arXiv subjects
Publications and source records attributed to Robert Knegjens.
We summarize the performance of recently-proposed methods for achieving fault tolerant fusions-based quantum computation with high tolerance to qubit loss, specifically aimed at photonic implementations.
Restrictions imposed by existing infrastructure can make it hard to ensure an even spacing between the nodes of future fiber-based quantum networks. We here investigate the negative effects of asymmetric node placement by considering separately the placement of midpoint stations required for heralded entanglement generation, as well as of processing-node quantum repeaters in a chain. For midpoint stations, we describe the effect asymmetry has on the time required to perform one entangling attempt, the success probability of such attempts, and the fidelity of the entangled states created. This includes accounting for the effects of chromatic dispersion on photon indistinguishability. For quantum-repeater chains we numerically investigate how uneven spacing between repeater nodes leads to bottlenecks, thereby increasing both the waiting time and the time states are stored in noisy quantum memory. We find that while the time required to perform one entangling attempt may increase linearly with the midpoint's asymmetry, the success probability and fidelity of heralded entanglement generation and the distribution time and error rate for repeater chains all have vanishing first derivatives with respect to the amount of asymmetry. This suggests resilience of quantum-network performance against small amounts of asymmetry.
In order to bring quantum networks into the real world, we would like to determine the requirements of quantum network protocols including the underlying quantum hardware. Because detailed architecture proposals are generally too complex for mathematical analysis, it is natural to employ numerical simulation. Here we introduce NetSquid, the NETwork Simulator for QUantum Information using Discrete events, a discrete-event based platform for simulating all aspects of quantum networks and modular quantum computing systems, ranging from the physical layer and its control plane up to the application level. We study several use cases to showcase NetSquid's power, including detailed physical layer simulations of repeater chains based on nitrogen vacancy centres in diamond as well as atomic ensembles. We also study the control plane of a quantum switch beyond its analytically known regime, and showcase NetSquid's ability to investigate large networks by simulating entanglement distribution over a chain of up to one thousand nodes.
Quantum communication brings radically new capabilities that are provably impossible to attain in any classical network. Here, we take the first step from a physics experiment to a fully fledged quantum internet system. We propose a functional allocation of a quantum network stack and construct the first physical and link layer protocols that turn ad-hoc physics experiments producing heralded entanglement between quantum processors into a well-defined and robust service. This lays the groundwork for designing and implementing scalable control and application protocols in platform-independent software. To design our protocol, we identify use cases, as well as fundamental and technological design considerations of quantum network hardware, illustrated by considering the state-of-the-art quantum processor platform available to us (Nitrogen-Vacancy (NV) centers in diamond). Using a purpose built discrete-event simulator for quantum networks, we examine the robustness and performance of our protocol using extensive simulations on a super-computing cluster. We perform a full implementation of our protocol, where we successfully validate the physical simulation model against data gathered from the NV hardware. We first observe that our protocol is robust even in a regime of exaggerated losses of classical control messages with only little impact on the performance of the system.We proceed to study the performance of our protocols for 169 distinct simulation scenarios, including tradeoffs between traditional performance metrics such as throughput and the quality of entanglement. Finally, we initiate the study of quantum network scheduling strategies to optimize protocol performance for different use cases.
