Session J09: Quantum Information Science

Exponential quantum randomness expansion with trapped ions based on contextuality

We report a self-testing quantum random number generator (QRNG) achieving an exponential gain of randomness expansion. Similar to Bell's theorem [1], violation of Kochen and Specker theory, which also presents the intrinsic randomness of quantum mechanics and excludes noncontextual hidden variable models, is also used to certify generated randomness [2]. Here, we employ an extended Klyachko-Can-Binicoglu-Shumovsky (KCBS) inquality [4,5] on a qutrit system of a trapped 138Ba$+$ ion system. Furthermore, by applying a spot-checking protocol of [3], our self-testing QRNG realizes exponential randomness expansion without independent and identically distributed assumption considering the most general quantum adversary scenario. The system demonstrates 1.24×10\textasciicircum 8 trials and results in randomness extraction of 2.3×10\textasciicircum 5 bits realizing a 6.6×10\textasciicircum 4 bits exponential randomness expansion with the speed of 270 bits/s. [1] S. Pironio, et al., Nature 464, 1021 (2010). [2] Mark Um, et al., Sci. Rep. 3, 1627 (2013). [3] Carl Miller and Yaoyun Shi, Siam J. Comput. 46, 1304 (2017). [4] O. Guhne, et al., Phys. Rev. A 81, 022121 (2010). [5] J. Szangolies, et al., Phys. Rev. A. 87, 050101 (2013).   

Quantum repeaters based on two-species trapped ion systems

We consider the performance of quantum repeater architecture based on two-species co-trapped ion systems. One ion (Yb) provides a long-lived quantum memory while the other (Ba) serves as an optical communication qubit with high coupling efficiency. Our design includes quantum circuits that achieve intra-node Bell measurements between the ion-trap modules to perform entanglement-swapping locally within the nodes. Based on the fidelity of the required quantum operations and the currently available coupling efficiencies, we estimate the quantum key generation rates that can be achieved. We also analyze the dependence of the quantum key distribution rate on various experimental parameters, including coupling efficiency, gate infidelity, operation time and length of the elementary links.   

Intrinsic Retrieval Efficiency for Quantum Memory: A Three Dimensional Theory of Light Interaction with an Atomic Ensemble

Duan-Lukin-Cirac-Zoller (DLCZ) quantum repeater protocol, which was proposed to realize long distance quantum communication, requires usage of quantum memories. Atomic ensembles interacting with optical beams based on off-resonant Raman scattering serve as convenient on-demand quantum memories. Here, a complete free space, three-dimensional theory of the associated read and write process for this quantum memory is worked out with the aim of understanding intrinsic retrieval efficiency. We develop a formalism to calculate the transverse mode structure for the signal and the idler photons and use the formalism to study the intrinsic retrieval efficiency under various configurations. The effects of atomic density fluctuations and atomic motion are incorporated by numerically simulating this system for a range of realistic experimental parameters. We obtain results that describe the variation in the intrinsic retrieval efficiency as a function of the memory storage time for skewed beam configuration at a finite temperature, which provides valuable information for optimization of the retrieval efficiency in experiments.   

Fast, accurate, nondestructive quantum state detection of a single trapped neutral atom

In recent years, single neutral atom traps have emerged as a promising platform for the study of fundamental physics and quantum information protocols. Many of these applications can benefit greatly from fast, accurate, nondestructive quantum state detection. Here, we report our progress on achieving nondestructive quantum state detection in a single-atom trap constructed with an in-vacuum lens and off-the-shelf components. Through careful choice of the detection beam frequency and power, we have demonstrated a nondestructive state detection scheme with $>$$96 \%$ fidelity over $200$ $\mu s$ of integration time. To the best of our knowledge, this is faster than other reported results and of comparable fidelity. Furthermore, we report our investigations into the limiting features of this process. Our observations can be understood using a rate equation model.   

Comparing Zeeman qubits to hyperfine qubits in the context of the surface code: $^{171}$Yb$^{+}$ and $^{174}$Yb$^{+}$

Many systems used for quantum computing possess additional states beyond those defining the qubit. Leakage out of the qubit subspace must be considered when designing quantum error correction codes (QECC). Here we consider trapped ion qubits manipulated by Raman transitions. Zeeman qubits do not suffer from leakage errors but are sensitive to magnetic fields to first-order. Hyperfine qubits can be encoded in clock states that are insensitive to magnetic fields to first-order, but spontaneous scattering during the Raman transition can lead to leakage. Here we compare a Zeeman qubit ($^{174}$Yb$^+$) to a hyperfine qubit ($^{171}$Yb$^+$) in the context of the surface code. We find that the number of physical qubits required to reach a specific logical qubit error can be reduced by using $^{174}$Yb$^+$ if the magnetic field can be stabilized with fluctuations smaller than $10$ $\mu$G.   

