Quantum adiabatic passages can be greatly accelerated by a suitable control field, called a counter-diabatic field, which varies during the scan through resonance. Here, we implement this technique on the electron spin of a single nitrogen-vacancy center in diamond. We demonstrate two versions of this scheme. The first follows closely the procedure originally proposed by Demirplak and Rice [J. Phys. Chem. A 107, 9937 (2003)]. In the second scheme, we use a control field whose amplitude is constant but whose phase varies with time. This version, which we call the rapid-scan approach, allows an even faster passage through resonance and therefore makes it applicable also for systems with shorter decoherence times.
Quantum memories provide intermediate storage of quantum information until it
is needed for the next step of a quantum algorithm or a quantum communication
process. Relevant figures of merit are therefore the fidelity with which the
information can be written and retrieved, the storage time, and also the speed
of the read-write process. Here, we present experimental data on a quantum
memory consisting of a single $^{13}$C nuclear spin that is strongly coupled to
the electron spin of a nitrogen-vacancy (NV) center in diamond. The strong
hyperfine interaction of the nearest-neighbor carbon results in transfer times
of 300 ns between the register qubit and the memory qubit, with an overall
fidelity of 88 % for the write - storage - read cycle. The observed storage
times of 3.3 ms appear to be limited by the T$_1$ relaxation of the electron
spin. We discuss a possible scheme that may extend the storage time beyond this
limit.Comment: 7 pages, 6 figure
Electron spin resonance (ESR) of volume-limited samples or nanostructured materials can be made significantly more efficient by using microresonators whose size matches that of the structures under investigation. We describe a series of planar microresonators that show large improvements over conventional ESR resonators in terms of microwave conversion efficiency (microwave field strength for a given input power) and sensitivity (minimum number of detectable spins). We explore the dependence of these parameters on the size of the resonator and find that both scale almost linearly with the inverse of the resonator size. Scaling down the loops of the planar microresonators from 500 down to 20 mum improves the microwave efficiency and the sensitivity of these structures by more than an order of magnitude and reduces the microwave power requirements by more than two orders of magnitude.
Dynamical decoupling is a powerful technique for extending the coherence time (T2) of qubits. We apply this technique to the electron spin qubit of a single nitrogen-vacancy center in type IIa diamond. In a crystal with natural abundance of 13 C nuclear spins, we extend the decoherence time up to 2.2 ms. This is close to the T1 value of this NV center (4 ms). Since dynamical decoupling must perform well for arbitrary initial conditions, we measured the dependence on the initial state and compared the performance of different sequences with respect to initial state dependence and robustness to experimental imperfections.
Pulsed excitation of broad spectra requires very high field strengths if monochromatic pulses are used. If the corresponding high power is not available or not desirable, the pulses can be replaced by suitable low-power pulses that distribute the power over a wider bandwidth. As a simple case, we use microwave pulses with a linear frequency chirp. We use these pulses to excite spectra of single nitrogen-vacancy centres in a Ramsey experiment. Compared to the conventional Ramsey experiment, our approach increases the bandwidth by at least an order of magnitude. Compared to the conventional continuous wave-ODMR experiment, the chirped Ramsey experiment does not suffer from power broadening and increases the resolution by at least an order of magnitude. As an additional benefit, the chirped Ramsey spectrum contains 6
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