The decays $K^+\toπ^+ν\barν$ and $K_L\toπ^0ν\barν$, being the theoretically cleanest rare decays of mesons, are very sensitive probes of New Physics. In view of the excellent prospects of reaching the Standard Model sensitivity for $K^+\toπ^+ν\barν$ by the NA62 experiment at CERN and for $K_L\toπ^0ν\barν$ by the KOTO experiment at J-PARC, we study them in the simplest extensions of the SM in which stringent correlations between these two decays and other flavour observables are present. We first consider simple models with tree-level Z and Z' contributions in which either MFV or a $U(2)^3$ symmetry is imposed on the quark flavour-violating couplings. We then compare the resulting correlations with those present in generic models in which the latter couplings are arbitrary, subject to constraints from $ΔF=2$ processes, electroweak and collider data. Of particular interest are the correlations with $ε'/ε$ and $K_L\toμ^+μ^-$ which limit the size of NP contributions to $K^+\toπ^+ν\barν$ and $K_L\toπ^0ν\barν$, depending on the Dirac structure of couplings and the relevant operators. But in MFV also the constraint from $B_s\toμ^+μ^-$ turns out to be important. We take into account the recent results from lattice QCD and large N approach that indicate $ε'/ε$ in the SM to be significantly below the data. While in many models the enhancement of $ε'/ε$ implies a suppression of $K_L\toπ^0ν\barν$, we present two models in which these observables can be simultaneously enhanced relative to SM predictions. A correlation between $K^+\toπ^+ν\barν$ and $B\to K(K^*)μ^+μ^-$, found by us in the simple models considered here, should be of interest for NA62 and LHCb experimentalists at CERN in the coming years. The one with $B\to K(K^*)ν\barν$ will be tested at Belle II.
In view of the recent start of the NA62 experiment at CERN that is expected to measure the $K^+\toπ^+ν\barν$ branching ratio with a precision of 10%, we summarise the present status of this promising decay within the Standard Model (SM). We do likewise for the closely related $K_L\toπ^0ν\barν$, which will be measured by the KOTO experiment around 2020. As the perturbative QCD and electroweak corrections in both decays are under full control, the dominant uncertainties within the SM presently originate from the CKM parameters $V_{cb}$, $V_{ub}$ and $γ$. We show this dependence with the help of analytic expressions as well as accurate interpolating formulae. Unfortunately a clarification of the discrepancies between inclusive and exclusive determinations of $V_{cb}$ and $V_{ub}$ from tree-level decays will likely require results from the Belle II experiment available at the end of this decade. Thus we investigate whether higher precision on both branching ratios is achievable by determining $V_{cb}$, $V_{ub}$ and $γ$ by means of other observables that are already precisely measured. In this context $\varepsilon_K$ and $ΔM_{s,d}$, together with the expected progress in QCD lattice calculations will play a prominent role. We find $\mathcal{B}(K^+\toπ^+ν\barν) = (9.11\pm 0.72) \times 10^{-11}$ and $\mathcal{B}(K_L\toπ^0ν\barν) = (3.00\pm 0.30) \times 10^{-11}$, which is more precise than using averages of the present tree-level values of $V_{cb}$, $V_{ub}$ and $γ$. Furthermore, we point out the correlation between $\mathcal{B}(K^+\toπ^+ν\barν)$, $\overline{\mathcal{B}}(B_s\toμ^+μ^-)$ and $γ$ within the SM, that is only very weakly dependent on other CKM parameters. Finally, we also update the ratio $\varepsilon'/\varepsilon$ in the SM and present its correlation with $K_L\toπ^0ν\barν$.
In this talk I give a theoretical overview of the rare decays $B_{s} \to μ^+ μ^-$ and $B_{d} \to μ^+ μ^-$. The branching ratios of these decays are promising probes of New Physics, both independently and relative to each other. Recent experimental progress at the LHC has confirmed the existence of the $B_{s} \to μ^+ μ^-$ decay, and has not revealed any large signals of New Physics that may have been present. This raises the question of whether moderate New Physics effects can be identified in the LHC era. To that end I review several important developments in the Standard Model branching ratio predictions, and discuss how the latest measurements currently constrain New Physics. Furthermore, I highlight how a time-dependent analysis of $B_{s} \to μ^+ μ^-$, which may be feasible at the upgraded CMS and LHCb detectors, can complement the search for and identification of New Physics.
