Session S45: Atomic, Molecular and Optical Quantum Information and Metrology

Quantum Rabi Model in Quantum Technologies

We will discuss how to simulate a wide range of regimes of the Quantum Rabi Model (QRM) in quantum platforms as trapped ions and circuit QED. Directly accesible regimes of the QRM correspond to a very narrow set of values of the ratio between the coupling strength and the characteristic frequencies of the system, typically in the strong coupling regime or in the perturbative zone of the ultrastrong coupling regime. However, with analog and digital quantum simulation techniques we can access the most elusive regimes of the QRM. Recent theoretical developments have disclosed a plethora of physical phenomena appearing at these previously unexplored regimes of the QRM, making its experimental implementation timely and of high interest.   

What can we learn from the dynamics of entanglement and quantum discord in the Tavis-Cummings model?

We revisit the problem of the dynamics of quantum correlations in the exact Tavis-Cummings model. We show that many of the dynamical features of quantum discord attributed to dissipation are already present in the exact framework and are due to the well known non-linearities in the model and to the choice of initial conditions. Through a comprehensive analysis, supported by explicit analytical calculations, we find that the dynamics of entanglement and quantum discord are far from being trivial or intuitive. In this context, we find states that are indistinguishable from the point of view of entanglement and distinguishable from the point of view of quantum discord, states where the two quantifiers give opposite information and states where they give roughly the same information about correlations at a certain time. Depending on the initial conditions, this model exhibits a fascinating range of phenomena that can be used for experimental purposes such as: Robust states against change of manifold or dissipation, tunable entanglement states and states with a counterintuitive sudden birth as the number of photons increase. We furthermore propose an experiment called quantum discord gates where discord is zero or non-zero depending on the number of photons.   

Robust quantum state transfer with suppressed parametric noise

For opto-electro-mechanical transducers, there are undesirable parametric processes that introduce parametric noise, which will limit the fidelity of the transferred quantum state \footnote{R. W. Andrews, R. W. Peterson, T. P. Purdy, K. Cicak, R. W. Simmonds, C. A. Regal & K. W. Lehnert, Nature Physics \textbf{10}, 321-326 (2014)}. To overcome this imperfection, we propose a quantum state transfer scheme with squeezed input states and measurement dependent compensation to eliminate the parametric noise from the quantum state transfer. Besides parametric noise, we also investigate the sensitivity of our scheme to thermal noise, signal frequency detuning and imperfect impedance matching, and show a good quantum state fidelity and applicability to quantum state transfer.   

High efficiency in Mode Selective Frequency Conversion for Optical Quantum Information Processing

Mode selective Frequency conversion (FC) is an enabling process in many quantum information protocols \footnote{B. Brecht et al. Phys. Rev. X {\bf 5}, 041017 (2015)}. Recently, it has been observed that upconversion efficiencies in single-photon, mode-selective FC are limited to around 80\% \footnote{B. Brecht et al. Phys. Rev. A {\bf 90}, 030302 (2014)}. In this contribution we show that these limits can be understood as time ordering corrections (TOCs) that modify the joint conversion amplitude of the process \footnote{N. Quesada et al. Phys. Rev. A {\bf 90}, 063840 (2014).}. Furthermore we show, using a simple scaling argument, that recently proposed cascaded FC protocols \footnote{D.V. Reddy et al. Opt. Lett. {\bf 39}, 2924-2927 (2014).} that overcome the aforementioned limitations act as ``attenuators'' of the TOCs. This observation allows us to argue that very similar cascaded architectures can be used to attenuate TOCs in photon generation via spontaneous parametric down-conversion\footnote{N. Quesada et al., Phys. Rev. Lett. {\bf 114}, 093903 (2015).}. Finally, by using the Magnus expansion, we argue that the TOCs, which are usually considered detrimental for FC efficiency, can also be used to increase the efficiency of conversion in partially mode selective FC.   

