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Register a new userGeometric entangler via spin-electric coupling in molecular magnets
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Added by
Vahid Azimi Mousolou
Publication date
2015
fields
Physics
and research's language is
English
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A fundamental requirement in the circuit model of quantum information processing is the realization of fault-tolerant multi-qubit quantum gates with entangling capabilities. A key step towards this end is to achieve control of qubit states through geometric phases at very small spatial scales in an effective and feasible way. A spin-electric coupling present in antiferromagnetic triangular single-molecule magnets (SMMs) allows for manipulation of the spin (qubit) states with a great flexibility. Here, we establish an all-electrical two-qubit geometric phase shift gate acting on the four-fold ground state manifold of a triangular SMM, which represents an effective two-qubit state space. We show that a two-qubit quantum gate with arbitrary entangling power can be achieved through the Berry phase effect, induced by adiabatically varying an external electric field in the plane of the molecule.
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The ground state of frustrated (antiferromagnetic) triangular molecular magnets is characterized by two total-spin $S =1/2$ doublets with opposite chirality. According to a group theory analysis [M. Trif textit{et al.}, Phys. Rev. Lett. textbf{101}, 217201 (2008)] an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, $d$, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by textit{ab-initio} methods [M. F. Islam textit{et al.}, Phys. Rev. B textbf{82}, 155446 (2010)]. In this work we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a $d$-dependent splitting of the ground state manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters $t$ and $U$, and the spin-electric coupling constant $d$. This allows us to predict the regime in which the coupling constant $d$ can be extracted from experiment.
Using X-band pulsed electron spin resonance, we report the intrinsic spin-lattice ($T_1$) and phase coherence ($T_2$) relaxation times in molecular nanomagnets for the first time. In Cr$_7M$ heterometallic wheels, with $M$ = Ni and Mn, phase coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples $T_2$ reaches 3 $mu$s at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.
Coherent control of individual molecular spins in nano-devices is a pivotal prerequisite for fulfilling the potential promised by molecular spintronics. By applying electric field pulses during time-resolved electron spin resonance measurements, we measure the sensitivity of the spin in several antiferromagnetic molecular nanomagnets to external electric fields. We find a linear electric field dependence of the spin states in Cr$_7$Mn, an antiferromagnetic ring with a ground-state spin of $S=1$, and in a frustrated Cu$_3$ triangle, both with coefficients of about $2~mathrm{rad}, mathrm{s}^{-1} / mathrm{V} mathrm{m}^{-1}$. Conversely, the antiferromagnetic ring Cr$_7$Ni, isomorphic with Cr$_7$Mn but with $S=1/2$, does not exhibit a detectable effect. We propose that the spin-electric field coupling may be used for selectively controlling individual molecules embedded in nanodevices.
Heavy metals are key to spintronics because of their high spin-orbit coupling (SOC) leading to efficient spin conversion and strong magnetic interactions. When C60 is deposited on Pt, the molecular interface is metallised and the spin Hall angle in YIG/Pt increased, leading to an enhancement of up to 600% in the spin Hall magnetoresistance and 700% for the anisotropic magnetoresistance. This correlates with Density Functional Theory simulations showing changes of 0.46 eV/C60 in the SOC of Pt. This effect opens the possibility of gating the molecular hybridisation and SOC of metals.
We discuss Floquet engineering of dissipative molecular systems through periodic driving of an infrared-active vibrational transition, either directly or via a cavity mode. Following a polaron quantum Langevin equations approach, we derive correlation functions and stationary quantities showing strongly modified optical response of the infrared-dressed molecule. The coherent excitation of molecular vibrational modes, in combination with the modulation of electronic degrees of freedom due to vibronic coupling can lead to both enhanced vibronic coherence as well as control over vibrational sideband amplitudes. The additional coupling to an infrared cavity allows for the controlled suppression of undesired sidebands, an effect stemming from the Purcell enhancement of vibrational relaxation rates.
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