long (10 ms assuming the experiment is limited to the noise generated by the HEMT). By replacing the attenu- ators with properly designed isolators it will be possible to reduce the required acquisition times to 100 − 500 µ s, comparable to those using a SQUID based single-shot read out. Following standard phase qubit pulse sequences we are able to characterize the qubit. We show in Fig. 4 the results for Rabi oscillations and a T$_{1}$ experiment for a qubit frequency of ω$_{L}$/ 2 π = 4 . 46 GHz ( ω$_{R}$/ 2 π = 5 . 6 GHz). The energy relaxation time T$_{1}$ is found to be approximately 83 ns. We believe this value is limited by T$_{1}$$_{,feed}$ because a separate sample using a SQUID based read out gave T$_{1}$$_{,instrinsic}$ = 260 ns and because (1 /T$_{1}$$_{,intrinsic}$ + 1 /T$_{1}$$_{,feed}$ ) − 1 = 106 ns is close to the ob- served coherence time. An improvement in T$_{1}$ should therefore be possible by simply reducing the size of the coupling capacitor. We have clearly demonstrated the basic principle of a new microwave method of reading out phase qubits. It is instructive to discuss how the current implementa- tion relates to other experimental practicalities as well as to scalability. The most significant drawback of our technique is the fact that the T$_{1}$ times of the qubit are somewhat affected by the coupling to the feedline. This is perfectly acceptable, however, if one is interested in studying qubit spectroscopy [18]. However, because of the large size of frequency shifts for the L and R config- urations we believe that it should be possible to also cir- cumvent this limitation. By coupling the qubit to other linear elements we estimate it should still be possible to FIG. 4: Rabi oscillations and an energy relaxation curve of the qubit. Note that no absolute scale is shown as we are presently not performing a single shot measurement. observe a frequency shift dependent on the qubit config- uration without limiting coherence times [13]. We note that our proposed scheme is a step towards multiplexing many phase qubits off a single feedline, similar to readout schemes for photon detectors [19]. 3 [19] P. K. Day, H. G. LeDuc1, B. A. Mazin, A. Vayonakis, and J. Zmuidzinas, Nature 425 , 817 (2003). [18] J. S. Kline, H. Wang, S. Oh, J. M. Martinis, and D. P. Pappas, Supercond. Sci. Technol. 22 , 015004 (2009). [17] F. Milliken, J. Rozen, G. Keefe, and R. Koch, Rev. Sci. Instr. 78 , 024701 (2007). [16] M. Steffen, F. Brito, D. P. DiVincenzo, M. Farinelli, G. Keefe, M. Ketchen, S. Kumar, F. P. Milliken, M. B. Rothwell, J. Rozen, et al., Journal of Physics: Condensed Matter (2010). [15] A. O’Connell, M. Ansmann, R. Bialczak, M. Hofheinz, N. Katz, E. Lucero, C. McKenney, M. N. H. Wang, E. Weig, A. Cleland, et al., Appl. Phys. Lett. 92 , 112903 (2008). [14] M. Steffen, M. Ansmann, R. McDermott, N. Katz, R. C. Bialczak, E. Lucero, M. Neeley, E. M. Weig, A. N. Cle- land, and J. M. Martinis, Phys. Rev. Lett. 97 , 050502 (2006). [13] J.M. Martinis, private communication. [12] K. Cooper, M. Steffen, R. McDermott, R. Simmonds, S. Oh, D. Hite, D. Pappas, and J. Martinis, Phys. Rev. Lett. 93 , 18 (2004). [11] M. J. Farinelli, G. A. Keefe, S. Kumar, and M. Steffen, Microwave readout for flux-biased qubits , US Patent Ap- plication 20090289638 (05/23/2008). [10] M. Hofheinz, H. Wang, M. Ansmann, R. Bialczak, E. Lucero, M. Neeley, A. O’Connell, D. W. J. Sank, J. Martinis, and A. Cleland, Nature 459 , 546 (2009). [9] M. Ansmann, H. Wang, R. Bialczak, M. Hofheinz, E. Lucero, M. Neeley, A. O’Connell, D. Sank, M. Weides, J. Wenner, et al., Nature 461 , 504 (2009). [8] A. Palomaki, S. Dutta, H. Paik, H. Xu, J. Matthews, R. Lewis, R. Ramos, K. Mitra, P. Johnson, F. Strauch, et al., Phys. Rev. B 73 , 014520 (2006). [7] I. Siddiqi, R. Vijay, M. Metcalfe, E. Boaknin, L. Frunzio, R. J. Schoelkopf, and M. H. Devoret, Phys. Rev. B 73 , 054510 (2006). [6] A. Lupascu, C. Verwijs, R. Schouten, H. C.J.P.M., and J. Mooij, Phys. Rev. Lett. 93 , 177006 (2004). [5] A. Wallraff, D. Schuster, A. Blais, L. Frunzio, R. Huang, J. Majer, S. Kumar, S. Girvin, and R. Schoelkopf, Nature 431 , 162 (2004). [4] T. Robertson, B. Plourde, T. Hime, S. Linzen, P. Re- ichardt, F. Wilhelm, and J. Clarke, Phys. Rev. B 72 , 024513 (2005). [3] K. Lang, S. Nam, J. Aumentdao, C. Urbina, and J. Mar- tinis, IEEE Trans. Appl. Supercond. 13 , 989 (2003). [2] J. M. Martinis, S. Nam, J. Aumentado, and C. Urbina, Phys. Rev. Lettl 89 , 117901 (2002). ∗ Electronic address: msteffe@us.ibm.com [1] Y. Nakamura, Y. Pashkin, and J. Tsai, Nature 398 , 786 (1999).