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Sjöqvist, Erik
Publications (10 of 113) Show all publications
Ramberg, N. & Sjöqvist, E. (2019). Environment-assisted holonomic quantum maps. Physical Review Letters, 122, Article ID 140501.
Open this publication in new window or tab >>Environment-assisted holonomic quantum maps
2019 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, article id 140501Article in journal (Refereed) Published
Abstract [en]

Holonomic quantum computation uses non-Abelian geometric phases to realize error resilient quantum gates. Nonadiabatic holonomic gates are particularly suitable to avoid unwanted decoherence effects, as they can be performed at high speed. By letting the computational system interact with a structured environment, we show that the scope of error resilience of nonadiabatic holonomic gates can be widened to include systematic parameter errors. Our scheme maintains the geometric properties of the evolution and results in an environment-assisted holonomic quantum map that can mimic the effect of a holonomic gate. We demonstrate that the sensitivity to systematic errors can be reduced in a proof-of-concept spin-bath model.

Keywords
Quantum computation, geometric phase, open quantum systems
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-368733 (URN)10.1103/PhysRevLett.122.140501 (DOI)000463902800002 ()
Funder
Swedish Research Council, 2017-03832
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2019-05-03Bibliographically approved
Azimi Mousolou, V. & Sjöqvist, E. (2018). Entangling power of holonomic gates in atom-based systems. Journal of Physics A: Mathematical and Theoretical, 51(47), Article ID 475303.
Open this publication in new window or tab >>Entangling power of holonomic gates in atom-based systems
2018 (English)In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 51, no 47, article id 475303Article in journal (Refereed) Published
Abstract [en]

Entanglement is one of the main resources of quantum computation, and entangling power of a quantum system is a crucial element in the universality and efficiency of a proposed architecture for realization of quantum processing. Our goal here is to study the entangling power of holonomic gates in some particular systems. We explore the holonomy-induced entanglement, by means of nonadiabatic quantum holonomies, through different types of interactions in atom-based systems, namely, the tripod-type interaction induced by the quantum Zeno effect between three-level atoms, as well as the Λ-type interaction arising from dipole–dipole or van der Waals forces between high-lying states of two-level atoms in systems consisting of N optically trapped identical atoms. Our analysis shows that although the two schemes provide completely separate classes of entangling gates, both schemes permit for full entangling power and also in the sense of quantum efficiency both families of entanglers consist of holonomic gates that have the same efficiency in quantum algorithms. Besides, we observe that holonomy-induced entanglement characteristics remarkably depend on the interaction configuration of the system.

Keywords
Quantum gates, quantum holonomy, quantum entanglement, entangling power, holonomic gates, quantum computation, holonomy-induced entanglement
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-343839 (URN)10.1088/1751-8121/aae78b (DOI)000448961400001 ()
Funder
Swedish Research Council, 2017-03832
Available from: 2018-03-02 Created: 2018-03-02 Last updated: 2019-06-26Bibliographically approved
Xu, G., Tong, D. & Sjöqvist, E. (2018). Path-shortening realizations of nonadiabatic holonomic gates. Physical Review A. Atomic, Molecular, and Optical Physics, 98, Article ID 052315.
Open this publication in new window or tab >>Path-shortening realizations of nonadiabatic holonomic gates
2018 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 98, article id 052315Article in journal (Refereed) Published
Abstract [en]

Nonadiabatic holonomic quantum computation uses non-Abelian geometric phases to implement a universal set of quantum gates that are robust against control imperfections and decoherence. Until now, a number of three-level-based schemes of nonadiabatic holonomic computation have been put forward, and several of them have been experimentally realized. However, all these works are based on the same class of nonadiabatic paths, which originates from the first nonadiabatic holonomic proposal. Here, we propose a universal set of nonadiabatic holonomic gates based on an extended class of nonadiabatic paths. We find that nonadiabatic holonomic gates can be realized with paths shorter than the known ones, which provides the possibility of realizing nonadiabatic holonomic gates with less exposure to decoherence. Furthermore, inspired by the form of this new type of paths, we find a way to eliminate decoherence from nonadiabatic holonomic gates without resorting to redundancies.

Keywords
Quantum computation, quantum gates, quantum holonomy
National Category
Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics; Physics
Identifiers
urn:nbn:se:uu:diva-364265 (URN)10.1103/PhysRevA.98.052315 (DOI)000450138700002 ()
Funder
Swedish Research Council, 2017-03832Carl Tryggers foundation , 14:441
Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2019-01-22Bibliographically approved
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, article id 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.

