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Sjöqvist, Erik
Publications (10 of 107) Show all publications
Xu, G., Zhao, P., Xing, T., Sjöqvist, E. & Tong, D. (2017). Composite nonadiabatic holonomic quantum computation. Physical Review A. Atomic, Molecular, and Optical Physics, 95, Article ID 032311.
Open this publication in new window or tab >>Composite nonadiabatic holonomic quantum computation
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2017 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 95, 032311Article in journal (Refereed) Published
Abstract [en]

Nonadiabatic holonomic quantum computation has a robust feature in suppressing control errors because of its holonomic feature. However, this kind of robust feature is challenged since the usual way of realizing nonadiabatic holonomic gates introduces errors due to systematic errors in the control parameters. To resolve this problem, we here propose a composite scheme to realize nonadiabatic holonomic gates. Our scheme can suppress systematic errors while preserving holonomic robustness. It is particularly useful when the evolution period is shorter than the coherence time. We further show that our composite scheme can be protected by decoherence-free subspaces. In this case, the strengthened robust feature of our composite gates and the coherence stabilization virtue of decoherence-free subspaces are combined.

Keyword
Quantum computation, geometric phase
National Category
Physical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics; Physics
Identifiers
urn:nbn:se:uu:diva-316999 (URN)10.1103/PhysRevA.95.032311 (DOI)000395982600005 ()
Funder
Swedish Research Council, D0413201
Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-06-06
Zhao, P., Xu, G., Ding, Q., Sjöqvist, E. & Tong, D. (2017). Single-shot realization of nonadiabatic holonomic quantum gates in decoherence-free subspaces. Physical Review A: covering atomic, molecular, and optical physics and quantum information, 95(6), Article ID 062310.
Open this publication in new window or tab >>Single-shot realization of nonadiabatic holonomic quantum gates in decoherence-free subspaces
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2017 (English)In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 95, no 6, 062310Article in journal (Refereed) Published
Abstract [en]

Nonadiabatic holonomic quantum computation in decoherence-free subspaces has attracted increasing attention recently, as it allows for high-speed implementation and combines both the robustness of holonomic gates and the coherence stabilization of decoherence-free subspaces. Since the first protocol of nonadiabatic holonomic quantum computation in decoherence-free subspaces, a number of schemes for its physical implementation have been put forward. However, all previous schemes require two noncommuting gates to realize an arbitrary one-qubit gate, which doubles the exposure time of gates to error sources as well as the resource expenditure. In this paper, we propose an alternative protocol for nonadiabatic holonomic quantum computation in decoherence-free subspaces, in which an arbitrary one-qubit gate in decoherence-free subspaces is realized by a single-shot implementation. The present protocol not only maintains the merits of the original protocol but also avoids the extra work of combining two gates to implement an arbitrary one-qubit gate and thereby reduces the exposure time to various error sources.

Place, publisher, year, edition, pages
American Physical Society, 2017
Keyword
Quantum computation, geometric phase, decoherence free subspaces
National Category
Atom and Molecular Physics and Optics Other Physics Topics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-322383 (URN)10.1103/PhysRevA.95.062310 (DOI)000402794000002 ()
Funder
Swedish Research Council, D0413201Carl Tryggers foundation , 14:441
Available from: 2017-05-21 Created: 2017-05-21 Last updated: 2017-11-29Bibliographically approved
Bengtson, C. & Sjöqvist, E. (2017). The role of quantum coherence in dimer and trimer excitation energy transfer. New Journal of Physics, 19, Article ID 113015.
Open this publication in new window or tab >>The role of quantum coherence in dimer and trimer excitation energy transfer
2017 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 19, 113015Article in journal (Refereed) Published
Abstract [en]

Recent progress in resource theory of quantum coherence has resulted in measures to quantify coherence in quantum systems. Especially, the l1-norm and relative entropy of coherence have been shown to be proper quantifiers of coherence and have been used to investigate coherence properties in different operational tasks. Since long-lasting quantum coherence has been experimentally confirmed in a number of photosynthetic complexes, it has been debated if and how coherence is connected to the known efficiency of population transfer in such systems. In this study, we investigate quantitatively the relationship between coherence, as quantified by l1 norm and relative entropy of coherence, and efficiency, as quantified by fidelity, for population transfer between end-sites in a network of two-level quantum systems. In particular, we use the coherence averaged over the duration of the population transfer in order to carry out a quantitative comparision between coherence and fidelity. Our results show that although coherence is a necessary requirement for population transfer, there is no unique relation between coherence and the efficiency of the transfer process.

