J. Beck

Ultralong spin coherence time in isotopically engineered diamond

As quantum mechanics ventures into the world of applications and engineering, materials science faces the necessity to design matter to quantum grade purity. For such materials, quantum effects define their physical behaviour and open completely new (quantum) perspectives for applications. Carbon-based materials are particularly good examples, highlighted by the fascinating quantum properties of, for example, nanotubes or graphene. Here, we demonstrate the synthesis and application of ultrapure isotopically controlled single-crystal chemical vapour deposition (CVD) diamond with a remarkably low concentration of paramagnetic impurities. The content of nuclear spins associated with the (13)C isotope was depleted to 0.3% and the concentration of other paramagnetic defects was measured to be <10(13) cm(-3). Being placed in such a spin-free lattice, single electron spins show the longest room-temperature spin dephasing times ever observed in solid-state systems (T2=1.8 ms). This benchmark will potentially allow observation of coherent coupling between spins separated by a few tens of nanometres, making it a versatile material for room-temperature quantum information processing devices. We also show that single electron spins in the same isotopically engineered CVD diamond can be used to detect external magnetic fields with a sensitivity reaching 4 nT Hz(-1/2) and subnanometre spatial resolution.

Single-Shot Readout of a Single Nuclear Spin

Probed But Not Perturbed The processing and manipulation of quantum information holds great promise in terms of outperforming classical computers and secure communication. However, quantum information is delicate, and even reading the information is a destructive and probabilistic process requiring a number of measurements to home in on the information stored as a quantum state. For the nitrogen vacancy in diamond, Neumann et al. (p. 542, published online 1 July) show that these limitations can be eliminated. A measurement protocol was designed and implemented where the spin state of the nuclear spin of the vacancy could be mapped onto and read out from the surrounding electronic spins in a single-shot measurement nondestructively. The quantum state of a single nitrogen vacancy in diamond can be read out nondestructively in a single-shot measurement. Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin’s behavior. We demonstrated single-shot, projective measurement of a single nuclear spin in diamond using a quantum nondemolition measurement scheme, which allows real-time observation of an individual nuclear spin’s state in a room-temperature solid. Such an ideal measurement is crucial for realization of, for example, quantum error correction protocols in a quantum register.

Quantum register based on coupled electron spins in a room-temperature solid.

Nitrogen–vacancy centres in diamond have emerged as a promising platform for quantum information processing at room temperature. Now, coherent coupling between two electron spins separated by almost 10 nm has been demonstrated. At this distance, the spins can be addressed individually, which might enable the construction of a network of connected quantum registers.

Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature

We report a versatile method to polarize single nuclear spins in diamond, based on optical pumping of a single nitrogen-vacancy (NV) defect and mediated by a level anticrossing in its excited state. A nuclear-spin polarization higher than 98% is achieved at room temperature for the 15N nuclear spin associated with the NV center, corresponding to microK effective nuclear-spin temperature. We then show simultaneous initialization of two nuclear spins in the vicinity of a NV defect. Such robust control of nuclear-spin states is a key ingredient for further scaling up of nuclear-spin based quantum registers in diamond.

Excited-state spectroscopy of single NV defects in diamond using optically detected magnetic resonance.

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3 E excited state, which is invaluable for the development of diamond-based quantum information processing.

Coherence of single spins coupled to a nuclear spin bath of varying density

The dynamics of single electron and nuclear spins in a diamond lattice with different 13 C nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free-induction decays of nuclear spin Bell states and single nuclear coherences among 13 C nuclear spins are compared and analyzed. Reduction in a free-induction-decay time T2 and a coherence time T2 upon increase in nuclear spin concentration has been found. For pure diamond, T 2 as long as 30 s and T2 of up to 0.65 ms for the electron spin has been observed. The 13 C concentration dependence of T 2 is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that T2 decreases approximately as 1 / n, where n is 13 C concentration, which corresponds to the reported theoretical line of T2 for an electron spin interacting with a nuclear spin bath.

Universal enhancement of the optical readout fidelity of single electron spins at nitrogen-vacancy centers in diamond

Precise readout of spin states is crucial for any approach toward physical realization of a spin-based quantum computer and for magnetometry with single spins. Here, we report a method to strongly improve the optical readout fidelity of electron spin states associated with single nitrogen-vacancy (NV) centers in diamond. The signal-to-noise ratio is enhanced significantly by performing conditional flip-flop processes between the electron spin and the nuclear spin of the NV centers nitrogen atom. The enhanced readout procedure is triggered by a short preparatory pulse sequence. As the nitrogen nuclear spin is intrinsically present in the system, this method is universally applicable to any nitrogen-vacancy center. Besides the readout method, our studies included coherent control over a single nitrogen nuclear spin for the first time.

Dark states of single nitrogen-vacancy centers in diamond unraveled by single shot NMR.

The nitrogen-vacancy (NV) center in diamond is supposed to be a building block for quantum computing and nanometer-scale metrology at ambient conditions. Therefore, precise knowledge of its quantum states is crucial. Here, we experimentally show that under usual operating conditions the NV exists in an equilibrium of two charge states [70% in the expected negative (NV-) and 30% in the neutral one (NV0)]. Projective quantum nondemolition measurement of the nitrogen nuclear spin enables the detection even of the additional, optically inactive state. The nuclear spin can be coherently driven also in NV0 (T1≈90  ms and T2≈6  μs).

Sensing external spins with nitrogen-vacancy diamond

A single nitrogen-vacancy (NV) center is used to sense individual, as well as small ensembles of, electron spins placed outside the diamond lattice. Applying double electron–electron resonance techniques, we were able to observe Rabi nutations of these external spins as well as the coupling strength between the external spins and the NV sensor, via modulations and accelerated decay of the NV spin echo. Echo modulation frequencies as large as 600 kHz have been observed, being equivalent to a few nanometers distance between the NV and an unpaired electron spin. Upon surface modification, the coupling disappears, suggesting the spins to be localized at surface defects. The present study is important for understanding the properties of diamond surface spins so that their effects on NV sensors can eventually be mitigated. This would enable potential applications such as the imaging and tracking of single atoms and molecules in living cells or the use of NVs on scanning probe tips to entangle remote spins for scalable room temperature quantum computers.

Enhanced generation of single optically active spins in diamond by ion implantation

The nitrogen-vacancy (NV) centers in diamond are amongst the most promising candidates for quantum information applications. Up to now the creation of such defects was highly probabilistic, requiring many copies of the nanodevice. Here we show that by employing a two step implantation process which includes low dose N2+ molecular ion implantations followed by high dose C implantation can increase the generation efficiency of NV centers by over 50%. Moreover, we detected intrinsic N14 concentration as low as 0.07 ppb by converting the nitrogen impurities into NV and then counting the single centers by using a confocal microscope.