# Suter, Dieter Prof. Dr.

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Item Quantitative complementarity between local and nonlocal character of quantum states in a three-qubit system(The American Physical Society, 2008-05-08) Suter, Dieter; Peng, Xinhua; Zhang, Jingfu; Du, JiangfengShow more Local or nonlocal character of quantum states can be quantified and is subject to various bounds that can be formulated as complementarity relations. Here, we investigate the local vs nonlocal character of pure three-qubit states by a four-way interferometer. The complete entanglement in the system can be measured as the entanglement of a specific qubit with the subsystem consisting of the other two qubits. The quantitative complementarity relations are verified experimentally in an NMR quantum information processor.Show more Item Detection of Quantum Critical Points by a Probe Qubit(The American Physical Society, 2008-03-13) Suter, Dieter; Zhang, Jingfu; Peng, Xinhua; Rajendran, NageswaranShow more Quantum phase transitions occur when the ground state of a quantum system undergoes a qualitative change when an external control parameter reaches a critical value. Here, we demonstrate a technique for studying quantum systems undergoing a phase transition by coupling the system to a probe qubit. It uses directly the increased sensibility of the quantum system to perturbations when it is close to a critical point. Using an NMR quantum simulator, we demonstrate this measurement technique for two different types of quantum phase transitions in an Ising spin chain.Show more Item Quantum Computing(Wiley-VCH, 2008) Suter, Dieter; Stolze, JoachimShow more Item Experimental observation of a topological phase in the maximally entangled state of a pair of qubits(The American PHysical Society, 2007-10-22) Suter, Dieter; Du, Jiangfeng; Zhu, Jing; Shi, Mingjun; Peng, XinhuaShow more Quantum mechanical phase factors can be related to dynamical effects or to the geometrical properties of a trajectory in a given space—either parameter space or Hilbert space. Here, we experimentally investigate a quantum mechanical phase factor that reflects the topology of the SO(3) group: since rotations by pi around antiparallel axes are identical, this space is doubly connected. Using a pair of nuclear spins in a maximally entangled state, we subject one of the spins to a cyclic evolution. If the corresponding trajectory in SO(3) can be smoothly deformed to a point, the quantum state at the end of the trajectory is identical to the initial state. For all other trajectories the quantum state changes sign.Show more Item Measuring complete quantum states with a single observable(The American Physical Society, 2007-10-13) Suter, Dieter; Peng, Xinhua; Du, JiangfengShow more Experimental determination of an unknown quantum state usually requires several incompatible measurements. However, it is also possible to determine the full quantum state from a single, repeated measurement. For this purpose, the quantum system whose state is to be determined is first coupled to a second quantum system (the “assistant”) in such a way that part of the information in the quantum state is transferred to the assistant. The actual measurement is then performed on the enlarged system including the original system and the assistant. We discuss in detail the requirements of this procedure and experimentally implement it on a simple quantum system consisting of nuclear spins.Show more Item Evolution of Athletic Records(Routledge, 2007-07) Suter, Dieter; Gembris, Daniel; Taylor, John G.Show more Athletic records represent the best results in a given discipline, thus improving monotonically with time. As has already been shown, this should not be taken as an indication that the athletes' capabilities keep improving. In other words, a new record is not noteworthy just because it is a new record, instead it is necessary to assess by how much the record has improved. In this paper we derive formulae that can be used to show that athletic records continue to improve with time, even if athletic performance remains constant. We are considering two specific examples, the German championships and the world records in several athletic disciplines. The analysis shows that, for the latter, true improvements occur in 20-50% of the disciplines. The analysis is supplemented by an application of our record estimation approach to the prediction of the maximum body length of humans for a specified size of a population respectively population group from a representative sample.Show more Item Iterative quantum-state transfer along a chain of nuclear spin qubits(The American Physical Society, 2007-07-17) Suter, Dieter; Zhang, Jingfu; Rajendran, Nageswaran; Peng, XinhuaShow more Transferring quantum information between two qubits is a basic requirement for many applications in quantum communication and quantum-information processing. In the iterative quantum-state transfer proposed by Burgarth et al. [Phys. Rev. A 75, 062327 (2007)], this is achieved by a static spin chain and a sequence of gate operations applied only to the receiving