Readout for Phase Qubits without Josephson Junctions Matthias Steffen, ∗ Shwetank Kumar, David DiVincenzo, George Keefe, Mark Ketchen, Mary Beth Rothwell, and Jim Rozen IBM Watson Research Center, Yorktown Heights, NY 10598 (Dated: November 21, 2018) We present a novel readout scheme for phase qubits which eliminates the read-out SQUID so that the entire qubit and measurement circuitry only requires a single Josephson junction. Our scheme capacitively couples the phase qubit directly to a transmission line and detects its state after the measurement pulse by determining a frequency shift observable in the forward scattering parameter of the readout microwaves. This readout is extendable to multiple phase qubits coupled to a common readout line and can in principle be used for other flux biased qubits having two quasi-stable readout configurations. PACS numbers: The reliable identification of the quantum state occu- pation probability of a qubit with high fidelity without introducing detrimental system complexity has been at the center of attention in the design and operation of superconducting qubits in recent years[1–4]. Over the years these trade-offs were optimized for various types of qubits to enhance overall qubit performance [5–7]. Al- though some of these new methods could also apply to the phase qubit, measurement techniques for these qubits have remained largely unchanged. The switching of a two or three Josephson junction SQUID into its normal state remains the main technique to identify the qubit state of a phase qubit [8–10]. In this letter, we show a novel phase qubit read-out method [11] which eliminates the SQUID and its associ- ated dissipation entirely [3]. By removing the SQUID we also reduce the number of required Josephson junctions from three or four to only a single Josephson junction, thereby dramatically improving the overall device yield. The measurement of phase qubits is typically done in two steps (e.g. [12]). First, the quantum state is ”mea- sured” by taking advantage of the potential energy land- scape such that post measurement the system is in one of two quasi-stable configuration depending on the quantum state prior to measurement. This step has been demon- strated with large single-shot fidelities and reliable per- formance [9]. Finally, the qubit is ”read out” by identify- ing which of the quasi-stable configurations the system is in. By convention we refer to these configurations as the left (L) and right (R) configuration. Because the L and R configurations correspond to a large difference of the circulating current in the qubit, the obvious choice has been to flux couple a SQUID to the qubit and detect the current at which a SQUID switches to its voltage state. The configuration, L or R, can be identified with high confidence using the SQUID, usually without significant backaction. Recently, however there has been mounting evidence that the SQUID does indeed limit qubit coher- ence times or at least drastically impacts the repetition rate of the experiment [3, 13]. Because the L and R configurations in a phase qubit have dramatically different resonance frequencies we pro- pose to identify the system configuration by probing the qubit with microwave pulses using dispersive techniques [5–7]. We estimate that the correct identification of the L or R configuration is possible with high certainty given read-out times comparable to those using the SQUID. The new technique is made possible by directly coupling the qubit to a microwave feedline and probing the ampli- tude or phase response of a microwave pulse that passes by the qubit. The new readout is compatible with all ex- isting state-of-the-art phase qubit techniques, including reset and measurement. FIG. 1: Outline (a) and micrograph (b) of the qubit. The phase qubit consists of the standard circuit elements in par- allel - a Josephson junction, an inductor and a capacitor all capacitively coupled to a feedline via a coupling capacitor. An optical micrograph of the qubit depicts the actual layout of circuit. c) Shows the readout schematic, The qubit flux bias and microwave lines (S1,S2) are sufficiently filtered and attenuated to ensure sufficiently low electron temperatures. A HEMT at the 4K stage amplifies the outgoing microwave signal. arXiv:1001.1453v1 [cond-mat.supr-con] 9 Jan 2010