Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D38: Focus Session: Sensing and Entanglement Measures 
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Sponsoring Units: GQI Chair: Susanne Yelin, University of Connecticut Room: 212B 
Monday, March 2, 2015 2:30PM  3:06PM 
D38.00001: Optimal spectrum estimation of density operators with alkalineearth atoms Invited Speaker: Alexey Gorshkov The eigenspectrum $\vec{p} \equiv (p_1,p_2,..p_d)$ of the density operator $\hat{\rho}$ describing the state of a quantum system can be used to characterize the entanglement of this system with its environment. In the seminal paper [Phys. Rev. A 64, 052311 (2001)], Keyl and Werner present the optimal measurement scheme for inferring $\vec{p}$ given $n$ copies of an unknown state $\hat{\rho}$. Since this measurement uses a highly entangled basis over the full joint state $\hat{\rho}^{\otimes n}$ of all copies, it should naively be extremely difficult to implement in practice. In this talk, we give a simple experimental protocol to carry out the KeylWerner measurement for $\hat \rho$ on the nuclear spin degrees of freedom of $n$ alkalineearth atoms using standard Ramsey spectroscopy techniques. [Preview Abstract] 
Monday, March 2, 2015 3:06PM  3:18PM 
D38.00002: Testing ClauserHorneShimonyHolt inequalities using observables with arbitrary spectrum Andreas Ketterer, Arne Keller, Thomas Coudreau, P\'erola Milman The ClauserHorneShimony and Holt inequality applies when measurements with binary outcomes are performed on physical systems under the assumption of local realism. Testing such inequalities in the quantum realm involves either measurements of twovalued quantum observables or predefining a context dependent binning procedure. Here we establish the conditions to test the ClauserHorneShimony and Holt inequality using any quantum observable. Our result applies to observables with an arbitrary spectrum and no prior knowledge of their Hilbert space's dimension is required. Finally, we demonstrate the proposed general measurement strategy, that can be seen as positive operator valued measurements performed on the system, using the formalism of modular variables applied to the transverse degrees of freedom of photon pairs. [Preview Abstract] 
Monday, March 2, 2015 3:18PM  3:30PM 
D38.00003: Violation of Classical Inequalities in the Light Scattered by a Quantum Dot Manoj Peiris, Ben Petrak, Kumarasiri Konthasinghe, Ying Yu, Zhichuan Niu, Andreas Muller We report the measurement of the twophoton spectrum of the light scattered nearresonantly by a single InAs semiconductor quantum dot exposed to a monochromatic laser. In contrast to the ordinary (onedimensional) onephoton spectrum, the twophoton spectrum represents the probability of emitting two photons with two different colors. It is obtained experimentally using a pair of frequencytunable filters in a modified HanburyBrown and Twiss setup. We analyze the resulting twodimensional maps for different parameters including the Rabi frequency, the laser detuning, and the filter bandwidth. We find excellent agreement with the theory of Del Valle et al. In particular, our measurements reveal the interferences of different decay paths to yield overall antibunched photon statistics while the individual filtered pathways may exhibit photon bunching, photon antibunching or nearPoisson statistics. We further evidence how the radiative cascade can proceed via virtual intermediate states giving rise to transitions previously termed ``leapfrog transitions''. Highly nonclassical physics are seen, which can violate wellknown classical inequalities such as the CauchySchwarz inequality or Bells inequalities. [Preview Abstract] 
Monday, March 2, 2015 3:30PM  3:42PM 
D38.00004: Classical Model for Measurements of an Entanglement Witness Brian La Cour, E.C. George Sudarshan We describe a classical model that may serve as an analog for joint and local measurments of an entanglement witness. The analogous experimental procedure and data analysis protocol of the model follow those of a previous experiment to measure an entanglement witness with polarized photons prepared in a mixed state [M. Barbieri \textit{et al.}, Phys.\ Rev.\ Lett.\ \textbf{91}, 227901 (2003)]. Numerical simulations show excellent agreement with both experimental results and quantum mechanical predictions. [Preview Abstract] 
Monday, March 2, 2015 3:42PM  3:54PM 
