One-step implementation of multiqubit conditional phase gating with nitrogen-vacancy centers coupled to a high-Q silica microsphere cavity

Abstract

The diamond nitrogen-vacancy (NV) center is an excellent candidate for quantum information processing, whereas entangling separate NV centers is still of great experimental challenge. We propose an one-step conditional phase flip with three NV centers coupled to a whispering-gallery mode cavity by virtue of the Raman transition and smart qubit encoding. As decoherence is much suppressed, our scheme could work for more qubits. The experimental feasibility is justified.As a promising building block for room-temperature quantum computing, 1 the nitrogen-vacancy (NV) center consisting of a substitutional nitrogen atom and an adjacent vacancy in diamond can feature near-unity quantum efficiency, a homogeneous line width, and long electronic spin decoherence time at room-temperature, 2 . Readout of spin state and single qubit gating have been achieved in optical fashion in individual NV centers, 3 and quantum information swapping and entanglement are available between electronic and the nuclear spins. 4 However, scalability is the main obstacle in such a system because entanglement of NV centers in distant diamonds has never been accomplished experimentally. Recently, Benjamin et al 5 suggested to entangle different NV electron spins by detecting the emitted photons, but met some difficulties due to the particular characteristic of the NV centers, such as the fact that 96% of the emitted photons reside in broad photon sidebands to the resonant zero phonon line (ZPL) at 637 nm even in cryogenic situation. 5 It implies that the most photons emitted from the NV centers could not effectively interfere in the beam splitter.We study a potential idea to entangle separate NV centers using the quantized whispering-gallery mode (WGM) of a fused-silica high-Q microsphere cavity. So far there has been much development in WGM cavities with, such as the microtoroidal, 6 microcylinders, 7 microdisks, 8 and microspheres. 9 Especially, microsphere cavity had gained widespread attention because of their ultrahigh Q factor (≥ 10 8 even up to 10 10 ), 10 very small volume (V m ≤ 100 µm 3 ) 10 and simple fabrication technique. In the fused-silica microsphere cavity, WGMs form via total internal reflection along the curved boundary, and the small radius of 10 µm could lead to a vacuum electric field of 150 V /cm at the sphere surface (with wavelength 600 nm) and to the Q factor exceeding 10 9 . On the other hand, the lowest-order WGM correa) Electronic sponding to the light traveling around the equator of the microsphere 11 offers predominant conditions for reaching strong coupling regime. Recent experimental progresses about the nanocrystal-microsphere system also provide experimental evidence for strong coupling between NV centers and the WGM of silica microsphere 12 or polystyrene microsphere, 13 respectively.The key point of our proposal is a conditional phase flip (CPF) on the NV center electron-spins, based on recent experimental and theoretical progresses, e.g., the possible Λ-type configuration of the optical transitions in NV cente...

Simple scheme for multiqubit quantum controlled phase-flip gate with singlet-triplet spin qubits in nanowire double quantum dots

“…Before conclusion, we note that there are some relevant works on realization of three-qubit CPS gates (for example [38]) in other physical system. The scheme for three-qubit CPS gates in Ref.…”

“…The scheme for three-qubit CPS gates in Ref. [38] is based on the coupling between three NV centers and a whispering-gallery mode cavity by virtue of the Raman transition, in which the cavity field and the assistant classical field are required coupling the relative transition simultaneously. Unlike this scheme, our idea is quite straightforward because no classical laser field is involved, and thus the required interaction time is very short due the resonant interaction.…”

Fast quantum state engineering via universal SU(2) transformation

“…Two negatively charged NV centers are separately positioned near the equator of a high-Q microsphere cavity, in which the distance is much larger than the wavelength of the WGM and the direct coupling between NV centers is negligible. The NV centers can be modeled as Λ-type three level systems [73,78,79], where the states | 3 A, m s = 0 and | 3 A, m s = −1 serve as qubit states |g and |f , respectively, the state | 3 E, m s = 0 is labeled by |e and the metastable state 1 A is left aside for not fully understood [80]. In our case, the transition |g ↔ |e is resonantly coupled to the WGM with coupling constant λ and |f ↔ |e is resonantly driven through a timedependent laser pulse with Rabi frequency Ω(t) [74,84].…”

One-step implementation of the genuine Fredkin gate in high-Q coupled three-cavity arrays

“…Nevertheless, this kind of decomposition becomes inefficient as applied to a multi-qubit (three or more) gate, for the procedure will become more complicated and makes the quantum system further susceptible to the environment. Therefore, much attention has been paid on the direct implementation of multi-qubit gates both in theory and experiment [5][6][7][8][9][10][11][12][13][14][15][16]. …”