Quantum noise limited and entanglement-assisted magnetometry (original) (raw)

Magnetometry with entangled atomic samples

Physical Review A, 2005

We present a theory for the estimation of a scalar or a vector magnetic field by its influence on an ensemble of trapped spin-polarized atoms. The atoms interact off resonantly with a continuous laser field, and the measurement of the polarization rotation of the probe light, induced by the dispersive atom-light coupling, leads to spin squeezing of the atomic sample

Can a Quantum Nondemolition Measurement Improve the Sensitivity of an Atomic Magnetometer?

Physical Review Letters, 2004

We consider the limitations due to noise (e.g., quantum projection noise and photon shot-noise) on the sensitivity of an idealized atomic magnetometer that utilizes spin squeezing induced by a continuous quantum nondemolition measurement. Such a magnetometer measures spin precession of N atomic spins by detecting optical rotation of far-detuned light. We show that for very short measurement times, the optimal sensitivity scales as N ÿ3=4 ; if strongly squeezed probe light is used, the Heisenberg limit of N ÿ1 scaling can be achieved. However, if the measurement time exceeds rel =N 1=2 in the former case, or rel =N in the latter, where rel is the spin relaxation time, the scaling becomes N ÿ1=2 , as for a standard shot-noise-limited magnetometer.

Robust entanglement-based magnetic field sensor beyond the standard quantum limit

Recently, there have been significant developments in entanglement-based quantum metrology. However, entanglement is fragile against experimental imperfections, and quantum sensing to beat the standard quantum limit in scaling has not yet been achieved in realistic systems. Here, we show that it is possible to overcome such restrictions so that one can sense a magnetic field with an accuracy beyond the standard quantum limit even under the effect of decoherence, by using a realistic entangled state that can be easily created even with current technology. Our scheme could pave the way for the realizations of practical entanglement-based magnetic field sensors.

Quantum-Enhanced Magnetometry at Optimal Number Density

Physical Review Letters

We study the use of squeezed probe light and evasion of measurement back-action to enhance the sensitivity and measurement bandwidth of an optically-pumped magnetometer (OPM) at sensitivityoptimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement back-action evasion.

Sub-Projection-Noise Sensitivity in Broadband Atomic Magnetometry

Physical Review Letters, 2010

We demonstrate sub-projection-noise sensitivity of a broadband atomic magnetometer using quantum nondemolition spin measurements. A cold, dipole-trapped sample of rubidium atoms provides a longlived spin system in a nonmagnetic environment, and is probed nondestructively by paramagnetic Faraday rotation. The calibration procedure employs as known reference state, the maximum-entropy or ''thermal'' spin state, and quantitative imaging-based atom counting to identify electronic, quantum, and technical noise in both the probe and spin system. The measurement achieves a sensitivity 1.6 dB (2.8 dB) better than projection-noise (thermal state quantum noise) and will enable squeezing-enhanced broadband magnetometry.

Ultrahigh sensitivity magnetic field and magnetization measurements with an atomic magnetometer

2010

We describe an ultrasensitive atomic magnetometer based on optically pumped potassium atoms operating in a spin-exchange relaxation free regime. We demonstrate magnetic field sensitivity of 160 aT/ Hz 1/2 in a gradiometer arrangement with a measurement volume of 0.45 cm 3 and energy resolution per unit bandwidth of 44ប. As an example of an application enabled by such a magnetometer, we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10 −10 emu/ cm 3 / Hz 1/2 and temperatures up to 420°C.

Magnetometry via a double-pass continuous quantum measurement of atomic spin

Physical Review A, 2009

We argue that it is possible in principle to reduce the uncertainty of an atomic magnetometer by double-passing a far-detuned laser field through the atomic sample as it undergoes Larmor precession. Numerical simulations of the quantum Fisher information suggest that, despite the lack of explicit multi-body coupling terms in the system's magnetic Hamiltonian, the parameter estimation uncertainty in such a physical setup scales better than the conventional Heisenberg uncertainty limit over a specified but arbitrary range of particle number N. Using the methods of quantum stochastic calculus and filtering theory, we demonstrate numerically an explicit parameter estimator (called a quantum particle filter) whose observed scaling follows that of our calculated quantum Fisher information. Moreover, the quantum particle filter quantitatively surpasses the uncertainty limit calculated from the quantum Cramér-Rao inequality based on a magnetic coupling Hamiltonian with only single-body operators. We also show that a quantum Kalman filter is insufficient to obtain super-Heisenberg scaling, and present evidence that such scaling necessitates going beyond the manifold of Gaussian atomic states.

