Quantum speed limit (QSL) in a quantum system originates from the essential principle of the quantum mechanics. It gives a maximum speed of evolution or a minimum evolution time of the quantum system, which has potential applications in the fields of quantum information, quantum communication and quantum control and so on. In the last decades, the QSL bounds have been explored from the isolated quantum systems to the open quantum systems, several different geometric measures have been adopted to investigate the distinguishability between the initial and the evolved state. The QSL bounds in many systems have been discussed, indicating that the tightness of the QSL metric is related to the researched dynamical system. However, the QSL problem for the molecular system has rarely been reported. The study of the quantum speed limits in the different molecular systems is helpful for realizing the quantum information technology based on the molecules. In this paper, the generalized quantum speed limit metric for linear molecular dynamics is studied by the algebraic approach. The quantum Fisher information metric and the Wigner-Yanase information metric are both used to study the QSLs in the dynamical evolution of the two linear molecules. Here the dynamical evolutions begin with the two kinds of vibrational states, Fock initial state and coherent initial state. The results show that the quantum Fisher information metric is more appropriate than the Wigner-Yanase information metric for HCN and DCN molecules. The relative differences between the generalized geometric QSL and the two geodesic QSL metrics become bigger gradually with the increase of the initial vibrational quantum number. However, the relative difference for the DCN molecule is smaller than for the HCN molecule. The relative difference between the strong coherent states is smaller, which indicates that the generalized geometric quantum speed limit is suitable to describing high-coherent case. In conclusion, the different QSL metrics reveal the discrepancy in the evolution of the molecular system, and the relative difference is related to the initial state of the molecules and molecular parameters. More molecular systems need to be investigated in order to obtain the criteria between the QSL metrics and the molecular parameters.
Quantum speed limit (QSL) in a quantum system originates from the essential principle of the quantum mechanics. It gives a maximum speed of evolution or a minimum evolution time of the quantum system, which has potential applications in the fields of quantum information, quantum communication and quantum control and so on. In the last decades, the QSL bounds have been explored from the isolated quantum systems to the open quantum systems, several different geometric measures have been adopted to investigate the distinguishability between the initial and the evolved state. The QSL bounds in many systems have been discussed, indicating that the tightness of the QSL metric is related to the researched dynamical system. However, the QSL problem for the molecular system has rarely been reported. The study of the quantum speed limits in the different molecular systems is helpful for realizing the quantum information technology based on the molecules. In this paper, the generalized quantum speed limit metric for linear molecular dynamics is studied by the algebraic approach. The quantum Fisher information metric and the Wigner-Yanase information metric are both used to study the QSLs in the dynamical evolution of the two linear molecules. Here the dynamical evolutions begin with the two kinds of vibrational states, Fock initial state and coherent initial state. The results show that the quantum Fisher information metric is more appropriate than the Wigner-Yanase information metric for HCN and DCN molecules. The relative differences between the generalized geometric QSL and the two geodesic QSL metrics become bigger gradually with the increase of the initial vibrational quantum number. However, the relative difference for the DCN molecule is smaller than for the HCN molecule. The relative difference between the strong coherent states is smaller, which indicates that the generalized geometric quantum speed limit is suitable to describing high-coherent case. In conclusion, the different QSL metrics reveal the discrepancy in the evolution of the molecular system, and the relative difference is related to the initial state of the molecules and molecular parameters. More molecular systems need to be investigated in order to obtain the criteria between the QSL metrics and the molecular parameters.
As one of the fundamental questions, quantum speed limit (QSL) is of vital role in the community of quantum physics, which sets the ultimate, maximal speed with which a quantum system can evolve. QSL originated from the understanding of the Heisenberg uncertainty relation. In 1945, Mandelstam and Tamm found the time Δt in the celebrated relation ΔEΔt≥ℏ could be thought as the minimum time that a quantum system needs to evolve from an initial state to the orthogonal state for unitary evolution. QSL sets the lower bound of the evolution. They established the relationship between the minimum evolution time ( τQSL ) and the energy uncertainty of the system (ΔE), namely τQSL=πℏ/(2ΔE) , the Mandelstam-Tamm (MT) bound. Later, Margolus and Levitin derived another different bound for a closed system which connected the τQSL with the average energy of the system ( 〈E〉 ), i.e., τQSL=πℏ/(2〈E〉) , the Margolus-Levitin (ML) bound. Combining the MT and ML bounds together, we can have a unified bound for a unitary system and orthogonal states. In the case of open system dynamics, because there is energy and/or coherence exchange between the system and the environment, the orthogonal final state of the initial state can hardly be reached any longer. An investigation of the QSL on non-unitary quantum evolution needs to be considered. The generalized QSLs are established by employing the distance between the distinguishable states. Different metrics define different distances, leading to different QSL bounds. We show a few typical generalized QSL bounds based on different metrics. And the unified QSL bound for non-unitary evolution is obtained by combining the generalized MT and ML bounds under the same metric. The generations and developments of QSL have successfully generalized the MT bounds. Even the ML bounds have been attempted to generalize, the current studies of ML bound are thought as the significantly harder task, and keep as the open questions. The influences of external environment and quantum system dynamics which include the role of entanglement in QSL for open dynamics and many-body systems, the relation between the maximum interaction speed and QSL in quantum spin systems, the non-Markovianity effect of the environment on accelerating the speed of evolution have been investigated. Furthermore, by employing the gauge invariant distance in the Riemannian metric, we establish a distinct QSL bound, which relates the quantum speed limit time to the geometry phase, and presents the geometric and gauge invariant properties of it. The bound shows the impacts of the transmission modes of the state vectors on the evolution path in the manifold. Different from the established MT and ML bounds, our bound establishes the relation between the QSL and the Pancharatnam connection 1-form, and our results show that the QSL can be controlled via manipulating the generalized Pahcharatnam connection. Also, our results of QSL are holonomy via the generalized Pahcharatnam connection, which show us a possible technique to speed up the holonomic quantum computation. The bound is also interesting to the metrology and non-equilibrium thermodynamics, etc. QSL has also been studied in the quantum representation of Wigner function. In the future research of quantum speed limit, there are many issues that need to be considered, such as the generalized ML bound, the unified QSL bound under different metrics, the application of maximum rate of quantum information derived from the QSL, and the relation between the quantum speed limit and the quantum heat engine, etc.
