. G. (2012). Quantum memory: Phonons in diamond crystals. Nat. Photon., 6, 10–12.
Abstract: The demonstration that quantum information can be stored in a bulk-diamond crystal in the form of an optically excited phonon gives researchers a new type of mechanical solid-state quantum memory to explore.
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Johnson, B. R., Reed, M. D., Houck, A. A., Schuster, D. I., Bishop, L. S., Ginossar, E., et al. (2010). Quantum non-demolition detection of single microwave photons in a circuit. Nat. Phys., 6(9), 663–667.
Abstract: Thorough control of quantum measurement is key to the development of quantum information technologies. Many measurements are destructive, removing more information from the system than they obtain. Quantum non-demolition (QND) measurements allow repeated measurements that give the same eigenvalue. They could be used for several quantum information processing tasks such as error correction, preparation by measurement and one-way quantum computing. Achieving QND measurements of photons is especially challenging because the detector must be completely transparent to the photons while still acquiring information about them. Recent progress in manipulating microwave photons in superconducting circuits has increased demand for a QND detector that operates in the gigahertz frequency range. Here we demonstrate a QND detection scheme that measures the number of photons inside a high-quality-factor microwave cavity on a chip. This scheme maps a photon number, n, onto a qubit state in a single-shot by means of qubit-photon logic gates. We verify the operation of the device for n=0 and 1 by analysing the average correlations of repeated measurements, and show that it is 90% QND. It differs from previously reported detectors because its sensitivity is strongly selective to chosen photon number states. This scheme could be used to monitor the state of a photon-based memory in a quantum computer.
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Xu, X. A., & Wong, C. W. (2012). Quantum optics: Correlations on a chip. Nat. Photon., 6, 75–76.
Abstract: Researchers have developed a semiconductor structure capable of supporting quantum correlations between photons and strong single-photon nonlinearities, thus paving the way for the development of chip-based devices for quantum secure communications and quantum information processing.
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Kok, P. (2010). Quantum optics: Entangled photons report for duty. Nat. Photon., 4(8), 504–505.
Abstract: Entangled photons are a key ingredient in optical quantum technologies, but researchers have so far been unable to produce a single pair of entangled photons. Now, two groups from China and Austria independently report just that, with a technique that avoids the need to infer entanglement from detection signatures.
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Clerk, A. (2012). Quantum phononics: To see a SAW. Nat. Phys., 8(4), 256–257.
Abstract: Mechanical oscillations of microscopic resonators have recently been observed in the quantum regime. This idea could soon be extended from localized vibrations to travelling waves thanks to a sensitive probe of so-called surface acoustic waves.
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