Microwave readout scheme for a Josephson phase qubit

Abstract

We present experimental results on a dispersive scheme for reading out a Josephson phase qubit. A capacitively shunted dc-SQUID is used as a nonlinear resonator which is inductively coupled to the qubit. We detect the flux state of the qubit by measuring the amplitude and phase of a microwave pulse reflected from the SQUID resonator. By this low-dissipative method, we reduce the qubit state measurement time down to 25 µs, which is much faster than using the conventional readout performed by switching the SQUID to its non-zero dc voltage state. The demonstrated readout scheme allows for reading out multiple qubits using a single microwave line by employing frequency-division multiplexing. , consist of superconducting loops interrupted by one or more Josephson junctions. Since their readable states can be discriminated by the magnetic flux passing through the qubit loop, it is common to use inductively coupled dc-SQUIDs as sensitive detectors.The standard method to read out a Josephson phase qubit is to record the dc bias current at which the SQUID switches to its non-superconducting state [3][4][5][6]. This process generates heat directly on the chip and quasiparticles in the circuitry. Both effects are responsible for a relatively long cool-down time of about 1-2 ms that is required after each switching event. This, together with the time needed to ramp up the bias current of the SQUID, limits the repetition rate of the experiment.For the flux qubit, non-destructive dispersive readout schemes have successfully been realized already some time ago, either by coupling to a high quality LC-tank circuit [7], or to a dc-SQUID [8]. So far, most measurements of phase qubits were typically done by the above mentioned switching current measurement of an inductively coupled dc-SQUID. Recently, first experiment overcoming the limitations of the switching readout was reported [9]. In this approach, the phase qubit was capacitively coupled to a transmission line which allows for direct probing its resonance frequency with a microwave pulse. This approach eliminates a readout dc-SQUID, but in turn introduce decoherence via the line coupled directly to the qubit.In this letter, we present experiments on dispersive * Electronic address: ustinov@kit.edu readout of a SQUID weakly coupled to a phase qubit. By using weak coupling between the SQUID and the qubit, this scheme protects the qubit from decoherence sources introduced by the readout circuitry. Moreover, while preserving the intrinsic coherence of the qubit, this method is suitable for reading out many qubits using a single microwave line and frequency-division multiplexing addressing individual readout SQUID resonators. We couple the qubit to a capacitively shunted dc-SQUID which forms a tank circuit having a resonance frequency around 2 GHz. It is connected to a microwave line by a coupling capacitor C 0 shown in Fig. 1. Our sample was fabricated in a standard niobium-aluminium trilayer process. Measurement of the amplitude and phase of a reflected microwave pulse allows ...

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