Vanadium spin qubits as telecom quantum emitters in silicon carbide

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Science Advances  01 May 2020:
Vol. 6, no. 18, eaaz1192
DOI: 10.1126/sciadv.aaz1192
  • Fig. 1 Optical spectroscopy of V4+ in 4H-SiC and 6H-SiC.

    (A) SiC lattices and their inequivalent sites for the V impurities. (B) Expected energy diagram of the orbital d1 states from crystal field theory and spin-orbit (S-O) coupling according to (17, 24). The quasi-cubic (k) sites mainly have a tetrahedral (Td) symmetry, and the quasi-hexagonal (h) sites mainly have a C3v symmetry due to additional trigonal distortion. (C) Resonant PL spectroscopy at 3.3 K (blue) and 15 K (red) for all the available sites. The different transitions are partially identified from the difference in thermal population of the orbital states. The assigned transitions for the 6H-SiC γ site are resolved in fig. S9. The weak sharp peak at the center of the 6H-SiC γ site spectrum is an artifact from weak transient laser side modes at this exact wavelength. arb. units, arbitrary units. (D) Resonant PL spectroscopy of the 6H-SiC β site at 3.3 K fitted according to an isotope model for the mass shift from neighbor 28Si, 29Si, 30Si, 12C, and 13C. (E) Optical lifetimes at 3.3 K after laser excitation of the GS1-ES1 transition for each available site. All error bars are 1 SD from experimental acquisition.

  • Fig. 2 Single V4+ α site emitters implanted in 4H-SiC.

    All experiments are at 3.3 K. (A) Spatial (near-surface) PL mapping of single and few V defects by resonant excitation at 1278.8 nm. (B) and (C) are obtained at the circled bright spot with spatial feedback to prevent drifting. (B) g(2) autocorrelation measurement obtained with a single detector with 20-ns deadtime and 10-ns resolution. The autocorrelation signal is normalized using its value at long delay time, and the dark count contribution is calculated and subtracted (~3% of total). The red line is the fit [g(2)(0) = 0.1(1)], and the red shadowed area is the 95% confidence interval. In the inset, the autocorrelation intensity is shown for longer times. (C) Resonant spectrum taken over 100 acquisitions for a total duration of 15 hours, with averaged intensity shown in the bottom. The spectrum shows two maxima from the slightly resolved electron spin states. (D) Resonant spectrum taken for a variety of likely single emitters [not confirmed with g(2)]. Their fitted linewidths (right) remain consistent at about 750-MHz full width at half maximum (FWHM). All the single-defect experiments are conducted at around 700 G to narrow the linewidth (high field limit). A weak 365-nm continuous illumination helps stabilize the PL from charge conversion (possibly from two-photon ionization). (C) and (D) are calibrated using a wavemeter with below 50-MHz accuracy. All error bars are 1 SD from experimental acquisition.

  • Fig. 3 Magnetic resonance spectroscopy of the V4+ β site in 4H-SiC at 3.4 K.

    See figs. S4 to S7 for all the other sites in 4H-SiC and 6H-SiC. (A) Energy level structure including the electron spin (S) and nuclear spin (I) for the GS1, GS2, and ES1 levels. The static magnetic field (B0) dependence is simulated using fitted spin parameters from (B), (C), and (F). (B) ODMR of the lowest ground-state GS1 as a function of B0 and microwave drive frequency. Both B0 and the microwave drive (B1) are applied parallel to the c axis. The dashed lines in black are modeled from fitting the spin Hamiltonian in Eq. 1. The lines at low magnetic fields around 400 MHz are attributed to ES1. At high magnetic field under parallel excitation, the spin transitions become forbidden, and the ODMR contrast disappears. (C) ODMR of GS2 under similar conditions to (B). (D) ODMR as a function of the laser wavelength to correlate ground states and optical transitions. The red and blue curves are measured, respectively, at the red and blue circles in (B) and (C). The curves are fitted (using the isotope model line shape) to extract the individual transitions. (E) X-band ESR signal as a function of B0 and angles from the c axis. The blue and red lines are simulated from the spin parameters obtained by ODMR and are shown to compare derivative peak center positions. (F) ODMR of the excited state ES1 with B1 orthogonal to B0. Nonfitted signals in the background are attributed to GS1/2 and are much weaker owing to low g values. (G) Optical hole burning recovery using pump-probe excitations between GS1 and ES1 and detuned by the microwave frequency using an EOM. The transitions are modeled (black dashed lines) using the parameters obtained in (F).