Keyword
Quantum coherence, energy transport
National Category
Other Physics Topics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-327295 (URN)10.1088/1367-2630/aa916b (DOI)
Funder
Swedish Research Council, D0413201
Available from: 2017-08-08 Created: 2017-08-08 Last updated: 2017-11-29Bibliographically approved
Sjöqvist, E., Azimi Mousolou, V. & Canali, C. M. (2016). Conceptual aspects of geometric quantum computation. Quantum Information Processing, 15(10), 3995-4011.
Open this publication in new window or tab >>Conceptual aspects of geometric quantum computation
2016 (English)In: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 15, no 10, 3995-4011 p.Article in journal (Refereed) Published
Abstract [en]

Geometric quantum computation is the idea that geometric phases can be used to implement quantum gates, i.e., the basic elements of the Boolean network that forms a quantum computer. Although originally thought to be limited to adiabatic evolution, controlled by slowly changing parameters, this form of quantum computation can as well be realized at high speed by using nonadiabatic schemes. Recent advances in quantum gate technology have allowed for experimental demonstrations of different types of geometric gates in adiabatic and nonadiabatic evolution. Here, we address some conceptual issues that arise in the realizations of geometric gates. We examine the appearance of dynamical phases in quantum evolution and point out that not all dynamical phases need to be compensated for in geometric quantum computation. We delineate the relation between Abelian and non-Abelian geometric gates, and find an explicit physical example where the two types of gates coincide. We identify differencies and similarities between adiabatic and nonadiabatic realizations of quantum computation based on non-Abelian geometric phases. 

Keyword
Geometric phase, quantum computation
National Category
Other Physics Topics Atom and Molecular Physics and Optics Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-212119 (URN)10.1007/s11128-016-1381-1 (DOI)000383587100004 ()
Funder
Swedish Research Council, D0413201Swedish Research Council, 621-2014-4785
Available from: 2013-12-05 Created: 2013-12-05 Last updated: 2017-12-06Bibliographically approved
Andersson, O., Bengtsson, I., Ericsson, M. & Sjöqvist, E. (2016). Geometric phases for mixed states of the Kitaev chain. Philosophical Transactions. Series A: Mathematical, physical, and engineering science, 374(2069), Article ID 20150231.
Open this publication in new window or tab >>Geometric phases for mixed states of the Kitaev chain
2016 (English)In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 374, no 2069, 20150231Article in journal (Refereed) Published
Abstract [en]

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. 

Keyword
Fermions, geometric phase, thermal states
National Category
Condensed Matter Physics Other Physics Topics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-264295 (URN)10.1098/rsta.2015.0231 (DOI)000376159700001 ()
External cooperation:
Funder
Swedish Research Council, D0413201
Available from: 2015-10-08 Created: 2015-10-08 Last updated: 2017-12-01
Sjöqvist, E. (2016). Nonadiabatic holonomic single-qubit gates in off-resonant Λ systems [Letter to the editor]. Physics Letters A, 380(1-2), 65-67.
Open this publication in new window or tab >>Nonadiabatic holonomic single-qubit gates in off-resonant Λ systems
2016 (English)In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 380, no 1-2, 65-67 p.Article in journal, Letter (Refereed) Published
Abstract [en]

We generalize nonadiabatic holonomic quantum computation in a resonant Λ configuration proposed in [New J. Phys. 14 (2012) 103035] to the case of off-resonant driving lasers. We show that any single-qubit holonomic gate can be realized by separately varying the detuning, amplitude, and phase of the lasers. 

Keyword
Geometric phase; Quantum computation
National Category
Atom and Molecular Physics and Optics Other Physics Topics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-264256 (URN)10.1016/j.physleta.2015.10.006 (DOI)000365365300011 ()
Funder
Swedish Research Council, D0413201
Available from: 2015-10-07 Created: 2015-10-07 Last updated: 2017-12-01Bibliographically approved
Bengtson, C., Stenrup, M. & Sjöqvist, E. (2016). Quantum nonlocality in the excitation energy transfer in the Fenna-Matthews-Olson complex. International Journal of Quantum Chemistry, 116, 1763-1771.
Open this publication in new window or tab >>Quantum nonlocality in the excitation energy transfer in the Fenna-Matthews-Olson complex
2016 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 116, 1763-1771 p.Article in journal (Refereed) Published
Abstract [en]

The Fenna-Matthews-Olson (FMO) complex - a pigment protein complex involved in photosynthesis in green sulfur bacteria - is remarkably efficient in transferring excitation energy from light harvesting antenna molecules to a reaction center. Recent experimental and theoretical studies suggest that quantum coherence and entanglement may play a role in this excitation energy transfer (EET). We examine whether bipartite quantum nonlocality, a property that expresses a stronger-than-entanglement form of correlation, exists between different pairs of chromophores in the FMO complex when modeling the EET by the hierarchically coupled equations of motion method. We compare the results for nonlocality with the amount of bipartite entanglement in the system. In particular, we analyze in what way these correlation properties are affected by different initial conditions. It is found that bipartite nonlocality only exists when the initial conditions are chosen in an unphysiological manner and probably is absent when considering the EET in the FMO complex in its natural habitat. It is also seen that nonlocality and entanglement behave quite differently in this system. In particular, for localized initial states, nonlocality only exists on a very short time scale and then drops to zero in an abrupt manner. As already known from previous studies, quantum entanglement between chromophore pairs on the other hand is oscillating and exponentially decaying and follow thereby a pattern more similar to the chromophore population dynamics. The abrupt disappearance of nonlocality in the presence of nonvanishing entanglement is a phenomenon we call nonlocality sudden death; a striking manifestation of the difference between these two types of correlations in quantum systems. 

