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Journal of Mathematical Physics

Various pointer states approaches to polar modular values

Le Bin Ho, Nobuyuki Imoto

Published Apr 1, 2018• 4 citations
Publisher

Abstract

We theoretically analyze the polar decomposition for quantum modular values under various pointer states approaches. We consider both the finite-dimensional discrete pointer state and continuous pointer state cases. In contrast to that, a weak value of an observable is usually divided into its real and imaginary parts; here, we show that separation from the modulus and phase is necessary to a modular value. We show that the modulus of the modular value is related to the pointer post-selection conditional probability, and the phase of the modular value is connected to the summation of a geometric phase and an intrinsic phase. We also discuss a relationship between the modulus and phase, and therein, the derivative of the phase is related to the derivative of the logarithm of the modulus via a Berry-Simon-like connection which is in the form of a weak value. As a consequence, the modulus-phase relation allows us to obtain these polar components whenever the connection is specified. One of the possible applications of our results is to evaluate the weak value (the Berry-Simon-like connection) when the polar modular value is experimentally obtained.

DOI
10.1063/1.5000990

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We theoretically analyze the polar decomposition for quantum modular values under various pointer states approaches. We consider both the finite-dimensional discrete pointer state and continuous pointer state cases. In contrast to that, a weak value of an observable is usually divided into its real and imaginary parts; here, we show that separation from the modulus and phase is necessary to a modular value. We show that the modulus of the modular value is related to the pointer post-selection conditional probability, and the phase of the modular value is connected to the summation of a geometric phase and an intrinsic phase. We also discuss a relationship between the modulus and phase, and therein, the derivative of the phase is related to the derivative of the logarithm of the modulus via a Berry-Simon-like connection which is in the form of a weak value. As a consequence, the modulus-phase relation allows us to obtain these polar components whenever the connection is specified. One of the possible applications of our results is to evaluate the weak value (the Berry-Simon-like connection) when the polar modular value is experimentally obtained.
In contrast to that a weak value of an observable is usually divided into real and imaginary parts, here we show that separation into modulus and argument is important for modular values. We first show that modular values are expressed by the average of dynamic phase factors with complex conditional probabilities. We then relate, using the polar decomposition, the modulus of the modular value to the relative change in the qubit pointer post-selection probabilities, and relate the argument of the modular value to the summation of a geometric phase and an intrinsic phase.
We consider a generalized modular-value-based scheme based on the standard von Neumann measurement. We model the scheme as an interaction between a quantum system and a discrete quantum pointer where the pointer operator is a projection operator onto one of the states of the basis of the pointer Hilbert space. The interaction strength is made arbitrarily large. After post-selection onto the system, the results of the pointer measurement are the so-called conditional probabilities. We first explicitly derive the analytical expressions of the conditional probabilities, the expectation value, and the average displacement in the measured value of a pointer observable that we name as the pointer quantities. We also provide an expression for a generalized modular value and discuss the relationship between the generalized modular value and generalized weak values. The study then shows that the generalized modular value can characterize these pointer quantities. Then we give applications of our proposal to the cases of a spin-sss particle pointer and a semiclassical pointer state. One of the key results is that the amplification effect, similar to the weak-value case, is also observed in the case of the generalized modular value. Our study can also apply to the cases of nonclassical pointer states.
Abstract We express modular and weak values of observables of three- and higher-level quantum systems in their polar form. The Majorana representation of N -level systems in terms of symmetric states of N − 1 qubits provides us with a description on the Bloch sphere. With this geometric approach, we find that modular and weak values of observables of N -level quantum systems can be factored in N − 1 contributions. Their modulus is determined by the product of N − 1 ratios involving projection probabilities between qubits, while their argument is deduced from a sum of N − 1 solid angles on the Bloch sphere. These theoretical results allow us to study the geometric origin of the quantum phase discontinuity around singularities of weak values in three-level systems. We also analyze the three-box paradox (Aharonov and Vaidman 1991 J. Phys. A: Math. Gen . 24 2315–28) from the point of view of a bipartite quantum system. In the Majorana representation of this paradox, an observer comes to opposite conclusions about the entanglement state of the particles that were successfully pre- and postselected.
We present a procedure to completely determine the complex modular values of arbitrary observables of pre- and postselected ensembles, which works experimentally for all measurement strengths and all postselected states. This procedure allows us to discuss the physics of modular and weak values in interferometric experiments involving a qubit meter. We determine both the modulus and the argument of the modular value for any measurement strength in a single step, by simultaneously controlling the visibility and the phase in a quantum eraser interference experiment. Modular and weak values are closely related. Using entangled qubits for the probed and meter systems, we show that the phase of the modular and weak values has a topological origin. This phase is completely defined by the intrinsic physical properties of the probed system and its time evolution. The physical significance of this phase can thus be used to evaluate the quantumness of weak values.
We investigate general differential relations connecting the respective behaviors of the phase and modulus of probability amplitudes of the form 〈ψf|ψ〉, where |ψf〉 is a fixed state in Hilbert space and |ψ〉 is a variable state, treated as a section of a U(1) bundle over a complex subspace of the corresponding ray space R=CPn. Amplitude functions on such holomorphic line bundles, while not strictly holomorphic, nevertheless satisfy generalized Cauchy–Riemann conditions involving the U(1) Berry–Simon connection on the parameter space. These conditions entail invertible relations between the gradients of the phase and modulus, therefore allowing for the reconstruction of the phase from the modulus (or vice versa) and other conditions on the behavior of either polar component of the amplitude. As a special case, we consider amplitude functions valued on the space of pure states, the ray space R=CPn, where transition probabilities have a geometric interpretation in terms of geodesic distances as measured with the Fubini–Study metric. In conjunction with the generalized Cauchy–Riemann conditions, this geodesic interpretation leads to additional relations, in particular, a novel connection between the modulus of the amplitude and the phase gradient, somewhat reminiscent of the WKB formula. Finally, a connection with geometric phases is established.
Abstract Quantum measurements can be generalized to include complex quantities. It is possible to relate the quantum weak values of projection operators to the third order Bargmann invariants. The argument of the weak value becomes, up to a sign, equal to the Berry’s phase associated with the three state vectors. In case of symmetric informationally complete, positive operator valued measures, this relation takes a particularly simple form. Alternating strong and weak measurements can be used to determine Berry’s phase directly, which demonstrates that not only their real and imaginary parts but also moduli and arguments of weak values have a physical significance. For an arbitrary projection operator, weak value is real when the projector, pre- and post-selected states lie on a so-called null phase curve which includes the geodesic containing the three states as a special case.