arXiv subjects

Chandrasekhar Ramanathan

Publications and source records attributed to Chandrasekhar Ramanathan.

At least 37 records · Page 2Linked to original sources

arXiv2020

Observation of Floquet prethermalization in dipolar spin chains

Periodically driven Floquet quantum systems provide a promising platform to investigate novel physics out of equilibrium. Unfortunately, the drive generically heats up the system to a featureless infinite temperature state. For large driving frequency, the heat absorption rate is predicted to be exponentially small, giving rise to a long-lived prethermal regime which exhibits all the intriguing properties of Floquet systems. Here we experimentally observe Floquet prethermalization using nuclear magnetic resonance techniques. We first show the relaxation of a far-from-equilibrium initial state to a long-lived prethermal state, well described by the time-independent ''prethermal'' Hamiltonian. By measuring the autocorrelation of this prethermal Hamiltonian we can further experimentally confirm the predicted exponentially slow heating rate. More strikingly, we find that in the timescale when the effective Hamiltonian picture breaks down, the Floquet system still possesses other quasi-conservation laws. Our results demonstrate that it is possible to realize robust Floquet engineering, thus enabling the experimental observation of non-trivial Floquet phases of matter.

quant-ph↗

arXiv2020

Prethermal quasiconserved observables in Floquet quantum systems

Prethermalization, by introducing emergent quasiconserved observables, plays a crucial role in protecting Floquet many-body phases over exponentially long time, while the ultimate fate of such quasiconserved operators can signal thermalization to infinite temperature. To elucidate the properties of prethermal quasiconservation in many-body Floquet systems, here we systematically analyze infinite temperature correlations between observables. We numerically show that the late-time behavior of the autocorrelations unambiguously distinguishes quasiconserved observables from non-conserved ones, allowing to single out a set of linearly-independent quasiconserved observables. By investigating two Floquet spin models, we identify two different mechanism underlying the quasi-conservation law. First, we numerically verify energy quasiconservation when the driving frequency is large, so that the system dynamics is approximately described by a static prethermal Hamiltonian. More interestingly, under moderate driving frequency, another quasiconserved observable can still persist if the Floquet driving contains a large global rotation. We show theoretically how to calculate this conserved observable and provide numerical verification. Having systematically identified all quasiconserved observables, we can finally investigate their behavior in the infinite-time limit and thermodynamic limit, using autocorrelations obtained from both numerical simulation and experiments in solid state nuclear magnetic resonance systems.

quant-ph↗

arXiv2019

DNP-NMR of surface hydrogen on silicon microparticles

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent $^{1}$H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of $\sim 150-300$ Hz for both peaks, indicative of a sparse distribution of protons in both environments. The downfield peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The upfield peak was found to vary in position between -19 and -37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The upfield peak was also found to vary significantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles.

cond-mat.mtrl-sci↗

arXiv2019

Hyperpolarized relaxometry based nuclear T1 noise spectroscopy in hybrid diamond quantum registers

Understanding the origins of spin lifetimes in hybrid quantum systems is a matter of current importance in several areas of quantum information and sensing. Methods that spectrally map spin relaxation processes provide insight into their origin and can motivate methods to mitigate them. In this paper, using a combination of hyperpolarization and precision field cycling over a wide range (1mT-7T), we map frequency dependent relaxation in a prototypical hybrid system of 13C nuclear spins in diamond coupled to Nitrogen Vacancy centers. Nuclear hyperpolarization through the optically pumped NV electrons allows signal time savings for the measurements exceeding million-fold over conventional methods. We observe that 13C lifetimes show a dramatic field dependence, growing rapidly with field up to 100mT and saturating thereafter. Through a systematic study with increasing substitutional electron (P1 center) concentration as well as 13C enrichment levels, we identify the operational relaxation channels for the nuclei in different field regimes. In particular, we demonstrate the dominant role played by the 13C nuclei coupling to the interacting P1 electronic spin bath. These results pave the way for quantum control techniques for dissipation engineering to boost spin lifetimes in diamond, with applications ranging from engineered quantum memories to hyperpolarized 13C imaging.

