Elsevier

Diamond and Related Materials

Volume 16, Issue 11, November 2007, Pages 1887-1895
Diamond and Related Materials

Creating diamond color centers for quantum optical applications

https://doi.org/10.1016/j.diamond.2007.09.009Get rights and content

Abstract

Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state qubits. This is because of their large dipole moment, convenient level structure and very long room-temperature coherence times. In general, a combination of ion irradiation and subsequent annealing is used to create the centers, however for the rigorous demands of quantum computing all processes need to be optimized, and decoherence due to the residual damage caused by the implantation process itself must be mitigated. To that end we have studied photoluminescence (PL) from NV, NV0 and GR1 centers formed by ion implantation of 2 MeV He ions over a wide range of fluences. The sample was annealed at 600 °C to minimize residual vacancy diffusion, allowing for the concurrent analysis of PL from NV centers and irradiation induced vacancies (GR1). We find non-monotic PL intensities with increasing ion fluence, monotonic increasing PL in NV0/NV and GR1/(NV0 + NV1) ratios, and increasing inhomogeneous broadening of the zero-phonon lines with increasing ion fluence. All these results shed important light on the optimal formation conditions for NV qubits. We apply our findings to an off-resonant photonic quantum memory scheme using vibronic sidebands.

Introduction

The creation of single photoluminescence centers in diamond has promising applications in the development of hybrid optical solid state quantum devices. Diamond exhibits a large inventory of optically active centers, related to the vibrational and electronic states of impurities and defects in the crystal lattice [1]. The prime candidate for many such applications is the negatively charged nitrogen-vacancy (NV) center. It has proven to be a reliable source for single photons [2] suitable for quantum key distribution [3], and when used as a spin qubit has demonstrated coherent oscillations [4], qubit tomography [5], Stark shift control of the optical transition and coherent population trapping [6]. Coupling of NV centers to nearby nuclear [5] and electron [7], [8] spins has also been demonstrated. Furthermore, technology to sculpt optically important nano and micro-structures in diamond is progressing rapidly [9]. Based on such promising results, diamond seems a strong candidate platform for quantum information processing, and several different schemes have been proposed, including resonant dipole–dipole coupling, repeat-until-success and brokered graph states [10]. A recent review of this field can be found in Ref. [11].

The schemes mentioned above were all based on the formation and control of isolated qubits, however there are many protocols which benefit from ensembles of optically active centers. In ensemble protocols, the emphasis is usually on using the system as a medium to act on individual photons or photonic qubits, rather than focusing on the centers themselves as the qubits. Here we are explicitly thinking about protocols such as the weak nonlinear coupling schemes [12], or photon storage schemes [13], [14]. Again, NV appears to be an extremely suitable scheme for such applications, and electromagnetically induced transparency has been demonstrated at radio-frequency [15] and optical transitions [16] in ensembles. Our principal concern in the present work is the optimization of the formation methods of ensembles for these applications, with special focus on storage of light outlined in Ref. [14].

Beyond the obvious attraction of ensembles in increasing the density of particles over the single particle case, ensembles of emitters have other advantages. The electromagnetic interaction strength is strongly enhanced [17], which increases the fidelity of state transfer and storage of quantum information [18]. Of great pertinence to quantum memory schemes is that the distributed storage of quantum information provides more temporal stability. Moreover, ensembles account for the modal structure of interacting light and allow for the storage of the temporal shape of qubit photons [14].

The realization of scalable devices for the above-mentioned applications requires adequate control of the formation processes of active NV centers in diamond crystals. As the final applications of the NV centers differ from the previous motivations (which were usually into the fundamental properties of the centers), so too are the formation requirements. In particular, for quantum device applications, we require the ability to engineer spatially defined ensembles, perhaps in some patterned array, with minimal residual inhomogeneity, and compatibility with sculpted optical structures. These are demanding requirements indeed, and force a critical reappraisal of the existing methodologies.

At least two alternative routes are available for the creation of NV centers in single crystal diamond (a third, where NV centers are created during diamond growth is more pertinent to diamond formed by chemical vapor deposition, e.g. Ref. [19]). Firstly radiation damage from various sources (electrons, neutrons, ions, etc.) can be used to create vacancies in nitrogen-rich (i.e. type Ib) diamond, which combine to form NV post-anneal. Secondly, direct implantation of nitrogen in the purest (i.e. type IIa) crystals, where the NV center is formed by the vacancies due to the N implantation. While the latter strategy represents a suitable method to fabricate low density NV ensembles or isolated qubits in diamond, the first strategy provides a ready method to quickly and efficiently produce high density ensembles. Because of our focus on ensemble protocols, this is the approach that we have chosen in this study. Here, we present an extensive study on the effects of MeV He+ ion implantation in type Ib diamond crystals, as a candidate technique to create high-density NV ensembles in diamond. Irradiation with high energy ions creates vacancies as the ion looses energy to the lattice. Such processes can be easily and accurately modeled using the standard TRIM package [20]. Because the energy loss of the ions is non-uniform with distance into the material, with most damage created at the end of range of the ion, this perforce leads to a highly non-uniform depth profile of NV centers formed post-anneal.

Although NV ensembles in diamond are promising for optical quantum information processing (QIP), there are several material-related issues that need to be fully understood if ion irradiation is to be considered for the creation of NV center ensembles:

A high conversion efficiency from nitrogen and vacancy defects to optically active NV centers is crucial to reach high NV densities, increasing the probability of an interaction, but also minimizing the number of unconverted N which will contribute to decoherence without contributing to the interaction. An active NV center can be created in a diamond crystal containing nitrogen and vacancy defects with thermal annealing at temperature ≥ 600 °C. At such temperatures [21], the vacancies start migrating to the nearest substitutional nitrogen atoms, where their aggregation is energetically favorable [22]. The conversion efficiency is limited due to competitive processes such as the formation of other defects and vacancy-interstitial recombination.