end of the chain. The only requirement on the spin chain is that it transfers a finite part of the input amplitude to the end of the chain, where the gate operations accumulate the information. For an appropriate sequence of evolutions and gate operations, the fidelity of the transfer can asymptotically approach unity. We demonstrate the principle of operation of this transfer scheme by implementing it in a nuclear magnetic resonance quantum-information processor.Show more Item Factorizing numbers with the Gauss sum technique(The American Physical Society, 2007-06-04) Suter, Dieter; Mahesh, T. S.; Rajendran, Nageswaran; Peng, XinhuaShow more Several physics-based algorithms for factorizing large numbers were recently presented. A notable recent algorthm by Schleich et al. uses Gauss sums for distinguishing between factors and nonfactors. We demonstrate two NMR techniques that evaluate Gauss sums and thus implement their algorithm. The first one is based on differential excitation of a single spin magnetization by a cascade of rf pulses. The second method is based on spatial averaging and selective refocusing of magnetization for Gauss sums corresponding to factors. All factors of 16 637 and 52 882 363 are successfully obtained.Show more Item Effect of system level structure and spectral distribution of the environment on the decoherence rate(The American Physical Society, 2007-04-13) Suter, Dieter; Zhang, Jingfu; Peng, Xinhua; Rajendran, NageswaranShow more Minimizing the effect of decoherence on a quantum register must be a central part of any strategy to realize scalable quantum-information processing. Apart from the strength of the coupling to the environment, the decoherence rate is determined by the system level structure and by the spectral composition of the noise trace that the environment generates. Here, we discuss a relatively simple model that allows us to study these different effects quantitatively in detail. We evaluate the effect that the perturbation has on a nuclear magnetic resonance system while it performs a Grover search algorithm.Show more Item Decoherence in large quantum registers under variable interaction with the environment(The American Physical Society, 2007-04-03) Suter, Dieter; Lovric, Marko; Krojanski, Hans G.Show more Effective quantum-information processing requires coherent control of large numbers of qubits on a time scale that is short compared to the decoherence time of the system. It is therefore important to extrapolate and measure decoherence times for large quantum registers and to determine the effect of different couplings between system and environment on the decoherence rate. For this purpose, we have experimentally realized a system that allows one to generate model quantum registers with more than 100 qubits and measure the decay of the information in these states while adjusting the strength of the interaction between the quantum register and the environment. Our results indicate a power-law dependence of the decoherence rate on the number of qubits in the system, with an exponent of the order of 0.5. This behavior remains qualitatively unchanged when the coupling strength to the environment is reduced by about an order of magnitude.Show more Item Spins as probes of different electronic states(Wiley-VCH, 2007-03-01) Suter, Dieter; Klieber, RobertShow more Nuclear spins are efficient probes of electronic states. Because most NMR experiments are performed in thermal equilibrium, they probe the electronic ground state - the only state that is significantly populated under ambient conditions. Probing electronically excited states becomes possible, if magnetic resonance techniques are combined with optical (laser) excitation. Depending on the nature of the electronic state, drastic changes of the magnetic resonance parameters may be observed. We discuss the basic principles of this type of investigation. Depending on the lifetime of the electronically excited state, it is possible to measure separate spectra of ground and excited state if the lifetime is long on the NMR timescale, or an averaged spectrum if the lifetime is short. We present examples for both limiting cases using rare earth ions and semiconductor heterostructures.Show more Item Two-qubit gates between noninteracting qubits in endohedral-fullerene-based quantum computation(The American Physical Society, 2007-01-17) Suter, Dieter; Du, Jiangfeng; Ju, ChenyongShow more Item Experimental realization of 1 --> 2 asymmetric phase-covariant quantum cloning(The American Physical Society, 2007-01-17) Suter, Dieter; Cheng, Hongwei; Zhou, Xianyi; Du, JiangfengShow more While exact cloning of an unknown quantum state is prohibited by the linearity of quantum mechanics, approximate cloning is possible and has been used, e.g., to derive limits on the security of quantum communication protocols. In the case of asymmetric cloning, the information from the input state is distributed asymmetrically between the different output states. Here, we consider asymmetric phase-covariant cloning, where the goal is to optimally transfer the phase information from a