arXiv subjects

Masaya Kohda

Publications and source records attributed to Masaya Kohda.

At least 19 recordsLinked to original sources

arXiv2025

Quantum computation of a quasiparticle band structure with the quantum-selected configuration interaction

Quasiparticle band structures are fundamental for understanding strongly correlated electron systems. While solving these structures accurately on classical computers is challenging, quantum computing offers a promising alternative. Specifically, the quantum subspace expansion (QSE) method, combined with the variational quantum eigensolver (VQE), provides a quantum algorithm for calculating quasiparticle band structures. However, optimizing the variational parameters in VQE becomes increasingly difficult as the system size grows, due to device noise, statistical noise, and the barren plateau problem. To address these challenges, we propose a hybrid approach that combines QSE with the quantum-selected configuration interaction (QSCI) method for calculating quasiparticle band structures. QSCI may leverage the VQE ansatz as an input state but, unlike the standard VQE, it does not require full optimization of the variational parameters, making it more scalable for larger quantum systems. Based on this approach, we demonstrate the quantum computation of the quasiparticle band structure of a silicon using 16 qubits on an IBM quantum processor.

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arXiv2025

Hardness of classically sampling quantum chemistry circuits

Significant advances have been made in the study of quantum advantage both in theory and experiment, although these have mostly been limited to artificial setups. In this work, we extend the scope to address quantum advantage in tasks relevant to chemistry and physics. Specifically, we consider the unitary cluster Jastrow (UCJ) ansatz-a variant of the unitary coupled cluster ansatz, which is widely used to solve the electronic structure problem on quantum computers-to show that sampling from the output distributions of quantum circuits implementing the UCJ ansatz is likely to be classically hard. More specifically, we show that there exist UCJ circuits for which classical simulation of sampling cannot be performed in polynomial time, under a reasonable complexity-theoretical assumption that the polynomial hierarchy does not collapse. Our main contribution is to show that a class of UCJ circuits can be used to perform arbitrary instantaneous quantum polynomial-time (IQP) computations, which are already known to be classically hard to simulate under the same complexity assumption. As a side result, we also show that UCJ equipped with post-selection can generate the class post-BQP. Our demonstration, worst-case nonsimulatability of UCJ, would potentially imply quantum advantage in quantum algorithms for chemistry and physics using unitary coupled cluster type ansatzes, such as the variational quantum eigensolver and quantum-selected configuration interaction.

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arXiv2024

Quantum expectation value estimation by doubling the number of qubits

Expectation value estimation is ubiquitous in quantum algorithms. The expectation value of a Hamiltonian, which is essential in various practical applications, is often estimated by measuring a large number of Pauli strings on quantum computers and performing classical post-processing. In the case of $n$-qubit molecular Hamiltonians in quantum chemistry calculations, it is necessary to evaluate $O(n^4)$ Pauli strings, requiring a large number of measurements for accurate estimation. To reduce the measurement cost, we assess an existing idea that uses two copies of an $n$-qubit quantum state of interest and coherently measures them in the Bell basis, which enables the simultaneous estimation of the absolute values of expectation values of all the $n$-qubit Pauli strings. We numerically investigate the efficiency of energy estimation for molecular Hamiltonians of up to 12 qubits. The results show that, when the target precision is no smaller than tens of milli-Hartree, this method requires fewer measurements than conventional sampling methods. This suggests that the method may be useful for many applications that rely on expectation value estimation of Hamiltonians and other observables as well when moderate precision is sufficient.

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arXiv2023

Demonstrating Quantum Computation for Quasiparticle Band Structures

Understanding and predicting the properties of solid-state materials from first-principles has been a great challenge for decades. Owing to the recent advances in quantum technologies, quantum computations offer a promising way to achieve this goal. Here, we demonstrate the first-principles calculation of a quasiparticle band structure on actual quantum computers. This is achieved by hybrid quantum-classical algorithms in conjunction with qubit-reduction and error-mitigation techniques. Our demonstration will pave the way to practical applications of quantum computers.

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arXiv2023

Quantum-Selected Configuration Interaction: classical diagonalization of Hamiltonians in subspaces selected by quantum computers

We propose quantum-selected configuration interaction (QSCI), a class of hybrid quantum-classical algorithms for calculating the ground- and excited-state energies of many-electron Hamiltonians on noisy quantum devices. Suppose that an approximate ground state can be prepared on a quantum computer either by variational quantum eigensolver or by some other method. Then, by sampling the state in the computational basis, which is hard for classical computation in general, one can identify the electron configurations that are important for reproducing the ground state. The Hamiltonian in the subspace spanned by those important configurations is diagonalized on classical computers to output the ground-state energy and the corresponding eigenvector. The excited-state energies can be obtained similarly. The result is robust against statistical and physical errors because the noisy quantum devices are used only to define the subspace, and the resulting ground-state energy strictly satisfies the variational principle even in the presence of such errors. The expectation values of various other operators can also be estimated for obtained eigenstates with no additional quantum cost, since the explicit eigenvectors in the subspaces are known. We verified our proposal by numerical simulations, and demonstrated it on a quantum device for an 8-qubit molecular Hamiltonian. The proposed algorithms are potentially feasible to tackle some challenging molecules by exploiting quantum devices with several tens of qubits, assisted by high-performance classical computing resources for diagonalization.