quant-ph↗

arXiv2018

Emergent prethermalization signatures in out-of-time ordered correlations

How a many-body quantum system thermalizes --or fails to do so-- under its own interaction is a fundamental yet elusive concept. Here we demonstrate nuclear magnetic resonance observation of the emergence of prethermalization by measuring out-of-time ordered correlations. We exploit Hamiltonian engineering techniques to tune the strength of spin-spin interactions and of a transverse magnetic field in a spin chain system, as well as to invert the Hamiltonian sign to reveal out-of-time ordered correlations. At large fields, we observe an emergent conserved quantity due to prethermalization, which can be revealed by an early saturation of correlations. Our experiment not only demonstrates a new protocol to measure out-of-time ordered correlations, but also provides new insights in the study of quantum thermodynamics.

quant-ph↗

arXiv2017

Optical dependence of electrically-detected magnetic resonance in lightly-doped Si:P devices

Electrically-detected magnetic resonance (EDMR) provides a highly sensitive method for reading out the state of donor spins in silicon. The technique relies on a spin-dependent recombination (SDR) process involving dopant spins that are coupled to interfacial defect spins near the Si/SiO$_2$ interface. To prevent ionization of the donors, the experiments are performed at cryogenic temperatures and the mobile charge carriers needed are generated via optical excitation. The influence of this optical excitation on the SDR process and the resulting EDMR signal is still not well understood. Here, we use EDMR to characterize changes to both phosphorus and defect spin readout as a function of optical excitation using: a 980 nm laser with energy just above the silicon band edge at cryogenic temperatures; a 405 nm laser to generate hot surface-carriers; and a broadband white light source. EDMR signals are observed from the phosphorus donor and two distinct defect species in all the experiments. With near-infrared excitation, we find that the EDMR signal primarily arises from donor-defect pairs, while at higher photon energies there are significant additional contributions from defect-defect pairs. The optical penetration depth into silicon is also known to be strongly wavelength dependent at cryogenic temperatures. The energy of the optical excitation is observed to strongly modulate the kinetics of the SDR process. Careful tuning of the optical photon energy could therefore be used to control both the subset of spin pairs contributing to the EDMR signal as well as the dynamics of the SDR process.

cond-mat.mes-hall↗

arXiv2016

Exploring Localization in Nuclear Spin Chains

Characterizing out-of-equilibrium many-body dynamics is a complex but crucial task for quantum applications and the understanding of fundamental phenomena. A central question is the role of localization in quenching quantum thermalization, and whether localization survives in the presence of interactions. The localized phase of interacting systems (many-body localization, MBL) exhibits a long-time logarithmic growth in entanglement entropy that distinguishes it from the noninteracting Anderson localization (AL), but entanglement is difficult to measure experimentally. Here, we present a novel correlation metric, capable of distinguishing MBL from AL in high-temperature spin systems. We demonstrate the use of this metric to detect localization in a natural solidstate spin system using nuclear magnetic resonance (NMR). We engineer the natural Hamiltonian to controllably introduce disorder and interactions and observe the emergence of localization. In particular, while our correlation metric saturates for AL, it keeps increasing logarithmically for MBL, a behavior reminiscent of entanglement entropy, as we confirm by simulations. Our results show that our NMR techniques, akin to measuring out-of-time correlations, are well suited for studying localization in spin systems.

cond-mat.dis-nn↗

arXiv2016

Chemisorption of water on the surface of silicon microparticles measured by DNP-enhanced NMR

We use dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) at liquid helium temperatures to directly detect hydrogen attached to the surface of silicon microparticles. The proton NMR spectrum from a dry sample of polycrystalline silicon powder (1-5 $μ$m) shows a distinctively narrow Lorentzian-shaped resonance with a width of 6.2 kHz, indicative of a very sparse distribution of protons attached to the silicon surface. These protons are within a few atomic monolayers of the silicon surface. The high sensitivity NMR detection of surface protons from low surface area ($0.26 - 1.3 \: m^2/g$) particles is enabled by an overall signal enhancement of 4150 over the room temperature NMR signal at the same field. When the particles were suspended in a solvent with 80% H2O and 20% D2O, the narrow peak was observed to grow in intensity over time, indicating growth of the sparse surface proton layer. However, when the particles were suspended in a solvent with 20% H2O and 80% D2O, the narrow bound-proton peak was observed to shrink due to exchange between the surface protons and the deuterium in solution. This decrease was accompanied by a concomitant growth in the intensity of the frozen solvent peak, as the relative proton concentration of the solvent increased. When the particles were suspended in the organic solvent hexane, the proton NMR spectra remained unchanged over time. These results are consistent with the known chemisorption of water on the silicon surface resulting in the formation of hydride and hydroxyl species. Low-temperature DNP NMR can thus be used as a non-destructive probe of surface corrosion for silicon in aqueous environments. This is important in the context of using silicon MEMS and bioMEMS devices in such environments, for silicon micro- and nano-particle MRI imaging agents, and the use of nanosilicon for splitting water in fuel cells.