The precision expected for the rare $K\to πν\barν$ decays by the NA62 and KOTO experiments in the coming decade will rival their current SM predictions. In preparation for this upcoming opportunity, we review the SM predictions and discuss the sensitivity of these decays to models beyond the Standard Model, considering in particular simplified Z and Z' models as benchmarks. In the latter case we also discuss how these decays could ultimately probe distance scales as small as zeptometers i.e. peek into the Zeptouniverse.
In light of recent LHC results for the extraction of the $B_s$ mixing phase $ϕ_s$, we can already conclude that if New Physics (NP) is present in this observable, it is hiding pretty well. Thus, as our hunt continues, we must be weary not to confuse NP for penguin effects, or vice versa. In this talk the progress made towards addressing hadronic uncertainties in extractions of $ϕ_s$ from $B_s\to J/ψϕ$ is reviewed, and the nature of the scalar $f_0(980)$ state, which plays a dominant role in the extraction of $ϕ_s$ from the $B_s\to J/ψπ^+π^-$ decay, is discussed.
The Large Hadron Collider will directly probe distance scales as short as 10^{-19}m, corresponding to energy scales at the level of a few TeV. In order to reach even higher resolutions before the advent of future high-energy colliders, it is necessary to consider indirect probes of New Physics (NP), a prime example being Delta F=2 neutral meson mixing processes, which are sensitive to much shorter distance scales. However Delta F=2 processes alone cannot tell us much about the structure of NP beyond the LHC scales. To identify for instance the presence of new quark flavour-changing dynamics of a left-handed (LH) or right-handed (RH) nature, complementary results from Delta F=1 rare decay processes are vital. We therefore address the important question of whether NP could be seen up to energy scales as high as 200 TeV, corresponding to distances as small as 10^{-21}m -- the Zeptouniverse -- in rare K and B_{s,d} decays, subject to present Delta F=2 constraints and perturbativity. We focus in particular on a heavy Z' gauge boson. If restricted to purely LH or RH Z' couplings to quarks, we find that rare K decays, in particular KL-> pi^0 nu nubar and K^+->pi^+ nu nubar, allow us to probe the Zeptouniverse. On the other hand rare B_s and B_d decays, which receive stronger Delta F=2 constraints, allow us to reach about 15 TeV. Allowing for both LH and RH couplings a loosening of the Delta F=2 constraints is possible, and we find that the maximal values of M_Z' at which NP effects that are consistent with perturbative couplings could be found are approximately 2000 TeV for K decays and 160 TeV for rare B_{s,d} decays. Finally we present a simple idea for an indirect determination of M_Z' that could be realised at the next linear e^+e^- or mu^+ mu^- collider and with future precise flavour data.
The B_s to mu+ mu- decay plays an outstanding role in tests of the Standard Model and physics beyond it. The LHCb collaboration has recently reported the first evidence for this decay at the 3.5 sigma level, with a branching ratio in the ballpark of the Standard Model prediction. Thanks to the recently established sizable decay width difference of the B_s system, another observable, A^mumu_DeltaGamma, is available, which can be extracted from the time-dependent untagged B_s to mu+ mu- rate. If tagging information is available, a CP-violating asymmetry, S_mumu, can also be determined. These two observables exhibit sensitivity to New Physics that is complementary to the branching ratio. We define and analyse scenarios in which these quantities allow us to discriminate between model-independent effective operators and their CP-violating phases. In this context we classify a selection of popular New Physics models into the considered scenarios. Furthermore, we consider specific models with tree-level FCNCs mediated by a heavy neutral gauge boson, pseudoscalar or scalar, finding striking differences in the predictions of these scenarios for the observables considered and the correlations among them. We update the Standard Model prediction for the time-integrated branching ratio taking the subtle decay width difference effects into account. We find (3.56 +/- 0.18) x 10^-9, and discuss the error budget.
