Single-Shot State Measurement of Coupled Phase Qubits

John M. Martinis

G. P. Berman, D. I. Kamenev, D. Kinion +1 more

We have studied the possibility of a single-shot non-demolition measurement of a superconducting qubit using a microstrip SQUID amplifier (MSA). The Johnson noise generated by all resistors in the MSA is taken into consideration. We show that a single-shot non-demolition measurement is possible with six photons in the measurement resonator. For a phase qubit inductively coupled to a measurement resonator we have obtained the expression for the mutual inductance required for measurement of the qubit state.

R. McDermott, Raymond W. Simmonds, Matthias Steffen +6 more

One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits). We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit 01 and 10 states, consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization of multiqubit gates and elementary quantum algorithms.

Jürgen Lisenfeld, Alexander V. Lukashenko, Alexey Vladimirovich Ustinov

The states of superconducting quantum bits, as the Josephson phase qubits and flux qubits, are distinguished by magnetic flux generated by a persistent current in a superconducting loop. As the readout circuit contributes decoherence, its coupling to the qubit should be made as weak as possible. We demonstrate a method to maximize the measurement contrast of the two qubit states, thus allowing to significantly decrease the coupling to the readout. This technique is promising for multiqubit circuits since it allows to read out the states of several qubits by using a single readout circuit.