arXiv subjects

Morgan W. Mitchell

Publications and source records attributed to Morgan W. Mitchell.

At least 55 records · Page 3Linked to original sources

arXiv2021

Auto-heterodyne characterization of narrow-band photon pairs

We describe a technique to measure photon pair joint spectra by detecting the time-correlation beat note when non-degenerate photon pairs interfere at a beamsplitter. The technique implements a temporal analog of the Ghosh-Mandel effect with one photon counter and a time-resolved Hong-Ou-Mandel interference with two. It is well suited to characterize pairs of photons, each of which can interact with a single atomic species, as required to study recently predicted photon-photon interaction in sub-wavelength atomic arrays. With this technique, we characterize photon pairs from cavity-enhanced parametric downconversion with a bandwidth $\approx$ 5 MHz and frequency separation of $\sim$ 200 MHz near the D$_1$ line of atomic Rb.

quant-ph↗

arXiv2021

Manipulating and measuring single atoms in the Maltese cross geometry

We describe optical methods for trapping, cooling, and observing single $^{87}$Rb atoms in a four-lens "Maltese cross" geometry (MCG). The use of four high numerical-aperture lenses in the cardinal directions enables efficient collection of light from non-collinear directions, but also restricts the optical access for cooling and optical pumping tasks. We demonstrate three-dimensional atom localization with sub-wavelength precision, and present measurements of the trap lifetime, temperature and transverse trap frequency in this geometry. We observe a trap performance comparable to what has been reported for single-atom traps with one- or two-lens optical systems, and conclude that the additional coupling directions provided by the MCG come at little cost to other trap characteristics.

physics.atom-ph↗

arXiv2020

Criticality-enhanced quantum sensing in ferromagnetic Bose-Einstein condensates: role of readout measurement and detection noise

We theoretically investigate estimation of the control parameter in a ferromagnetic Bose-Einstein condensate near second order quantum phase transitions. We quantify sensitivity by quantum and classical Fisher information and using the error-propagation formula. For these different metrics, we find the same, beyond-standard-quantum-limit (SQL) scaling with atom number near critical points, and SQL scaling away from critical points. We find that both depletion of the $m_f=0$ Zeeman sub-level and transverse magnetization provide signals of sufficient quality to saturate the sensitivity scaling. To explore the effect of experimental imperfections, we study the scaling around criticality at nonzero temperature and with nonzero detection noise. Our results suggest the feasibility of sub-SQL sensing in ferromagnetic condensates with current experimental capabilities.

cond-mat.quant-gas↗

arXiv2020

Scale-invariant spin dynamics and the quantum limits of field sensing

We describe quantum limits to field sensing that relate noise, geometry and measurement duration to fundamental constants, with no reference to particle number. We cast the Tesche and Clarke (TC) bound on dc-SQUID sensitivity as such a limit, and find analogous limits for volumetric spin-precession magnetometers. We describe how randomly-arrayed spins, coupled to an external magnetic field of interest and to each other by the magnetic dipole-dipole interaction, execute a spin dynamics that depolarizes the spin ensemble even in the absence of coupling to an external reservoir. We show the resulting spin dynamics are scale invariant, with a depolarization rate proportional to spin number density and thus a number-independent quantum limit on the energy resolution per bandwidth $E_R$. Numerically, we find $E_R \ge α\hbar$, $α\sim 1$, in agreement with the TC limit, for paradigmatic spin-based measurements of static and oscillating magnetic fields.

quant-ph↗

arXiv2020

Measurement-induced nonlocal entanglement in a hot, strongly-interacting atomic system