In many extensions of the Standard Model flavour changing neutral current processes can be mediated by tree-level heavy neutral scalars and/or pseudo-scalars H^0(A^0). This generally introduces new sources of flavour violation and CP violation as well as left-handed and right-handed scalar currents. These new physics contributions imply a pattern of deviations from SM expectations for FCNC processes that depends only on the couplings of H^0(A^0) to fermions and on their masses. In situations in which a single H^0 or A^0 dominates NP contributions stringent correlations between Delta F=2 and Delta F=1 observables exist. Anticipating the Flavour Precision Era ahead of us we illustrate this by searching for allowed oases in the landscape of a given model assuming significantly smaller uncertainties in CKM and hadronic parameters than presently available. To this end we analyze Delta F=2 observables in B^0_{s,d}-bar B^0_{s,d} and K^0-bar K^0 systems and rare B and K decays with charged leptons in the final state including both left-handed and right-handed scalar couplings of H^0 and A^0 to quarks in various combinations. We identify a number of correlations between various flavour observables that could test and distinguish these different scenarios. The prominent role of the decays B_{s,d}-> mu^+ mu^- in these studies is emphasized. Imposing the existing flavour constraints, a rich pattern of deviations from the SM expectations in rare B_{s,d} decays emerges provided M_H< 1 TeV. NP effects in rare K decays are very small. Neutral SM Higgs contributions to rare B and K decays turn out to be negligible once the constraints from Delta F=2 processes are taken into account. Finally, we point out striking differences between the correlations found here and in scenarios in which tree-level FCNC are mediated by a new neutral gauge boson Z'.
The experimentally established non-zero decay width difference of the B_s meson system gives us access to a mass eigenstate rate asymmetry for each B_s transition. This observable is not only the key ingredient in converting between differing definitions of a B_s branching ratio, but can also be a sensitive probe of New Physics that does not require flavour tagging. We discuss how a pair of effective lifetimes for CP even and odd final states, which probe this asymmetry, can constrain the parameters of B_s mixing. We then shift our focus to the rare decay B_s to mu+ mu-, for which the Standard Model branching ratio prediction receives a sizable correction due to a maximal asymmetry. We present how this asymmetry, which can be extracted from an untagged time-dependent measurement, serves as a new observable, complementary to the branching ratio, for constraining New Physics. Further, we analyse types of models beyond the Standard Model that this pair of observables for B_s to mu+ mu- can discriminate between.
The $B_s \to D_s^{(*)\pm} K^\mp$ decays allow a theoretically clean determination of $ϕ_s+γ$, where $ϕ_s$ is the $B^0_s$-$\bar B^0_s$ mixing phase and $γ$ the usual angle of the unitarity triangle. A sizable $B_s$ decay width difference $ΔΓ_s$ was recently established, which leads to subtleties in analyses of the $B_s \to D_s^{(*)\pm} K^\mp$ branching ratios but also offers new "untagged" observables, which do not require a distinction between initially present $B^0_s$ or $\bar B^0_s$ mesons. We clarify these effects and address recent measurements of the ratio of the $B_s\to D_s^\pm K^\mp$, $B_s\to D_s^\pmπ^\mp$ branching ratios. In anticipation of future LHCb analyses, we apply the SU(3) flavour symmetry of strong interactions to convert the $B$-factory data for $B_d\to D^{(*)\pm}π^\mp$, $B_d\to D_s^{\pm}π^\mp$ decays into predictions of the $B_s \to D_s^{(*)\pm} K^\mp$ observables, and discuss strategies for the extraction of $ϕ_s+γ$, with a special focus on untagged observables and the resolution of discrete ambiguities. Using our theoretical predictions as a guideline, we make simulations to estimate experimental sensitivities, and extrapolate to the end of the planned LHCb upgrade. We find that the interplay between the untagged observables, which are accessible thanks to the sizable $ΔΓ_s$, and the mixing-induced CP asymmetries, which require tagging, will play the key role for the experimental determination of $ϕ_s+γ$.
