Quasideterministic realization of a universal quantum gate in a single scattering process

Ciccarello, Francesco; Browne, Dan E.; Kwek, Leong Chuan; Schomerus, Henning; Zarcone, M.; Bose, Sougato

doi:10.1103/physreva.85.050305

Cited by 63 publications

(73 citation statements)

References 59 publications

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“…Novel correlations among injected near-resonant photons result from the nonlinearity of the qubits, and intriguing interference effects occur because of the 1D confinement of the light. The field has focused on qubits in a local region for which these correlation and interference effects can be used for local quantum information purposes such as single-photon routing [6], rectification of photonic signals [7][8][9][10], and quantum gates [11][12][13]. This regime of waveguide QED involves neglecting delay times: the time taken by photons to travel between qubits is far shorter than all other characteristic times.…”

Section: Introductionmentioning

confidence: 99%

Non-Markovian dynamics of a qubit due to single-photon scattering in a waveguide

2018

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We investigate the open dynamics of a qubit due to scattering of a single photon in an infinite or semi-infinite waveguide. Through an exact solution of the time-dependent multi-photon scattering problem, we find the qubitʼs dynamical map. Tools of open quantum systems theory allow us then to show the general features of this map, find the corresponding non-Linbladian master equation, and assess in a rigorous way its non-Markovian nature. The qubit dynamics has distinctive features that, in particular, do not occur in emission processes. Two fundamental sources of non-Markovianity are present: the finite width of the photon wavepacket and the time delay for propagation between the qubit and the end of the semi-infinite waveguide.Clarifying the importance of NM effects and the mechanisms behind their onset is thus pivotal in waveguide QED. At the same time, the theory of OQS [34,35] is currently making major advances, yielding a more accurate understanding of NM effects [36][37][38][39]. Through an approach often inspired by quantum information concepts [40], a number of physical properties such as information back-flow [41] and divisibility [42] have been spotlighted as distinctive manifestations of quantum NM behavior and then used to formulate corresponding quantum non-Markovianity measures. These tools have been effectively applied to dynamics in various scenarios [37,38], including in waveguide QED with regard to emission processes [26, 32] 5 such as a single atom emitting into a semi-infinite waveguide [26].Motivated by the need to quantify NM effects in photon scattering from qubits, we present a case study of a qubit undergoing single-photon scattering in an infinite or semi-infinite waveguide (see figure 1), the latter of which is the basis of the proposed controlled-Z [12] and controlled-NOT [13] gates. We aim at answering two main questions: What are the essential features of the qubit open dynamics during scattering? Is such dynamics NM?The key task is to find the dynamical map (DM) of the qubit in the scattering process, which fully describes the open dynamics and is needed in order to apply OQS tools [38]. A distinctive feature of our open dynamics is that the bath (the waveguide field) is initially in a well-defined single-photon state [43,44]. Toward this task, we tackle in full the time evolution of multiple excitations (in contrast to those limited to the one-excitation sector [43,[45][46][47][48]), a problem that has become important recently [30,31,33,[49][50][51][52][53][54][55][56][57].Intuitively, one may expect that the dynamics is fully Markovian in the infinite-waveguide case and NM in the semi-infinite case due to the atom-mirror delay time. We show that this expectation is inaccurate in general, mostly because it does not account for a fundamental source of NM behavior namely the wavepacket bandwidth. This NM mechanism is present in an infinite waveguide, while in a semi-infinite waveguide it augments the natural NM behavior coming from the photon delay time.Recently, NM effects in infi...

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Section: Introductionmentioning

confidence: 99%

Non-Markovian dynamics of a qubit due to single-photon scattering in a waveguide

2018

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“…are the projector operators associated with the singlet and triplet subspaces, respectively, of the f -SQ 1 system. Note that in the computational basis {|α f α 1 } (α f , α 1 =↑, ↓), a matrix element α f α 1 |R f 1 |α f α 1 yields the probability amplitude that, given the initial joint spin state |α f α 1 , f is reflected back and the final spin state is |α f α 1 [8,9] (an analogous statement holds forT f 1 ). Via the identities (4) and (5), one can easily verify that equation (7) immediately entails the proper normalization condition…”

