*Citation:* Hanson, R., O. Gywat, and D. D. Awschalom. "Room-temperature manipulation and decoherence of a single spin in diamond." Physical Review B 74.16 (2006): 161203.

*Web:* https://arxiv.org/abs/quant-ph/0608233

*Tags:* Hardware

T his paper was reviewed as part of a quantum information science class. It studies spin manipulations of the nitrogen-vacancy (N-V) center in diamond. The key fact about this center is that it is coherent on the scale of seconds, and it operates at room temperature. While other features of the N-V center make it not amenable to quantum computation, the N-V center has found use in other areas of quantum information science such as magnetic field detection. Coherent manipulation of large groups of N-V centers has been performed since the 80s,

> E. van Oort, N. B. Manson and M. Glasbeek, J. Phys. C 21, 4385 (1988).

but the manipulation of single N-V centers proved more elusive, and had been first performed two years prior to this paper:

> F. Jelezko, T. Gaebel, I. Popa, A. Gruber and J. Wrachtrup, Phys. Rev. Lett. 92, 076401 (2004).

The goal of this paper was to put single N-V center manipulation on more solid ground by using single-center spectroscopy to report the behavior of N-V centers while they are being manipulated.

The experimental setup was as follows. A large magnetic field is shot through the diamond. Then, an AC magnetic field is induced perpendicular to the static magnetic field by sending radio-frequency AC through a 25 micrometer thick gold wire. This gives the main form of manipulation in the setup. By changing the pulse width of the AC current through the wire as an independent variable, one can examine how the N-V center reacts. The easiest dependent variable to analyze is the rate of radiative transition between the mS=1 and mS=0 level of the center. This is quantified as an intensity of phospholuminesce, and is plotted in Figure 2(b). The resulting plots are (exponentially normalized) sine waves. As the AC current pulse length increases, the resulting intensity of phospholuminesce oscillates. The rate of this oscillation is known as the Rabi frequency.

The next experiment performed in this paper is relevant to how one would physically implement the CNOT gate. To recall, the way that one uses spin qubits to perform CNOT is by first rotating one spin pi/2 radians away from the magnetic field, then waiting a prescribed amount of time, then flipping all the way to the other side with a pi radian rotation, then waiting the same amount of time as before, and then applying a final pi/2 rotation. These rotations away from the axis of the magnetic field are performed with a procedure known as a Hahn echo. In the second part of this paper, they show that these Hahn echos can be performed on the N-V center with the desired results.