Measurements of the $B_s^0$--$\bar{B}_s^0$ mixing phase $ϕ_s$ appear to be converging towards the SM prediction, suggesting that contributions from New Physics (NP), if present, are small. This poses the question of whether smallish NP in $ϕ_s$ can be distinguished from the hadronic uncertainties present in the interfering $B_s$ decay mode. In this paper we discuss the potential of extracting $ϕ_s$ from the decay modes $B_s\to J/ψη^{(\prime)}$ and $B_s\to J/ψf_0(980)$ and how their respective hadronic uncertainties can be controlled. In addition, we demonstrate how the branching ratios of the former decays can be used to estimate the $η$--$η'$ mixing angles. Finally, we point out that effective lifetime measurements of decay modes such as the $B_s\to J/ψf_0(980)$ can be used to constrain both $ϕ_s$ and the $B_s$ decay width difference $ΔΓ_s$, complementary to the usual time-dependent tagged analysis.
We have just entered an era of precision measurements for $B_s$-decay observables. A characteristic feature of the $B_s$-meson system is $B^0_s$--$\bar B^0_s$ mixing, which exhibits a sizable decay width difference. The latter feature leads to a subtle complication for the extraction of branching ratios of $B_s$ decays from untagged data samples, leading to systematic biases as large as O(10%) that depend on the dynamics of the considered decay. We point out that this effect can only be corrected for using information from a time-dependent analysis and suggest the use of the effective $B_s$ decay lifetime, which can already be extracted from the untagged data sample, for this purpose. We also address several experimental issues that can play a role in the extraction of effective lifetimes at a hadron collider, and advocate the use of the $B_s$ branching ratios, as presented in this note, for consistent comparisons of theoretical calculations and experimental measurements in particle listings.
We have recently seen new upper bounds for $B^0_s\to μ^+μ^-$, a key decay to search for physics beyond the Standard Model. Furthermore a non-vanishing decay width difference $ΔΓ_s$ of the $B_s$ system has been measured. We show that $ΔΓ_s$ affects the extraction of the $B^0_s\to μ^+μ^-$ branching ratio and the resulting constraints on the New Physics parameter space, and give formulae for including this effect. Moreover, we point out that $ΔΓ_s$ provides a new observable, the effective $B^0_s\to μ^+μ^-$ lifetime $τ_{μ^+μ^-}$, which offers a theoretically clean probe for New Physics searches that is complementary to the branching ratio. Should the $B^0_s\to μ^+μ^-$ branching ratio agree with the Standard Model, the measurement of $τ_{μ^+μ^-}$, which appears feasible at upgrades of the LHC experiments, may still reveal large New Physics effects.
The $B^0_{s,d} \to J/ψη^{(\prime)}$ decays provide new terrain for exploring CP violation. After briefly discussing $η$-$η'$ mixing, we analyse the effective lifetimes and CP-violating observables of the $B_s$ channels, which allow us to probe New-Physics effects in $B^0_s$-$\bar B^0_s$ mixing. We have a critical look at these observables and show how hadronic corrections can be controlled by means of the $B_d$ decays. Using measurements of the $B^0_{s,d}\to J/ψη^{(\prime)}$ branching ratios by the Belle collaboration, we discuss tests of the $SU(3)_F$ flavour symmetry of strong interactions, obtain the first constraints on the hadronic parameters entering the $B^0_{s,d} \to J/ψη$ system, and predict the $B^0_d\to J/ψη'$ branching ratio at the $5\times10^{-6}$ level. Furthermore, we present strategies for the determination of the $η$-$η'$ mixing parameters from the $B^0_{s,d} \to J/ψη^{(\prime)}$ observables. We also observe that the $B^0_{s,d} \to J/ψη$ and $B^0_{s,d} \to J/ψη'$ decays are - from a formal point of view - analogous to the quark-antiquark and tetraquark descriptions of the $f_0(980)$ in the $B^0_{s,d} \to J/ψf_0(980)$ channels, respectively.