Abstract The quantum speed limit (QSL) is a fundamental lower bound on the evolution time for quantum systems, and its tightness has been observed to be dependent on the properties of the physical process. However, experimental studies exploring the QSL in open quantum systems are still missing. Here, we studied geometric quantum speed limits of a qubit subject to decoherence in an ensemble of chloroform molecules in a Nuclear Magnetic Resonance experiment. We controlled the system-reservoir interaction and the spin relaxation rates by adding a paramagnetic salt, allowing the observation of both Markovian and non-Markovian open system dynamics for the qubit. We used two distinguishability measures of quantum states to assess the speed of the qubit evolution: the quantum Fisher information (QFI) and Wigner-Yanase skew information (WY). For non-Markovianity and low salt concentrations, we found crossovers between QSLs related to those metrics. The WY metric sets the tighter QSL for high concentrations and Markovian dynamics. We also show that QSLs are sensitive even to small fluctuations in spin magnetization.
According to the Heisenberg uncertainty principle between time and energy fluctuation, a concept of the quantum speed limit (QSL) has been established to determine the minimum evolutionary time between quantum states. Considerable theoretical and experimental efforts are invested in obtaining the QSL time bounds in various scenarios. However, it remains a long-standing goal to derive a meaningful QSL bound for a general quantum problem. Here, we propose a geometrical approach to derive a QSL bound for closed and open quantum systems. By solving a quantum brachistochrone problem in the framework of the Riemannian metric, we show that the QSL between a given initial state and a final state is determined not only by the entire dynamics of the system but also by the individual dynamics of a critical parameter. We exemplify the utility of the bound in three representative scenarios, demonstrating a pronounced advantage in finding a tight and meaningful QSL bound of a general quantum evolution problem.
The quantum speed limit (QSL) refers to the maximum speed of a quantum system to evolve from an initial state to its orthogonal states. The bound on the QSL for Hermitian systems, for example the Mandelstam-Tamm (MT) and Margolus-Levitin (ML) as well as Sun-Zheng(SZ) bound, was studied respectively from the perspectives of average value and variance of the system Hamiltonian as well as the geometry of the system. While the compactness of the MT-type, ML-type and SZ-type bounds has been examined well for Hermitian systems, a compact QSL for non-Hermitian systems has not been well studied. In this work, based on the biorthogonal basis theory we derive two distinct and tighter bounds on the QSL for non-Hermitian systems, which correspond to the MT and ML bounds for Hermitian systems. We show that the shortest evolution time corresponding to the two bounds of the non-Hermitian system can be attained by certain initial states, showing the compactness and tightness of our bounds. These initial states dubbed fastest initial states(FIS) are different from that in Hermitian systems. A bound close to QSL for non-FIS is presented and comparison of our bound with others in literature is performed. To illustrate our results, we present a minimal non-Hermitian system to show QSL, and the condition for the shortest evolution time is derived analytically using the present theory.
We develop an intuitive geometric picture of quantum states, define a particular state distance, and derive a quantum speed limit (QSL) for open systems. Our QSL is attainable because any initial state can be driven to a final state by the particular dynamics along the geodesic. We present the general condition for dynamics along the geodesic for our QSL. As evidence, we consider the generalized amplitude damping dynamics and the dephasing dynamics to demonstrate the attainability. In addition, we also compare our QSL with others by strict analytic processes as well as numerical illustrations, and show our QSL is tight in many cases. It indicates that our paper is significant in tightening the bound of evolution time.