Measurement Of Quantum States Research Articles

Properties of excited state of polaron in quantum disk in finite depth parabolic potential well

Studies of single quantum state measurement and the relevant physics are very important for the fields of quantum information and quantum coupution. In recent years, quantum dots as information carrier have become a hotpoint of research. The study on quantum dot properties has atracted a lot of attetion and made a series of progress.#br#In this paper, we formulate a theoretical method that can be used to investigate polaron properties in low-dimensional structures in finite depth potential well. We assume that an electron in a quantum disk which is in other medium is in parabolic potential field, but the effect of the medium on the electron in quantum disk is equivalent to a potential barrier with height V1 and width d. By expanding the finite height potential barrier as plane waves and Lee-Low-Pines unitary transformation for Hamiltonian, as well as variation for expectation value of Hamiltonian where trial wave functions are obtained by solving the energy eigen-value equation, the ground state energy, the first excited state energy, and excitation energy of polaron are drived.#br#Numerical calculation by using polaron unit, numerical results indicate that the first excited state energy and excitation energy of polaron increase with increasing the width or height of the potential barrier, because the probability of electron penetrating potential barrier will decrease as the width or height of potential barrier increases, so that electronic energy, the first excited state energy and excitation energy of polaron all increase. Numerical results also show that energies mentioned earlier decrease with increasing radius of quantum disk, which illustrates that the quantum disk has obvious quantum size effect.#br#It is also found from numerical results that the first excited state energy of polaron decreases with increasing effective confine length, it falls quickly when effective confine length is less than 0.3 and is a little change when effective confine length is more than 0.3. The longer the effective confine length, the more weakly the electron is bounded and the smaller the potential energy is, so that the first excited state energy of polaron decreases. Oppositely, the excitation energy of polaron increases with increasing effective confine length, because the first excited state energy decreases more slowly than the ground state energy.

Exploring entanglement with the help of quantum state measurement

We have performed a series of experiments using a spontaneous parametric down-conversion source to produce pairs of photons in either entangled or non-entangled polarization states. We determine the full quantum mechanical polarization state of one photon, conditioned on the results of measurements performed on the other photon. For non-entangled states, we find that the measured state of one photon is independent of measurements performed on the other. However, for entangled states, the measured state does depend on the results of measurements performed on the other photon. This is possible because of the nonlocal nature of entangled states. These experiments are suitable for an undergraduate teaching laboratory.

Photon-number-resolving detectors for quantum-state engineering: Introduction to the feature issue

Generation and measurement of quantum states of light represent a hot and widely discussed topic in the physical community for their possible applications to quantum optics and quantum information. To this aim, it is desirable that optical states are robust with respect to losses and can be produced and addressed at high rates. In fact, mesoscopic pulsed optical states containing few photons per pulse seem to be the ideal candidates for applications to quantum communication protocols. For all these reasons, during the last two decades many efforts have been devoted to the development of new kinds of detectors that can operate in such a photon-number-resolving domain.

Necessary and sufficient optimality conditions for classical simulations of quantum communication processes

We consider the process consisting of preparation, transmission through a quantum channel, and subsequent measurement of quantum states. The communication complexity of the channel is the minimal amount of classical communication required for classically simulating it. Recently, we reduced the computation of this quantity to a convex minimization problem with linear constraints. Every solution of the constraints provides an upper bound on the communication complexity. In this paper, we derive the dual maximization problem of the original one. The feasible points of the dual constraints, which are inequalities, give lower bounds on the communication complexity, as illustrated with an example. The optimal values of the two problems turn out to be equal (zero duality gap). By this property, we provide necessary and sufficient conditions for optimality in terms of a set of equalities and inequalities. We use these conditions and two reasonable but unproven hypotheses to derive the lower bound $n 2^{n-1}$ for a noiseless quantum channel with capacity equal to $n$ qubits. This lower bound can have interesting consequences in the context of the recent debate on the reality of the quantum state.

Hybrid Quantum-Classical Protocol for Storage and Retrieval of Discrete-Valued Information

In this paper we present a hybrid (i.e., quantum-classical) adaptive protocol for the storage and retrieval of discrete-valued information. The purpose of this paper is to introduce a procedure that exhibits how to store and retrieve unanticipated information values by using a quantum property, that of using different vector space bases for preparation and measurement of quantum states. This simple idea leads to an interesting old wish in Artificial Intelligence: the development of computer systems that can incorporate new knowledge on a real-time basis just by hardware manipulation.

Fault-tolerant quantum cryptographic protocols with collective detection over the collective amplitude damping channel

In this paper, a quantum key distribution (QKD) protocol, which can be immune to collective amplitude damping noise, is proposed with collective detection strategy. Then a multi-party quantum secret sharing (MQSS) protocol and a quantum private comparison (QPC) protocol are introduced as two applications of the proposed QKD protocol. Except for one participant who is responsible for preparing and measuring quantum states, the rest of the users in each of these protocols only need to perform certain unitary operations due to the utilization of collective detection. Therefore, in addition to the advantage of being secure against collective amplitude damping noise, the proposed protocols still have the advantages of higher qubit efficiency and lower cost for implementation. Moreover, the security of these protocols is guaranteed by theorems on quantum operation discrimination.

