Single-shot qubit readout in circuit Quantum Electrodynamics

Methods

The sample was fabricated using standard lithography techniques. In a first step, a $120\,$nm-thick niobium film is sputtered on an oxidized high-resistivity silicon chip. It is patterned by optical lithography and reactive ion etching of the niobium to form the coplanar waveguide resonator. The transmon and the Josephson junction of the CJBA are then patterned by e-beam lithography and double-angle evaporation of two aluminum thin-films, the first one being oxidized to form the junction tunnel barrier. The chip is glued on and wire-bonded to a microwave printed-circuit board enclosed in a copper box, which is thermally anchored to the mixing chamber of a dilution refrigerator at typically $20\,$mK.

.2 Electrical lines and signals

Qubit control and readout microwave pulses are generated by mixing the output of a microwave source with “DC” pulses generated by arbitrary waveform generators, using DC coupled mixers. They are then sent to the input microwave line that includes bandpass filters and attenuators at various temperatures. The powers given in dB in this letter are arbitrarily refered to $1\,$mW (on $50\,\Omega$) at the input of the dilution refrigerator; the total attenuation down to the sample is about $-77\,$dB. The pulses are routed to the resonator through a circulator to separate the input and output waves.

.3 Sample characterization

.4 Qubit state preparation

We prepare the qubit in its ground state with a high fidelity at the beginning of each experimental sequence by letting it relax during about $20$µs. We estimate at about $1\%$ the equilibrium population in state $|1\rangle$ due to residual noise coming from measurement lines.

To prepare the qubit in its excited state $|1\rangle$ or $|2\rangle$, one or two successive resonant square-shaped pulses of length $t_{\pi}\sim$ 20 ns are applied prior to the readout pulse. The dotted blue S-curve of Fig. 1 was recorded with a single resonant $\pi$ pulse at $f_{12}$ (see text): it reveals that this pulse induces a spurious population of the $|1\rangle$ state of order $1\%$. We checked that this effect is corrected by using gaussian-shaped pulses Martinis1 (data not shown).

.5 Readout Pulses

.6 Coherence times measurement

References