D38.00005: Realtime adaptive quantum measurements on a single spin in diamond for sensing and quantum information protocols Machiel Blok, Cristian Bonato, Ronald Hanson Realtime feedback based on quantum measurements is a crucial ingredient for many proposed quantum information and sensing technologies. Implementation requires highfidelity measurements as well as fast electronics that perform the control operation. A single electron spin, associated with the NitrogenVacancy defect in diamond, forms an excellent test bed for realtime feedback protocols, since it can be read out with high fidelity in a single shot using optical transitions and maintain coherence for long times. Here we demonstrate an adaptive phase estimation protocol to sense DC magnetic fields with very high precision. The magnetic field is measured with a Ramsey interferometry sequence which is repeated many times. When adjusting the parameters of the Ramsey sequence in real time based on the outcome of previous measurements, the sensitivity of our magnetometer shows scaling close to the fundamental Heisenberg limit ($\sim$ 1/N) as the number of measurements N is increased. [Preview Abstract] 
Monday, March 2, 2015 3:54PM  4:06PM 
D38.00006: Joint estimation of phase and phase diffusion for quantum metrology Mihai Vidrighin, Gaia Donati, Marco Genoni, XianMin Jin, Steven Kolthammer, Myungshik Kim, Animesh Datta, Marco Barbieri, Ian Walmsley Phase estimation, at the heart of many quantum metrology and communication schemes, is affected by noise. We have investigated joint estimation of a phase and the amplitude of phase diffusion. The motivation is that the two parameters are not independent, phase estimation depending on calibration of the diffusion amplitude, which may drift in experiments. Also, joint estimation could provide an advantage in estimating the two parameters. For several instances, describing relevant probe statessplit singlephotons, coherent states or N00N states, we found that the multiparameter estimation problem can be modelled in a twodimensional Hilbert space. For these cases, we obtained a tradeoff bound on statistical variances of the phase and diffusion amplitude. We found an optimum measurement scheme for states with fixed photon numbers, which has some useful properties; it is robust to calibration imperfections and does not need adaptive phase control. We also used our bound to quantify the effectiveness of an experimental setup for joint estimation in classical light polarimetry. By numerical searches, we found that joint measurements on subsequent probes can improve joint estimation and we found indications that our bound holds for general states at small diffusion amplitudes. [Preview Abstract] 
Monday, March 2, 2015 4:06PM  4:18PM 
D38.00007: Noiseresilient quantum metrology for singlemolecule spectroscopy with low light levels Felipe Herrera, Alan AspuruGuzik Continuous observation of biological processes over long timescales exceeding seconds is challenging using standard fluorescence techniques due to technical issues such as photodamage. Current photonic technology can be exploited to overcome those challenges while preserving sensitivity at the single molecule level. We show that using a simple quantum metrology scheme involving periodic driving for optical state preparation, it is possible to perform spectroscopy of a single chiral molecule in a condensed phase environment, with low photon fluxes. We show that for certain nonclassical optical probes and measurement settings, it is possible to exceed the standard quantum limit of precision for a range of driving parameters, even in the presence of high transmission losses due to background absorption. We compare the proposed scheme with fluorescence spectroscopy for single molecule detection, and discuss possible applications of quantum metrology in systems biology. [Preview Abstract] 
Monday, March 2, 2015 4:18PM  4:30PM 
D38.00008: Quantum Computation and Quantum Metrology based on Single Electron Spin in Diamond Jiangfeng Du It is of great challenge to perform the accurate controlling the electron spin qubits in realistic system, due to the noises aroused from the noisy spin bath and the driving field. Firstly, we adopted dynamically corrected gates to realize robust and highfidelity quantum gates. In this work, the quantum gate's performance was pushed to T1r limit [PRL 2014, 112, 050503]. Then, a new Rabi Oscillations (ROs) resulting from LandauZener (LZ) transitions is observed useful to suppress the fluctuations of the driving field [PRL 2014, 112, 010503]. Besides, quantum error correction is experimentally employed to overcome the noise effect in diamonds [Nature 2014, 506, 204207]. Precise quantum control and effectively supressing noise of the environment are of great importance for quantum metrology. We succeeded in sensing and atomicscale analysis of single nuclear spin clusters in diamond at room temperature [Nature Physics 2014, 10, 2125], and also have succeed to detect a few nuclear spins with single spin sensitivity [Nature Commu., 2014, 4:4703]. [Preview Abstract] 
Monday, March 2, 2015 4:30PM  4:42PM 