Quantum technologies use entanglement to outperform classical technologies, and often employ strong cooling and isolation to protect entangled entities from decoherence by random interactions. Here we show that the opposite strategy - promoting random interactions - can help generate and preserve entanglement. We use optical quantum non-demolition measurement to produce entanglement in a hot alkali vapor, in a regime dominated by random spin-exchange collisions. We use Bayesian statistics and spin-squeezing inequalities to show that at least $1.52(4)\times 10^{13}$ of the $5.32(12) \times 10^{13}$ participating atoms enter into singlet-type entangled states, which persist for tens of spin-thermalization times and span thousands of times the nearest-neighbor distance. The results show that high temperatures and strong random interactions need not destroy many-body quantum coherence, that collective measurement can produce very complex entangled states, and that the hot, strongly-interacting media now in use for extreme atomic sensing are well suited for sensing beyond the standard quantum limit.

quant-ph↗

arXiv2020

Bose-Einstein Condensate Comagnetometer

We describe a comagnetometer employing the $f=1$ and $f=2$ ground state hyperfine manifolds of a $^{87}$Rb spinor Bose-Einstein condensate as co-located magnetometers. The hyperfine manifolds feature nearly opposite gyromagnetic ratios and thus the sum of their precession angles is only weakly coupled to external magnetic fields, while being highly sensitive to any effect that rotates both manifolds in the same way. The $f=1$ and $f=2$ transverse magnetizations and azimuth angles are independently measured by non-destructive Faraday rotation probing, and we demonstrate a $44.0(8)\text{dB}$ common-mode rejection in good agreement with theory. We show how spin-dependent interactions can be used to inhibit $2\rightarrow 1$ hyperfine relaxing collisions, extending to $\sim 1\text{s}$ the transverse spin lifetime of the $f=1,2$ mixtures. The technique could be used in high sensitivity searches for new physics on sub-millimeter length scales, precision studies of ultra-cold collision physics, and angle-resolved studies of quantum spin dynamics.

physics.atom-ph↗

arXiv2020

Strong quantum correlations of light emitted by a single atom in free space

We present a novel approach to engineer the photon correlations emerging from the interference between an input field and the field scattered by a single atom in free space. Nominally, the inefficient atom-light coupling causes the quantum correlations to be dominated by the input field alone. To overcome this issue, we propose the use of separate pump and probe beams, where the former increases the atomic emission to be comparable to the probe. Examining the second-order correlation function $g^{(2)}(τ)$ of the total field in the probe direction, we find that the addition of the pump formally plays the same role as increasing the coupling efficiency. We show that one can tune the correlation function $g^{(2)}(0)$ from zero (perfect anti-bunching) to infinite (extreme bunching) by a proper choice of pump amplitude. We further elucidate the origin of these correlations in terms of the transient atomic state following the detection of a photon.

quant-ph↗

arXiv2020

Quantum limits to the energy resolution of magnetic field sensors

The energy resolution per bandwidth $E_R$ is a figure of merit that combines the field resolution, bandwidth or duration of the measurement, and size of the sensed region. Several different dc magnetometer technologies approach $E_R = \hbar$, while to date none has surpassed this level. This suggests a technology-spanning quantum limit, a suggestion that is strengthened by model-based calculations for nitrogen-vacancy centres in diamond, for superconducting quantum interference device (SQUID) sensors, and for some optically-pumped alkali-vapor magnetometers, all of which predict a quantum limit close to $E_R = \hbar$. Here we review what is known about energy resolution limits, with the aim to understand when and how $E_R$ is limited by quantum effects. We include a survey of reported sensitivity versus size of the sensed region for more than twenty magnetometer technologies, review the known model-based quantum limits, and critically assess possible sources for a technology-spanning limit, including zero-point fluctuations, magnetic self-interaction, and quantum speed limits. Finally, we describe sensing approaches that appear to be unconstrained by any of the known limits, and thus are candidates to surpass $E_R = \hbar$.

