| 1. |
- Andersson, Ole, et al.
(författare)
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Geometric phases for mixed states of the Kitaev chain
- 2016
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 374:2069
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Tidskriftsartikel (refereegranskat)abstract
- The Berry phase has found applications in building topological order parameters for certain condensed matter systems. The question whether some geometric phase for mixed states can serve the same purpose has been raised, and proposals are on the table. We analyze the intricate behaviour of Uhlmann’s geometric phase in the Kitaev chain at finite temperature, and then argue that it captures quite different physics from that intended. We also analyze the behaviour of a geometric phase introduced in the context of interferometry. For the Kitaev chain, this phase closely mirrors that of the Berry phase, and we argue that it merits further investigation.
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| 2. |
- Dorell, Åsa, et al.
(författare)
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Experiences With Family Health Conversations at Residential Homes for Older People
- 2016
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Ingår i: Clinical Nursing Research. - : SAGE Publications. - 1054-7738 .- 1552-3799. ; 25:5, s. 560-582
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to highlight family members' experiences of participating in Family Health Conversation (FamHC), based on families in which a family member was living in a residential home for older people. A total of 10 families and 22 family members participated in evaluating family interviews 1 month after participating in FamHC. The interviews were analyzed by qualitative content analysis. The main finding was being a part of FamHC increased family members' insights, understanding, and communication within the family. Getting confirmation from nurses was essential to cope with the new life situation, which also meant that they felt comfortable to partly hand over the responsibility for the older person who moved to the residential home. By being open and expressing their feelings, a bad conscience could be relieved. These findings showed that FamHC could be helpful for family members in adapting to this novel situation.
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| 3. |
- Ericsson, Marie, et al.
(författare)
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Har vi redan skapat kvantdatorn?
- 2015
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Annan publikation (populärvet., debatt m.m.)abstract
- Vissa menar att kvantdatorn redan är verklighet, och syftar på en gåtfull låda som kallas D-wave. Men uppfyller den verkligen kraven? Frågan diskuteras livligt i forskarvärlden. Vad är egentligen en riktig kvantdator? Här hjälper Uppsalaforskarna Marie Ericsson och Erik Sjöqvist till att reda ut begreppen.
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| 4. |
- Johansson, Markus, et al.
(författare)
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Classification scheme of pure multipartite states based on topological phases
- 2014
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 89:1, s. 012320-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the connection between the concept of a-balancedness introduced in [Phys. Rev A. 85, 032112 (2012)] and polynomial local SU invariants and the appearance of topological phases respectively. It is found that different types of a-balancedness correspond to different types of local SU invariants analogously to how different types of balancedness as defined in [New J. Phys. 12, 075025 (2010)] correspond to different types of local SL invariants. These different types of SU invariants distinguish between states exhibiting different topological phases. In the case of three qubits the different kinds of topological phases are fully distinguished by the three-tangle together with one more invariant. Using this we present a qualitative classification scheme based on balancedness of a state. While balancedness and local SL invariants of bidegree $(2n,0)$ classify the SL-semistable states [New J. Phys. 12, 075025 (2010), Phys. Rev. A 83, 052330 (2011)], a-balancedness and local SU invariants of bidegree (2n-m,m) gives a more fine grained classification. In this scheme the a-balanced states form a bridge from the genuine entanglement of balanced states, invariant under the SL-group, towards the entanglement of unbalanced states characterized by U invariants of bidegree (n,n). As a by-product we obtain generalizations to the W-state, states that are entangled, but contain only globally distributed entanglement of parts of the system.
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| 5. |
- Ericsson, Marie, et al.
(författare)
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Comment on `Detecting non-Abelian geometric phases with three-level Λ systems'
- 2013
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 87:3, s. 036101-
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Tidskriftsartikel (refereegranskat)abstract
- In their recent paper, Yan-Xiong Du {\it et al.} [Phys. Rev. A {\bf 84}, 034103 (2011)] claim to have found a non-Abelian adiabatic geometric phase associated with the energy eigenstates of a large-detuned $\Lambda$ three-level system. They further propose a test to detect the non-commutative feature of this geometric phase. On the contrary, we show that the non-Abelian geometric phase picked up by the energy eigenstates of a $\Lambda$ system is trivial in the adiabatic approximation, while, in the exact treatment of the time evolution, this phase is very small and cannot be separated from the non-Abelian dynamical phase acquired along the path in parameter space.
