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Bason, M. G., Viteau, M., Malossi, N., Huillery, P., Arimondo, E., Ciampini, D., et al. (2012). High-fidelity quantum driving. Nat. Phys., 8(2), 147–152.
Abstract: Accurately controlling a quantum system is a fundamental requirement in quantum information processing and the coherent manipulation of molecular systems. The ultimate goal in quantum control is to prepare a desired state with the highest fidelity allowed by the available resources and the experimental constraints. Here we experimentally implement two optimal high-fidelity control protocols using a two-level quantum system comprising Bose-Einstein condensates in optical lattices. The first is a short-cut protocol that reaches the maximum quantum-transformation speed compatible with the Heisenberg uncertainty principle. In the opposite limit, we realize the recently proposed transitionless superadiabatic protocols in which the system follows the instantaneous adiabatic ground state nearly perfectly. We demonstrate that superadiabatic protocols are extremely robust against control parameter variations, making them useful for practical applications.
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Bozyigit, D., Lang, C., Steffen, L., Fink, J. M., Eichler, C., Baur, M., et al. (2011). Antibunching of microwave-frequency photons observed in correlation measurements using linear detectors. Nat. Phys., 7(2), 154–158.
Abstract: At optical frequencies the radiation produced by a source, such as a laser, a black body or a single-photon emitter, is frequently characterized by analysing the temporal correlations of emitted photons using single-photon counters. At microwave frequencies, however, there are no efficient single-photon counters yet. Instead, well-developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate first- and second-order correlation function measurements of a pulsed microwave-frequency single-photon source integrated on the same chip with a 50/50 beam splitter followed by linear amplifiers and quadrature amplitude detectors. We clearly observe single-photon coherence in first-order and photon antibunching in second-order correlation function measurements of the propagating fields.
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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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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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Mineev, V. P. (2012). Superfluid helium: Order in disorder. Nat. Phys., 8, 253–254.
Abstract: Confining liquid 3He in porous silica aerogel prepared with strong anisotropy stabilizes a state of axial superfluidity.
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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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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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Gustafsson, M. V., Santos, P. V., Johansson, G., & Delsing, P. (2012). Local probing of propagating acoustic waves in a gigahertz echo chamber. Nat. Phys., 8(4), 338–343.
Abstract: In the same way that micro-mechanical resonators resemble guitar strings and drums, surface acoustic waves resemble the sound these instruments produce, but moving over a solid surface rather than through air. In contrast with oscillations in suspended resonators, such propagating mechanical waves have not before been studied near the quantum mechanical limits. Here, we demonstrate local probing of surface acoustic waves with a displacement sensitivity of 30amRMSHz-1/2 and detection sensitivity on the single-phonon level after averaging, at a frequency of 932MHz. Our probe is a piezoelectrically coupled single-electron transistor, which is sufficiently fast, non-destructive and localized to enable us to track pulses echoing back and forth in a long acoustic cavity, self-interfering and ringing the cavity up and down. We project that strong coupling to quantum circuits will enable new experiments, and hybrids using the unique features of surface acoustic waves. Prospects include quantum investigations of phonon-phonon interactions, and acoustic coupling to superconducting qubits for which we present favourable estimates.
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Baumert, T. (2011). Quantum technology: Wave packets get a kick. Nat. Phys., 7(5), 373–374.
Abstract: Intense femtosecond pulses of infrared light can manipulate molecules. It is now shown that such control even extends to making different molecular eigenstates interfere with each other in a way never considered before -- a potential tool for optically engineered chemical reactions and for ultrafast information encoding and manipulation.
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Ma, X. - S., Dakic, B., Naylor, W., Zeilinger, A., & Walther, P. (2011). Quantum simulation of the wavefunction to probe frustrated Heisenberg spin systems. Nat. Phys., 7(5), 399–405.
Abstract: 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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