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Heralded entanglement of two distant quantum dot spins via optical interference

We present a proposal for heralded entanglement between two quantum dots via Hong--Ou--Mandel effect. Each of the quantum dots, drived off-resonance by two lasers, can be entangled with the coherent cavity mode. The output photons from the two coherent cavity modes interfering by a beamsplitter, we could entangle the two QDs with nearly unit success probability. Our scheme requires neither direct coupling between qubits nor the detection of single photons. Moreover the quantum dots do not need to have the same frequencies and coupling constants.


I. INTRODUCTION
Quantum entanglement is treated as a crucial resource in many quantum information tasks, such as quantum teleportation [1], quantum dense coding [2], quantum cryptography and quantum computation [3]. Most of the tasks require generating entanglement among distant quantum nodes. However, it is not easy to generate entanglement between distant nodes, as interaction between qubit is generally local. In order to solve the problem, many schemes of creating entanglement between spatially separated nodes have been proposed [4][5][6][7][8][9][10][11][12] and some experimentally demonstrated [13][14][15][16][17]. These schemes are either probabilistic or have yet to be demonstrated experimentally.
Recently, spin qubits in semiconductor quantum dots (QDs) attract much interest because of their potential for a compact and scalable quantum information architecture, and relatively long coherence time [18]. It has been demonstrated that double quantum dots can be entangled by directly coupling [19]. However, the method cannot be used to entangle the distant quantum dots. It is found that semiconductor QDs can be strongly coupled to photonic crystal cavity systems [20,21]. However, due to the dot size variation, semiconductor QDs usually have different radiative properties, which make it difficult to generate entanglement by indistinguishable emitting photons, as those have been done in atomic systems.
There are two approaches to overcome the problem: (i) tuning the QDs into resonance by using externally applied strain [22,23], or controlling the Overhauser field [24,25] or (ii) making a detuning between quantum dots and the same frequency output cavity modes [26][27][28], or using quantum frequency conversion [29,30]. There has, to our best knowledge, been no reported experimental realization of entanglement between two distant QDs. In Ref. [26] QDs in low Q cavities are coupled to a common high-Q cavity mode. There are no reliable device structures like this yet. In Ref. [27,28] weak coherent fields being reflected from the Cavity-QDs system have low efficiency or success possibility. Recently, schemes for robust multiphoton entanglement creation using coherent (or Gaussian) state light were put forward [31,32]. The rate is much larger than the single-photon entanglement creation schemes [13][14][15]17].
In this paper, we describe a protocol for generating entanglement between two quantum dots via Hong-Ou-Mandel effect [33]. The QDs are strongly driven by V polarized laser fields, generating H polarized coherent cavity modes which separately entangle with the two QDs. The output photons from the two coherent cavity modes interfering by a beamsplitter, we could entangle the two QDs with nearly unit efficiency and fidelity. The scheme does not need the quantum dots to have the same radiation frequency. Compared with the entanglement distributing schemes reported previously [13][14][15], our scheme combines both the advantages that appear in direct coupling method (high efficiency) and single photon interference method (high fidelity) [34]. We believe that our protocol is a promising route to a scalable quantum computation in solid system.

II. ENTANGLEMENT OF TWO QUANTUM DOTS
Suppose that two quantum dots are seperately coupled to two cavities with same cavity mode frequency. The internal level configuration of each quantum dot is shown in the left part of Fig. 1 [24,26,35]. The |X+ i → |T + i transition of dot i is driven by a V polarized laser pulse with Rabbi frequency Ω i + and detuning ∆ i under the condition and choose Ω i the Hamiltonian (1) simplifies to here α i = −iλ i t, and |m i c represents the state of the cavity mode a i . If our initial state of the dot-cavity combined system is i=1,2 |X− i |0 i c , under the Hamiltonian H eff , the state of the system evolve to The mode a out 1 ( a out 2 ) output from the cavity 1 (2) reaches the beamsplitter (BS). The BS then applies the transformations where c and d are the bosonic modes monitored by detector c and detector d respectively.
In the case of α 1 = α 2 = α, the state of the system becomes where The state |vac represents the vacuum state of field mode. Conditioned on photons detection event at detector c, the state of the QDs collapses onto photons detection event at detector d, the state of the QDs collapses onto It should be noted that, so long as detector is triggered, a perfect entangled state can be generated heraldedly even when the QDs have different resonant frequencies.

III. SIMULATIONS WITH THE LINDBLAD MASTER EQUATION
The system consists of two cavity modesâ 1 ,â 2 and two quantum dots. In the absence of any photon detection, the system evolution obeys the Lindblad master equatioṅ where Γ i denotes the relaxation of the two QDs and κ i the damping of the two cavities.
where the vacuum fluctuation partb 1,in (t) andb 2,in (t) have been neglected, since they have no effect on the photon counter. The output field of the beam splitter iŝ During [t, t + dt] (dt → 0 is an infinitesimal interval), photon detection c occurs with prob- after which the system collapses to the state (un-normalized)ρ (1) (t+dt) =ĉρ ss (t)ĉ † dt. Fig. 2 plots of the average detecting photon number N vs λt for different values of κ/λ, here we choose λ 1 = λ 2 = λ, κ 1 = κ 2 = κ, Γ 1 = Γ 2 = Γ, and Γ = 0.05λ. In the case of the detector c (or d) is triggered, Fig. 2 (2) and (3) are fulfilled and value λ = 2.2 GHz is obtained. In realistic experiments, the spontaneous decay rate of the trion state |T ± is about Γ T = 2π × 130 MHz [38]. Fig. 4 plots the population of the trion state |T ± by using the above parameters. Due to large detuning, the effective decay rate could be estimated as Γ T Ωg/∆ 2 ≈ 2π × 2.6 MHz [39,40], which is much smaller than λ. This implies that the influence from the spontaneous decay in our scheme. The main source of error is the depahsing of elecron spin. The typical dephasing rate of the InAs QD electron spin has been measured to be about Γ = (10ns) −1 ≈ 0.05λ [37], the fidelity of the entanglemnt state is larger than 0.95 as shown in Fig. 3. From the Fig. 3. and 4 we can neglect the influence the trion state |T ± .

IV. CONCLUSION
In conclusion, we have generated deterministic entanglement between two distant quantum dots using classical interference; that is, there is no inherent probabilistic nature to our quantum entangling source. The method is robust to the difference of the two quantum dots and cavities. Our scheme does not need to control coupling between qubits nor to detection of single photons. Using this larger arrays of quantum dot qubits could be linked together for scale-up to a quantum computer [41].