Entangling homogeneously broadened matter qubits in the weak-coupling cavity-QED regime

Abstract

In distributed quantum information processing, flying photons entangle matter qubits confined in cavities. However, when a matter qubit is homogeneously broadened, the strong-coupling regime of cavity QED is typically required, which is hard to realize in actual experimental setups. Here, we show that a high-fidelity entanglement operation is possible even in the weak-coupling regime in which dampings (dephasing, spontaneous emission, and cavity leakage) overwhelm the coherent coupling between a qubit and the cavity. Our proposal enables distributed quantum information processing to be performed using much less demanding technology than previously.Distributed architecture is a promising approach for realizing scalable quantum computation [1][2][3][4][5][6]. Elementary nodes composed of a few qubits are networked to achieve scalable quantum computation. The node separation can potentially suppress decoherence induced by uncontrollable interactions between qubits. Moreover, since the nodes are spatially separated, individual qubits can be easily addressed by the optical field.A critical operation for realizing distributed quantum computation is the entanglement operation (EO) [1][2][3][4][5][6][7][8][9]. To construct an entire network, qubits in distant nodes have to be coupled by EOs. Most EOs are based on photon interference, and the successful execution of an EO can be heralded by detecting a photon at the target port. This approach has been experimentally demonstrated using an ion trap system [10]. If the EO fails, the two qubits involved should be initialized, which risks destroying the entanglement of other qubits generated by previous EOs. Although EOs typically have such probabilistic properties, previous studies have revealed that only polynomial steps are required to construct large entangled states [2,[11][12][13][14], such as a cluster state [15]. Moreover, by introducing a quantum memory to each node, EOs can be repeatedly performed until they are successful without destroying prior entanglement [16].EOs involve optical excitations of matter qubits. However, the excited states are inherently noisy and significantly degrade the target entanglement. For example, nitrogen vacancy (NV) centers in diamond have promising properties such as a long coherence time at room temperature and optical addressability. Entanglement between an NV center and an emitted photon has been demonstrated at a low temperature of about 7 K [17]. However, at room temperature, this otherwise attractive system suffers from strong environmental dephasing originating from interactions with phonons when the system is optically excited. Consequently, it acquires a large homogeneous broadening of the order of THz [18]. Therefore, in such an approach, NV centers can be used for distributed quantum computation only at low temperatures.One way to overcome homogeneous broadening is to employ high-Q cavities. Previous theoretical proposals of EOs require strong coupling between a matter qubit and the cavity when the matter qubit has large hom...

Error-rejecting quantum computing with solid-state spins assisted by low-Qoptical microcavities

“…In fact, every single photon pulse is of finite linewidth, i.e., a polarized photon of pulse shape in Gaussian function f (ω) = exp(−ω/∆) 2 /( √ π∆) with bandwidth ∆. This finite-linewidth character usually introduces some additional infidelity in the previous EG protocols [42][43][44][45][46][47][48][49][50], while it has little harmful effect on the fidelity of our EG. When one constitutes our EG with a polarized single-photon pulse p of Gaussian shape,…”

“…DISCUSSION AND SUMMARY Our scheme of error-rejecting EG can work efficiently with almost unity fidelity in the strong coupling regime, g > κ T , γ, the Purcell regime, C > 1/4, or even the resonantly scattering regime, C = 1/4. It is robust to the imperfections involved in the practical input-output process, i.e., the nonzero bandwidth, QD or cavity decay, and the finite coupling g/κ, since the fidelity of our EG is independent of the reflective coefficients r j (ω) and thus independent of the cooperativity C, which is far different from other schemes that depends on C [43][44][45][46][47][48]. The original low fidelity or error items originating from the practical input-output process are converted into a relatively lower efficiency in our EG.…”

“…It is a practical proposal for performing efficient solid-state quantum computing that overcomes the existing limitations, since the fidelity of the EG for two QDs is always towards unity and the efficiency of the EG can, in principle, also approach unity. Compared with previous EGs for QDs based on cavity QED [43][44][45][46][47][48], the present scheme also has several other advantages. First, it does not require the strong-coupling limit and can work efficiently in low-Q cavities or even in the regime of resonance scattering [48] where the modified spontaneous emission parameter of QDs coupled resonantly to a microcavity is matched to that in the bulk dielectric.…”

“…Most existing quantum computing schemes based on single photons and single spins in QDs are performed in a strong coupling regime as a result of cavity quantum electrodynamics (QED) [35,[42][43][44][45]. However, the strong coupling regime remains a challenge and some EG proposals for QDs in a low-Q cavity are proposed at the price of a decrease in the fidelity and the efficiency of the EG [46][47][48][49][50]. Meanwhile, the solidstate spins used in quantum computing are supposed to be homogeneous and the inhomogeneity of the spins will further decrease the feasibility of the proposed solid-state spin quantum computing [30].…”

High-fidelity spin measurement on the nitrogen-vacancy center

“…Recently an alternative approach based on dipole-induced transparency has been proposed [118][119][120][121][122] wherein the state of the NV center changes the resonance properties of an optical cavity. More specifically, if the electronic spin in its 0 m s ñ | state, an incident photon is reflected from the cavity, while for the 1 m s + ñ | electronic spin state the photon enters the cavity where it is scattered but not absorbed by the NV center.…”