November 30, 2010

Nanodiamond qubits and a path to large scale quantum computer systems

Arxiv - Enhancement of the zero phonon line emission from a single NV-center in a nanodiamond via coupling to a photonic crystal cavity

Using a nanomanipulation technique a nanodiamond with a single nitrogen vacancy center is placed directly on the surface of a gallium phosphide photonic crystal cavity. A Purcell-enhancement of the fluorescence emission at the zero phonon line (ZPL) by a factor of 12:1 is observed. The ZPL coupling is a first crucial step towards future diamond-based integrated quantum optical devices.

A further enhancement of the emission into the ZPL can be achieved by improving the Q-factor or by performing experiments at cryogenic temperatures. At 4 Kelvin the ZPL can be nearly Fourier-limited and about 3% of the light is emitted into the ZPL. In this case we estimate that coupling to a similar PCC with a Q-factor of 600 should allow channeling of almost 30% of the emission into the cavity mode.

In conclusion, we have demonstrated the deterministic coupling of the zero phonon line of a single nitrogen-vacancy center in a nanodiamond to a photonic crystal cavity. This is a major step towards the realization of integrated quantum optical devices. With the presented pick-and-place technique and the selective cavity tuning, even more complex systems, involving two cavities and emitters, can be assembled in a controlled way. Simple quantum gates, integrated on a single photonic crystal chip, are within reach.

2. Arxiv - Unconventional geometric quantum controlled phase gates with two nonresonant quantum dots in a photonic crystal cavity

We study the system of two nonresonant quantum dots trapped in two coupled photonic crystal cavity, and propose the two-qubit quantum phase gates based on the conventional geometric quantum computation. During the gate operation, the quantum dots undergo no transitions, while the cavity mode is displaced along a closed path in the phase space. In this way, the system can acquire a geometric phase conditional upon the states of the quantum dots and realize the phase gate.

In conclusion, we have shown that in a single-mode PC cavity, two non-identical and spatially separated QDs can be used to construct the unconventional geometric two-qubit controlled phase gate with the application of the classical light fields. During the gate operation, the QDs remain in their ground states, while the cavity mode is displaced along a circle in the phase space, and gets a geometric phase conditional upon the states of QDs. After the gate operation is finished, the QDs will disentangle with the cavity mode. The distinct advantages of the proposed scheme are as follows: firstly, as the evolution of the system is dependent on the laser fields, this system is controllable; secondly, as the system is consisted with the QDs and PC cavity, it can be integrated; thirdly, as the QDs are non-identical, it is more practical; fourthly, as the quantum information is encoded on the two ground states, this gate is insensitive to the spontaneous emission of the QDs. Moreover, as the number of different QDs in the cavity can be increased, this system is scalable. Finally, as this controlled phase gate is a unconventional geometric quantum phase gate, it is insensitive to the quantum fluctuation. Therefore, we could use this scheme to construct a kind of solid-state optical logical devices which is controlled, integrated and is insensitive to the quantum fluctuation. In addition, as the controlled phase gate is a universal gate, this system also can realize the controlled entanglement and interaction between the two nonidentical and spatially separated QDs.

3. Arxiv - Design and Focused Ion Beam Fabrication of Single Crystal Diamond Nanobeam Cavities

We present the design and fabrication of nanobeam photonic crystal cavities in single crystal diamond for applications in cavity quantum electrodynamics. First, we describe three-dimensional finite-difference time-domain simulations of a high quality factor (Q ~ 10^6) and small mode volume (V ~ 0.5 (λ/n)3) device whose cavity resonance corresponds to the zero-phonon transition (637nm) of the Nitrogen-Vacancy (NV) color center in diamond. This high Q/V structure, which would allow for strong light-matter interaction, is achieved by gradually tapering the size of the photonic crystal holes between the defect center and mirror regions of the nanobeam. Next, we demonstrate two different focused ion beam (FIB) fabrication strategies to generate thin diamond membranes and nanobeam photonic crystal resonators from a bulk crystal. These approaches include a diamond crystal “side-milling” procedure as well as an application of the “lift-off” technique used in TEM sample preparation. Finally, we discuss certain aspects of the FIB fabrication routine that are a challenge to the realization of the high-Q/V designs.

We have observed that the FIB is a flexible and versatile tool to perform three-dimensional sculpting of nanobeam photonic crystal structures from a bulk diamond crystal. There are still several challenges to the successful implementation of this technique in high Q/V designs such as the one described here. Substantial rounding of critical nanobeam cavity features, such as in the definition of the waveguide and in the patterning of the linear array of holes, results from the wide FIB beam that extends beyond the intended direct-write area. This presents a major obstacle to the fabrication of high-Q photonic crystal cavities that are exponentially sensitive to cavity spacing at the ~1nm level. Moreover, realization of these complex photonic structures is time consuming and few structures can be made within a reasonable amount of time due to the serial nature of the FIB-based fabrication. This limits probability of:
(i) having a NV center within a given cavity, unless sophisticated implantation approaches are used;
(ii) achieving the high Q/V design for any resonance wavelength; and finally
(iii) achieving cavity resonance at the zero-phonon line wavelength of the NV center without implementing an additional tuning mechanism.

We therefore expect that, with this purely FIB-based fabrication approach to generate the nanobeam cavity, it will be challenging to achieve strong light-matter interactions with a NV center.

One approach that could be used in the future is to utilize the FIB in less-complex photonic structures. For example, we have fabricated both diamond nanowire waveguides as well as solid-immersion lenses using two-dimensional FIB patterning techniques (images not shown). This approach is far simpler to implement than complex three-dimensional sculpting, and these devices have already been shown to be useful for increasing the number of single photons collected from an individual NV center. Another strategy would be to substitute other diamond nanofabrication techniques such as lithography and reactive ion etching wherever possible in the “lift-off” and “side-milling” schemes presented here. This could result in greater device yield and improved device quality. Finally, devices based on thin single crystal diamond films promise many exciting applications and would likely offer the greatest opportunity for fabricating high quality diamond photonic systems.

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