| 1. |
- André, Tomas, et al.
(författare)
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Dark path holonomic qudit computation
- 2022
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 106:6
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Tidskriftsartikel (refereegranskat)abstract
- Nonadiabatic holonomic quantum computation is a method used to implement high-speed quantum gates with non-Abelian geometric phases associated with paths in state space. Due to their noise tolerance, these phases can be used to construct error resilient quantum gates. We extend the holonomic dark path qubit scheme in [M.-Z. Ai et al., Fundam. Res. 2, 661 (2022)] to qudits. Specifically, we demonstrate one- and two-qudit universality by using the dark path technique. Explicit qutrit (d = 3) gates are demonstrated and the scaling of the number of loops with the dimension d is addressed. This scaling is linear and we show how any diagonal qudit gate can be implemented efficiently in any dimension.
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| 2. |
- Eriksson, Anders, et al.
(författare)
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Monopole Field Textures in Interacting Spin Systems
- 2020
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society (APS). - 1050-2947 .- 1094-1622. ; 101:5
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic monopoles can appear as emergent structures in a wide range of physical settings, ranging from spin ice to Weyl points in semimetals. Here, a distribution of synthetic (Berry) monopoles in parameter space of a slowly changing external magnetic field is demonstrated in a system of interacting spin-1/2 particles with broken spherical symmetry. These monopoles can be found at points where the external field is nonzero. The spin-spin interaction provides a mechanism for splitting the synthetic local magnetic charges until their magnitude reach the smallest allowed value 1/2. For certain states, a nonzero net charge can be created in an arbitrarily large finite region of parameter space. The monopole field textures contain non-monopolar contributions in the presence of spin-spin interaction.
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| 3. |
- Hammar, Karl, et al.
(författare)
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Error-rate-agnostic decoding of topological stabilizer codes
- 2022
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Ingår i: Physical Review A. - 2469-9934 .- 2469-9926. ; 105
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Tidskriftsartikel (refereegranskat)abstract
- Efficient high-performance decoding of topological stabilizer codes has the potential to crucially improve the balance between logical failure rates and the number and individual error rates of the constituent qubits. High-threshold maximum-likelihood decoders require an explicit error model for Pauli errors to decode a specific syndrome, whereas lower-threshold heuristic approaches such as minimum-weight matching are error agnostic. Here we consider an intermediate approach, formulating a decoder that depends on the bias, i.e., the relative probability of phase-flip to bit-flip errors, but is agnostic to error rate. Our decoder is based on counting the number and effective weight of the most likely error chains in each equivalence class of a given syndrome. We use Metropolis-based Monte Carlo sampling to explore the space of error chains and find unique chains that are efficiently identified using a hash table. Using the error-rate invariance, the decoder can sample chains effectively at an error rate which is higher than the physical error rate and without the need for thermalization between chains in different equivalence classes. Applied to the surface code and the XZZX code, the decoder matches maximum-likelihood decoders for moderate code sizes or low error rates. We anticipate that, because of the compressed information content per syndrome, it can be taken full advantage of in combination with machine-learning methods to extrapolate Monte Carlo-generated data.
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| 4. |
- Kjellsson, Ludvig, et al.
(författare)
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Resonant inelastic x-ray scattering at the N2 π*-resonance: Lifetime-vibrational interference, radiative electron rearrangement, and wave-function imaging
- 2021
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 103
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Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic x-ray scattering spectra excited at the pi*-resonance of the nitrogen molecule are presented. Well-resolved vibrational excitations in the electronic ground state, and in the 3 sigma g(-1 )1 pi(1)(g) a(1) Pi(g) state are observed. The spectra are analyzed within the Kramers-Heisenberg formalism, and the importance of lifetime-vibrational interference effects is highlighted. In addition, strongly dissociative multiply excited final states populated in radiative electron rearrangement are found in the valence ionization continua. The vibrational wave functions of the core-excited state are imaged on the strongly dissociative final state potentials.
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| 5. |
- Lindroth, Eva, et al.
(författare)
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Electron-impact recombination and excitation rates for charge-state-selected highly charged Si ions
- 2020
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 101:6
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Tidskriftsartikel (refereegranskat)abstract
- Charge-state selective recombination rate coefficients were measured by time of flight (TOF) analyzed highly charged Si ions extracted from an electron-beam ion trap. Additionally, the combination of simultaneous TOF and x-ray measurements and a separation of the dielectronic recombination contribution in the x-ray spectra is used for extracting electron-impact excitation rate coefficients for several overlaying charge states. Experimentally derived dielectronic recombination spectra for XIII and XIV Si are compared and found in excellent agreement with the results of relativistic many-body perturbation theory calculations.
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| 6. |
- Mavrogordatos, Themistoklis K., et al.
