arXiv subjects

Kae Nemoto

Publications and source records attributed to Kae Nemoto.

At least 73 records · Page 4Linked to original sources

arXiv2020

Aggregating Quantum Networks

Quantum networking allows the transmission of information in ways unavailable in the classical world. Single packets of information can now be split and transmitted in a coherent way over different routes. This aggregation allows information to be transmitted in a fault tolerant way between different parts of the quantum network (or the future internet) - even when that is not achievable with a single path approach. It is a quantum phenomenon not available in conventional telecommunication networks either. We show how the multiplexing of independent quantum channels allows a distributed form of quantum error correction to protect the transmission of quantum information between nodes or users of a quantum network. Combined with spatial-temporal single photon multiplexing we observe a significant drop in network resources required to transmit that quantum signal - even when only two channels are involved. This work goes far beyond the concepts of channel capacities and shows how quantum networking may operate in the future. Further it shows that quantum networks are likely to operate differently from their classical counterparts which is an important distinction as we design larger scale ones.

quant-ph↗

arXiv2020

Ergodic-localized junctions in a periodically-driven spin chain

We report the analogue simulation of an ergodiclocalized junction by using an array of 12 coupled superconducting qubits. To perform the simulation, we fabricated a superconducting quantum processor that is divided into two domains: a driven domain representing an ergodic system, while the second is localized under the effect of disorder. Due to the overlap between localized and delocalized states, for small disorder there is a proximity effect and localization is destroyed. To experimentally investigate this, we prepare a microwave excitation in the driven domain and explore how deep it can penetrate the disordered region by probing its dynamics. Furthermore, we performed an ensemble average over 50 realizations of disorder, which clearly shows the proximity effect. Our work opens a new avenue to build quantum simulators of driven-disordered systems with applications in condensed matter physics and material science

quant-ph↗

arXiv2019

Continuous-time quantum walk spatial search on the Bollobás scale-free network

The scale-free property emerges in various real-world networks and is an essential property which characterizes the dynamics or features of such networks. In this work we investigate the effect of this scale-free property on a quantum information processing task of finding a marked node in the network, known as the quantum spatial search. We analyze the quantum spatial search algorithm using continuous-time quantum walk on the Bollobás network, and evaluate the time $T$ to localize the quantum walker on the marked node starting from an unbiased initial state. Our main finding is that $T$ is determined by the global structure around the marked node, while some local information of the marked node such as degree does not identify $T$. We discuss this by examining the correlation between $T$ and some centrality measures of the network, and show that the closeness centrality of the marked node is highly correlated with $T$. We also characterize the distribution of $T$ by marking different nodes in the network, which displays a multi-mode lognormal distribution. Especially on the Bollobás network, $T$ is magnitude of orders shorter depending whether the marked node is adjacent to the largest degree hub node or not. However, as $T$ depends on the property of the marked node, one requires some amount of prior knowledge about such property of the marked node in order to identify the optimal time to measure the quantum walker and achieve fast search. These results indicate that the existence of the hub node in the scale-free network is playing a crucial role on the quantum spatial search.

quant-ph↗

arXiv2019

Decoding Quantum Error Correction Codes with Local Variation

In this paper we investigate the role of local information in the decoding of the repetition and surface error correction codes for the protection of quantum states. Our key result is an improvement in resource efficiency when local information is taken into account during the decoding process: the code distance associated with a given logical error rate is reduced with a magnitude depending on the proximity of the physical error rate to the accuracy threshold of the code. We also briefly discuss an averaged approach with local information for table-lookup and localised decoding schemes, an expected breakdown of these effects for large-scale systems, and the importance of this resource reduction in the near-term.

quant-ph↗

arXiv2018

Ergodic-localized junctions in periodically-driven systems

Quantum phases of matter have many relevant applications in quantum computation and quantum information processing. Current experimental feasibilities in diverse platforms allow us to couple two or more subsystems in different phases. In this letter, we investigate the situation where one couples two domains of a periodically-driven spin chain where one of them is ergodic while the other is fully localized. By combining tools of both graph and Floquet theory, we show that the localized domain remains stable for strong disorder, but as this disorder decreases the localized domain becomes ergodic.

quant-ph↗

arXiv2018

Characterizing twin-particle entanglement in double-well potentials

We consider a pair of twin atoms trapped in double-well potentials. For each atom, two orthogonal spatial modes are accessible: the states $ |L\rangle$ and $|R\rangle$ spatially localized in the left and right wells respectively. Furthermore the twin atoms are distinguishable thanks to an additional degree of freedom. We propose a method for experimentally quantifying the particle entanglement between these atoms which allows us to probe a violation of Bell's inequality. It is based on measuring the correlations in the atoms' momentum distribution. If the tunneling and the energy difference between the wells are tunable, then full state tomography is achievable.