Layout Generation with Decoherence Estimation for Gate-Model Quantum Computer Architectures

We define a method for quantum circuit layout generation for gate-model quantum computer architectures. We propose an algorithm for the optimal placement of the quantum computational blocks of gate-model quantum circuits with arbitrary number of entangled connections in the layout. We introduce a method for the decoherence estimation in superconducting quantum computers with multilayer quantum gate structures.   

Quantum-assisted dimension estimation of an interacting qubit system

The dimension estimation of an interacting qubit system is a crucial task since the dimension determines the quantum computation complexity or the precision in quantum sensing. Various protocols have been developed for dimension witnesses, which put a lower bound in the dimension by the measurement of the collective observables. However, their experiment implementation is practically demanding especially in multiple qubit systems. In our recent work [Phys. Rev. A 96, 062334 (2017)], we proposed a more practical strategy to estimate the dimension of an interacting qubit system, which relies on the coupling between a local quantum probe and the target system, instead of directly measuring the unknown target system. We assume that the interaction model is priorly given and all qubits are correlated with the probe through the time evolution. We employed realization theory to verify that the dimension of the target system can be exactly estimated from the model order of the system. We also derived the exact relation between dimension and model order for finite one-dimensional spin chain systems with nearest-neighboring interaction.   

Reduced-Density-Matrix Description of Decoherence and Relaxation Processes for Electron-Spin Systems

Electron-spin systems are investigated using a quantum-open-systems description. Applications of interest include trapped atomic systems in optical lattices, semiconductor quantum dots, and vacancy defect centers in solids. Time-domain and frequency-domain formulations are developed. The general non-perturbative and non-Markovian formulations provide a fundamental framework for systematic investigations of corrections to the standard Born and Markov approximations. Attention is given to decoherence and relaxation processes, as well as spectral-line broadening phenomena, that are induced by interactions with photons, phonons, nuclear spins, and external electric and magnetic fields. These phenomena are described either as coherent interactions or as environmental interactions. The environmental interactions are incorporated by means of the general expressions derived for the time-domain and frequency-domain Liouville-space self-energy operators, for which the tetradic-matrix elements are explicitly evaluated in the diagonal-resolvent, lowest-order, and Markov (short-memory time) approximations.   

g$^{\mathrm{(2)\thinspace }}$Measurement of Neutral Rubidium Wavelength Single Photons from a Trapped Barium Ion via Quantum Frequency Conversion

Networks using quantum memories and photonic interconnects [1] rely on detection of photons and performance is improved when photon transmission loss is minimized. Additionally, networking different types of quantum memories for hybrid networking requires overcoming the disparate photon frequencies emitted by each memory. Here, we convert 493 nm photons (50 dB/km fiber loss) emitted from a single trapped $^{\mathrm{138}}$Ba$^{\mathrm{+}}$ ion to a 780 nm wavelength (3 dB/km fiber loss) resonant with the D2 transition in $^{\mathrm{87}}$Rb atoms. The frequency conversion is performed using difference frequency generation in a PPLN waveguide into which 493 nm photons from the ion and a high intensity pump at 1343 nm are coupled. An end-to-end conversion efficiency of 19{\%} is obtained with CW light and 2.3{\%} with ion photons, with the difference attributable to running at a lower pump power to maximize the SNR. We performed a g$^{\mathrm{(2)}}(\tau )$ ~measurement of the non-converted and converted photons and, in both cases, observed g$^{\mathrm{(2)}}$(0) \textless 1 [2]. This work and QFC work reported in Ca$^{\mathrm{+}}$ [3,4] provides a pathway for hybrid neutral-ion atomic quantum networks utilizing features of both systems. [1] J. D. Siverns et. al, J. Quant. Info. 16, 314 (2017) [2] J.D. Siverns et. al, arXiv:1801.01193 (2018) [3] T. Walker, et. al., arXiv:1711.09644 (2017) [4] M. Bock, et. al., arXiv:1710.04866 (2017)