single input qubit to different output qubits. We construct an optimal quantum cloning machine for two qubits that does not require ancilla qubits and implement it on an NMR quantum information processor.Show more Item Decoherence in large NMR quantum registers(The American Physical Society, 2006-12-26) Suter, Dieter; Krojanski, Hans GeorgShow more Item Quantum-information processing using strongly dipolar coupled nuclear spins(The American Physical Society, 2006-12-18) Suter, Dieter; Mahesh, T. S.Show more Dipolar coupled homonuclear spins present challenging, yet useful systems for quantum-information processing. In such systems, the eigenbasis of the system Hamiltonian is the appropriate computational basis and coherent control can be achieved by specially designed strongly modulating pulses. In this paper we describe the first experimental implementation of the quantum algorithm for numerical gradient estimation by nuclear magnetic resonance, using the eigenbasis of a four spin system.Show more Item Reduced Decoherence in Large Quantum Registers(The American Physical Society, 2006-10-11) Suter, Dieter; Krojanski, Hans G.Show more Among the biggest obstacles for building larger (and thus more powerful) quantum-information processors is decoherence, the decay of quantum-information by the coupling between the quantum register and its environment. Procedures for reducing decoherence processes will be essential for successful operation of larger quantum processors. We study model quantum registers consisting of up to 4900 qubits and measure their decay as a function of the register size. We demonstrate that appropriate sequences of qubit rotations reduce the coupling between system and environment for all sizes of the quantum register, thus preserving the quantum-information 50 times longer than without decoupling.Show more Item Speedup of quantum-state transfer by three-qubit interactions(The American Physical Society, 2006-06-19) Suter, Dieter; Zhang, Jingfu; Peng, XinhuaShow more Universal quantum information processing requires single-qubit rotations and two-qubit interactions as minimal resources. A possible step beyond this minimal scheme is the use of three-qubit interactions. We consider such three-qubit interactions and show how they can reduce the time required for a quantum state transfer in an XY spin chain. For the experimental implementation, we use liquid-state nuclear magnetic resonance, where three-qubit interactions can be implemented by sequences of radio-frequency pulses.Show more Item Laser-assisted magnetic resonance: principles and applications(Springer, 2006-09-11) Suter, Dieter; Gutschank, J.Show more Laser radiation can be used in various magnetic resonance experiments. This chapter discusses a number of cases, where laser light either improves the information content of conventional experiments or makes new types of experiments possible, which could not be performed with conventional means. Sensitivity is often the main reason for using light, but it also allows one to become more selective, e.g. by selecting signals only from small parts of the sample. Examples are given for NMR, NQR, and EPR spectra that use were taken with the help of coherent optical radiation.Show more Item Adding salt to an aqueous solution of t-butanol(American Institute of Physics, 2006-04-21) Suter, Dieter; Paschek, Dietmar; Geiger, Alfons; Herve, Momo J.Show more Recent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. 89, 215508 (2002); J. Chem. Phys. 118, 8357 (2003)] suggest the formation of t-butanol pairs, bridged by a chloride ion via O–H[centered ellipsis]Cl– hydrogen bonds, leading to a reduced number of intermolecular hydrophobic butanol-butanol contacts. Here we present a joint experimental/theoretical study on the same system, using a combination of molecular dynamics (MD) simulations and nuclear magnetic relaxation measurements. Both MD simulation and experiment clearly support the more classical scenario of an enhanced number of hydrophobic contacts in the presence of salt, as it would be expected for purely hydrophobic solutes. [T. Ghosh et al., J. Phys. Chem. B 107, 612 (2003)]. Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental error bars of the Bowron and Finney data.Show more Item Time-resolved coherent double Raman detection of nuclear spin transitions(The American Physical Society, 2006-03-08) Suter, Dieter; Klieber, RobertShow more Coherent Raman scattering can be used for detection of nuclear spin transitions in solids and atomic vapors if both nuclear spin states of the spin transition to be detected are connected to a single nuclear spin state of a different electronic state by allowed optical transitions. This is not the case in crystals with high symmetry. Here, we introduce the coherent double Raman experiment, where the difference between nuclear spin transitions in two different electronic states is observed. In contrast to the conventional Raman scattering experiment, this scheme is applicable also to systems with high symmetry, where the nuclear spin does not change during an optical transition.Show more