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Optimal non-demolition quantum measurements

Final Report Summary - ONDEQUAM (Optimal non-demolition quantum measurements)

Quantum science and technology have made enormous progress in recent years. The ongoing, so-called second quantum revolution is expected to bring transformative changes not only to the scientific domain but to industry and society as well. The recent decision of the European Commission to endorse this field by labeling quantum science a flagship program well illustrates the importance of the domain as a current and future pillar of the scientific and economic development of Europe.

For the further technological and commercial exploitation of quantum systems the efficient control of quantum states of growing size and complexity as well as effective protection of these states against decoherence will be quintessential.

The goal of ONDEQUAM in this context was to develop and evaluate novel, enhanced techniques for the quantum nondemolition measurement of photon fields. In order to implement the various schemes, we used a superconducting ultrahigh-finesse cavity to store the photon field and circular Rydberg atoms to probe it. As part of the ONDEQUAM project, the setup then underwent a complete overhaul to accommodate a second cavity to allow for the adaptation of the developed procedures to the case of non-local states as well.

In order to perform a nondemolition measurement of the photon number in a cavity, we utilize a Ramsey interferometer to evaluate the phase shift the individual probe atoms encounter while passing through the cavity. The amount of information each probe atom extracts, depends both on the phase shift that the atom experiences in the cavity field and on the orientation of the phase of the Ramsey interferometer with respect to the atomic phase. As part of ONDEQUAM we enhanced this existing scheme by evaluating all available information that is available on the cavity field in real-time in order to accordingly adapt the phase setting of the Ramsey interferometer such as to always maximize the information extraction [1].

The novel adaptive method provided a speedup of the measurement of up to 45% over the conventional scheme. This is particularly important when applied in the case of a decaying photon field, where a speedup of the measurement process allows the measurement (and potentially the stabilization) of large photon fields which would remain inaccessible with the conventional method.

As another way to improve the efficiency of quantum measurements, ONDEQUAM implemented a so-called forward-backward analysis of the photon-number evolution [2]. By evaluating, a posteriori, also probe data which are located in the future of the current state, we were able to significantly reduce the noise level of the measurement. In addition, the forward-backward evaluation lifts the ambiguities in the photon number which are present in the standard QND measurement as a result of the periodicity of the interferometric read-out of the phase of the probe atoms. As a third major asset the novel procedure further offers a highly improved time-resolution of photon jumps.

In the course of ONDEQUAM, we also demonstrated the effectiveness of using an entangled mesoscopic state as a resource for quantum metrology [3]. We employed the quantum superposition generated and probed by the interaction of a single circular Rydberg atom with an initially coherent field in the cavity, to measure the amplitude of a small microwave field acting on the cavity. The scheme enabled us to achieve an experimental precision of 2.4 dB below the standard quantum limit.

After completion of these experiments, ONDEQUAM entered the second phase of the project: the installation of a second cavity to allow for experiments with non-local photon fields. In the course of moving towards a dual-cavity setup, major elements of the experiment had to be overhauled or completely replaced. Most prominently, we developed and installed a new, improved source for circular Rydberg atoms which relies on a much better defined RF-field. Furthermore, new detectors were installed, boosting detection efficiency.

In a lengthy process, a multitude of superconducting mirrors were manufactured using the clean-room facilities of the Laboratoire Kastler-Brossel. The mirrors were all tested at liquid Helium-3 temperatures and the best sets of mirrors, featuring lifetimes of 30ms and 60ms, respectively, were selected and installed. With the mirrors currently undergoing fine-tuning of their resonance frequencies, we expect to be in a position to create and experiment with non-local states within the next months following the end of the ONDEQUAL project.

[1] Adaptive Quantum Nondemolition Measurement of a Photon Number
B. Peaudecerf, T. Rybarczyk, S. Gerlich, S. Gleyzes, J.M. Raimond, S. Haroche, I. Dotsenko, and M. Brune
Phys. Rev. Lett. 112, 080401 (2014)

[2] Forward-backward analysis of the photon-number evolution in a cavity
T. Rybarczyk, M. Penasa, S. Gerlich, B. Julsgaard, K. Molmer, S. Gleyzes, M. Brune, J.M. Raimond, S. Haroche, and I. Dotsenko
Phys. Rev. A 91, 062116 (2015)

[3] Measurement of a microwave field amplitude beyond the standard quantum limit
M. penasa, S. Gerlich, V. metillon, T. Rybarczyk, M. Brune, J.M. Raimond, S. Haroche, L. Davidovich, and I. Dotsenko
Phys Rev. A (submitted)

Contact data: Jean-Michel Raimond