Maximal adaptive-decision speedups in quantum-state readout

The average time $T$ required for high-fidelity readout of quantum states can be significantly reduced via a real-time adaptive decision rule. An adaptive decision rule stops the readout as soon as a desired level of confidence has been achieved, as opposed to setting a fixed readout time $t_f$. The performance of the adaptive decision is characterized by the ``adaptive-decision speedup'', $t_f/T$. In this work, we reformulate this readout problem in terms of the first-passage time of a particle undergoing stochastic motion. This formalism allows us to theoretically establish the maximum achievable adaptive-decision speedups for several physical two-state readout implementations. We show that for two common readout schemes (the Gaussian latching readout and a readout relying on state-dependent decay), the speedup is bounded by $4$ and $2$, respectively, in the limit of high single-shot readout fidelity. We experimentally study the achievable speedup in a real-world scenario by applying the adaptive decision rule to a readout of the nitrogen-vacancy-center (NV-center) charge state. We find a speedup of $\approx 2$ with our experimental parameters. Our results should lead to immediate improvements in nano-scale magnetometry based on spin-to-charge conversion of the NV-center spin.   

Negatively-charged NV-center in SiC: Electronic structure properties

Deep defects with high-spin states in semiconductors are promising candidates as solid-state systems for quantum computing applications. The charged NV-center in diamond is the best-known and most-studied defect center, and has proven to be a good proof-of-principle structure for demonstrating the use of such defects in quantum technologies. Increasingly, however, there is an interest in exploring deep defects in alternative semiconductors such as SiC. This is due to the challenges posed by diamond as host material for defects, as well as the attractive properties of SiC. In this density functional theory work, we study the spin-1 structure of the negatively charged NV-center in two polytypes: 3C-SiC and 4H-SiC. The calculated zero phonon line for the excited state of the defect is in telecom range (~0.90eV), making it a very good candidate for quantum technologies. This work provides basic ingredients required to understand the physics of this color center at a quantitative and qualitative level. We also design quantum information applications, such as a spin-photon interface and multi-photon entanglement.   

Optical patterning of trapped charge in nitrogen-doped diamond

The nitrogen-vacancy (NV) center in diamond is emerging as a promising platform for solid-state quantum information processing and nanoscale metrology. Of interest in these applications is the manipulation of the NV charge state, which can be attained by optical illumination. Here we use two-color optical microscopy to investigate the dynamics of NV photo-ionization, charge diffusion, and trapping in type-1b diamond. We combine fixed-point laser excitation and scanning fluorescence imaging to locally alter the concentration of negatively charged NVs and to subsequently probe the corresponding redistribution of charge. We uncover the formation of various spatial patterns of trapped charge, which we semi-quantitatively reproduce via a model of the interplay between photo-excited carriers and atomic defects in the diamond lattice. Further, by using the NV as a local probe, we map the relative fraction of positively charged nitrogen upon localized optical excitation. These observations may prove important to various technologies, including the transport of quantum information between remote NVs and the development of three-dimensional, charge-based memories.   

Towards High Density 3-D Memory in Diamond

The nitrogen-vacancy (NV) center in diamond is presently the focus of widespread attention for applications ranging from quantum information processing to nanoscale metrology. Of great utility is the ability to optically initialize the NV charge state, which has an immediate impact on the center's light emission properties. Here, we use two-color microscopy in NV-rich, type-1b diamond to demonstrate fluorescence-encoded long-term storage of classical information. As a proof of principle, we write, reset, and rewrite various patterns with 2-D binary bit density comparable to present DVD-ROM technology. The strong fluorescence signal originating from the diffraction-limited bit volume allows us to transition from binary to multi-valued encoding, which translates into a significant storage capacity boost. Finally, we show that our technique preserves information written on different planes of the diamond crystal and thus serves as a platform for three-dimensional storage. Substantial enhancement in the bit density could be achieved with the aid of super resolution microscopy techniques already employed to discriminate between NVs with sub-diffraction, nanometer accuracy, a regime where the storage capacity could exceed 10$^{\mathrm{17}}$ bytes/cm$^{\mathrm{3}}$   

Quantum memory enhanced nuclear magnetic resonance of nanometer-scale samples with a single spin in diamond

Recently nuclear magnetic resonance (NMR) of nanoscale samples at ambient conditions has been achieved with nitrogen-vacancy (NV) centers in diamond. So far the spectral resolution in the NV NMR experiments was limited by the sensor's coherence time, which in turn prohibited revealing the chemical composition and dynamics of the system under investigation. By entangling the NV electron spin sensor with a long-lived memory spin qubit we increase the spectral resolution of NMR measurement sequences for the detection of external nuclear spins. Applying the latter sensor-memory-couple it is particularly easy to track diffusion processes, to identify the molecules under study and to deduce the actual NV center depth inside the diamond. We performed nanoscale NMR on several liquid and solid samples exhibiting unique NMR response. Our method paves the way for nanoscale identification of molecule and protein structures and dynamics of conformational changes.   