Keywords
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
Sjöqvist, E. (2017). Proposed neutron interferometry test of Berry's phase for a circulating planar spin. Physical Review A. Atomic, Molecular, and Optical Physics, 96, Article ID 052117.
Open this publication in new window or tab >>Proposed neutron interferometry test of Berry's phase for a circulating planar spin
2017 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 96, article id 052117Article in journal (Refereed) Published
Abstract [en]

The energy eigenstates of a spin-1/2 particle in a magnetic field confined to a plane, define a planar spin. If the particle moves adiabatically around a loop in this plane, it picks up a topological Berry phase that can only be an integer multiple of π. We propose a neutron interferometry test of the Berry phase for a circulating planar spin induced by a magnetic field caused by a very long current-carrying straight wire perpendicular to the plane. This Berry phase causes destructive interference in the direction of the incoming beam of thermal neutrons moving through a triple-Laue interferometer. 

Keywords
Neutron interferometry, topological phase, Berry phase
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-330865 (URN)10.1103/PhysRevA.96.052117 (DOI)000415167000001 ()
Funder
Swedish Research Council, D0413201
Available from: 2017-10-05 Created: 2017-10-05 Last updated: 2018-02-22Bibliographically approved
Xu, G., Zhao, P., Tong, D. & Sjöqvist, E. (2017). Robust paths to realize nonadiabatic holonomic gates. Physical Review A. Atomic, Molecular, and Optical Physics, 95(5), Article ID 052349.
Open this publication in new window or tab >>Robust paths to realize nonadiabatic holonomic gates
2017 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 95, no 5, article id 052349Article in journal (Refereed) Published
Abstract [en]

To realize one desired nonadiabatic holonomic gate, various equivalent evolution paths can be chosen. However, in the presence of errors, these paths become inequivalent. In this paper, we investigate the difference of these evolution paths in the presence of systematic Rabi frequency errors and aim to find paths with optimal robustness to realize one-qubit nonadiabatic holonomic gates. We focus on three types of evolution paths in the $\Lambda$ system: paths belonging to the original two-loop scheme [New J. Phys. 14, 103035 (2012)], the single-loop multiple-pulse scheme [Phys. Rev. A 94, 052310 (2016)], and the off-resonant single-shot scheme [Phys. Rev. A 92, 052302 (2015); Phys. Lett. A 380, 65 (2016)]. Whereas both the single-loop multiple-pulse and single-shot schemes aim to improve the robustness of the original two-loop scheme by shortening the exposure to decoherence, we here find that the two-loop scheme is more robust to systematic errors in the Rabi frequencies. More importantly, we derive conditions under which the resilience to this kind of error can be optimized, thereby strengthening the robustness of nonadiabatic holonomic gates.

Keywords
Quantum computation, geometric phase
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-321939 (URN)10.1103/PhysRevA.95.052349 (DOI)000402463200008 ()
Funder
Swedish Research Council, D0413201Carl Tryggers foundation , 14:441
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2018-09-14Bibliographically approved
Zhao, P., Cui, X.-D., Xu, G., Sjöqvist, E. & Tong, D. (2017). Rydberg-atom-based scheme of nonadiabatic geometric quantum computation. Physical Review A. Atomic, Molecular, and Optical Physics, 96(5), Article ID 052316.
Open this publication in new window or tab >>Rydberg-atom-based scheme of nonadiabatic geometric 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. 96, no 5, article id 052316Article in journal (Refereed) Published
Abstract [en]

Nonadiabatic geometric quantum computation provides a means to perform fast and robust quantum gates. It has been implemented in various physical systems, such as trapped ions, nuclear magnetic resonance and superconducting circuits. Another system being adequate for implementation of nonadiabatic geometric quan- tum computation may be Rydberg atoms, since their internal states have very long coherence time and the Rydberg-mediated interaction facilitates the implementation of two-qubit gate. Here, we propose a scheme of nonadiabatic geometric quantum computation based on Rydberg atoms, which combines the robustness of nonadiabatic geometric gates with the merits of Rydberg atoms. 

Keywords
Geometric phase, Rydberg atoms, quantum computation
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-332818 (URN)10.1103/PhysRevA.96.052316 (DOI)000414951500011 ()
Funder
Swedish Research Council, D0413201Carl Tryggers foundation , CTS 14:441
Available from: 2017-11-01 Created: 2017-11-01 Last updated: 2018-03-19Bibliographically approved
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, article id 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
Keywords
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, article id 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.

Keywords
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)000415196900003 ()
Funder
Swedish Research Council, D0413201Swedish National Infrastructure for Computing (SNIC), snic2017-7-17
Available from: 2017-08-08 Created: 2017-08-08 Last updated: 2018-02-21Bibliographically 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, p. 3995-4011Article 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. 

Keywords
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
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