Gravity resonance spectroscopy constrains dark energy and dark matter scenarios.

We report on precision resonance spectroscopy measurements of quantum states of ultracold neutrons confined above the surface of a horizontal mirror by the gravity potential of Earth. Resonant transitions between several of the lowest quantum states are observed for the first time. These measurements demonstrate that Newton's inverse square law of gravity is understood at micron distances on an energy scale of 10-14 eV. At this level of precision, we are able to provide constraints on any possible gravitylike interaction. In particular, a dark energy chameleon field is excluded for values of the coupling constant β>5.8×108 at 95% confidence level (C.L.), and an attractive (repulsive) dark matter axionlike spin-mass coupling is excluded for the coupling strength gsgp>3.7×10-16 (5.3×10-16) at a Yukawa length of λ=20 μm (95%C.L.).

An indirect measurement process parameterized by a nonunitary system–pointer interaction

An indirect quantum state measurement process is under investigation. Common evolution of the detector's microscopic part (pointer) and the measured system (target) is considered to be nonunitary due to the interaction between the pointer and the environment. The system of differential equations for superoperators, which map the initial target state into its conditional final state, is obtained. A developed formalism is applied to the problem of cavity quantum mode photodetection to investigate the basic information characteristics of this measurement process.

Gallium arsenide (GaAs) quantum photonic waveguide circuits

Integrated quantum photonics is a promising approach for future practical and large-scale quantum information processing technologies, with the prospect of on-chip generation, manipulation and measurement of complex quantum states of light. The gallium arsenide (GaAs) material system is a promising technology platform, and has already successfully demonstrated key components including waveguide integrated single-photon sources and integrated single-photon detectors. However, quantum circuits capable of manipulating quantum states of light have so far not been investigated in this material system. Here, we report GaAs photonic circuits for the manipulation of single-photon and two-photon states. Two-photon quantum interference with a visibility of 94.9±1.3% was observed in GaAs directional couplers. Classical and quantum interference fringes with visibilities of 98.6±1.3% and 84.4±1.5% respectively were demonstrated in Mach–Zehnder interferometers exploiting the electro-optic Pockels effect. This work paves the way for a fully integrated quantum technology platform based on the GaAs material system.

Preparation and Measurement of Quantum State between Two-Dimensional Dots in Quantum Network

Currently, researches on the preparation and measurement of quantum state between two-dimensional dots in quantum network are mainly dot-to-dot or dot-to-multi-dot transmission schemes. The existing experimental and theoretical work is relatively scattered, and the overall knowledge is incomplete. The article proposes the preparation of multiple quantum entanglement state, the measurement and classification of multi-state and multi-body quantum entanglement, the design of the preparation and measurement scheme of quantum state between any two one-dimensional dots and proposes the transmission scheme of quantum state between two-dimensional dots in quantum network. Theoretical guidance is provided for relevant experiments through deep study on the problem.

Quantum state measurement in double quantum dots with a radio-frequency quantum point contact

We study the dynamics of two electron spins in coupled quantum dots (CQDs) monitored by a quantum point contact (QPC) detector. Their quantum state can be measured by embedding the QPC in an LC circuit. We derive the Bloch-type rate equations of the reduced density matrix for CQDs. Special attention is paid to the numerical results for the weak measurement condintion under a strong Coulomb interaction. It is shown that the evolution of QPC current always follows that of electron occupation in the right dot. In addition, we find that the output voltage of the circuit can reflect the evolution of QPC current when the circuit and QPC are approximately equal in frequency. In particular, the wave shape of the output voltage can be improved by adjusting the circuit resonance frequency and bandwidth.

High-Capacity Quantum Summation with Single Photons in Both Polarization and Spatial-Mode Degrees of Freedom

In this paper, we employ single photons in both polarization and spatial-mode degrees of freedom to design a quantum summation protocol. We assume that the third party, i.e. TP, is semi-honest in our protocol. That TP is semi-honest means TP executes the protocol loyally, keeps a record of all its intermediate computations and might try to steal the participants’ private inputs from the record, but he cannot be corrupted by the adversary. Participants can independently encode their private inputs on the polarization states and the spatial-mode states of single photons. Thus our protocol doubles the capacity of quantum communication compared with those based on single photons with only one degree of freedom. In addition, our protocol is feasible as the preparation and the measurement of single-photon quantum states in both the polarization and the spatial-mode degrees of freedom are available with current quantum techniques. We also analyze its security in this paper.

Optimal probabilistic measurement of phase

When measuring the phase of quantum states of light, the optimal single-shot measurement implements a projection on the unphysical phase states. If we want to improve the precision further we need to accept a reduced probability of success, either by implementing a probabilistic measurement or by probabilistically manipulating the measured quantum state by means of noiseless amplification. We analyze the limits of this approach by finding the optimal probabilistic measurement that, for a given rate of success, maximizes the precision with which the phase can be measured.