Single molecule spin resonance spectroscopy and imaging by diamond-sensor

Single-molecule magnetic resonance spectroscopy and imaging is one of the ultimate goals in magnetic resonance and will has great applications in a broad range of scientific areas, from life science to physics and chemistry. The spin of a single nitrogen vacancy (NV) center in diamond is a highly sensitive magnetic-field sensor, which has been proposed for detection of single molecules or nanoscale targets. We and co-workers have successfully obtained the first single-protein spin resonance spectroscopy under ambient conditions [1], high-resolution vector microwave imaging [2], and realized atomic-scale structure analysis of single nuclear-spin clusters in diamond [3]. Moreover, we have tried to improve the quantum control technique and succeed to achieve fault-tolerant universal quantum gates [4]. As the last part, I will briefly introduce our most recently work on single protein imaging in situ in cell. References: [1] Fazhan Shi, et al., Science, 347, 1135 (2015) [2] Pengfei Wang, et al., Nature Commu.,6, 6631 (2015) [3] Fazhan Shi, et al., Nature Physics, 10, 21 (2014) [4] Xing Rong, et al., Nature Commu., In press (2015)   

Beating the Shot-Noise Limit with Partially-Distinguishable Photons

Quantum metrology promises high-precision measurements beyond the capability of any classical techniques. This has the potential to be an integral part of investigative techniques, utilised across all areas of science and technology. However, all sensors must be able to operate despite imperfections to be of practical use. Proposals for photonic quantum sensors typically exploit quantum interference between photons which are perfectly indistinguishable, but achieving this indistinguishability can be a major technical challenge in practice, in particular with immature but promising approaches to photon sources. Here we show that highly indistinguishable photons are not required for quantum-enhanced measurements, nor do partially distinguishable photons have to be engineered to mitigate the effects of distinguishability. We conduct an experiment to verify the utility of two- and four-photon states containing partially distinguishable particles by performing quantum-enhanced measurements with low-visibility quantum interference. This demonstrates that sources producing spectrally-mixed single photons can be readily applied in quantum metrology systems.   

How zero light intensity can exert a nonzero force on a charged particle

A classical electromagnetic field is deterministic and fully specified by a single temporal boundary condition. In contrast, a quantum electromagnetic field is irreducibly stochastic, such that only its average corresponds to a classical field for large ensembles of measurements. Such a field-average may be further refined by a second temporal boundary condition, which can expose fundamentally different classical fields in the same classical averaging limit. To demonstrate this, we consider an ensemble of coherent laser pulses that interact with identically prepared test charges before being collected at an intensity meter. Isolating only the pulses with zero collected intensity reveals a nonzero average classical force on the charge from those pulses. The charge is affected with no light collected.   

Sub-Cycle Quantum Optics: Direct Access to Electric Field Vacuum Fluctuations.

Vacuum fluctuations are fundamental to a variety of physical aspects ranging from spontaneous photon emission via the Casimir force all the way to cosmology. Study and manipulation of the ground state of the radiation field is a central subject in quantum optics. In common approaches, such as for example homodyne detection, the information is averaged over multiple cycles of light and amplification to finite intensity is mandatory. Usually, ultrashort pulses are applied for quantum measurements within a slowly-varying envelope approximation. We demonstrate direct detection of the vacuum fluctuations of the local electric field amplitude in free space. Broadband electro-optic sampling with sub-6 femtosecond gate pulses enables quantum-statistic readout [1]. Distinction from the detector shot noise is achieved by modification of the sampled space-time volume. Measuring with a bandwidth matching the 70 THz center frequency maximizes the vacuum amplitude since the ground-state energy approaches half a photon per optical cycle. Our findings open up a new avenue to quantum analysis and manipulation of light working in the time domain and with sub-cycle access to the electric field quadrature. \newline [1] C. Riek et al, Science \textbf{350}, 420 (2015).