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arXiv2022

Quantum expectation-value estimation by computational basis sampling

Measuring expectation values of observables is an essential ingredient in variational quantum algorithms. A practical obstacle is the necessity of a large number of measurements for statistical convergence to meet requirements of precision, such as chemical accuracy in the application to quantum chemistry computations. Here we propose an algorithm to estimate the expectation value based on its approximate expression as a weighted sum of classically-tractable matrix elements with some modulation, where the weight and modulation factors are evaluated by sampling appropriately prepared quantum states in the computational basis on quantum computers. Each of those states is prepared by applying a unitary transformation consisting of at most N CNOT gates, where N is the number of qubits, to a target quantum state whose expectation value is evaluated. Our algorithm is expected to require fewer measurements than conventional methods for a required statistical precision of the expectation value when the target quantum state is concentrated in particular computational basis states. We provide numerical comparisons of our method with existing ones for measuring electronic ground state energies (expectation values of electronic Hamiltonians for the lowest-energy states) of various small molecules. Numerical results show that our method can reduce the numbers of measurements to obtain the ground state energies for a targeted precision by several orders of magnitudes for molecules whose ground states are concentrated. Our results provide another route to measure expectation values of observables, which could accelerate the variational quantum algorithms.

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arXiv2019

Enhanced $B \to μ\barν$ Decay at Tree Level as Probe of Extra Yukawa Couplings

With no New Physics seen at the LHC, a second Higgs doublet remains attractive and plausible. The ratio ${\cal R}_B^{μ/τ} = {\cal B}(B \to μ\barν)/{\cal B}(B \to τ\barν)$ is predicted at 0.0045 in both the Standard Model and the type II two Higgs doublet model, but it can differ if extra Yukawa couplings exist in Nature, which we deem an experimental issue. Considering recent Belle update on $B \to μ\barν$, we show that in the general two Higgs doublet model, the ratio could be up by a factor of two, which can be probed by the Belle~II experiment with just a few ab$^{-1}$.

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arXiv2019

Implications of Four-Top and Top-Pair Studies on Triple-Top Production

Multi-top quark production is a staple program at the LHC. Single-top and $t\bar t$ productions are studied extensively, while current efforts are zooming in on four-top search, where the Standard Model (SM) cross section is at ${\cal O}(10)$ fb. In contrast, only at the fb level in SM, triple-top production has not been targeted for study so far. But such a small cross section makes it a unique probe for New Physics. Without the usual discrete $Z_2$ symmetry, the general two Higgs doublet model (g2HDM) can naturally raise the triple-top production to pb level. We illustrate how certain signal regions of four-top search can be utilized to constrain triple-top production, but urge a dedicated search. As an aside, we note that the CMS study at 13 TeV of scalar $t\bar t$ resonance interfering with QCD production background indicate some activity at 400 GeV. We comment that this could be explained in principle in g2HDM via the extra top Yukawa coupling.

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arXiv2019

Flavor Changing Heavy Higgs Interactions with Leptons at Hadron Colliders

In a general two Higgs doublet model, we study flavor changing neutral Higgs (FCNH) decays into leptons at hadron colliders, $pp \to ϕ^0 \to τ^\mpμ^\pm +X$, where $ϕ^0$ could be a CP-even scalar ($h^0$, $H^0$) or a CP-odd pseudoscalar ($A^0$). The light Higgs boson $h^0$ is found to resemble closely the Standard Model Higgs boson at the Large Hadron Collider. In the alignment limit of $\cos(β-α) \cong 0$ for $h^0$--$H^0$ mixing, FCNH couplings of $h^0$ are naturally suppressed, but such couplings of the heavier $H^0, A^0$ are sustained by $\sin(β-α) \simeq 1$. We evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. We find promising results for $\sqrt{s} = 14$ TeV, which we extend further to $\sqrt{s} = 27$ TeV and 100 TeV future pp colliders.