Section: Read-out Of a Single Static Memory Qubitmentioning

confidence: 98%

Selective writing and read-out of a register of static qubits

et al. 2013

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We propose a setup comprising an arbitrarily large array of static qubits (SQs), which interact with a flying qubit (FQ). The SQs work as a quantum register, which can be written or read out by means of the FQ through quantum state transfer (QST). The entire system, including the FQ's motional degrees of freedom, behaves quantum mechanically. We demonstrate a strategy allowing for selective QST between the FQ and a single SQ chosen from the register. This is achieved through a perfect mirror located beyond the SQs and suitable modulation of the inter-SQ distances.A prominent paradigm in quantum information processing (QIP) [1] is to employ flying qubits (FQs) and static qubits (SQs) as the carriers and registers of quantum information, respectively [2]. Key to such an idea is the ability to write and read out the information content of a SQ by means of a FQ. By this, here we mean that an efficient quantum state transfer (QST) between these two types of qubits must be possible on demand. In this picture, control over memory allocation appears to be a desirable if not indispensable requirement. For instance, one can envisage the situation where only one or a few SQs are available, e.g. because the remaining ones are encoding some information to save. On the other hand, one may need to carry away only the information saved in certain specific SQs. Alternatively, only a restricted area of the register of SQs may be interfaced with some external processing network where one would like to eventually convey information or from which output data are to be received. In such cases, the ability of selecting the exact location where the information content of the FQ should be uploaded or downloaded is demanded. Ideally, according to the schematics in figure 1, one would like the FQ to reach the specific target SQ, then fully transfer its quantum state to this and eventually fly away. Evidently, this picture is implicitly based on the assumption that, firstly, the motional degrees of freedom (MDOFs) of the FQ are in fact fully classical and, secondly, these can be accurately controlled. Despite its simplicity, although interesting research along these lines is being carried out mostly through the so-called surface acoustic waves (see e.g. [3] and references therein), such an approach calls for a very high level of control.If set within a fully quantum framework, the most natural situation to envisage is the one where the FQ, besides bearing an internal spin, moves in a quantum mechanical way and hence propagates as a wave-like object. Such a circumstance substantially complicates the dynamics in that, besides the complex spin-spin interactions, intricate wave-like effects such as multiple reflections between the many SQs occur as well. This appears to be an adverse environment to accomplish selective QST: while ideally one would like to focus the FQ's wave packet right on the target SQ, the former is expected to spread throughout the SQs' register. Thus, not only is it non-trivial what strategy would enable selective QST but ...

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“…There is growing evidence that the rich physics of waveguide QED can be harnessed for a number of promising applications in photonics, e.g. light switches [15] and single-photon transistors [16], as well as QIP, such as quantum gates [23,24]. The scheme to be presented here further witnesses the potential of waveguide QED.…”

Section: Introductionmentioning

confidence: 97%

“…To make the language simpler, in what follows we refer to W(ω) as the effective potential (this is reminiscent of Refs. [23,27], where however R was absent). Note that this is frequency-dependent, such dependance occurring through function ε(ω) which is associated with J(ω) [cf.…”

Section: Photon Scattering From Smentioning

confidence: 99%

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Waveguide-QED-based measurement of a reservoir spectral density

Ciccarello

2015

Phys. Rev. A

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The spectral density (SD) function has a central role in the study of open quantum systems (OQSs). We discover a method allowing for a "static" measurement of the SD -i.e., it requires neither the OQS to be initially excited nor its time evolution tracked in time -which is not limited to the weak-coupling regime. This is achieved through one-dimensional photon scattering for a zero-temperature reservoir coupled to a two-level OQS via the rotating wave approximation. We find that the SD profile is a universal simple function of the photon's reflectance and transmittance. As such, it can be straightforwardly inferred from photon's reflection and transmission spectra.

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Quasideterministic realization of a universal quantum gate in a single scattering process

Cited by 63 publications

References 59 publications

Non-Markovian dynamics of a qubit due to single-photon scattering in a waveguide

Non-Markovian dynamics of a qubit due to single-photon scattering in a waveguide

Selective writing and read-out of a register of static qubits

Waveguide-QED-based measurement of a reservoir spectral density

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