The NV defect can exist in two charge-states (NV and NV0) that have been correlated with λ = 638 nm and λ = 575 nm luminescence emissions, respectively [1]. Charge transfer [23], [24], [25], [26] mechanisms lead to the conversion between NV and NV0 centers (photo-ionization). This process appears experimentally as photo-bleaching. The equilibrium between NV and NV0 concentrations is determined by the presence of nitrogen donors in the lattice [27], [28]. Only the NV center has an easily accessible Λ-shaped energy desired for many quantum computation applications, and a high NV/NV0 ratio is desirable.

The inhomogeneous broadening of the zero phonon line (ZPL) NV emission is usually reported as 750 GHz [1], which significantly exceeds the homogeneous linewidth corresponding to a transition lifetime ∼ 12 ns. Such large broadening is commonly attributed to variations in strain and electric fields within the diamond crystal, due both to impurities and structural defects [11], although recent work in low N diamond where NV concentrations less than the canonical 750 GHz have been observed seem to indicate that this linewidth can be altered by suitable preparation. Inhomogeneous broadening can lead to dephasing or undesired transitions in resonant QIP schemes, while off-resonant [14] or frequency-selective [6] schemes are less vulnerable to it.

The above mentioned issues are related with the fabrication of NV centers in diamond. They might impose limits on the suitability of NV centers for certain QIP schemes. In the present work, we compare our findings on NV center ensembles with the requirements for a photonic quantum memory scheme (Qmem) that uses broadband pulses and an off-resonant transition [14]. This scheme utilizes a strong classical control pulse to absorb the qubit photon into a dark state in the material, which is a collective state of the ensemble. Unlike the typical QIP schemes that use the spin-state of the NV center, here, the first excited phonon state has been chosen as the dark state (see Fig. 1). The thermal population of this state is 10 80 at T 4 K, making state preparation, e.g. by optical pumping, unnecessary. The read-out of the stored photon can be achieved using another strong classical control pulse.

Raman-like transitions are typically insensitive to inhomogeneous broadening of the upper level, provided that the energy separation between the ground states is well-defined throughout the ensemble. Certainly this is the case for the phonon sidebands considered here. However, with increasing inhomogeneity, a larger detuning is necessary and the absolute strength of the two-photon transition decreases, and a correspondingly higher number of absorbers is necessary to achieve a high quantum memory fidelity (the minimum number of NV centers per ensemble is 107 for a Δλ = 10 nm detuned Λ-transition). On the other hand, a large detuning allows for the off-resonant use of truly broadband pulses. Due to the high refractive index of diamond, the storage is optimum for irradiation at the Brewster angle, to avoid reflection-loss of the qubit photon.

Taking into account the practical considerations for light storage in a realistic sample of NV diamond, we find that at a fluence F = 1 × 1015 ions cm 2, a λ = 648 nm photon with picosecond bandwidth could be stored using a 16 μW ultrafast control laser at 80 MHz repetition rate (transition occurs at 10 nm detuning from the ZPL resonance). Our storage scheme is not affected by the strong phonon sideband emission typical for the NV center, as the Λ-transition is mediated by stimulated emission at the control pulse frequency, and as the transition is truly off-resonant and the excited state is never populated. It has to be stressed that, although we calculate this scheme for the NV center only, it can in principle be applied to any optically active defect in diamond with a Λ-shaped energy level structure, for example the GR1 defect, which also has an optically accessible phonon-sideband. This implies that these phononic schemes for light storage are considerably more flexible than the more conventional approach using existing atomic or molecular levels. The current sample therefore has been annealed at a comparatively low temperature (see next chapter), to allow for the PL-analysis of both NV and GR1.

Section snippets

Implantation and annealing

The employed sample was an artificial diamond produced by Sumitomo. The crystal was cut and polished from a large single crystal which was synthesized under high pressure and temperature (HPHT). All sides of the sample have [100] face orientation, and the crystal is classified as type Ib; the nitrogen concentration is ∼ 100 ppm, as reported by the manufacturers. All irradiated regions were in the central [100] growth sector of the sample, where negligible fluctuations in the nitrogen

Results

Fig. 3 shows PL spectra from regions implanted at 14 different ion fluences, ranging from 2 × 1017 ions cm 2 to 1 × 1013 ions cm 2. The NV and GR1 peaks (λ = 638 nm and λ = 742 nm, respectively) are clearly visible for most regions, although the GR1 doublet structure is not resolved due to an overlap of the two peaks attributed to inhomogeneous broadening. The NV0 emission is at λ = 575 nm and much weaker, but visible for some regions. The broad peaks on the red side of each ZPL peak arise from vibronic

Conclusion

We have analyzed the photoluminescence properties of a type-Ib diamond sample, which has been systematically He implanted over a wide range of fluences. At low and medium fluences, the PL signal from NV centers increases with the fluence, whereas at high fluences absorption and competing processes lead to a decline of the NV luminescence. The ratio of NV/NV0 fluorescence decreases for high implantation densities, as the density of residual nitrogen atoms which act as electron donors decreases.

Acknowledgments

The authors gratefully acknowledge support by the QIPIRC and EPSRC (grant number GR/S82176/01), the Australian Research Council, the Australian government, the US National Security Agency (NSA), Advanced Research and Development Activity (ARDA), and the Army Research Office (ARO) under contract number W911NF-05-1-0284 and DARPA QUIST. FW thanks Toshiba Research Europe for their support. We thank Martin Castell, Andrew Briggs and Chris Salter supporting us in the analysis of the sample.

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