Dynamic Distance Measure on Spaces of Isospectral Mixed Quantum States

Distance measures are used to quantify the extent to which information is preserved or altered by quantum processes, and thus are indispensable tools in quantum information and quantum computing. In this paper we propose a new distance measure for mixed quantum states, which we call the dynamic distance measure, and we show that it is a proper distance measure. The dynamic distance measure is defined in terms of a measurable quantity, which makes it suitable for applications. In a final section we compare the dynamic distance measure with the well-known Bures distance measure.

Entanglement and Complexity of Measurement

Notion about complexity of composite systems is applied to the problem of measurement of quantum states. An expression for the determination of the value of complexity of measurement is given. Its practical meaning is illustrated in the example how this value differs when measuring complexity of authorized and non-authorized receiving of information transferred by quantum channels in protocol of direct coding.Unconditional security of information in direct coding protocol with information masking by uniformly distributed state of carrier is shown.

Experimental analysis of the measurement strength dependence of superconducting qubit readout using a Josephson bifurcation readout method

We succeeded in controlling the measurement strength of a macroscopic quantum system, namely a persistent current quantum bit (qubit), by using a transmission line type Josephson bifurcation amplifier (JBA: an ac-driven superconducting quantum interferometer device) as a probe. By employing a special pulse sequence, we found that the weighted average of the Ramsey fringe visibility (α-value) can be used as an indicator of quantum state projection. The sudden change in the α-value magnitude at around the threshold suggests that an entangled state of the qubit–JBA composite system settles in one of the two possible classically correlated qubit–JBA states caused by strong decoherence. This is an important result in terms of understanding the mechanisms of quantum state measurement.

Analysis of a photon number resolving detector based on fluorescence readout of an ion Coulomb crystal quantum memory inside an optical cavity

The ability to detect single photons with a high efficiency is a crucial requirement for various quantum information applications. By combining the storage process of a quantum memory for photons with fluorescence-based quantum state measurement, it is, in principle, possible to achieve high-efficiency photon counting in large ensembles of atoms. The large number of atoms can, however, pose significant problems in terms of noise stemming from imperfect initial state preparation and off-resonant fluorescence. We identify and analyse a concrete implementation of a photon number resolving detector based on an ion Coulomb crystal inside a moderately high-finesse optical cavity. The cavity enhancement leads to an effective optical depth of 15 for a finesse of 3000 with only about 1500 ions interacting with the light field. We show that these values allow for essentially noiseless detection with an efficiency larger than 93%. Moderate experimental parameters allow for repetition rates of about 3 kHz, limited by the time needed for fluorescence collection and re-cooling of the ions between trials. Our analysis may lead to the first implementation of a photon number resolving detector in atomic ensembles.

An arbitrated quantum signature scheme based on entanglement swapping with signer anonymity

In this paper an arbitrated quantum signature scheme based on entanglement swapping is proposed. In this scheme a message to be signed is coded with unitary operators. Combining quantum measurement with quantum encryption, the signer can generate the signature for a given message. Combining the entangled states generated by the TTP's Bell measurement with the signature information, the verifier can verify the authentication of a signature through a single quantum state measurement. Compared with previous schemes, our scheme is more efficient and less complex, furthermore, our scheme can ensure the anonymity of the signer.

Experimental realization of an approximate transpose operation for qutrit systems using a structural physical approximation

Although important for detecting entanglement, the transpose operation cannot be directly realized in laboratory because it is a nonphysical operation. It is, however, possible to find an approximate transpose operation using the method known as the structural physical approximation (SPA); recently, SPA-based implementations of the transpose and partial transpose have been demonstrated for a single-qubit [Phys. Rev. A 83, 020301(R) (2011)] and an entangled two-qubit system [Phys. Rev. Lett. 107, 160401 (2011)]. In this work, we expand SPA-transpose to a three-dimensional quantum system: a qutrit. The photonic qutrit state is encoded in the polarization, and path degrees of freedom of a single-photon and the SPA-transpose operation, which is based on measurement and preparation of quantum states, is implemented with linear optics. Our work paves the way toward entanglement detection for higher-dimensional quantum systems.

Generalized quantum tomographic maps

Some nonlinear generalizations of classical Radon tomography were introduced by Asorey et al (2008 Phys. Rev. A 77 042115), where the straight lines of the standard Radon map are replaced by quadratic curves (ellipses, hyperbolas and circles) or quadratic surfaces (ellipsoids, hyperboloids and spheres). We consider here the quantum version of this novel nonlinear approach and obtain, by the systematic use of the Weyl map, a tomographic encoding approach to quantum states. Nonlinear quantum tomograms admit a simple formulation within the framework of the star-product quantization scheme and the reconstruction formulae of the density operators are explicitly given in a closed form, with an explicit construction of quantizers and dequantizers. The role of symmetry groups behind the generalized tomographic maps is analyzed in some detail. We also introduce new generalizations of the standard singular dequantizers of the symplectic tomographic schemes, where the Dirac delta-distributions of operator-valued arguments are replaced by smooth window functions, giving rise to the new concept of thick quantum tomography. Applications in quantum state measurements of photons and matter waves are discussed.