  • Fig. 4 Lifetimes and coherent properties at 3.3 K.

    (A) Transient detection of hole burning and recovery in the 6H-SiC γ site. The transition GS1-ES1 is pumped (0 to 2 μs), followed by a moving delay before a second pump measures the intensity of the peak signal. (B) Optical decays obtained from the sequence in (A) where the amplitude is the hole depth [differential between maximum and steady-state amplitudes in (A)]. For 6H-SiC, the error bars are within the data point size. For 4H-SiC, the decay has some additional fluctuations that are not modeled here. (C) Left: Coherent Rabi oscillations obtained in the 6H-SiC β site at 200 G for two different microwave drive powers. Right: Corresponding Fourier transform spectrum with a 15-MHz Rabi frequency for the highest power. Fit (line) is from the equation in the main text. FFT, fast Fourier transform. (D) Pulsed-ODMR spectrum using an inversion pulse calibrated from (C). All error bars are 1 SD from experimental acquisition.

  • Table 1 Optical and spin properties of V4+ defects in 4H-SiC and 6H-SiC around 3.3 K.

    The k1 site is assigned to the 6H-SiC β site on the basis of having the closest crystal configuration and properties to the 4H-SiC β site. k2 is the most cubic-like site and therefore assigned to γ with the smallest GS1-GS2 splitting. The Debye-Waller (DW) factor is a coarse estimation as long-wavelength contributions cannot be observed (see fig. S1). τ is the optical lifetime. The spin parameters (absolute values for the diagonal components xx, yy, and zz) are given in their principal axis with g being the g-factor and A being the hyperfine interaction. xx and yy components are interchangeable, we cannot distinguish gxx from gyy, and a good fit is obtained when Axx and Ayy are equal for GS2 and ES1. θxx, θyy, and θzz are the angles between the principal and the c-axis basis. “-” indicates unresolved parameters.

    Site assignmenthkhk1k2
    ES1-GS1 (nm)1278.808(6)1335.331(6)1308.592(6)1351.845(6)1387.806(6)
    GS2-GS1 (GHz)529(1)43(1)524(1)25(1)16(1)
    ES2-ES1 (GHz)181(1)-167(1)628(1)6(1)
    ES3-ES2 (GHz)---72(1)-
    DW (%)~25~50~45~50~40
    τ (ns)167(1)45(1)108(1)11(1)31(1)
    GS1: gxx,yy, gzz0*,1.748*0 < g < 1,1.870(5)0*,1.749*-,1.95(2)0 < g < 1,1.933(5)
    GS1: Axx, Ayy, Azz (MHz)165,165,232(5)103,188,174(5)165,165,232(5)114,166,171(5)45,215,175(10)
    GS2: gxx,yy, gzz-0 < g < 1,2.035(5)--,2.00(2)0 < g < 1,1.972(5)
    GS2: Axx,yy, Azz-0,257(5)-0,258(5)0,265(5)
    GS2: θxx, θyy, θzz (°)-0,52(2),0-0,50(2),00,51(2),0
    ES1: gxx,yy, gzz-,2.24†-,2.03(2)-,2.24*-,2.0(1)-,2.03(2)
    ES1: Axx,yy, Azz (MHz)20,220(20)112,52(5)20,200(20)80,20(20)110,50(20)†

    * Parameters taken from literature (24, 25).

    †Partially resolved parameters obtained by comparison with other sites.

    Supplementary Materials

    • Supplementary Materials

      Vanadium spin qubits as telecom quantum emitters in silicon carbide

      Gary Wolfowicz, Christopher P. Anderson, Berk Diler, Oleg G. Poluektov, F. Joseph Heremans, David D. Awschalom

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      • Section S1
      • Figs. S1 to S10

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