cond-mat.quant-gas↗

arXiv2018

Environmental engineering for quantum energy transport

Transport phenomena are ubiquitous throughout the science, engineering and technology disciplines as it concerns energy, mass, charge and information exchange between systems. In particular, energy transport in the nanoscale regime has attracted significant attention within the physical science community due to its potential to explain complex phenomena like the electronic energy transfer in molecular crystals or the Fenna-Matthews-Olson / light harvesting complexes in photosynthetic bacteria with long time coherences. Energy transport in these systems is highly affected by environmental noise but surprisingly not always in a detrimental way. It was recently found that situations exist where noise actually enhances the transport phenomena. Such noise can take many forms, but can be characterised in three basic behaviours: quantum, coloured or nonlocal. All have been shown potential to offer an energy transport enhancement. The focus of this work is on quantum transport caused by stochastic environment with spatio-temporal correlation. We consider a multi-site nearest neighbour interaction model with pure dephasing environmental noise with coloured and nonlocal character and show how an accelerated rate for the energy transfer results especially under anti-correlation. Negative spatial correlations provide another control parameter to help one establish the most efficient transfer of energy and may provide new insights into the working of exciton transport in photosynthetic complexes. Further the usage of spatio-temporal correlated noise may be a beneficial resource for efficient transport in large scale quantum networks.

quant-ph↗

arXiv2018

Quantum multiplexing

Distributing entangled pairs is a fundamental operation required for many quantum information science and technology tasks. In a general entanglement distribution scheme, a photonic pulse is used to entangle a pair of remote quantum memories. Most applications require multiple entangled pairs between remote users, which in turn necessitates several photonic pulses (single photons) being sent through the channel connecting those users. Here we present an entanglement distribution scheme using only a single photonic pulse to entangle an arbitrary number of remote quantum memories. As a consequence the spatial temporal resources are dramatically reduced. We show how this approach can be simultaneously combined with an entanglement purification protocol to generate even higher fidelity entangled pairs. The combined approach is faster to generate those high quality pairs and requires less resources in terms of both matter qubits and photons consumed. To estimate the efficiency of our scheme we derive a normalized rate taking into account the raw rate at which the users can generate purified entangled pairs divided by the total resources used. We compare the efficiency of our system with the Deutsch protocol in which the entangled pairs have been created in a traditional way. Our scheme outperforms this approach both in terms of generation rate and resources required. Finally we show how our approach can be extended to more general error correction and detection schemes with higher normalized generation rates naturally occurring.

quant-ph↗

arXiv2018

Robust quantum sensing with strongly interacting probe systems

In the field of quantum metrology and sensing, a collection of quantum systems (e.g. spins) are used as a probe to estimate some physical parameter (e.g. magnetic field). It is usually assumed that there are no interactions between the probe systems. We show that strong interactions between them can increase robustness against thermal noise, leading to enhanced sensitivity. In principle, the sensitivity can scale exponentially in the number of probes -- even at non-zero temperatures -- if there are long-range interactions. This scheme can also be combined with other techniques, such as dynamical decoupling, to give enhanced sensitivity in realistic experiments.

quant-ph↗

arXiv2018

Negative-Temperature State Relaxation and Reservoir-Assisted Quantum Entanglement in Double Spin Domain Systems

Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two novel collective behaviors: the negative-temperature state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.

quant-ph↗

arXiv2018

Superradiant Hybrid Quantum Devices

Superradiance is the archetypical collective phenomenon where radiation is amplified by the coherence of emitters. It plays a prominent role in optics, where it enables the design of lasers with substantially reduced linewidths, quantum mechanics, and is even used to explain cosmological observations like Hawking radiation from black holes. Hybridization of distinct quantum systems allows to engineer new quantum metamaterials pooling the advantages of the individual systems. Superconducting circuits coupled to spin ensembles are promising future building blocks of integrated quantum devices and superradiance will play a prominent role. As such it is important to study its fundamental properties in hybrid devices. Experiments in the strong coupling regime have shown oscillatory behaviour in these systems but a clear signature of Dicke superradiance has been missing so far. Here we explore superradiance in a hybrid system composed of a superconducting resonator in the fast cavity limit inductively coupled to an inhomogeneously broadened ensemble of nitrogen-vacancy (NV) centres. We observe a superradiant pulse being emitted a trillion of times faster than the decay for an individual NV centre. This is further confirmed by the non-linear scaling of the emitted radiation intensity with respect to the ensemble size. Our work provides the foundation for future quantum technologies including solid state superradiant masers.

quant-ph↗

arXiv2018

Making the most of time in quantum metrology: concurrent state preparation and sensing

A quantum metrology protocol for parameter estimation is typically comprised of three stages: probe state preparation, sensing and then readout, where the time required for the first and last stages is usually neglected. In the present work we consider non-negligible state preparation and readout times, and the tradeoffs in sensitivity that come when a limited time resource $τ$ must be divided between the three stages. To investigate this, we focus on the problem of magnetic field sensing with spins in one-axis twisted or two-axis twisted states. We find that (accounting for the time necessary to prepare a twisted state) by including entanglement, which is introduced via the twisting, no advantage is gained unless the time $τ$ is sufficiently long or the twisting sufficiently strong. However, we also find that the limited time resource is used more effectively if we allow the twisting and the magnetic field to be applied concurrently which is representative of a more realistic sensing scenario. We extend this result into the optical regime by utilizing the exact correspondence between a spin system and a bosonic field mode as given by the Holstein-Primakoff transformation.

