Progetto comune di ricerca

Immagini con risoluzione alla scala atomica dello stato condensato superconduttivo nel composto Sr2RuO4: una piattaforma per calcoli quantistici topologici?

Responsabili di progetto
Andrea Gerbi, Peter Wahl
REGNO UNITO - RSE - The Royal Society of Edinburgh (Scotland)
CNR/RSE biennio 2019-2020 (SCOZIA) 2019-2020
Scienze fisiche e tecnologie della materia
Area tematica
Scienze fisiche e tecnologie della materia
Stato del progetto

Proposta di ricerca

Achieving quantum computation with truly entangled qubits promises transformative advances in current information technologies. A growing worldwide research effort is oriented towards its viable implementation [e.g. Riedel17]. A central problem in realizing quantum computers is dealing with decoherence effects, which can destroy the quantum states of the internal registers. Several schemes have been developed to reduce decoherence, through error correction schemes or exploitation of protected quantum states to achieve "fault-tolerant" quantum computation. One such proposal is through topological quantum computation (TQC) [Kitaev97]. In the fault-tolerant TQC, the unitary quantum gates result from the braiding of topological quantum objects called 'anyons' [Sarma15]. Only anyons with non-Abelian braiding statistics can be used as robust building blocks for TQC. The simplest realization of a non-Abelian anyon is a quasiparticle or defect in an ordered system supporting a Majorana zero mode (MZM). Abrikosov vortices in a topological superconductor are one possible realization. MZMs are expected to exist in the vortex cores of a chiral p-wave superconductor. In such a superconductor, vortices can exist with half of the usual superconducting flux quantum and obey non-abelian braiding statistics when moved around each other. The extra stability afforded through such topological states, in principle, could be used to encode qubits with unusually long decoherence time.
The realization of topological quantum computation in this scheme depends critically on identifying a suitable material basis. One of the few materials which have been argued to host the required symmetry of the superconducting condensate is Sr2RuO4, which has been argued to exhibit a superconducting state with chiral p-wave symmetry [Kallin12]. Both, the superconducting order and the stabilization of Majorana fermions in this compound are still under debate though and demand novel targeted studies. Background knowledge indicates a number of exotic properties of Sr2RuO4, mostly originated from theoretical investigations and macroscopic characterization. Among them:
1. Theoretical investigations indicate that Sr2RuO4 can host topologically protected quantum states, such as an unconventional pairing similar to the one realized in the A phase of superfluid 3He. The order parameters of the p-wave pairing state is multicomponent due to the extra orbital and spin degree of freedom with time- reversal breaking symmetry [Kallin12, Rice95];
2. Half-quantum vortices exist and are due to the extra spin freedom in the order parameter [Jang11]. The half-quantum vortices are expected to host MZMs at exactly zero energy as bound states inside the vortex cores [Read00].
3. A distinctive feature of Sr2RuO4 is to have multiple bands crossing the Fermi energy. Therefore, the multiband nature of the system, the interplay between the different degree of freedom (spin and lattice) and the strong k-dependent of the spin-orbit coupling make the description of the superconductivity in this compound quite complex. [Iwasawa00, Veenstra14].
This proposal plans to make a step forward into these crucial issues, by taking advantage of the complementary competences and instrumentation facilities hosted by the two research partners from CNR and the University of St Andrews in the fields of quantum technologies (with an approved CNR project QUANTOX in the QuantERA Consortium), superconductivity, strongly correlated electron materials and experimental characterization through advanced, atomically-resolved Scanning Tunneling Microscopy/Spectroscopy (STM/STS) techniques. In particular, the project synergistically combines the synthesis of high quality fully-characterized Sr2RuO4 single-crystal samples provided by the CNR partner, with the high spatial and energy resolution offered by ultra-low-temperature STM/STS and theoretical modeling at the University of St Andrews. In fact, STM proved to be an important tool in determining band structure and its relationship to the superconductivity, including the superconducting symmetry in cuprate superconductors. Only a few STM studies have been reported up to now on Sr2RuO4 samples in the superconducting state [Firmo13, Suderow09, Upward02]. This project involves cutting-edge STM experiments to study the superconducting condensate of Sr2RuO4, in search for specific signatures discriminating among different superconducting orders and providing evidences for the existence of MZMs, namely by means of Bogoliubov quasiparticle interference (QPI) and spin-polarized STM at the vortex core. The proposed experimental methodology has the potential to greatly contribute to the current scientific debate.

[Akbari13] A. Akbari, P. Thalmeier, Phys. Rev. B 88, 134519 (2013)
[Firmo13] I. A. Firmo et al. Phys. Rev. B 88, 134521 (2013)
[Huo13] J. W. Huo, F-C. Zhang, Phys. Rev. B 87, 134501 (2013)
[Iwasawa00] H. Iwasawa et al., Phys. Rev. Lett. 105, 226406 (2010)
[Jang11] J. Jang et al., Science 331, 186 (2011)
[Kallin12] C. Kallin, Rep. Prog. Phys. 75, 042501 (2012)
[Kawakami15] T. Kawakami and X. Hu Phys. Rev. Lett. 115, 177001 (2015)
[Kitaev97] A. Kitaev, Ann. Physics 303, 2 (1997)
[Read00] N. Read and D. Green, Phys. Rev. B 61, 10267 (2000)
[Rice95] T. M. Rice and M. Sigrist, J. Phys.: Condens. Matter 7, L643 (1995).
[Riedel17] M. F. Riedel, T. Calarco, The European Quantum Technologies Roadmap, arXiv:1712.03773
[Sarma15] S-D. Sarma et al., npj Quantum Information 1, 15001 (2015)
[Suderow09] H. Suderow et al., New J. of Phys. 11, 093004 (2009)
[Sun16] H-H. Sun et al., Phys. Rev. Lett. 116, 257003 (2016)
[Thalmeier16] P. Thalmeier, A. Akbari, J. Magnetism Magnetic Materials 400, 23 (2016)
[Upward02] M. D. Upward et al., Phys. Rev. B 65, 220512(R) (2002)
[Veenstra14] C. N. Veenstra et al., Phys. Rev. Lett. 112, 127002 (2014)
[Wang17] Z. Wang et al., Nature Physics 13, 799 (2017)
[Xu15] J-P. Xu et al., Phys. Rev. Lett. 114, 017001 (2015)

Obiettivi della ricerca

The project intends to establish a new link and seed a synergetic and mutually beneficial cooperation between two institutions (St. Andrews University and CNR-SPIN Institute). Both have a track record in the study of quantum technologies (QUANTOX, an approved CNR-SPIN project), superconductivity and strongly correlated materials. The long experience and combined expertise of the involved groups enables to target important scientific goals. Results will be published in joint papers in high impact journals, as well as through dissemination in oral presentations at national and international conferences.
We envision strong bilateral impact, with creation of new opportunities for fundamental and applied research in the TQC field thanks to the effective exchange of complementary competences and knowledge.
From the scientific point of view the two main expected goals are:

1. To determine the symmetry of the Sr2RuO4 superconducting state by means QPI STM experiments, that have been successfully used to investigate the strongly correlated unconventional superconductors and very recently the normal state of the Sr2RuO4 [Wang17];

2. To detect and characterize MZMs by mapping the spatial and energy evolution of the local density of states (LDOS) within individual Abrikosov vortices. The existence of MZMs should provide signatures detectable by selecting the spin-sensitive part of the LDOS across the vortex cores.

Ultimo aggiornamento: 12/08/2020