Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDirect Measurement of the Quantum Density Matrix without Ancillary Pointers
87
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Direct measurement protocol allows reconstructing specific elements of the density matrix of a quantum state without using quantum state tomography. However, the direct measurement protocols to date are primarily based on weak or strong measurements with ancillary pointers, which interacts with the investigated system to extract information about the specified elements. Here we present a new direct measurement protocol based on phase-shifting technique which do not need ancillary pointers. In this protocol, at most six different projective measurements suffice to determine any specific element of an unknown quantum density matrix. A concrete quantum circuit to implement the phase-shifting measurement protocol for multi-qubit states is provided, where the circuit is composed of just single-qubit gates and two multi-qubit controlled-phase gates. This protocol is also extended to the continuous-variable cases for directly measuring the Wigner function. Furthermore, we show that the protocol has the advantage of reducing measurement and device complexity in the task of measuring the complete density matrix compared to quantum state tomography in some quantum experiments. Our method provides an efficient way to characterize arbitrary quantum systems, which may be used to predict various properties of a quantum system and find applications in quantum information processing.
rate research
Read More
One drawback of conventional quantum state tomography is that it does not readily provide access to single density matrix elements, since it requires a global reconstruction. Here we experimentally demonstrate a scheme that can be used to directly measure individual density matrix elements of general quantum states. The scheme relies on measuring a sequence of three observables, each complementary to the last. The first two measurements are made weak to minimize the disturbance they cause to the state, while the final measurement is strong. We perform this joint measurement on polarized photons in pure and mixed states to directly measure their density matrix. The weak measurements are achieved using two walk-off crystals, each inducing a polarization-dependent spatial shift that couples the spatial and polarization degree of freedom of the photons. This direct measurement method provides an operational meaning to the density matrix and promises to be especially useful for large dimensional states.
Entanglement and wave function description are two of the core concepts that make quantum mechanics such a unique theory. A method to directly measure the wave function, using Weak Values, was demonstrated by Lundeen et al., Nature textbf{474}, 188(2011). However, it is not applicable to a scenario of two disjoint systems, where nonlocal entanglement can be a crucial element since that requires obtaining the Weak Values of nonlocal observables. Here, for the first time, we propose a method to directly measure a nonlocal wave function of a bipartite system, using Modular Values. The method is experimentally implemented for a photon pair in a hyper-entangled state, i.e. entangled both in polarization and momentum degrees of freedom.
The time-symmetric formalism endows the weak measurement and its outcome, the weak value, many unique features. In particular, it allows a direct tomography of quantum states without resort to complicated reconstruction algorithms and provides an operational meaning to wave functions and density matrices. To date the direct tomography only takes the forward direction of the weak measurement. Here we propose the direct tomography of a measurement apparatus by combining the backward direction of weak measurement and retrodictive description of quantum measurement. As an experimental demonstration, the scheme is applied to the characterization of both projective measurements and general positive operator-valued measures with a photonic setup. Our work provides new insight on the symmetry between quantum states and measurements, as well as an efficient method to characterize a measurement apparatus.
Recently in 2018, Niu et al. proposed a measurement-device-independent quantum secure direct communication protocol using Einstein-Podolsky-Rosen pairs and generalized it to a quantum dialogue protocol (Niu et al., Science bulletin 63.20, 2018). By analyzing these protocols we find some security issues in both these protocols. In this work, we show that both the protocols are not secure against information leakage, and a third party can get half of the secret information without any active attack. We also propose suitable modifications of these protocols to improve the security.
Quantum secure direct communication (QSDC) is the technology to transmit secret information directly through a quantum channel without neither key nor ciphertext. It provides us with a secure communication structure that is fundamentally different from the one that we use today. In this Letter, we report the first measurement-device-independent(MDI) QSDC protocol with sequences of entangled photon pairs and single photons. It eliminates security loopholes associated with the measurement device. In addition, the MDI technique doubles the communication distance compared to those without using the technique. We also give a protocol with linear optical Bell-basis measurement, where only two of the four Bell-basis states could be measured. When the number of qubit in a sequence reduces to 1, the MDI-QSDC protocol reduces to a deterministic MDI quantum key distribution protocol, which is also presented in the Letter.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
