5 [5] J. S. Neergaard-Nielsen, B. M. Nielsen, H. Takahashi, A. I. Vistnes, and E. S. Polzik, Opt. Express 15 , 7940 (2007). [6] A. Ourjoumtsev, H. Jeong, R. Tualle-Brouri, and P. Grangier, Nature (London) 448 , 784 (2007). [7] L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Na- ture (London) 414 , 413 (2001). [8] A. Kuzmich, W. P. Bowen, A. D. Boozer, A. Boca, C. W. Chou, L.-M. Duan, and H. J. Kimble, Nature (London) 423 , 731 (2003). [9] S. Chen, Y.-A. Chen, T. Strassel, Z.-S. Yuan, B. Zhao, J. Schmiedmayer, and J.-W. Pan, Phys. Rev. Lett. 97 , 173004 (2006). [10] D. Porras and J. I. Cirac, arXiv:0704.0641. [11] E. Brion, K. Mølmer, and M. Saffman, Phys. Rev. Lett. 99 , 260501 (2007). [12] D. Jaksch, J. I. Cirac, P. Zoller, S. L. Rolston, R. Cˆ ot´e, and M. D. Lukin, Phys. Rev. Lett. 85 , 2208 (2000). [13] M. D. Lukin, M. Fleischhauer, R. Cote, L. M. Duan, D. Jaksch, J. I. Cirac, and P. Zoller, Phys. Rev. Lett. 87 , 037901 (2001). [14] A. Ga¨ etan, Y. Miroshnychenko, T. Wilk, A. Chotia, M. Viteau, D. Comparat, P. Pillet, A. Browaeys, and P. Grangier, Nature Physics 5 , 115 (2009). [15] E. Urban, T. A. Johnson, T. Henage, L. Isenhower, D. [16] R. Heidemann, U. Raitzsch, V. Bendkowsky, B. Butscher, R. L¨ ow, L. Santos, and T. Pfau, Phys. Rev. Lett. 99 , 163601 (2007). [17] M. Saffman and T. G. Walker, Phys. Rev. A 66 , 065403 (2002). [18] L. H. Pedersen and K. Mølmer, Phys. Rev. A 79 , 012320 (2009). [19] D. Porras and J. I. Cirac, Phys. Rev. A 78 , 053816 (2008). [20] G. Tabia and B.-G. Englert, arXiv:0910.5375. [21] N. H. Lindner and T. Rudolph, Phys. Rev. Lett. 103 , 113602 (2009). [22] After submission of the present paper, a related quantum dot proposal to produce photonic 2D cluster states has been described in S. E. Economou, N. Lindner, and T. Rudolph, arXiv:1003.2410. [23] M. A. Nielsen, Reports on Mathematical Physics 57 , 147 (2006). [24] M. Saffman and K. Mølmer, Phys. Rev. A 78 , 012336 (2008). [25] T. Pfau, private communication. D. Yavuz, T. G. Walker, M. Saffman, Nature Physics 5 , 110 (2009).