Electrical control of single hole spins in nanowire quantum dots

Journal name:
Nature Nanotechnology
Volume:
8,
Pages:
170–174
Year published:
DOI:
doi:10.1038/nnano.2013.5
Received
Accepted
Published online

Abstract

The development of viable quantum computation devices will require the ability to preserve the coherence of quantum bits (qubits)1. Single electron spins in semiconductor quantum dots are a versatile platform for quantum information processing, but controlling decoherence remains a considerable challenge1, 2, 3, 4. Hole spins in III–V semiconductors have unique properties, such as a strong spin–orbit interaction and weak coupling to nuclear spins, and therefore, have the potential for enhanced spin control5, 6, 7, 8 and longer coherence times8, 9, 10, 11, 12. A weaker hyperfine interaction has previously been reported in self-assembled quantum dots using quantum optics techniques10, 11, 12, but the development of hole–spin-based electronic devices in conventional III-V heterostructures has been limited by fabrication challenges13. Here, we show that gate-tunable hole quantum dots can be formed in InSb nanowires and used to demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tunable between hole and electron quantum dots, which allows the hyperfine interaction strengths, g-factors and spin blockade anisotropies to be compared directly in the two regimes.

At a glance

Figures

  1. Ambipolar transport in an InSb nanowire.
    Figure 1: Ambipolar transport in an InSb nanowire.

    a, Schematic of a device used to demonstrate ambipolar transport in InSb nanowires. b, Current through an InSb nanowire as a function of voltage VBG applied to the silicon backgate, for a device as shown in a (source–drain bias VSD = 10 mV). The nanowire is separated from the backgate by 285 nm of SiO2, and the spacing between the source and drain Ti/Au contacts is ~300 nm. Right (left) insets: when the Fermi level is tuned to the conduction (valence) band, current transport in the nanowire is mediated by electrons (holes). Hole conductance is typically one to two orders of magnitude lower than electron conductance. This could indicate reduced transparency of the nanowire–metal contact interface for holes16 or lower hole mobility14. The measured bandgap is ~0.2 eV, in agreement with the bulk value for InSb (ref 14; Supplementary Fig. S1).

  2. Gate tuning between electron transport and hole quantum dot.
    Figure 2: Gate tuning between electron transport and hole quantum dot.

    a, Scanning electron microscopy image of a typical gated InSb nanowire device used for studying quantum dots. The gate widths and intergate spacing are ~30 nm; the source and drain contacts are made of Ti/Al. A wider metallic gate is used to control the charge density in the nanowire segments between the narrow gates and the source and drain electrodes. The wide gate is separated from the narrow gates by a 50 nm layer of Si3N4. The fine gates are covered by an additional 25 nm layer of Si3N4. b, Schematic of band diagram showing a gate-defined hole quantum dot. Tunnelling between the dot and the n-type leads occurs via p–n junctions. c, Charge stability diagram (device d1) of a device as in a, shown as a function of source–drain bias VSD and plunger gate voltage VG. For less negative VG, the nanowire is n-type and current is carried by electrons. For more negative VG, transport is suppressed due to the bandgap of the nanowire. For even more negative VG, a hole quantum dot is formed above the plunger gate. In this regime a finite transport current is observed as a result of tunnelling via discrete hole states in the dot. d, Hole Coulomb diamonds for device d1 (the dot potential is tuned slightly differently than in c). The fluctuating diamond sizes and the absence of any further transitions to the right of the diamond labelled ‘0 h’ are consistent with the few-hole regime. However, unambiguous identification of the number of holes would require a charge sensor. From the size of the Coulomb diamonds we estimate that charging energies Ec of single holes are on the scale of 20 meV and orbital energies Eorb are between 3 and 8 meV.

  3. Gate-defined few-hole double quantum dot.
    Figure 3: Gate-defined few-hole double quantum dot.

    a, Schematic band diagram showing a hole double quantum dot and n-type leads. Tunnelling onto and off the double dot occurs via p–n junctions. b, Charge stability diagram of a hole double quantum dot as a function of the left and right plunger gate voltages (VLG and VRG) (device d2, first cooldown). The larger, dimmer triangles are attributed to an additional quantum dot located in series to the right of the hole double dot and strongly coupled to the drain reservoir (Supplementary Section S2). We estimate the size of the dots to be ~15 nm from . Inset: stability diagram over a larger area of gate space (device d1, VSD = 12 mV), showing the transition from electron transport (upper-right corner) to the hole double dot regime (lower-left region).

  4. Electric-dipole spin resonance and hole g-factor anisotropy.
    Figure 4: Electric-dipole spin resonance and hole g-factor anisotropy.

    a, Interdot tunnelling is suppressed by spin blockade whenever unpaired holes in each dot form a triplet state (T). A microwave-frequency electric field of amplitude Eac applied to the right plunger gate induces spin rotation to a singlet (S) by means of EDSR, lifting the spin blockade. b, EDSR for weak interdot coupling (device d2, second cooldown). The line cut is at f = 3.4 GHz. In addition to the EDSR resonances, we observe a lifting of the spin blockade near B = 0, attributed to the hyperfine interaction. c, Anisotropy of the hole g-factor extracted from EDSR measurements for different angles ϕ between the applied magnetic field B and the nanowire axis in the plane of the sample surface. Weak coupling g-factor data is from the second cooldown of device d2 and strong coupling data from the third cooldown of the same device.

  5. Hole spin blockade for strong interdot coupling.
    Figure 5: Hole spin blockade for strong interdot coupling.

    a, Double-dot current versus detuning ε and magnetic field B in the strong-coupling regime for three different angles (device d2, third cooldown, VSD = −6 mV). Spin blockade is observed as a dip near B = 0. b, Cuts along the dotted lines at ε = 0 in a. The applied magnetic field is scaled by the effective g-factors at each angle (see Fig. 4). For clarity, the data for ϕ = 45° and ϕ = 0° are offset vertically by 2.5 pA and 5.0 pA, respectively. Offsets of several mT due to the magnet were subtracted from the data in a and b.

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Author information

Affiliations

  1. Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands

    • V. S. Pribiag,
    • S. Nadj-Perge,
    • S. M. Frolov,
    • J. W. G. van den Berg,
    • I. van Weperen,
    • E. P. A. M. Bakkers &
    • L. P. Kouwenhoven
  2. Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

    • S. R. Plissard &
    • E. P. A. M. Bakkers

Contributions

V.S.P., S.N., S.M.F., J.W.G.B. and I.W. performed the measurements. V.S.P., S.N., S.M.F. and J.W.G.B. analysed the data. V.S.P., S.N. and J.W.G.B. fabricated the devices. S.R.P. and E.P.A.M.B. provided the nanowires. L.P.K. supervised the project. All authors contributed to writing the manuscript.

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