Molecular Qubits Research Articles

Monotonic Convergent Optimal Control Theory with Strict Limitations on the Spectrum of Optimized Laser Fields

We present a modified optimal control scheme based on the Krotov method, which allows for strict limitations on the spectrum of the optimized laser fields. A frequency constraint is introduced and derived mathematically correct, without losing monotonic convergence of the algorithm. The method guarantees a close link to learning loop control experiments and is demonstrated for the challenging control of nonresonant Raman transitions, which are used to implement a set of global quantum gates for molecular vibrational qubits.

Molecular dipolar crystals as high-fidelity quantum memory for hybrid quantum computing

We study collective excitations of rotational and spin states of an ensemble of polar molecules, which are prepared in a dipolar crystalline phase, as a candidate for a high-fidelity quantum memory. While dipolar crystals are formed in the high-density limit of cold clouds of polar molecules under one- and two-dimensional trapping conditions, the crystalline structure protects the molecular qubits from detrimental effects of short-range collisions. We calculate the lifetime of the quantum memory by identifying the dominant decoherence mechanisms, and estimate their effects on gate operations, when a molecular ensemble qubit is transferred to a superconducting strip line cavity (circuit QED). In the case of rotational excitations coupled by dipole-dipole interactions we identify phonons as the main limitation of the lifetime of qubits. We study specific setups and conditions, where the coupling to the phonon modes is minimized. Detailed results are presented for a one-dimensional dipolar chain.

OH hyperfine ground state: From precision measurement to molecular qubits

We perform precision microwave spectroscopy---aided by Stark deceleration---to reveal the low-magnetic-field behavior of OH in its $^{2}\ensuremath{\Pi}_{3∕2}$ rovibronic ground state, identifying two field-insensitive hyperfine transitions suitable as qubits and determining a differential Land\'e $g$ factor of $1.267(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ between opposite-parity components of the $\ensuremath{\Lambda}$ doublet. The data are successfully modeled with an effective hyperfine Zeeman Hamiltonian, which we use to make a tenfold improvement of the magnetically sensitive, astrophysically important $\ensuremath{\Delta}F=\ifmmode\pm\else\textpm\fi{}1$ satellite-line frequencies, yielding $1\phantom{\rule{0.2em}{0ex}}720\phantom{\rule{0.2em}{0ex}}529\phantom{\rule{0.2em}{0ex}}887(10)\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$ and $1\phantom{\rule{0.2em}{0ex}}612\phantom{\rule{0.2em}{0ex}}230\phantom{\rule{0.2em}{0ex}}825(15)\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$.

Implications for Cognitive Quantum Computation and Decoherence Limits in the Presence of Large Extra Dimensions

An interdisciplinary physical theory of emergent consciousness has previously been proposed, stemming from quantum computation-like behavior between 109 or more entangled molecular qubit states (microtubulin). This model relies on the Penrose-Diosi gravity-driven wavefunction collapse framework, and thus is subject to any secondary classical and quantum gravity effects. Specifically, if large extra spatial dimensions exist in the Universe, then the resulting corrections to Newtonian gravity cause this model to suffer serious difficulties. It is shown that if the extra dimensions are larger than 100 fm in size, then this model of consciousness is unphysical. If the dimensions are on the order of 10 fm in size, then a significantly smaller number of microtubulin than originally predicted are required to satisfy experimental constraints. Some speculation on evolution of consciousness is also offered, based on the possibility that the size of these extra dimensions may have been changing over the history of the Universe.

The role of phases and their interplay in molecular vibrational quantum computing with multiple qubits

Within the scope of molecular quantum computing with vibrational qubits, we analyse the impact of phases that are present during the quantum computation processes. While the phase relation in superposition states and its temporal evolution are crucial to any implementation of quantum computing, we elucidate the special challenge that emerges for phase control of qubits encoded in molecular vibrational eigenstates. Phase correctly prepared superposition states in general exist only for a finite time and with the inherent entanglement in molecular vibrational qubit systems their development displays a complex pattern. We show that the free relative phase evolution in such qubit systems can be utilized for the implementation of quantum phase gates. Moreover, a practical experimental realization of phase correct quantum gates acting on molecular vibrational qubits could be accomplished by a decomposition into laser-induced population transfer and free evolution phase gates. This concept adds to the flexibility in the implementation of quantum gate sequences and algorithms. A modification, where only a reduced number of selected relative phases needs to be adjusted, will make this scheme more robust and versatile. Finally, we also disclose and discuss another key feature for the implementation of phase correct quantum gates, i.e. the dependence of the quantum gate fidelity on the absolute or carrier-envelope phase of the driving femtosecond laserfield.

Manganese pentacarbonyl bromide as candidate for a molecular qubit system operated in the infrared regime

Our concept for a quantum computational system is based on qubits encoded in vibrational normal modes of polyatomic molecules. The quantum gates are implemented by shaped femtosecond laser pulses. We adopt this concept to the new species manganese pentacarbonyl bromide [MnBr(CO)5] and show that it is a promising candidate in the mid-infrared (IR) frequency range to connect theory and experiment. As direct reference for the ab initio calculations we evaluated experimentally the absorption bands of MnBr(CO)5 in the mid-IR as well as the related transition dipole moments. The two-dimensional potential-energy surface spanned by the two strongest IR active modes and the dipole vector surfaces are calculated with density-functional theory. The vibrational eigenstates representing the qubit system are determined. Laser pulses are optimized by multitarget optimal control theory to form a set of global quantum gates: NOT, CNOT, Pi, and Hadamard. For all of them simply structured pulses with low pulse energies around 1 microJ could be obtained. Exemplarily for the CNOT gate we investigated the possible transfer to experimental shaping, based on the mask function for pulse shaping in the frequency regime as well as decomposition into a train of subpulses.

Preparation and addressability of molecular vibrational qubit states in the presence of anharmonic resonance

Recently, our group has exemplified the possibility of quantum computing with molecular vibrations in a 2D model of acetylene. Specially shaped fs-laser pulses in the IR-regime represent the global quantum gates and can be acquired by optimal control theory. With regard to leakage from the qubit basis used for quantum computation, the influence of dark, non-IR-active modes coupled via anharmonic resonances is an important issue. In a 3D model we demonstrate the selective preparation of a vibrational eigenstate in near anharmonic resonance. Furthermore, a Hadamard gate is realized, the first step in the preparation of a Bell state, also demonstrated.