Control over quantum dynamics of open systems is one of the central challenges in quantum science and engineering. Coherent optical techniques, such as coherent population trapping involving dark resonances1, 2, are widely used to control quantum states of isolated atoms and ions. In conjunction with spontaneous emission, they allow for laser cooling of atomic motion3, preparation and manipulation of atomic states4, and rapid quantum optical measurements that are essential for applications in metrology5, 6, 7. Here we show that these techniques can be applied to monitor and control individual atom-like impurities, and their local environment8, 9, 10, 11, in the solid state. Using all-optical manipulation of the electronic spin of an individual nitrogen–vacancy colour centre in diamond, we demonstrate optical cooling, real-time measurement and conditional preparation of its nuclear spin environment by post-selection. These methods offer potential applications ranging from all-optical nanomagnetometry to quantum feedback control of solid-state qubits, and may lead to new approaches for quantum information storage and processing
Figures at a glance
Figure 1: Coherent population trapping in NV centres. 
a, The Λ-type transitions between the ground states |±1
and excited states |A1 and |A2 of a single NV centre are addressed with a CPT laser, while a recycling laser drives the |0 
|Ey transition. An external magnetic field, B, is applied using a solenoid. b, Photon count from NV centre NVa in a 300-μs window are plotted versus the applied field for three different powers of a laser addressing the state |A2: blue, 10 μW; pink, 3 μW; yellow, 0.1 μW. The blue and pink data sets are shifted vertically by five and, respectively, two counts for clarity. μB, Bohr magneton; g, gyromagnetic ratio. c, Width of individual 14N CPT lines versus CPT laser power when the |A1 (blue) or |A2 (pink) state is used. Solid curves represent the theoretical model discussed in the main text and Supplementary Information. Error bars, s.d.
Figure 2: Optical control and conditional preparation of the proximal 14N nuclear spin. 
a, Mechanism for optical pumping of 14N states. b, Pulse sequence for 14N optical pumping using the laser addressing the state |A1
(A1 laser) and a fixed read-out of the prepared state using the A2 laser. To ensure that the nitrogen–vacancy was not ionized for all subsequent data runs, we turn on all three lasers at the end of each run so that there is no dark state, and only keep data from runs in which we obtain a high number of counts during this verification step. c, Counts collected with NVa during the read-out step versus the read-out magnetic field when there is no preparation step (blue) and when there is preparation with optical pumping using 100 nW of A1 laser power for 1.9 ms (pink). The yellow curve shows the results of 14N polarization through measurement-based preparation by selecting the read-out events in which the number of counts collected during the last 500 μs of preparation is zero (Supplementary Information, section 4.2). d, Steady-state population in the mI = 0 state after optical pumping for varying powers of the A1 laser, with theoretical model described in Supplementary Information (solid line). Error bars, s.d.
Figure 3: Observation of instantaneous Overhauser field from the 13C spin bath. 
a, Pulse sequence for real-time measurement of the 13C nuclear configuration. The applied magnetic field is ramped over a single 14N CPT line over 5 ms while counts are collected in 80-μs bins. b, Counts from 200 successive runs are shown on horizontal lines for NVb. Runs in which the verification step fails are blacked out. The centres of constrained Lorentzian fits (Supplementary Information, section 7) to individual runs are indicated with green dots. c, Two such individual runs are shown with their fits (pink and blue), along with an average of scans that passed verification (yellow). d, Autocorrelation, R(τ), of counts with magnetic field fixed at the mI = 1 14N line. The fit is to a bi-exponential decay.
Figure 4: Measurement-based preparation of 13C spin bath. 
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