Quantum Non-demolition Detection of Single Microwave Photons in a Circuit (1003.2734v1)

Abstract: Thorough control of quantum measurement is key to the development of quantum information technologies. Many measurements are destructive, removing more information from the system than they obtain. Quantum non-demolition (QND) measurements allow repeated measurements that give the same eigenvalue. They could be used for several quantum information processing tasks such as error correction, preparation by measurement, and one-way quantum computing. Achieving QND measurements of photons is especially challenging because the detector must be completely transparent to the photons while still acquiring information about them. Recent progress in manipulating microwave photons in superconducting circuits has increased demand for a QND detector which operates in the gigahertz frequency range. Here we demonstrate a QND detection scheme which measures the number of photons inside a high quality-factor microwave cavity on a chip. This scheme maps a photon number onto a qubit state in a single-shot via qubit-photon logic gates. We verify the operation of the device by analyzing the average correlations of repeated measurements, and show that it is 90% QND. It differs from previously reported detectors because its sensitivity is strongly selective to chosen photon number states. This scheme could be used to monitor the state of a photon-based memory in a quantum computer.

• The paper demonstrates a quantum non-demolition method where single microwave photon counts are non-invasively detected using qubit-photon logic gates.
• It leverages controlled-NOT operations and quasi-dispersive interactions to map photon numbers onto qubit states with approximately 90% fidelity.
• The technique enables repeated, high-selectivity measurements in superconducting circuits, advancing quantum error correction and photon-based memory applications.

Quantum Non-demolition Detection of Single Microwave Photons in a Circuit

The field of quantum information processing requires precise measurement techniques, specifically those that can be non-invasive. The paper "Quantum Non-demolition Detection of Single Microwave Photons in a Circuit" presents a methodology to achieve Quantum Non-demolition (QND) measurements for single microwave photons—a crucial development within the ambit of quantum computing technologies. The paper is significant in the regime of microwave photons manipulated in superconducting circuits, where detecting the presence or absence of photons without destroying them is technically challenging.

The authors report a QND detection technique wherein the number of photons within a microwave cavity is mapped onto a qubit state using photon-sensitive qubit-photon logic gates. This innovative approach leverages programmable controlled-NOT (CNOT) operations between Fock states and a qubit. Each interrogation employs quasi-dispersive interactions such that the qubit transition frequency varies with the photon number, effectuating a conditional operation that relies on the photon number within the cavity. By adiabatically decoupling the qubit and cavity, it preserves the quantum state during measurement, ensuring that the measurement itself does not disrupt the photon state in the cavity.

Experimental results demonstrate the efficacy and QND nature of this methodology. Tests reveal approximately 90% QND nature, with measurements indicating high selectivity towards specific photon number states. The framework extends circuit-based cavity QED by coupling a transmon qubit to dual cavities, optimizing one for rapid readout and the other for the prolonged storage of photons. The configuration was demonstrated to enable repeated measurements in excess of the photon lifetime within the storage cavity, with reported transition probabilities $\gamma_0\: = 1\%$ and $\gamma_1\: = 10\%$, and $\delta_0\: = 7\%$ and $\delta_1\: = 3\%$.

The QND method improves upon current single photon detection methods by leveraging high-selectivity operations, proving crucial for photon-based memories that could be integral to quantum computers. The paper opens pathways to prepare non-classical states of light and possibly observe quantum jumps of photons.

In theoretical terms, the implications of this work stress the need and capability for high-level control over quantum systems. Practically, it underscores the growing feasibility of implementing more sophisticated quantum error correction schemes and photon-number-resolving detection technologies. Future developments could explore extensions to higher photon numbers and improve real-time, single-shot measurements incorporating better qubit readout techniques. With quantum technologies evolving rapidly, the methodologies described serve as an essential building block in the broader quantum computing landscape.