D38.00009: HighSensitivity Temperature Sensing Using an Implanted Single NitrogenVacancy Center Array in Diamond Guanzhong Wang, Junfeng Wang We present a highsensitivity temperature detection using an implanted single NitrogenVacancy center array in diamond. The highorder Thermal CarrPurcellMeiboomGill (TCPMG) method was performed on the implanted single nitrogen vacancy (NV) center in diamond in a static magnetic field. We demonstrated that under small detunings for the two driving microwave frequencies, the oscillation frequency of the induced fluorescence of the NV center equals approximately to the average of the detunings of the two driving fields. On basis of the conclusion, the zerofield splitting D for the NV center and the corresponding temperature could be determined. The experiment showed that the coherence time for the highorder TCPMG was effectively extended, particularly up to 108 $\mu $s for TCPMG8, about 14 times of the value 7.7 $\mu $s for thermal Ramsey method. This coherence time corresponded to a thermal sensitivity of 10.1 mK/Hz$^{1/2}$. We also detected the temperature distribution on the surface of a diamond chip by using the implanted NV center array with the TCPMG3. Our approach implies the feasibility for using implanted NV centers in highquality diamonds to detect temperatures with highsensitivity and nanoscale resolution. [Preview Abstract] 
Monday, March 2, 2015 4:42PM  4:54PM 
D38.00010: Nitrogen Vacancy centers in diamond as $\theta ^{2}$ sensors Shonali Dhingra, Brian D'Urso Nitrogenvacancy (NV) centers are naturally occurring defects in diamond, which feature an electronic spin with welldefined quantum behavior, including long quantum coherence times and optical addressability. In this work, we show how to align an external magnetic field to the direction of the internallydefined spin axis of the NV center. We further show that this capability can be explored to use an NV center as a $\theta^{2}$ sensor in presence of this external magnetic field. The sensitivity of this sensor is shown to increase very rapidly with the external magnetic field, diverging as the external field approaches a value predefined by the NV center parameters. We show that the measured sensitivity has excellent agreement with the theoretical predictions. These results show that NV centers may be useful for coupling to torsional nanomechanical oscillators (NMO), to make quantum nondemolition measurements of the number states of the NMO. [Preview Abstract] 
Monday, March 2, 2015 4:54PM  5:06PM 
D38.00011: Optimal sensing at the nanoscale with diamond nitrogen vacancy center Wen Yang, Ping Wang Diamond nitrogenvacancy center is a leading platform for ultraprecise sensing at the nanoscale. The sensing is essentially a parameter estimation problem: (1) Encoding the unknown parameters into the NV state; (2) Readout the NV state from the fluorescence; (3) Process the data to infer the unknown parameters; (4) Adaptation of (1)(3) based on the updated knowledge about the unknown parameters. Recently, dynamical decoupling and adaptive measurement have significantly improved the sensitivity and dynamic range by improving steps (1) and (4), respectively. However, a full optimization of all the steps remains lacking, e.g., the widely used approach to step (3) based on averaging the data from repeated fluorescence measurements or from singleshot fluorescence binarized into 0 and 1 is suboptimal. Here we apply quantum metrology techniques, developed in optical phase estimation, to construct a general framework for optimal sensing using the NV center, incorporating the finite detection efficiency and decoherence. It can be readily applied to various sensing tasks, such as dc/ac magnetometry, noise spectroscopy, and singlemolecular NMR. For illustration, we demonstrate significant sensitivity improvement in tracking a timevarying magnetic field with the NV center. [Preview Abstract] 

D38.00012: ABSTRACT WITHDRAWN 
Monday, March 2, 2015 5:18PM  5:30PM 
D38.00013: Tensor network approach to quantum feedback dynamics Arne Grimsmo In this talk I consider the problem of a quantum system coupled to a bosonic reservoir creating a coherent feedback loop. Since the system can be strongly correlated with the inloop field, this is in general a highly nonMarkovian quantum problem where no perturbative approach can be expected to work well. I will present the first practical approach to model this type of dynamics for general quantum systems and large delay times. Borrowing ideas from the intersection of condensed matter and quantum information theory, I will show that a formal dynamical solution can be found as the continuum limit of a tensor network, much like the recently introduced continuous matrix product states for onedimensional field theories. This gives rise to a practical and efficient integration scheme in discrete time, which is numerically exact as the timestep goes to zero. Besides opening the possibility to study a new regime of quantum feedback control, this is also a novel application of tensor network techniques that could pave the way for a new approach to nonMarkovian quantum dynamics in a broader context as well. [Preview Abstract] 
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