cond-mat.mtrl-sci↗

arXiv2016

Superadiabatic Control of Quantum Operations

Adiabatic pulses are used extensively to enable robust control of quantum operations. We introduce a new approach to adiabatic control that uses the superadiabatic quality or $Q$-factor as a performance metric to design robust, high fidelity pulses. This approach permits the systematic design of quantum control schemes to maximize the adiabaticity of a unitary operation in a particular time interval given the available control resources. The interplay between adiabaticity, fidelity and robustness of the resulting pulses is examined for the case of single-qubit inversion, and superadiabatic pulses are demonstrated to have improved robustness to control errors. A numerical search strategy is developed to find a broader class of adiabatic operations, including multi-qubit adiabatic unitaries. We illustrate the utility of this search strategy by designing control waveforms that adiabatically implement a two-qubit entangling gate for a model NMR system.

quant-ph↗

arXiv2015

Design and characterization of a W-band system for modulated DNP experiments

Magnetic-field and microwave-frequency modulated DNP experiments have been shown to yield improved enhancements over conventional DNP techniques, and even to shorten polarization build-up times. The resulting increase in signal-to-noise ratios can lead to significantly shorter acquisition times in signal-limited multi-dimensional NMR experiments and pave the way to the study of even smaller sample volumes. In this paper we describe the design and performance of a broadband system for microwave frequency- and amplitude-modulated DNP that has been engineered to minimize both microwave and thermal losses during operation at liquid helium temperatures. The system incorporates a flexible source that can generate arbitrary waveforms at 94 GHz with a bandwidth greater than 1 GHz, as well as a probe that efficiently transmits the millimeter waves from room temperature outside the magnet to a cryogenic environment inside the magnet. Using a thin-walled brass tube as an overmoded waveguide to transmit a hybrid HE11 mode, it is possible to limit the losses to 1 dB across a 2 GHz bandwidth. The loss is dominated by the presence of a quartz window used to isolate the waveguide pipe. This performance is comparable to systems with corrugated waveguide or quasi-optical components. The overall excitation bandwidth of the probe is seen to be primarily determined by the final antenna or resonator used to excite the sample and its coupling to the NMR RF coil. Understanding the instrumental limitations imposed on any modulation scheme is key to understanding the observed DNP results and potentially identifying the underlying mechanisms. We demonstrate the utility of our design with a set of triangular frequency-modulated DNP experiments.

physics.ins-det↗

arXiv2011

Equivalent Hamiltonians for State to State Transfer in the Case of Partial Quantum Control

Given a fixed initial state, a desired Hamiltonian, and an amount of time, we provide a complete characterization of the set of Hamiltonians which perform the same action as the desired Hamiltonian on the state of interest. An example is provided where the desired Hamiltonian lies outside of the control Lie algebra, but implementable Hamiltonians exist within the characterization.

quant-ph↗

arXiv2011

High-field Overhauser DNP in silicon below the metal-insulator transition

Single crystal silicon is an excellent system in which to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of 29Si in n-doped silicon crystals, with doping concentrations in the range of 1-3 x 10^17 /cc. In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the 29Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is the rate-limiting step. Since only about 2 % of the silicon nuclei are located within one Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of $244 +/- 84 resulting in a silicon polarization of $10.4 +/- 3.4 % following two hours of microwave irradiation.

cond-mat.mtrl-sci↗

arXiv2011

Experimental characterization of coherent magnetization transport in a one-dimensional spin system