Tunable Atomic Magnetometer for Detection of Radio-Frequency Magnetic Fields

Physical Review Letters, 2005

We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of 2 fT=Hz 1=2 at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about 0:01 fT=Hz 1=2 can be achieved in a practical system with a measurement volume of 200 cm 3 .

Improving cold-atom sensors with quantum entanglement: Prospects and challenges

Applied Physics Letters

Quantum entanglement has been generated and verified in cold-atom experiments and used to make atominterferometric measurements below the shot-noise limit. However, current state-of-the-art cold-atom devices exploit separable (i.e. unentangled) atomic states. This Perspective piece asks the question: can entanglement usefully improve cold-atom sensors, in the sense that it gives new sensing capabilities unachievable with current state-of-the-art devices? We briefly review the state-of-the-art in precision cold-atom sensing, focussing on clocks and inertial sensors, identifying the potential benefits entanglement could bring to these devices, and the challenges that need to be overcome to realize these benefits. We survey demonstrated methods of generating metrologically-useful entanglement in cold-atom systems, note their relative strengths and weaknesses, and assess their prospects for near-to-medium term quantum-enhanced cold-atom sensing. Atom interferometry is a leading precision measurement technology that harnesses the wave-like interference of atoms to make precise measurements of time 1 , accelerations 2 , rotations 3 , gravity 4 , gravity gradients 5 , magnetic fields 6 , the fine structure constant 7 , and Newton's gravitational constant 8. Future applications of atom interferometry include inertial navigation 9-11 , mineral exploration and recovery 12,13 , groundwater monitoring 14 , satellite gravimetry 15-21 , and space-based experiments that test general relativity and candidate theories of quantum gravity 22-26. These applications require a new generation of atom interferometers capable of highly precise, stable measurements in compact, low-weight configurations, that can also operate in real-world field conditions 27-31. Consequently, improvements to cold-atom sensors are not solely aimed at improving precision 32 : increased stability 28,33-35 , increased accuracy 36 , increased dynamic range 37 , increased measurement rate 38 , and decreased size, weight, and power (SWaP) 39-41 are all desirable traits, alongside improved performance in the presence of technical and environmental noise (e.g. due to vehicle motion). Quantum entanglement offers a promising route to improved atom interferometry, since certain entangled atomic states allow relative phase measurements below the shot-noise limit (SNL) (the ultimate sensitivity limit achievable by uncorrelated sources) 42. Such quantum-enhanced atom interferometry could be beneficial when the atom number of the atomic source cannot be increased further due to technical issues or operational requirements. Spin-squeezed states 43 have been the focus of most experimental quantum-enhanced atom interferometry research to date, since they are relatively easy to generate, characterize, and directly incorporate into existing atom interferometry schemes. The first experimental demonstrations of metrological spin squeezing in cold atoms occurred little more than a decade ago 44-48 , and were quickly a) Electronic mail:

Quantum memory, entanglement and sensing with room temperature atoms

Journal of Physics: Conference Series, 2011

Room temperature atomic ensembles in a spin-protected environment are useful systems both for quantum information science and metrology. Here we utilize a setup consisting of two atomic ensembles as a memory for quantum information initially encoded in the polarization state of two entangled light modes. We also use the ensembles as a radio frequency entanglement-assisted magnetometer with projection noise limited sensitivity below femtoTesla/. The performance of the quantum memory as well as the magnetometer was improved by spin-squeezed or entangled atomic states generated by quantum non demolition measurements. Finally, we present preliminary results of long lived entangled atomic states generated by dissipation. With the method presented, one should be able to generate an entangled steady state.