Abstract
We introduce and analyze a dispersive qubit readout scheme where two-mode squeezing is generated directly in the measurement cavities. The resulting suppression of noise enables fast, high-fidelity readout of naturally weakly coupled qubits, and the possibility to protect strongly coupled qubits from decoherence by weakening their coupling. Unlike other approaches exploiting squeezing, our setup avoids the difficult task of transporting and injecting with high fidelity an externally generated squeezed state. Our setup is also surprisingly robust against unwanted non-QND backaction effects, as interference naturally suppresses Purcell decay: the system acts as its own Purcell filter. Our setup is compatible with the experimental state-of-the-art in circuit QED systems, but the basic idea could also be realized in other systems.
Highlights
Quantum states of radiation that have quadrature noise suppressed below the vacuum level, the so called squeezed states [1, 2], have a long history in quantum optics and quantum information science
We introduce a twocavity setup for dispersive qubit readout where two-mode squeezed radiation is generated in situ
Our scheme is effective in the case of a weak coupling between the readout cavity and the qubit: here, we find that our scheme fundamentally changes the scaling of the signalto-noise ratio (SNR) with coupling strength, such that it becomes independent of coupling strength at long times
Summary
Quantum states of radiation that have quadrature noise suppressed below the vacuum level, the so called squeezed states [1, 2], have a long history in quantum optics and quantum information science Their key utility has been in interferometric measurements, where they can be used to dramatically improve precision [3]. One potential drawback of generating squeezing in the measurement cavity is the possibility that the complementary amplified quadrature fluctuations in the cavity will cause extraneous backaction on the qubit Such effects are greatly suppressed in our system, as is standard Purcell decay of the qubit due to simple cavity vacuum fluctuations. This is because of a subtle interference effect, which causes the two-cavity system to act like a built-in Purcell filter
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