Five-second coherence of a single spin with single-shot readout in silicon carbide

Abstract

An outstanding hurdle for defect spin qubits in silicon carbide (SiC) is single-shot readout, a deterministic measurement of the quantum state. Here, we demonstrate single-shot readout of single defects in SiC via spin-to-charge conversion, whereby the defect's spin state is mapped onto a long-lived charge state. With this technique, we achieve over 80% readout fidelity without pre- or postselection, resulting in a high signal-to-noise ratio that enables us to measure long spin coherence times. Combined with pulsed dynamical decoupling sequences in an isotopically purified host material, we report single-spin T₂ > 5 seconds, over two orders of magnitude greater than previously reported in this system. The mapping of these coherent spin states onto single charges unlocks both single-shot readout for scalable quantum nodes and opportunities for electrical readout via integration with semiconductor devices.

Fig. 1.. Control and readout of spin and charge states of the divacancy.

(A) Optical and charge transitions of the divacancy. Excitation of the E_x and E_1,2 spin-selective optical transitions is performed using ~1131-nm light (yellow). Spin-flips (Γ_flip) prevent indefinite readout of the spin state with laser pumping. Microwave (MW) manipulation is used to induce ground-state spin-sublevel transitions. SCC is performed by excitation of the E_x transition followed by an ejection of a hole by the 1151-nm ionization laser (green). The dashed line represents the VV⁰ excited state. The 705-nm light (red) resets the divacancy from VV⁻ to VV⁰. The individual lasers used for SCC and charge resetting do not have enough energy to induce other charge transitions via a one-photon process (denoted by an “X”). (B) PL excitation spectrum of a single divacancy reveals its six spin-selective optical transitions at T = 5 K and B = 18 G with continuous microwave driving of the m_s = 0↔m_s = +1 transition. Detuning is relative to a center laser frequency of 265.1408 THz, and the transverse strain splitting is 9.78 GHz. The E_x and E_y optical transitions are m_s = 0 character, while the E_1,2, A₁, and A₂ transitions are m_s = ±1 character (5). (C) Mapping of the spin state onto the charge state. Pumping of the E_x transition allows for ionization of m_s = 0 with the 1151-nm laser. The SCC step is followed by charge readout via pumping of both the E_x and E_1,2 transitions. Detection of PL signifies that the divacancy is in its bright, neutral (dark, ionized) state and therefore was prepared to m_s = +1 (m_s = 0). (D) Typical experimental pulse sequence. After the charge and spin initialization and microwave manipulation of the spin state, single-shot readout of the spin state is performed with SCC followed by readout of the charge state.

Fig. 2.. Single-shot readout of the divacancy charge state.

(A) Charge readout PL signal dependence on delay time between the charge initialization and readout follows an exponential decay $e^{(-\frac{t}{{\mathrm{\tau }}_{\text{ch}}})}$, where τ_ch is the charge lifetime. We find that τ_ch is 6.9(0.9) s. (B) Log-scale histogram of photon number distributions collected during a charge readout for preparation into the neutral bright state and ionized dark state. We use a 20-ms readout window with 4.05-μW combined resonant laser power, selected to maximize the readout fidelity. For a cutoff of N = 4 photons, the single-shot readout fidelity of the charge state is 98.7(1.3)%. The false-positive rate p_0|1 = 1.17%, and false-negative rate p_1|0 = 1.26%. (C) Extracted photons per shot from observed PL rate and charge state decay for various combined resonant laser powers. The maximal extracted photons per shot is N = 1529(117). The line is a fit from a model (Supplementary Materials). All data are taken at B = 18 G and T = 5 K. All reported errors represent 1 SE from the fit, and all error bars represent 1 SD of the raw data.

Fig. 3.. Optical charge reset and ionization processes.

(A) Power dependence of the charge reset rate using the 705-nm laser. The reset rate is linear (red line fit) with power as 993 ± 17 Hz/μW. (B) Ionization rate dependence on combined resonant laser power (E_x and E_1,2 lines). The solid line is a fit using a saturating two-photon ionization model (Supplementary Materials). (C) Ionization rate dependence on 1151-nm laser alone. The ionization rate is linear with power (solid line fit) as 95.7 ± 3.7 kHz/W. (D) Spin-agnostic ionization rate dependence on the 1151-nm laser ionization power. The resonant laser excitation is beyond saturation at 15 μW. The solid blue line is a fit from our model including the effect of stimulated emission (Supplementary Materials), where the low-power ionization rate is 37.4 ± 0.7 MHz/W. All data are taken at B = 18 G and T = 5 K. All reported errors represent 1 SE from the fit, and all error bars represent 1 SD of the raw data.

Fig. 4.. Single-shot readout of the spin state with SCC.

(A) Charge readout signal following SCC step for preparation into m_s = 0 and m_s = +1. The maximum fitted contrast is 68.2(0.4)% at an ionization laser power of 71 mW and a SCC pulse duration of t_ion = 1.39 μs using 14.95 μW of resonant power. cps, counts per second. (B) Charge readout photon number distribution after SCC step for preparation into m_s = 0 and m_s = +1. The end-to-end process fidelity is 80.8(0.6)% for a cutoff of N = 2 photons. (C) Dependence of SCC fidelity on SCC pulse duration. (D) Dependence of SCC contrast with 1151-nm ionization laser power. The contrast follows a saturation behavior (red line fit; Supplementary Materials). (E) Calculated stimulated emission rate dependence on 1151-nm ionization laser power. The stimulated emission rate increases linearly as 13.3 MHz/W (yellow line fit). All data are taken at B = 18 G and T = 5 K. All reported errors represent 1 SE from the fit, and all error bars represent 1 SD of the raw data.

Fig. 5.. Ultralong spin coherence and lifetime for a single divacancy.

(A) T₁ spin relaxation time of divacancy using single-shot readout. Using a goodness of fit test with a 95% confidence interval, we estimate that T₁ ≥ 103 s (Supplementary Materials). The fit (solid line) is for a T₁ of 103 s. (B) T₂ decay curves measured after applying dynamical decoupling pulses sequences of increasing pulse number, N. We avoid electron spin echo envelope modulation oscillations by enforcing pulse spacing requirements as in (28), eliminating sharp dips, and smoothing to find the coherence function envelope. The envelope is fit to a stretched exponential function ${\mathrm{Ae}}^{-{\left(\frac{t}{\mathrm{\tau }}\right)}^n}$, where n is a stretch factor. (C) Extension of T₂ coherence time with total decoupling pulse number. We fit in log space the low (blue) and high (red) pulse number regimes as T₂ ~ N^ψ. All data are taken at B = 18 G and T = 5 K. All reported errors represent 1 SE from the fit, and all error bars represent 1 SD of the raw data.