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arXiv2019

Top-Assisted Di-Higgs boson Production Motivated by Baryogenesis

We study top-assisted di-Higgs production via $cg \to tH \to thh$, where $h$ is the 125 GeV scalar boson, and $H$ is the $CP$-even heavy Higgs. The context is the two Higgs doublet model without a $Z_2$ symmetry, where the extra Yukawa coupling $ρ_{tc}$ generates $tH$ production, with the extra top Yukawa $ρ_{tt} \simeq 0$ to avoid $gg \to H$ constraints. We find that discovery is possible for $m_H$ around 300 GeV or so at the LHC, but would need finite $h$-$H$ mixing angle $\cosγ$ to allow for finite $λ_{Hhh}$ coupling, and $ρ_{tc}$ also needs to be not too small. A sizable $ρ_{tc}$ could drive electroweak baryogenesis, which further motivates the search.

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arXiv2018

Probing for Extra Top Yukawa Couplings in Light of $t\bar th(125)$ Observation

The observation of $t\bar th(125)$ production at the Large Hadron Collider (LHC) is the first direct measurement of the top Yukawa coupling. It opens the window on an extra top Yukawa coupling, $ρ_{tt}$, from a second Higgs doublet, without a $Z_2$ symmetry to forbid flavor changing neutral Higgs couplings. We show that $t\bar th$ and Higgs property measurements at the High Luminosity LHC can constrain the ${\rm Re}\,ρ_{tt}$--${\rm Im}\,ρ_{tt}$ parameter space that could drive electroweak baryogenesis, but the $Γ_h$ width measurement must be considerably improved beyond current projections.

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arXiv2018

Constraining a Lighter Exotic Scalar via Same-sign Top

It was shown recently that, in two Higgs doublet models without $Z_2$ symmetry, extra Yukawa couplings such as $ρ_{tc}$, $ρ_{tt}$ can fuel enough $CP$ violation for electroweak baryogenesis (EWBG). We revisit an old proposal where a pseudoscalar $A^0$ has mass between $t\bar c$ and $t\bar t$ thresholds. With $ρ_{tt}$ small, it evades $gg \to A^0 \to h^0(125)Z$ constraints, where approximate alignment also helps. We find this scenario with relatively light $A^0$ is not yet ruled out, and $cg \to tA^0 \to tt\bar c$ can probe sizable $ρ_{tc}$ at the LHC, giving access to the second mechanism of EWBG provided by such models.

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arXiv2018

Unraveling the couplings of a Drell-Yan produced $Z'$ with heavy-flavor tagging

Despite no new physics so far at the LHC, a $Z'$ boson with $m_{Z'} \sim 100$ GeV could still emerge via Drell-Yan (DY) production, $q \bar q \to Z' \to μ^+ μ^-$, in the next few years. To unravel the nature of the $Z'$ coupling, we utilize the $c$- and $b$-tagging algorithms developed by ATLAS and CMS to investigate $cg \to c Z'$ at 14 TeV LHC. While light-jet contamination can be eliminated, mistagged $b$-jets cannot be rejected in any of the tagging schemes we adopt. On the other hand, for nonzero $bbZ'$ coupling, far superior $b$-tagging could discover the $bg \to b Z'$ process, where again light-jet mistag can be ruled out, but mistagged $c$-jets cannot yet be excluded. Provided that DY production is discovered soon enough, we find that a simultaneous search for $c g \to c Z'$ and $b g \to b Z'$ can conclusively discern the nature of $Z'$ couplings involved.

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arXiv2018

Identifying a $Z'$ behind $b \to s \ell \ell$ anomalies at the LHC

Recent $b\to s\ell\ell$ anomalies may imply the existence of a new $Z'$ boson with left-handed $Z'bs$ and $Z'μμ$ couplings. Such a $Z'$ may be directly observed at LHC via $b \bar s \to Z' \to μ^+μ^-$, and its relevance to $b\to s\ell\ell$ may be studied by searching for the process $gs \to Z'b \to μ^+μ^- b$. In this paper, we analyze the capability of the 14 TeV LHC to observe the $Z'$ in the $μ^+ μ^-$ and $μ^+μ^- b$ modes based on an effective model with major phenomenological constraints imposed. We find that both modes can be discovered with 3000 fb$^{-1}$ data if the $Z'bs$ coupling saturates the latest $B_s-\bar B_s$ mixing limit from UTfit at around $2σ$. Besides, a tiny right-handed $Z'bs$ coupling, if it exists, opens up the possibility of a relatively large left-handed counterpart, due to cancellation in the $B_s-\bar B_s$ mixing amplitude. In this case, we show that even a data sample of $\mathcal{O}(100)$ fb$^{-1}$ would enable discovery of both modes. We further study the impact of a $Z'bb$ coupling as large as the $Z'bs$ coupling. This scenario enables discovery of the $Z'$ in both modes with milder effects on the $B_s-\bar B_s$ mixing, but obscures the relevance of the $Z'$ to $b \to s\ell\ell$. Discrimination between the $Z'bs$ and $Z'bb$ couplings may come from the production cross section for the $Z'b\bar{b}$ final state. However, we do not find the prospect for this to be promising.

