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1. Preparation of Metrological States in Dipolar-Interacting Spin Systems

   Zheng, Tian-Xing ; Li, Anran ; Rosen, Jude ; Zhou, Sisi ; Koppenhöfer, Martin ; Ma, Ziqi ; Chong, Frederic T. ; Clerk, Aashish A. ; Jiang, Liang ; Maurer, Peter C. ( March 2022 , ArXivorg)

   Spin systems are an attractive candidate for quantum-enhanced metrology. Here we develop a variational method to generate metrological states in small dipolar-interacting ensembles with limited qubit controls and unknown spin locations. The generated states enable sensing beyond the standard quantum limit (SQL) and approaching the Heisenberg limit (HL). Depending on the circuit depth and the level of readout noise, the resulting states resemble Greenberger-Horne-Zeilinger (GHZ) states or Spin Squeezed States (SSS). Sensing beyond the SQL holds in the presence of finite spin polarization and a non-Markovian noise environment. 

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2. Back action evasion in optical lever detection

   https://doi.org/10.1364/OPTICA.500036  

   Hao, Shan ; Purdy, Thomas P. ( January 2024 , Optica)

   The optical lever is a centuries old and widely used detection technique employed in applications ranging from consumer products and industrial sensors to precision force microscopes used in scientific research. However, despite the long history, its quantum limits have yet to be explored. In general, any precision optical measurement is accompanied by optical force induced disturbance to the measured object (termed as back action) leading to a standard quantum limit (SQL). Here, we give a simple ray optics description of how such back action can be evaded in optical lever detection. We perform a proof-of-principle experiment demonstrating the mechanism of back action evasion in the classical regime, by developing a lens system that cancels extra tilting of the reflected light off a silicon nitride membrane mechanical resonator caused by laser-pointing-noise-induced optical torques. We achieve a readout noise floor two orders of magnitude lower than the SQL, corresponding to an effective optomechanical cooperativity of 100 without the need for an optical cavity. As the state-of-the-art ultralow dissipation optomechanical systems relevant for quantum sensing are rapidly approaching the level where quantum noise dominates, simple and widely applicable back action evading protocols will be crucial for pushing beyond quantum limits.

    

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3. Entanglement-enhanced optical atomic clocks

   https://doi.org/10.1063/5.0121372  

   Colombo, Simone ; Pedrozo-Peñafiel, Edwin ; Vuletić, Vladan ( November 2022 , Applied Physics Letters)

   Recent developments in atomic physics have enabled the experimental generation of many-body entangled states to boost the performance of quantum sensors beyond the Standard Quantum Limit (SQL). This limit is imposed by the inherent projection noise of a quantum measurement. In this Perspective article, we describe the commonly used experimental methods to create many-body entangled states to operate quantum sensors beyond the SQL. In particular, we focus on the potential of applying quantum entanglement to state-of-the-art optical atomic clocks. In addition, we present recently developed time-reversal protocols that make use of complex states with high quantum Fisher information without requiring sub-SQL measurement resolution. We discuss the prospects for reaching near-Heisenberg limited quantum metrology based on such protocols. 

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4. Enhancing the sensitivity of an atom interferometer to the Heisenberg limit using increased quantum noise

   https://doi.org/10.1364/JOSAB.396358  

   Fang, Renpeng ; Sarkar, Resham ; Shahriar, Selim M. ( June 2020 , Journal of the Optical Society of America B)

   In a conventional atomic interferometer employing$N$atoms, the phase sensitivity is at the standard quantum limit:$1/\sqrt{N}$. Under usual spin squeezing, the sensitivity is increased by lowering the quantum noise. It is also possible to increase the sensitivity by leaving the quantum noise unchanged while producing phase amplification. Here we show how to increase the sensitivity, to the Heisenberg limit of$1/N$, while increasing the quantum noise by$\sqrt{N}$and amplifying the phase by a factor of$N$. Because of the enhancement of the quantum noise and the large phase magnification, the effect of excess noise is highly suppressed. The protocol uses a Schrödinger cat state representing a maximally entangled superposition of two collective states of$N$atoms. The phase magnification occurs when we use either atomic state detection or collective state detection; however, the robustness against excess noise occurs only when atomic state detection is employed. We show that for one version of the protocol, the signal amplitude is$N$when$N$is even, and is vanishingly small when$N$is odd, for both types of detection. We also show how the protocol can be modified to reverse the nature of the signal for odd versus even values of$N$. Thus, for a situation where the probability of$N$being even or odd is equal, the net sensitivity is within a factor of$\sqrt{2}$of the Heisenberg limit. Finally, we discuss potential experimental constraints for implementing this scheme via one-axis-twist squeezing employing the cavity feedback scheme, and show that the effects of cavity decay and spontaneous emission are highly suppressed because of the increased quantum noise and the large phase magnification inherent to the protocol. As a result, we find that the maximum improvement in sensitivity can be close to the ideal limit for as many as 10 million atoms.

    

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5. Integrated quantum optical phase sensor in thin film lithium niobate

   https://doi.org/10.1038/s41467-023-38246-6  

   Stokowski, Hubert S. ; McKenna, Timothy P. ; Park, Taewon ; Hwang, Alexander Y. ; Dean, Devin J. ; Celik, Oguz Tolga ; Ansari, Vahid ; Fejer, Martin M. ; Safavi-Naeini, Amir H. ( June 2023 , Nature Communications)

   The quantum noise of light, attributed to the random arrival time of photons from a coherent light source, fundamentally limits optical phase sensors. An engineered source of squeezed states suppresses this noise and allows phase detection sensitivity beyond the quantum noise limit (QNL). We need ways to use quantum light within deployable quantum sensors. Here we present a photonic integrated circuit in thin-film lithium niobate that meets these requirements. We use the second-order nonlinearity to produce a squeezed state at the same frequency as the pump light and realize circuit control and sensing with electro-optics. Using 26.2 milliwatts of optical power, we measure (2.7 ± 0.2)% squeezing and apply it to increase the signal-to-noise ratio of phase measurement. We anticipate that photonic systems like this, which operate with low power and integrate all of the needed functionality on a single die, will open new opportunities for quantum optical sensing.

    

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