Using a single quantum probe to sense other quantum objects offers distinct advantages but suffers from some limitations that may degrade the sensing precision severely, especially when the probe-target coupling is weak. Here we propose a strategy to improve the sensing precision by using the quantum probe to engineer the evolution of the target. We consider an exactly solvable model, in which a qubit is used as the probe to sense the frequency of a harmonic oscillator. We show that by applying adaptive periodic quantum control on the qubit, the sensing precision can be enhanced from 1/T scaling with the total time cost T to 1/T^{2} scaling, thus improving the precision by several orders of magnitudes. Such improvement can be achieved without any direct access to the oscillator and the improvement increases with decreasing probe-target coupling. This provides a useful routine to ultrasensitive quantum sensing of weakly coupled quantum objects.
Using a single quantum probe to sense other quantum objects offers distinct advantages but suffers from some limitations that may degrade the sensing precision severely, especially when the probe-target coupling is weak. Here we propose a strategy to improve the sensing precision by using the quantum probe to engineer the evolution of the target. We consider an exactly solvable model, in which a qubit is used as the probe to sense the frequency of a harmonic oscillator. We show that by applying adaptive periodic quantum control on the qubit, the sensing precision can be enhanced from 1/T scaling with the total time cost T to 1/T^{2} scaling, thus improving the precision by several orders of magnitudes. Such improvement can be achieved without any direct access to the oscillator and the improvement increases with decreasing probe-target coupling. This provides a useful routine to ultrasensitive quantum sensing of weakly coupled quantum objects.
Sensing periodic fields using quantum sensors has been an active field of research. In many of these scenarios, the quantum state of the probe is flipped regularly by the application of $\ensuremath{\pi}$ pulses to accumulate information about the target periodic field. The emergence of a discrete time crystalline phase, as a nonequilibrium phase of matter, naturally provides oscillations in a many-body system with an inherent controllable frequency. They benefit from long coherence time and robustness against imperfections, which makes them excellent potential quantum sensors. In this paper, through theoretical and numerical analysis, we show that a disorder-free discrete time crystal probe can reach the ultimate achievable precision for sensing a periodic field. As the amplitude of the periodic field increases, the discrete time crystalline order diminishes, and the performance of the probe decreases remarkably. Nevertheless, the obtained quantum enhancement in the discrete time crystal phase, which is experimentally accessible using standard projective measurements, shows robustness against different imperfections and dephasing noise in the protocol. Finally, we propose the implementation of our protocol in ultracold atoms in optical lattices.
In this paper, we consider the system where a superconducting qubit is coupled to a resonator acting as a harmonic oscillator. Unlike the existing work regarding estimation of the coupling strength or oscillator frequency, we aim to sense the amplitude of a microwave drive by utilizing the qubit as a quantum probe. Furthermore, in order to improve the estimation precision, we employ a periodic control protocol that is easy to be implemented in practice. Such improvement does not require any manipulation of the oscillator. The proposed strategy sheds light on a systematic study on estimation of the driving field in superconducting circuits by means of adding control.
Quantum sensors take advantage of the inherent sensitivity of quantum systems to external environment perturbations to precisely measure target signals. The same interaction with the environment, however, limits the device sensitivity by reducing the coherence of the quantum states. Quantum control can play a crucial role in improving the sensor performance. We explore quantum control tools to improve the performance of a NitrogenVacancy (NV) center as a single-qubit sensor of ultra-weak magnetic fields in noisy environments. We present a method to obtain a precise spectroscopic characterization of the NV environment, instrumental to design robust sensing strategies with improved noise protection. Then, we exploit optimal quantum control based on a set of numerical methods to optimize the temporal dependence of the sensor driving field adapted to quantum sensing purposes, and prove its advantage to achieve best compromises between signal acquisition and noise cancellation.
Quantum sensing uses nonclassical states to improve measurement sensitivity, but the same states that provide metrological gain also decohere more rapidly. This limits the usable interrogation time and, in practice, often precludes improvement in ultimate sensitivity - realized useful quantum advantage has consequently remained rare. Here, we restore persistent quantum advantage by embedding Fock-state enhancement within a quantum heterodyne (Qdyne) protocol, decoupling sensitivity from the decoherence-limited interrogation time τ. Measurements are acquired at short τ where the quantum-enhanced gain is optimal, while precision accumulates with the total measurement time. Demonstrated on the motional mode of a trapped 40Ca+ ion, we observe quantum-enhanced precision that persists to measurement times seven orders of magnitude beyond the dephasing limit, scaling as T^{-3/2} with no indication of saturation. Using the n=3 Fock state, we reach a frequency precision of 0.5uHz relative to an 86MHz carrier, achieving a fractional precision ~6x10^{-15}. This represents a quantum-enhanced gain of 7.1(10) dB over the n=0 state, in agreement with Fisher information predictions. This is the first demonstration of Qdyne beyond solid-state spin-defects. Further, by using the quantum harmonic oscillator to perform frequency mixing, we extend operation beyond 1GHz, two orders of magnitude above the ceiling of pulsed dynamical-decoupling implementations. These results recover quantum advantage at the long timescales required to improve ultimate sensitivity, with direct implications for nanoscale NMR and quantum logic spectroscopy.
Small, controllable quantum systems, known as quantum probes, have been proposed to estimate various parameters characterizing complex systems such as the environments of quantum systems. These probes, prepared in some initial state, are allowed to interact with their environment, and subsequent measurements reveal information about different quantities characterizing the environment, such as the system-environment coupling strength, the cutoff frequency, and the temperature. These estimates have generally been made by considering only the way that the probe undergoes decoherence. However, we show that information about the environment is also imprinted on the probe via the probe and environment correlations that exist before the probe state preparation. This information can then be used to improve our estimates for any environment. We apply this general result to the particular case of a two-level system probe undergoing pure dephasing, due to a harmonic-oscillator environment, to show that a drastic increase in the quantum Fisher information, and hence the precision of our estimates, can indeed be obtained. We also consider applying periodic control pulses to the probe to show that with a combination of the two---the effect of the control pulses as well as the initial correlations---the quantum Fisher information can be increased by orders of magnitude.
We investigate the quantum sensing performance of a dephasing qubit as a probe in Ohmic environments, characterized by the coupling strength η, the Ohmicity parameter s, and the cutoff frequency ωc to be estimated. The performance is quantified by the dimensionless quantum signal-to-noise ratio Q. We show that the evolution of Q with the scaled time ωct is independent of ωc, and peaks at an optimal time topt, yielding optimal sensitivity Qopt. We analyze how Qopt depends on η, s and the temperature T. Our results demonstrate that, for any Ohmic environment, provided that ωctopt≪1, Qopt always reaches the upper bound: Qmax=0.648 at zero temperature, and consistently attains Qmax/4 at high temperatures. Remarkably, we find that increasing the scaled temperature T/ωc can enhance Qopt by nearly two orders of magnitude compared to its zero-temperature counterpart for certain Ohmic environments. Our work reveals that temperature can serve as a resource to enhance sensing precision, as it accelerates the encoding of the cutoff frequency information into the probe state, thereby enabling optimal measurement within a short time window.