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# 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. 

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]. 

PACS numbers: 

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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. 

