Ground-state cooling of quantum systems via a one-shot measurement (original) (raw)
Protective Measurements: Probing Single Quantum Systems
Current Science, 2015
Making measurements on single quantum systems is considered difficult, almost impossible if the state is a-priori unknown. Protective measurements suggest a possibility to measure single quantum systems and gain some new information in the process. Protective measurement is described, both in the original and generalized form. The degree to which the system and the apparatus remain entangled in a protective measurement, is assessed. Possible experimental tests of protective measurements are discussed.
Physical Review A, 2008
We introduce a method for analyzing ground state properties of quantum many body systems, based on the characterization of separability and entanglement by single subsystem unitary operations. We apply the method to the study of the ground state structure of several interacting spin-1/2 models, described by Hamiltonians with different degrees of symmetry. We show that the approach based on single qubit unitary operations allows to introduce "entanglement excitation energies", a set of observables that can characterize ground state properties, including the quantification of single-site entanglement and the determination of quantum critical points. The formalism allows to identify the existence and location of factorization points, and a purely quantum "transition of entanglement" that occurs at the approach of factorization. This kind of quantum transition is characterized by a diverging ratio of excitation energies associated to single-qubit unitary operations.
Theoretical Setting of Inner Reversible Quantum Measurements
Modern Physics Letters A, 2006
We show that any unitary transformation performed on the quantum state of a closed quantum system describes an inner, reversible, generalized quantum measurement. We also show that under some specific conditions it is possible to perform a unitary transformation on the state of the closed quantum system by means of a collection of generalized measurement operators. In particular, given a complete set of orthogonal projectors, it is possible to implement a reversible quantum measurement that preserves the probabilities. In this context, we introduce the concept of…
The quantum measurement process: an exactly solvable model
arXiv preprint cond-mat/0309188, 2003
An exactly solvable model for a quantum measurement is discussed, that integrates quantum measurements with classical measurements. The z-component of a spin-1 2 test spin is measured with an apparatus, that itself consists of magnet of N spin-1 2 particles, coupled to a bath. The initial state of the magnet is a metastable paramagnet, while the bath starts in a thermal, gibbsian state. Conditions are such that the act of measurement drives the magnet in the up or down ferromagnetic state, according to the sign of sz of the test spin. The quantum measurement goes in two ...
Protecting entanglement from decoherence using weak measurement and quantum measurement reversal
Nature Physics, 2011
Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence 1 . For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death 2,3 . Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing 4 , quantum cryptography 5 , quantum teleportation 6-8 and quantum metrology 9 . Here, we propose and demonstrate a scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death.
Measuring complete quantum states with a single observable
Experimental determination of an unknown quantum state usually requires several incompatible measurements. However, it is also possible to determine the full quantum state from a single, repeated measurement. For this purpose, the quantum system whose state is to be determined is first coupled to a second quantum system (the "assistant") in such a way that part of the information in the quantum state is transferred to the assistant. The actual measurement is then performed on the enlarged system including the original system and the assistant. We discuss in detail the requirements of this procedure and experimentally implement it on a simple quantum system consisting of nuclear spins.
Quantum thermometry by single-qubit dephasing
The European Physical Journal Plus, 2019
We address the dephasing dynamics of a qubit as an effective process to estimate the temperature of its environment. Our scheme is inherently quantum, since it exploits the sensitivity of the qubit to decoherence, and does not require thermalization with the system under investigation. We optimize the quantum Fisher information with respect to the interaction time and the temperature in the case of Ohmic-like environments. We also find explicitly the qubit measurement achieving the quantum Cramér-Rao bound to precision. Our results show that the conditions for optimal estimation originate from a non-trivial interplay between the dephasing dynamics and the Ohmic structure of the environment. In general, optimal estimation is achieved neither when the qubit approaches the stationary state, nor for full dephasing.
All quantum measurements can be simulated using projective measurements and postselection
2018
Implementation of generalized quantum measurements is often experimentally demanding, as it requires performing a projective measurement on a system of interest extended by the ancilla. We report an alternative scheme for implementing generalized measurements that uses solely: (a) classical randomness and post-processing, (b) projective measurements on a relevant quantum system and (c) postselection on non-observing certain outcomes. The method implements arbitrary quantum measurement in d dimensional system with success probability 1/d. It is optimal in the sense that for any dimensionn d there exist measurements for which the success probability cannot be higher. We apply our results to bound the relative power of projective and generalised measurements for unambiguous state discrimination. Finally, we test our scheme experimentally on IBM quantum processor. Interestingly, due to noise involved in the implementation of entangling gates, the quality with which our scheme implements...
Unifying quantum computation with projective measurements only and one-way quantum computation
Quantum Informatics 2004, 2005
Quantum measurement is universal for quantum computation (Nielsen [4], Raussendorf [7,). Two models for performing measurement-based quantum computation exist: the one-way quantum computer was introduced by Briegel and Raussendorf , and quantum computation via projective measurements only by Nielsen . The more recent development of this second model is based on state transfers [6] instead of teleportation. From this development, a finite but approximate quantum universal family of observables is exhibited, which includes only one two-qubit observable, while others are one-qubit observables . In this article, an infinite but exact quantum universal family of observables is proposed, including also only one two-qubit observable. The rest of the paper is dedicated to compare these two models of measurement-based quantum computation, i.e. one-way quantum computation and quantum computation via projective measurements only. From this comparison, which was initiated by Cirac and Verstraete [9], closer and more natural connections appear between these two models. These close connections lead to a unified view of measurement-based quantum computation.