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| 6. |
- Johansson, Markus, 1982-
(författare)
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Entanglement and Quantum Computation from a Geometric and Topological Perspective
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we investigate geometric and topological structures in the context of entanglement and quantum computation.A parallel transport condition is introduced in the context of Franson interferometry based on the maximization of two-particle coincidence intensity. The dependence on correlations is investigated and it is found that the holonomy group is in general non-Abelian, but Abelian for uncorrelated systems. It is found that this framework contains a parallel transport condition developed by Levay in the case of two-qubit systems undergoing local SU(2) evolutions.Global phase factors of topological origin, resulting from cyclic local SU(2) evolution, called topological phases, are investigated in the context of multi-qubit systems. These phases originate from the topological structure of the local SU(2)-orbits and are an attribute of most entangled multi-qubit systems. The relation between topological phases and SLOCC-invariant polynomials is discussed. A general method to find the values of the topological phases in an n-qubit system is described.A non-adiabatic generalization of holonomic quantum computation is developed in which high-speed universal quantum gates can be realized by using non-Abelian geometric phases. It is shown how a set of non-adiabatic holonomic one- and two-qubit gates can be implemented by utilizing transitions in a generic three-level Λ configuration. The robustness of the proposed scheme to different sources of error is investigated through numerical simulation. It is found that the gates can be made robust to a variety of errors if the operation time of the gate can be made sufficiently short. This scheme opens up for universal holonomic quantum computation on qubits characterized by short coherence times.
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| 7. |
- Johansson, Markus, et al.
(författare)
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Robustness of nonadiabatic holonomic gates
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 86:6, s. 062322-
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Tidskriftsartikel (refereegranskat)abstract
- The robustness to different sources of error of the scheme for non-adiabatic holonomic gates proposed in [arXiv:1107.5127v2] is investigated. Open system effects as well as errors in the driving fields are considered. It is found that the gates can be made more error resilient by using sufficiently short pulses. The principal limit of how short the pulses can be made is given by the breakdown of the quasi-monochromatic approximation. A comparison with the resilience of adiabatic gates is carried out.
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| 8. |
- Johansson, Markus, et al.
(författare)
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Topological phases and multiqubit entanglement
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 85:3, s. 032112-1-032112-11
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Tidskriftsartikel (refereegranskat)abstract
- Global phase factors of topological origin, resulting from cyclic local $\rm{SU}$ evolution, called topological phases, were first described in [Phys. Rev. Lett. {\bf 90}, 230403 (2003)], in the case of entangled qubit pairs. In this paper we investigate topological phases in multi-qubit systems as the result of cyclic local $\rm{SU(2)}$ evolution. These phases originate from the topological structure of the local $\rm{SU(2)}$-orbits and are an attribute of most entangled multi-qubit systems. We discuss the relation between topological phases and SLOCC-invariant polynomials and give examples where topological phases appear. A general method to find the values of the topological phases in an $n$-qubit system is described and a complete list of these phases for up to seven qubits is given.
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| 9. |
- Johansson, Markus, et al.
(författare)
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Correlation-induced non-Abelian quantum holonomies
- 2011
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Ingår i: Journal of Physics A. - : IOP Publishing. - 0305-4470 .- 1361-6447. ; 44:14, s. 145301-
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Tidskriftsartikel (refereegranskat)abstract
- In the context of two-particle interferometry, we construct a parallel transport condition that is based on the maximization of coincidence intensity with respect to local unitary operations on one of the subsystems. The dependence on correlation is investigated and it is found that the holonomy group is generally non-Abelian, but Abelian for uncorrelated systems. It is found that our framework contains the Lévay geometric phase (2004 J. Phys. A: Math. Gen. 37 1821) in the case of two-qubit systems undergoing local SU(2) evolutions.
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| 10. |
- Tidström, Jonas, et al.
(författare)
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Open system effects on slow light and electromagnetically induced transparency
- 2011
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 44:13, s. 135501-
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Tidskriftsartikel (refereegranskat)abstract
- The coherence properties of a three-level $\Lambda$-system influenced by a Markovian environment are analyzed. A coherence vector formalism is used and a vector form of the Lindblad equation is derived. Together with decay channels from the upper state, open system channels acting on the subspace of the two lower states are investigated, i.e., depolarization, dephasing, and amplitude damping channels. We derive an analytic expression for the coherence vector and the concomitant optical susceptibility, and analyze how the different channels influence the optical response. This response depends non-trivially on the type of open system interaction present, and even gain can be obtained. We also present a geometrical visualization of the coherence vector as an aid to understand the system response.
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