Forthcoming quantum devices will require high-fidelity information transfer across a many-body system. We formulate the criterion for lossless signal propagation and show that a single qubit can play the role of an antenna, collecting large amounts of information from a complex system. We derive the condition under which the antenna, far from the source and embedded in a many-body interacting medium, can still collect the complete information. A striking feature of this setup is that a single-qubit antenna can receive even the full signal amplified by the entanglement of the source. As a consequence, the recovery of this information can be performed with simple single-qubit operations on the antenna (which we fully characterize) rather than with multiqubit measurements of the source. Finally, we discuss the control of the system parameters necessary for lossless signal propagation. A method discussed here could improve the precision of quantum devices and simplify metrological protocols.
Forthcoming quantum devices will require high-fidelity information transfer across a many-body system. We formulate the criterion for lossless signal propagation and show that a single qubit can play the role of an antenna, collecting large amounts of information from a complex system. We derive the condition under which the antenna, far from the source and embedded in a many-body interacting medium, can still collect the complete information. A striking feature of this setup is that a single-qubit antenna can receive even the full signal amplified by the entanglement of the source. As a consequence, the recovery of this information can be performed with simple single-qubit operations on the antenna (which we fully characterize) rather than with multiqubit measurements of the source. Finally, we discuss the control of the system parameters necessary for lossless signal propagation. A method discussed here could improve the precision of quantum devices and simplify metrological protocols.
The Lieb-Robinson bound (LRB) states that the range and strength of interactions between the constituents of a complex many-body system impose upper limits to how fast the signal can propagate. It manifests in a light conelike growth of correlation function connecting two distant subsystems. Here, we employ the techniques of quantum information to demonstrate that the LRB can be determined from local measurements performed on a single qubit that is connected to a many-body system. This formulation provides an operational recipe for estimating the LRB in complex systems, replacing the measurement of the correlation function with simple single-particle manipulations. We demonstrate the potency of this approach by deriving the upper limit to the speed of signal propagation in the XY spin chain.
Abstract The transfer of quantum information between different locations is key to many quantum information processing tasks. Whereas, the transfer of a single qubit state has been extensively investigated, the transfer of a many-body system configuration has insofar remained elusive. We address the problem of transferring the state of n interacting qubits. Both the exponentially increasing Hilbert space dimension, and the presence of interactions significantly scale-up the complexity of achieving high-fidelity transfer. By employing tools from random matrix theory and using the formalism of quantum dynamical maps, we derive a general expression for the average and the variance of the fidelity of an arbitrary quantum state transfer protocol for n interacting qubits. Finally, by adopting a weak-coupling scheme in a spin chain, we obtain the explicit conditions for high-fidelity transfer of three and four interacting qubits.
Fundamental scientific advances are often facilitated by breakthroughs in calibration and readout techniques that increase the measurement precision of sensors. The betterment of sensing technologies enables the validation of existing scientific theories and congruently, may also pave the way for identifying breakdowns in these frameworks, signalling the necessity for new theoretical developments. A significant facet of the growing interest in quantum metrology is that it represents a prime example in which quantum features such as entanglement, squeezing, and criticality, can be harnessed to enhance the precision of parameter estimation well beyond the capacity of their classical counterparts. We begin by exploring the paradigmatic problem of temperature estimation using impurity probes embedded in a host environment. The physical setup considered provides a natural mechanism for the probe-environment unit to equilibrate and thermalize. Furthermore the nature of probe-environment interactions allows us to investigate the role that entanglement plays in equilibrium temperature estimation problems. Our analysis reveal that spectral features of the environment are imprinted on the probe system due to established entanglement and uncover a low temperature regime with a universal thermometric response that is independent of the microscopic details of the environment. Physical incarnations of nanoelectronic devices are operated in the linear-response regime with transport properties representing the measurement of choice. Motivated by this, we next propose nanoelectronic devices operated in this way as a novel paradigm for quantum sensing. We derive general expressions for the quantum Fisher information --- allowing us to provide a precision benchmark for quantum transport coefficients in the linear-response regime. The latter half of this thesis is concerned with aspects of quantum critical metrology and multiparameter estimation. Our first contribution is to propose an alternative platform for critical quantum metrology, the two-impurity Kondo model --- which does not rely on the preparation of exotic critical states, but instead can be passively tuned to criticality. Employing local measurements of the in situ probe at finite temperature, we find enhanced precision at criticality in estimating inter-impurity coupling, but low precision when utilized as a thermometer. Considering uncertainty in both of these parameters, we find the quantum Fisher information matrix to be singular, despite these parameters being physically independent. We demonstrate that application of a known control field can lift the singularity. In practical settings, an external parameter is tuned to bring a system to its quantum critical point. Therefore, we expect that experimental uncertainties in this control parameter to play a significant role in fully exploiting criticality as a resource for sensing. Motivated by this, we next propose a general framework interpolating between the single and multi-parameter settings. Applied to the Ising and Lipkin-Meshkov-Glick models, we establish a fundamental trade-off between the amount of uncertainty a critical system can withstand while still maintaining a quantum advantage for sensing. In the final chapter of the thesis, we investigate the physical origin of quantum fisher information matrix singularities. A singularity signals a fundamental breakdown in our ability to estimate parameters and bound their precision. We attribute these situations to emergent metrological symmetries whereby the same set of measurement outcomes are obtained for different combinations of the system parameters. We demonstrate that by using Bayesian estimation, deep insights can be revealed and estimation of effective parameters is possible by employing a parameter transformation of the Bayesian posterior distribution.