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arXiv2018

Searching for new scalar bosons via triple-top signature in $cg \to tS^0 \to tt\bar t$

The alignment phenomenon, that the 125 GeV $h^0$ boson so resembles the Standard Model Higgs boson, can be understood in a two Higgs doublet model without discrete symmetry. New Yukawa couplings $ρ_{tt}$ and $ρ_{tc}$ offer new avenues to discover the extra scalar $H^0$ and pseudoscalar $A^0$. We propose to search for $cg \to tH^0$, $tA^0$ followed by $H^0$, $A^0 \to t\bar t$, $t\bar c$, where same-sign dileptons could be the harbinger, with triple-top, in the signature of three leptons plus three $b$-jets, as confirmation. Discovery could touch upon the origin of baryon asymmetry of the Universe.

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arXiv2017

Search for $tZ'$ associated production induced by $tcZ'$ couplings at the LHC

The $P'_5$ and $R_K$ anomalies, recently observed by the LHCb collaboration in $B \to K^{(*)}$ transitions, may indicate the existence of a new $Z'$ boson, which may arise from gauged $L_μ- L_τ$ symmetry. Flavor-changing neutral current $Z'$ couplings, such as $tcZ'$, can be induced by the presence of extra vector-like quarks. In this paper we study the LHC signatures of the induced right-handed $tcZ'$ coupling that is inspired by, but not directly linked to, the $B \to K^{(*)}$ anomalies. The specific processes studied are $cg \to tZ'$ and its conjugate process each followed by $Z'\toμ^+μ^-$. By constructing an effective theory for the $tcZ'$ coupling, we first explore model-independently the discovery potential of such a $Z'$ at the 14 TeV LHC with 300 and 3000 fb$^{-1}$ integrated luminosities. We then reinterpret the model-independent results within the gauged $L_μ- L_τ$ model. In connection with $tcZ'$, the model also implies the existence of a flavor-conserving $ccZ'$ coupling, which can drive the $c \bar c \to Z' \to μ^+μ^-$ process. Our study shows that existing LHC results for dimuon resonance searches already constrain the $ccZ'$ coupling, and that the $Z'$ can be discovered in either or both of the $cg \to tZ'$ and $c \bar c \to Z'$ processes. We further discuss the sensitivity to the left-handed $tcZ'$ coupling and find that the coupling values favored by the $B \to K^{(*)}$ anomalies lie slightly below the LHC discovery reach even with 3000 fb$^{-1}$.

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arXiv2016

Z'-induced FCNC decays of top, beauty and strange quarks

Anomalous b --> s transitions from LHCb data may suggest a new massive gauge boson Z' that couples to the left-handed b --> s current, which in turn implies a coupling to the t --> c current. In this paper, we study flavor-changing neutral current (FCNC) decays of the top quark induced by a Z' boson, namely t --> c Z', based on a model of the gauged L_mu - L_tau symmetry (the difference between the muon and tauon numbers) with vector-like quarks, which was introduced to explain the anomalous LHCb data. We illustrate that searching for t --> c Z' via Z' --> mu^+ mu^- with LHC Run 1 data can already probe a parameter region which is unexplored by B physics for the Z' mass around O(10) GeV or more. We further extend the model to very light Z' with mass below 400 MeV, which is motivated by the muon g-2 anomaly. Taking rare B and K meson decay data into account, we give upper limits on the t --> c Z' branching ratio for the light Z' case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay K_L --> pi^0 Z' with Z' --> nu nubar may lead to apparent violation of the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9.

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arXiv2015

Correlating $B_q^0 \to μ^+μ^-$ and $K_L \to π^0ν\barν$ Decays with Four Generations

The long-awaited $B_s\to μ^+μ^-$ mode has finally been observed at rate consistent with Standard Model, albeit lower by 1.2$σ$. There is some hint for New Physics in the rarer $B_d^0 \to μ^+μ^-$ decay, especially if the currently 2.2$σ$-enhanced central value persists with more data. The measurement of $CP$ violating phase $ϕ_s$, via both $B_s\to J/ψK\bar K$ and $J/ψππ$ modes, has reached Standard Model sensitivity. These measurements stand major improvement when LHC enters Run 2. Concurrently, the $K_L\toπ^0ν\barν$ and $K^+\toπ^+ν\barν$ modes are being pursued in a similar time frame. We illustrate the possible correlations between New Physics effects in these four modes, using the fourth generation as example. While correlations may or may not exist in other New Physics models, the four generation model can accommodate enhancements in both $B_d^0 \to μ^+μ^-$ and $K_L\toπ^0ν\barν$.

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