We experimentally characterize the non-equilibrium, room-temperature magnetization dynamics of a spin chain evolving under an effective double-quantum Hamiltonian. We show that the Liouville space operators corresponding to the magnetization and the two-spin correlations evolve 90 degrees out of phase with each other, and drive the transport dynamics. For a nearest-neighbor-coupled N-spin chain, the dynamics are found to be restricted to a Liouville operator space whose dimension scales only as N^2, leading to a slow growth of multi-spin correlations. Even though long-range couplings are present in the real system, we find excellent agreement between the analytical predictions and our experimental results, confirming that leakage out of the restricted Liouville space is slow on the timescales investigated. Our results indicate that the group velocity of the magnetization is 6.04 +/- 0.38 um/s, corresponding to a coherent transport over N ~ 26 spins on the experimental timescale. As the double-quantum Hamiltonian is related to the standard one-dimensional XX Hamiltonian by a similarity transform, our results can be directly extended to XX quantum spin chains, which have been extensively studied in the context of both quantum magnetism and quantum information processing.

quant-ph↗

arXiv2010

Coherent state transfer via highly mixed quantum spin chains

Spin chains have been proposed as quantum wires in many quantum information processing architectures. Coherent transmission of quantum information over short distances is enabled by their internal dynamics, which drives the transport of single-spin excitations in perfectly polarized chains. Given the practical challenge of preparing the chain in a pure state, we propose to use a chain that is initially in the maximally mixed state. We compare the transport properties of pure and mixed-state chains, finding similarities that enable the experimental study of pure-state transfer by its simulation via mixed-state chains, and demonstrate protocols for the perfect transfer of quantum information in these chains. Remarkably, mixed-state chains allow the use of Hamiltonians which do not preserve the total number of excitations, and which are more readily obtainable from the naturally occurring magnetic dipolar interaction. We propose experimental implementations using solid-state nuclear magnetic resonance and defect centers in diamond.

quant-ph↗

arXiv2009

NMR multiple quantum coherences in quasi-one-dimensional spin systems: Comparison with ideal spin-chain dynamics

The 19F spins in a crystal of fluorapatite have often been used to experimentally approximate a one-dimensional spin system. Under suitable multi-pulse control, the nuclear spin dynamics may be modeled to first approximation by a double-quantum one-dimensional Hamiltonian, which is analytically solvable for nearest-neighbor couplings. Here, we use solid-state nuclear magnetic resonance techniques to investigate the multiple quantum coherence dynamics of fluorapatite, with an emphasis on understanding the region of validity for such a simplified picture. Using experimental, numerical, and analytical methods, we explore the effects of long-range intra-chain couplings, cross-chain couplings, as well as couplings to a spin environment, all of which tend to damp the oscillations of the multiple quantum coherence signal at sufficiently long times. Our analysis characterizes the extent to which fluorapatite can faithfully simulate a one-dimensional quantum wire.

quant-ph↗

arXiv2008

Dynamic nuclear polarization and spin-diffusion in non-conducting solids

There has been much renewed interest in dynamic nuclear polarization (DNP), particularly in the context of solid state biomolecular NMR and more recently dissolution DNP techniques for liquids. This paper reviews the role of spin diffusion in polarizing nuclear spins and discusses the role of the spin diffusion barrier, before going on to discuss some recent results.

cond-mat.other↗

arXiv2007

Fidelity enhancement by logical qubit encoding

We demonstrate coherent control of two logical qubits encoded in a decoherence free subspace (DFS) of four dipolar-coupled protons in an NMR quantum information processor. A pseudo-pure fiducial state is created in the DFS, and a unitary logical qubit entangling operator evolves the system to a logical Bell state. The four-spin molecule is partially aligned by a liquid crystal solvent, which introduces strong dipolar couplings among the spins. Although the system Hamiltonian is never fully specified, we demonstrate high fidelity control over the logical degrees of freedom. In fact, the DFS encoding leads to higher fidelity control than is available in the full four-spin Hilbert space.

quant-ph↗

arXiv2007

Signatures of incoherence in a quantum information processor

Incoherent noise is manifest in measurements of expectation values when the underlying ensemble evolves under a classical distribution of unitary processes. While many incoherent processes appear decoherent, there are important differences. The distribution functions underlying incoherent processes are either static or slowly varying with respect to control operations and so the errors introduced by these distributions are refocusable. The observation and control of incoherence in small Hilbert spaces is well known. Here we explore incoherence during an entangling operation, such as is relevant in quantum information processing. As expected, it is more difficult to separate incoherence and decoherence over such processes. However, by studying the fidelity decay under a cyclic entangling map we are able to identify distinctive experimental signatures of incoherence. This result is demonstrated both through numerical simulations and experimentally in a three qubit nuclear magnetic resonance implementation.

quant-ph↗