Keyword
Photosynthesis, quantum nonlocality, open quantum systems
National Category
Other Physics Topics Theoretical Chemistry
Research subject
Physics with specialization in Quantum Chemistry; Biology
Identifiers
urn:nbn:se:uu:diva-246119 (URN)10.1002/qua.25221 (DOI)000385587200001 ()
Funder
Swedish Research Council, D0413201eSSENCE - An eScience CollaborationSwedish National Infrastructure for Computing (SNIC), snic2014-3-66
Available from: 2015-03-02 Created: 2015-03-02 Last updated: 2017-12-04Bibliographically approved
Herterich, E. & Sjöqvist, E. (2016). Single-loop multiple-pulse nonadiabatic holonomic quantum gates. Physical Review A. Atomic, Molecular, and Optical Physics, 94(5), Article ID 052310.
Open this publication in new window or tab >>Single-loop multiple-pulse nonadiabatic holonomic quantum gates
2016 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 94, no 5, 052310Article in journal (Refereed) Published
Abstract [en]

Nonadiabatic holonomic quantum computation provides the means to perform fast and robust quantum gates by utilizing the resilience of non-Abelian geometric phases to fluctuations of the path in state space. While the original scheme [New J. Phys. 14, 103035 (2012)] needs two loops in the Grassmann manifold (i.e., the space of computational subspaces of the full state space) to generate an arbitrary holonomic one-qubit gate, we propose single-loop one-qubit gates that constitute an efficient universal set of holonomic gates when combined with an entangling holonomic two-qubit gate. Our one-qubit gate is realized by dividing the loop into path segments, each of which is generated by a Λ-type Hamiltonian. We demonstrate that two path segments are sufficient to realize arbitrary single-loop holonomic one-qubit gates. We describe how our scheme can be implemented experimentally in a generic atomic system exhibiting a three-level Λ-coupling structure, by utilizing carefully chosen laser pulses. 

Keyword
Quantum computation, quantum gates, geometric phase
National Category
Other Physics Topics Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-301984 (URN)10.1103/PhysRevA.94.052310 (DOI)000387531300004 ()
Funder
Swedish Research Council, D0413201
Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2017-11-21Bibliographically approved
Azimi Mousolou, V., Canali, C. M. & Sjöqvist, E. (2016). Spin-electric Berry phase shift in triangular molecular magnets. Physical Review B. Condensed Matter and Materials Physics, 94(23), Article ID 235423.
Open this publication in new window or tab >>Spin-electric Berry phase shift in triangular molecular magnets
2016 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 94, no 23, 235423Article in journal (Refereed) Published
Abstract [en]

We propose a Berry phase effect on the chiral degrees of freedom of a triangular magnetic molecule. The phase is induced by adiabatically varying an external electric field in the plane of the molecule via a spin-electric coupling mechanism present in these frustrated magnetic molecules. The Berry phase effect depends on spin-orbit interaction splitting and on the electric dipole moment. By varying the amplitude of the applied electric field, the Berry phase difference between the two spin states can take any arbitrary value between zero and π, which can be measured as a phase shift between the two chiral states by using spin-echo techniques. Our result can be used to realize an electric-field-induced geometric phase-shift gate acting on a chiral qubit encoded in the ground-state manifold of the triangular magnetic molecule.

Keyword
Molecular magnets, Berry phase
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-302656 (URN)10.1103/PhysRevB.94.235423 (DOI)000394546100004 ()
Funder
Swedish Research Council, 621-2014-4785Swedish Research Council, D0413201
Available from: 2016-09-07 Created: 2016-09-07 Last updated: 2017-08-08Bibliographically approved
Gürkan, N., Sjöqvist, E., Hessmo, B. & Grémaud, B. (2016). Towards a measurement of the effective gauge field and the Born-Huang potential with atoms in chip traps. .
Open this publication in new window or tab >>Towards a measurement of the effective gauge field and the Born-Huang potential with atoms in chip traps
2016 (English)Article in journal (Other academic) Submitted
Abstract [en]

We study magnetic traps with very high trap frequencies where the spin is coupled to the motion of the atom. This allows us to investigate how the Born-Oppenheimer approximation fails and how effective magnetic and electric fields appear as the consequence of the non-adiabatic dynamics. The results are based on exact numerical diagonalization of the full Hamiltonian describing the coupling between the internal and external degrees of freedom. The position in energy and the decay rate of the trapping states correspond to the imaginary part of the resonances of this Hamiltonian and are computed using the complex rotation method. 

Keyword
Cold atoms, atom chip, Born-Oppenheimer, effective gauge fields
National Category
Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-294927 (URN)
External cooperation:
Funder
Swedish Research Council, D0413201
Available from: 2016-05-30 Created: 2016-05-30 Last updated: 2016-09-07
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