Transport Signatures of Quasiparticle Poisoning in a Majorana Island

Abstract

We investigate effects of quasiparticle poisoning in a Majorana island with strong tunnel coupling to normal-metal leads. In addition to the main Coulomb blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage, consistent with transport through an excited (poisoned) state of the island. Comparison to a simple model yields an estimate of parity lifetime for the strongly coupled island (∼ 1 µs) and sets a bound for a weakly coupled island (> 10 µs). Fluctuations in the gate-voltage spacing of Coulomb peaks at high field, reflecting Majorana hybridization, are enhanced by the reduced lever arm at strong coupling. In energy units, fluctuations are consistent with previous measurements.Hybrid semiconductor-superconductor nanowire devices have been the focus of intense research in recent years [1-6] primarily because they are expected to support Majorana zero modes [7,8]. Of particular relevance to schemes for Majorana fusion-rule testing, braiding, and Majorana-based quantum computation [9][10][11][12] is the Majorana island geometry, in which the topological hybrid nanowire acquires a charging energy that lifts the degeneracy between occupied and empty Majorana states [6,[13][14][15][16], allowing for charge readout of the state parity.A fundamental bound to the coherence of Majorana based qubits is the parity lifetime of the Majorana state, limited by quasiparticle poisoning [9,17,18]. Studies on metallic superconductors have explored the associated poisoning rates in great detail [19][20][21][22][23][24][25][26][27][28], while experiments on semiconductor-superconductor hybrids have only established bounds on the relaxation rate of quasiparticles into the subgap state [29], with quantitative estimates for poisoning from external sources still pending.In this Letter, we use Coulomb blockade spectroscopy to quantify the quasiparticle poisoning time of a Majorana island. We find the poisoning time to a state with one extra quasiparticle in the BCS continuum to be ∼ 1 µs in the regime of relatively strong coupling between the island and the leads, and bounded from below by 10 µs in the less strongly coupled regime investigated in Ref. [29]. Our results demonstrate transport signatures of quasiparticle poisoning in Majorana islands up to the topological phase transition and place constraints on a relevant timescale for topological quantum computation and Majorana braiding.The device we investigate consisted of an MBE-grown [0001] wurtzite InAs nanowire with epitaxial Al on two of six facets [ Fig. 1(a)], which induces a hard superconducing gap in the nanowire [30,31]. The Al shell was removed on both ends using a chemical etch, leaving an Al island of length L ∼ 400 nm. Uncovered InAs segments at the wire ends are electrically contacted using normal-metal (Ti/Au) ohmic contacts. Lithographically patterned electrostatic gates near the ∼ 50 nm exposed segments next to the ohmic contacts were used to deplete carriers, bringing the device into the Coulomb blockade regime. Magnetic fields were applied per...

Probing electron-hole components of subgap states in Coulomb blockaded Majorana islands

“…For further details on the master-equation model and the calculation, see e.g. the supplementary material of [21] [22].…”

Dynamic current susceptibility as a probe of Majorana bound states in nanowire-based Josephson junctions

“…Following theoretical proposals [6,7], several experiments [8][9][10][11][12][13][14][15] have reported characteristic transport signatures in the form of a zerobias conductance peak compatible with the presence of zero-energy MBS. Despite this evidence, however, the nagging question of whether zero-bias peaks are due to MBS is still under debate [16].…”

Josephson effect in multiterminal topological junctions

“…As a first example for this approach, we study the local density of states at the boundary of the wire, 11 , where the indices now refer to spin-Nambu space. Figure 5 illustrates the evolution of ρ(E) as one varies the Zeeman scale V x .…”

“…In particular, presently available devices with a hard superconducting gap [8][9][10][11] and disorder levels close to the ballistic limit [12,13] exhibit characteristic transport signatures as expected for Majorana bound states (MBSs). The latter are robust states localized at the wire ends within the topological phase.…”

“…The possibility to induce topological superconductivity in semiconducting nanowires with strong spin-orbit coupling (e.g., InAs or InSb) by proximity coupling to a conventional superconductor has paved the way to exciting novel developments [1][2][3][4][5][6][7][8][9][10][11][12][13]. In particular, presently available devices with a hard superconducting gap [8][9][10][11] and disorder levels close to the ballistic limit [12,13] exhibit characteristic transport signatures as expected for Majorana bound states (MBSs).…”