Most quantum communication tasks need to rely on the transmission of quantum signals over long distances. Unfortunately, transmission of such signals is most often limited by losses in the channel, the same issue that affects classical communication. Simple signal amplification provides an elegant solution for the classical world, but this is not possible in the quantum world, as the no-cloning theorem forbids such an operation and, thus, an alternative approach, a quantum repeater, is needed. Quantum repeaters enable one to create a known maximally entangled state between the end points of the network by first segmenting the network into pieces, creating entanglement between the segments, and then, connecting those entanglement to create the required long range entanglement. Quantum teleportation then allows an unknown quantum message to be transmitted between them using the long-range entangled state. This form of quantum communication will be at the heart of the future quantum Internet. In this review, we will detail various approaches to quantum repeaters, and discuss their expected performance and limitations.
Quantum-enhanced metrology relies on entanglement to achieve sensitivities beyond the standard quantum limit. While remarkable progress has been made in generating highly entangled many-body states, extracting their metrological advantage remains a central challenge because the encoded information is often inaccessible to realistic measurements. A key development of the past decade has been the realization that many-body interactions can play a dual role: they can be used not only to generate entanglement, but also to decode it. This idea underlies interaction-based readout and time-reversal protocols, in which controlled non-linear dynamics transform weakly encoded signals into experimentally accessible observables. Cavity quantum electrodynamics (QED) provides a particularly powerful setting for these approaches because it combines collective enhancement, tunable interactions, and controllable reversibility within a single platform. In this review, we discuss the emergence of time-reversal protocols in cavity QED, from their conceptual roots in Loschmidt echoes to modern implementations of signal amplification through a time-reversed interaction (SATIN), scrambling-enhanced metrology, and more general interaction-based readout schemes. We examine the physical mechanisms that enable reversible many-body dynamics, review key experimental demonstrations, and discuss future directions involving complex entangled states, nonlinear decoding, and emerging quantum platforms. Together, these developments suggest that the ability to decode quantum information may become as important as the ability to generate it, establishing reversible many-body dynamics as a central resource for quantum-enhanced sensing.
The development of quantum information in the last decades has opened new horizons in science and technology.It turns out that quantum physics, apart from playing a fundamental role in the structure of matter, also opens new opportunities for computation and communication, if one can harness the quantum superposition and entanglement.This started a new scientific challenge to find the platform that can offer two well isolated discrete levels (a qubit), that are a standard building block for many quantum devices, like bits are those for a classical computer.In this thesis we focus on one such platform, the negatively charged nitrogen-vacancy (NV -) center in diamond.It uses the triplet states of two fermionic spin-1/2 particles (holes) to encode the information.In order to build a working device, be it a quantum computer or a communication hardware, one needs to have a very good control over the quantum system.Usually, the NV -centers are controlled with the microwave magnetic field of an antenna, that is placed in the vicinity of the NV -.One of the issues we address in this thesis is whether we can replace the magnetic field with electric field or strain to control the system.We derive the corresponding coupling Hamiltonian and discuss the schemes to measure its parameters.Another approach we are investigating to simplify the control of an NV -based device is replacing the antenna magnetic field with spin-waves, that couple to the NV -centers.We show that for a magnetic material with low damping, the spin waves can propagate over very long distances with no dissipation and can provide the control of NV -centers even if the microwave antenna is far away and can not reach the center.We develop the theoretical formalism that allows to calculate the coupling enhancement, offered with this approach with respect to a pure antenna driving.Another essential ingredient of a quantum device is the ability to generate state superpositions and entanglement between different qubits.This is done by means of quantum gates, that must be performed with extremely high fidelity in order for the system to work.We investigate the possibility to perform geometric quantum gates, that are tolerant to certain types of errors, on the spin of NV -centers in diamond.We are interested in both adiabatic and non-adiabatic geometric quantum computation.Inspired by a recent observation of Berry phase with NV -centers, we develop an effective Hamiltonian description of adiabatic geometric gates, alternative to conventional Berry connection approach.For non-adiabatic geometric gates, we analyze the fidelities and error channels, when the gates are performed optically through the orbital excited state of the NV -centers.We also develop a scheme to apply all-microwave universal non-adiabatic geometric gates on small registers of coupled NV -and nuclear spins.Finally, we develop an entanglement protocol, that generates a universal two-qubit gate on two NV -centers, coupled to a common mode of a high Q cavity.We show that it is possible to implement this gate fast and at a fidelity, that is high enough to apply error-correction protocols.