quant-ph↗

arXiv2017

Fast macroscopic-superposition-state generation by coherent driving

We propose a scheme to generate macroscopic superposition states in spin ensembles, where a coherent driving field is applied to accelerate the generation of macroscopic superposition states. The numerical calculation demonstrates that this approach allows us to generate a superposition of two classically distinct states of the spin ensemble with a high fidelity above 0.96 for 300 spins. For the larger spin ensemble, though the fidelity slightly decline, it maintains above 0.85 for an ensemble of 500 spins. The time to generate a macroscopic superposition state is also numerically calculated, which shows that the significantly shortened generation time allows us to achieve such macroscopic superposition states within a typical coherence time of the system.

quant-ph↗

arXiv2017

High-Fidelity Spin Measurement on the Nitrogen-Vacancy Center

Nitrogen-vacancy (NV) centers in diamond are versatile candidates for many quantum information processing tasks, ranging from quantum imaging and sensing through to quantum communication and fault-tolerant quantum computers. Critical to almost every potential application is an efficient mechanism for the high fidelity readout of the state of the electronic and nuclear spins. Typically such readout has been achieved through an optically resonant fluorescence measurement, but the presence of decay through a meta-stable state will limit its efficiency to the order of 99%. While this is good enough for many applications, it is insufficient for large scale quantum networks and fault-tolerant computational tasks. Here we explore an alternative approach based on dipole induced transparency (state-dependent reflection) in an NV center cavity QED system, using the most recent knowledge of the NV center's parameters to determine its feasibility, including the decay channels through the meta-stable subspace and photon ionization. We find that single-shot measurements above fault-tolerant thresholds should be available in the strong coupling regime for a wide range of cavity-center cooperativities, using a majority voting approach utilizing single photon detection. Furthermore, extremely high fidelity measurements are possible using weak optical pulses.

quant-ph↗

arXiv2017

Taming an Optical Schroedinger's cat - A quantum non-demolition approach

An interferometric experiment is described that characterizes an optical cat state in a cavity mode. Our method describes how to measure the amplitude and phase of the different coherent states that make up the cat states. We show that using a recently proposed quantum nondemolition measurement technique, we can circumvent the problem of decoherence inherent in measurement involving superposition of coherent states.

quant-ph↗

arXiv2017

Fault-Tolerant High Level Quantum Circuits: Form, Compilation and Description

Fault-tolerant quantum error correction is a necessity for any quantum architecture destined to tackle interesting, large-scale problems. Its theoretical formalism has been well founded for nearly two decades. However, we still do not have an appropriate compiler to produce a fault-tolerant, error corrected description from a higher level quantum circuit for state of the art hardware models. There are many technical hurdles, including dynamic circuit constructions that occur when constructing fault-tolerant circuits with commonly used error correcting codes. We introduce a package that converts high level quantum circuits consisting of commonly used gates into a form employing all decompositions and ancillary protocols needed for fault-tolerant error correction. We call this form the (I)initialisation, (C)NOT, (M)measurement form (ICM) and consists of an initialisation layer of qubits into one of four distinct states, a massive, deterministic array of CNOT operations and a series of time ordered $X$- or $Z$-basis measurements. The form allows a more flexbile approach towards circuit optimisation. At the same time, the package outputs a standard circuit or a canonical geometric description which is a necessity for operating current state-of-the-art hardware architectures using topological quantum codes.

quant-ph↗

arXiv2017

Dynamical Exploration of Amplitude Bistability in Engineered Quantum Systems

Nonlinear systems, whose outputs are not directly proportional to their inputs, are well known to exhibit many interesting and important phenomena which have profoundly changed our technological landscape over the last 50 years. Recently the ability to engineer quantum metamaterials through hybridisation has allowed to explore these nonlinear effects in systems with no natural analogue. Here we investigate amplitude bistability, which is one of the most fundamental nonlinear phenomena, in a hybrid system composed of a superconducting resonator inductively coupled to an ensemble of nitrogen-vacancy centres. One of the exciting properties of this spin system is its extremely long spin life-time, more than ten orders of magnitude longer than other relevant timescales of the hybrid system. This allows us to dynamically explore this nonlinear regime of cavity quantum electrodynamics (cQED) and demonstrate a critical slowing down of the cavity population on the order of several tens of thousands of seconds - a timescale much longer than observed so far for this effect. Our results provide the foundation for future quantum technologies based on nonlinear phenomena.

quant-ph↗

arXiv2016

Relaxation to Negative Temperatures in Double Domain Systems

We investigate the relaxation of two collective spins in double domain system, which are individually coupled to a single bosonic reservoir, by varying the total number of spins in each domain and their initial spin configurations. A particularly interesting situation occurs when the spin domains are set in an antiparallel configuration. Further for an unbalanced configuration where the number of spins are in the excited state initially is much greater than that in the ground state, the spin ensemble prepared in the ground state relaxes towards a negative-temperature state.

quant-ph↗