quant-ph↗

arXiv2019

Experimental Low-Latency Device-Independent Quantum Randomness

Applications of randomness such as private key generation and public randomness beacons require small blocks of certified random bits on demand. Device-independent quantum random number generators can produce such random bits, but existing quantum-proof protocols and loophole-free implementations suffer from high latency, requiring many hours to produce any random bits. We demonstrate device-independent quantum randomness generation from a loophole-free Bell test with a more efficient quantum-proof protocol, obtaining multiple blocks of $512$ bits with an average experiment time of less than $5$ min per block and with a certified error bounded by $2^{-64}\approx 5.42\times 10^{-20}$.

quant-ph↗

arXiv2019

Narrowband photon pairs with independent frequency tuning for quantum light-matter interactions

We describe a cavity-enhanced spontaneous parametric down-conversion (CE-SPDC) source for narrowband photon pairs with filters such that over 97% of the correlated photons are in a single mode of 4.3(4) MHz bandwidth. Type-II phase matching, a tuneable-birefringence resonator, MHz-resolution pump tuning, and tuneable Fabry-Perot filters are used to achieve independent signal and idler tuning. We map the CE-SPDC spectrum using difference frequency generation to precisely locate the emission clusters, and demonstrate CE-SPDC driven atomic spectroscopy. The generated photon pairs efficiently interact with neutral rubidium, a well-developed system for quantum networking and quantum simulation. The techniques are readily extensible to other material systems.

quant-ph↗

arXiv2019

Maltese cross coupling to individual cold atoms in free space

We report on the simultaneous observation from four directions of the fluorescence of single $^{87}$Rb atoms trapped at the common focus of four high numerical aperture (${\rm NA}=0.5$) aspheric lenses. We use an interferometrically-guided pick-and-place technique to precisely and stably position the lenses along the four cardinal directions with their foci at a single central point. The geometry gives right angle access to a single quantum emitter, and will enable new trapping, excitation, and collection methods. The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

physics.atom-ph↗

arXiv2019

Interferometric measurement of interhyperfine scattering lengths in $^{87}$Rb

We present interferometeric measurements of the $f=1$ to $f=2$ inter-hyperfine scattering lengths in a single-domain spinor Bose-Einstein condensate of $^{87}$Rb. The inter-hyperfine interaction leads to a strong and state-dependent modification of the spin-mixing dynamics with respect to a non-interacting description. We employ hyperfine-specific Faraday-rotation probing to reveal the evolution of the transverse magnetization in each hyperfine manifold for different state preparations, and a comagnetometer strategy to cancel laboratory magnetic noise. The method allows precise determination of inter-hyperfine scattering length differences, calibrated to intra-hyperfine scattering length differences. We report $(a_{3}^{(12)}-a_{2}^{(12)})/(a_{2}^{(1)}-a_{0}^{(1)})=-1.27(15)$ and $(a_{1}^{(12)}-a_{2}^{(12)})/(a_{2}^{(1)}-a_{0}^{(1)})=-1.31(13)$, limited by atom number uncertainty. With achievable control of atom number, we estimate precisions of $ \approx 0.3\%$ should be possible with this technique.

physics.atom-ph↗

arXiv2018

Experimental test of measurement dependent local Bell inequality with human free will

A Bell test can rule out local realistic models, and has potential applications in communications and information tasks. For example, a Bell inequality violation can certify the presence of intrinsic randomness in measurement outcomes, which then can be used to generate unconditional randomness. A Bell test requires, however, measurements that are chosen independently of other physical variables in the test, as would be the case if the measurement settings were themselves unconditionally random. This situation seems to create a "bootstrapping problem" that was recently addressed in The BIG Bell Test, a collection of Bell tests and related tests using human setting choices. Here we report in detail our experimental methods and results within the BIG Bell Test. We perform a experimental test of a special type of Bell inequality - the measurement dependent local inequality. With this inequality, even a small amount of measurement independence makes it possible to disprove local realistic models. The experiment uses human-generated random numbers to select the measurement settings in real time, and implements the measurement setting with space-like separation from the distant measurement. The experimental result shows a Bell inequality violation that cannot be explained by local hidden variable models with independence parameter (as defined in [Putz et al. Phys. Rev. Lett. 113, 190402 (2014).] ) l > 0.10 +/- 0.05. This result quantifies the degree to which a hidden variable model would need to constrain human choices, if it is to reproduce the experimental results.