(författare)
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Monitoring the resonantly driven Jaynes-Cummings oscillator by an external two-level emitter : A cascaded open-systems approach
- 2020
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 101:5
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Tidskriftsartikel (refereegranskat)abstract
- We address the consequences of back action in the unidirectional coupling of two cascaded open quantum subsystems connected to the same reservoir at different spatial locations. In the spirit of H. J. Carmichael [Phys. Rev. Lett. 70, 2273 (1993)], the second subsystem is a two-level atom, while the first transforms from a driven empty cavity to a perturbative QED configuration and ultimately to a driven Jaynes-Cummings (JC) oscillator through a varying light-matter coupling strength. For our purpose, we appeal at first to the properties of resonance fluorescence in the statistical description of radiation emitted along two channels-those of forward and sideways scattering-comprising the monitored output. In the simplest case of an empty cavity coupled to an external atom, we derive analytical results for the nonclassical fluctuations in the fields occupying the two channels, pursuing a mapping to the bad-cavity limit of the JC model to serve as a guide for the description of the more involved dynamics. Finally, we exemplify a conditional evolution for the composite system of a critical JC oscillator on resonance coupled to an external monitored two-level target, showing that coherent atomic oscillations of the target probe the onset of a second-order dissipative quantum phase transition in the source.
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| 7. |
- Mommers, Cornelis, J. G., et al.
(författare)
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Universal quantum computation and quantum error correction using discrete holonomies
- 2022
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 105:2
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Tidskriftsartikel (refereegranskat)abstract
- Holonomic quantum computation exploits a quantum state's nontrivial, matrix-valued geometric phase (holonomy) to perform fault-tolerant computation. Holonomies arising from systems where the Hamiltonian traces a continuous path through parameter space have been well researched. Discrete holonomies, on the other hand, where the state jumps from point to point in state space, have had little prior investigation. Using a sequence of incomplete projective measurements of the spin operator, we build an explicit approach to universal quantum computation. We show that quantum error correction codes integrate naturally in our scheme, providing a model for measurement-based quantum computation that combines the passive error resilience of holonomic quantum computation and active error correction techniques. In the limit of dense measurements we recover known continuous-path holonomies.
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| 8. |
- Salén, Peter, et al.
(författare)
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Resonant Auger electron-ion-coincidence spectroscopy of N-methyltrifluoroacetamide : Site-specific fragmentation studies
- 2020
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : AMER PHYSICAL SOC. - 2469-9926 .- 2469-9934. ; 102:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical and experimental study of the electronic structure of N-methyltrifluoroacetamide (FNMA) and examine the resonant Auger (RA) decay upon site- and state-specific excitation at the C 1s, N 1s, and O is edges. Based on the calculated energies for cationic states of 1 h and 2h-lp configurations, we assign the peaks of recorded VUV photoelectron and RA spectra. Specifically, we identify the spectral features associated with participator and spectator RA decay. To aid in a deeper understanding of previous observations of limited site-specific bond breaking in FNMA, the influence of the site-specific excitation and the character of the Auger decay on the fragmentation is investigated using Auger electron ion coincidence experiments. Moreover, to investigate the potentially different capacity for selective bond cleavage of participator and spectator Auger states, we apply this technique to the spectator Auger state with the main electronic configuration of (31a')(-1) (32e '')(-1) (34a '')(1) which is well resolved in the experimental spectrum. Finally, the influence of the molecular excess energy and the electronic character of the RA final states on fragmentation is addressed. Based on our findings, we argue that the excess energy the FNMA molecule gains upon RA decay is the dominant factor determining the fragmentation pattern.
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| 9. |
- Schiffmann, Sacha, et al.
(författare)
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Natural orbitals in multiconfiguration calculations of hyperfine-structure parameters
- 2020
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 101:6
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Tidskriftsartikel (refereegranskat)abstract
- We are reinvestigating the hyperfine structure of sodium using a fully relativistic multiconfiguration approach. In the fully relativistic approach, the computational strategy somewhat differs from the original nonrelativistic counterpart used by P. Jonsson et al., Phys. Rev. A 53, 4021 (1996). Numerical instabilities force us to use a layer-by-layer approach that has some broad unexpected effects. Core correlation is found to be significant and therefore should be described in an adequate orbital basis. The natural-orbital basis provides an interesting alternative to the orbital basis from the layer-by-layer approach, allowing us to overcome some deficits of the latter, giving rise to magnetic dipole hyperfine structure constant values, in excellent agreement with observations. Effort is made to assess the reliability of the natural-orbital bases and to illustrate their efficiency.
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| 10. |
- Ström, Petter, et al.
(författare)
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Energy deposition by nonequilibrium charge states of MeV 127I in Au
- 2021
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 103
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Tidskriftsartikel (refereegranskat)abstract
- The energy loss of iodine ions at initial charge states up to 25+ and energies up to 36 MeV in self-supporting gold foils between 37 and 107 nm of thickness was measured with an electron mirror type time-of-flight detector. An excess energy loss of 130 keV was observed at 36MeV for charge state 25+ compared to 16+, and an energy loss deficit of 100 keV was observed for charge state 8+. The charge state equilibration length for 36-MeV iodine was estimated to lie between 3 and 7 nm, corresponding to an equilibration time between 0.4 and 0.9 fs. This result is relevant both for nanostructure fabrication with MeV ion beams and for depth profiling based on ion beam analysis data in cases where the charge state of the primary ion is far from the equilibrium value in the sample under study. A comparison to published data on charge state equilibration for various projectile-target combinations and energies from 10 keV to 6 GeV indicated that the energy scaling of the equilibration length observed at high energy is invalid for projectile velocities on the order of the Bohr velocity and below. The measurement further provided equilibrium values of the electronic stopping power for iodine in gold at ten energies between 10 and 36 MeV.
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