quant-ph↗

arXiv2018

Sensitivity, quantum limits, and quantum enhancement of noise spectroscopies

We study the fundamental limits of noise spectroscopy using estimation theory, Faraday rotation probing of an atomic spin system, and squeezed light. We find a simple and general expression for the Fisher information, which quantifies the sensitivity to spectral parameters such as resonance frequency and linewidth. For optically-detected spin noise spectroscopy, we find that shot noise imposes "local" standard quantum limits for any given probe power and atom number, and also "global" standard quantum limits when probe power and atom number are taken as free parameters. We confirm these estimation theory results using non-destructive Faraday rotation probing of hot Rb vapor, observing the predicted optima and finding good quantitative agreement with a first-principles calculation of the spin noise spectra. Finally, we show sensitivity beyond the atom- and photon-number-optimized global standard quantum limit using squeezed light.

quant-ph↗

arXiv2018

Quantum enhanced measurements without entanglement

Quantum-enhanced measurements exploit quantum mechanical effects for increasing the sensitivity of measurements of certain physical parameters and have great potential for both fundamental science and concrete applications. Most of the research has so far focused on using highly entangled states, which are, however, difficult to produce and to stabilize for a large number of constituents. In the following we review alternative mechanisms, notably the use of more general quantum correlations such as quantum discord, identical particles, or non-trivial hamiltonians; the estimation of thermodynamical parameters or parameters characterizing non-equilibrium states; and the use of quantum phase transitions. We describe both theoretically achievable enhancements and enhanced sensitivities, not primarily based on entanglement, that have already been demonstrated experimentally, and indicate some possible future research directions.

quant-ph↗

arXiv2018

Phase diffusion quantum entropy source on a silicon chip

We report an accelerated laser phase diffusion quantum entropy source with all non-laser optical and optoelectronic elements implemented in silicon photonics. The device uses efficient and robust single-laser accelerated phase diffusion methods, and implements the whole quantum entropy source scheme including an unbalanced Mach-Zehnder interferometer with optimized splitting ratio, in a 0.5 mmx1 mm footprint. We demonstrate Gbps raw entropy-generation rates in a technology compatible with conventional CMOS fabrication techniques.

quant-ph↗

arXiv2018

Signal tracking beyond the time resolution of an atomic sensor by Kalman filtering

We study causal waveform estimation (tracking) of time-varying signals in a paradigmatic atomic sensor, an alkali vapor monitored by Faraday rotation probing. We use Kalman filtering, which optimally tracks known linear Gaussian stochastic processes, to estimate stochastic input signals that we generate by optical pumping. Comparing the known input to the estimates, we confirm the accuracy of the atomic statistical model and the reliability of the Kalman filter, allowing recovery of waveform details far briefer than the sensor's intrinsic time resolution. With proper filter choice, we obtain similar benefits when tracking partially-known and non-Gaussian signal processes, as are found in most practical sensing applications. The method evades the trade-off between sensitivity and time resolution in coherent sensing.

quant-ph↗

arXiv2018

A self-tuning optical resonator

We demonstrate a nonlinear optical resonator that tunes itself onto resonance with an input beam. In a monolithic Fabry-Perot cavity implemented in rubidium-doped periodically-poled potassium titanyl phosphate, an intensity-dependent refractive index produces line-pulling by multiple free-spectral ranges (FSRs). In this condition, the cavity passively maintains optical resonance in the face of FSR-scale excursions of the drive laser frequency: when one resonant operating-point becomes unstable, the resonator rapidly transitions to another resonant operating point. We demonstrate stable second-harmonic generation with no active feedback to laser or cavity. The self-tuning effect appears to be supported by a very strong, previously unreported optical nonlinearity.

physics.optics↗