Classical and quantum physics on phase space: The state spaces

Eloi Descamps, Astghik Saharyan, Arne Keller +1 more

Phase-space representations can be used to reveal resources for quantum computational advantage. In odd-dimensional qudit systems, states and operations admitting non-negative quasi-probability representations are efficiently classically simulable, and analogous conclusions hold for bosonic systems, usually represented in quadrature phase space. However, when quantum information is encoded in bosonic systems, the link between physical phase-space representations and computational resources becomes nontrivial: large negativities may arise in the physical space even when the encoded model remains efficiently simulable. We establish a general framework connecting physical phase spaces of bosonic systems to the effective computational phase spaces associated with arbitrary encodings. We show that bosonic Hilbert spaces with finitely many modes can always be exactly encoded on a quantum computer, without truncation or approximations, and that for any encoding there exists an adapted physical phase-space representation in which all provably classically simulable states and protocols are non-negative. Conversely, negativity can always persist in physical representations for arbitrary encodings. Our results formalize the operational meaning of phase-space negativity in bosonic quantum information and its relation with computational resources.

Yiannis N. Kaznessis

Nicolaas P. Landsman

Alexey A. Kryukov

It has been recently shown that Newtonian dynamics is the Schrodinger dynamics of the system whose state is constrained to a submanifold in the space of states of the system. Thus defined, the submanifold can be identified with the classical phase space of the system. The classical space is then also embedded into the space of states in a physically meaningful way. The resulting unified geometric framework establishes a new connection between classical and quantum physics. The framework is rigid in the sense that the Schrodinger dynamics is a unique extension of the Newtonian one from the classical phase space submanifold to the space of states. Quantum observables in the framework are identified with vector fields on the space of states. The commutators of canonical conjugate observables are expressed through the curvature of the sphere of normalized states. The velocity and acceleration of a particle in Newtonian dynamics are components of the velocity of state under the corresponding Schrodinger evolution. The metric properties of the embedding of the classical space into the space of states result in a relationship between the normal distribution of the position of a particle and the Born rule for the probability of transition of quantum states. In this paper the implications of the obtained mathematical results to the process of measurement in quantum physics are investigated. It is argued that interaction with the environment constrains the state of a macroscopic body to the classical space. The notion of collapse of a quantum state is analyzed. The double-slit, EPR and Schrodinger cat type experiments are reviewed anew. It is shown that, despite reproducing the usual results of quantum theory, the framework is not simply a reformulation of the theory. New experiments to discover the predicted effects are proposed.

Manuel Gadella

Arnold Neumaier

Coherent quantum physics is quantum physics in terms of a coherent space and an appropriate coherent product. The kinematical structure and the meaning of the quantities are given by the symmetries (invertible coherent maps) of the coherent space. If Z is a coherent manifold, a canonical symplectic form is often canonically determined by the coherent product through the coherent action principle discussed in Section 6.1. In a quantum mechanical context, Z is indeed a classical phase space or an extended phase space-typically a symplectic manifold, a Poisson manifold, or a circle or line bundle over such a manifold that incorporates the classical action variable (encoding the Berry phase under quantization). For example, the Aharonov-Bohm effect [3] needs the bundle formulation. A coherent space and its quantum space provide a classical and a quantum view of the same physical system, discussed in Section 6.2. Section 6.3 shows that symmetries of coherent spaces can be quantized, that is, promoted to linear operators on the quantum space. This leads to quantum dynamics, which in special (completely integrable) situations can be solved in closed form in terms of classical motions on the underlying coherent space, if the latter has a compatible manifold structure. Close relations to concepts from geometric quantization and Kähler manifolds are pointed out in Section 6.4. In order that the quantum space of a coherent space Z can describe physics, one needs not only a distinguished class of coherent states but a more general concept of state. In Sections 6.5-6.6, we give a general abstract setting for states in Lie ∗-algebras, emphasize the essential mathematical features and the close analogy between classical and quantum physics. We illustrate these generalities with a number of examples in Section 6.7. The coherent action principle is put to use in Section 6.8 for numerical quantum physics and an application to quantum chaos. Spectral issues can, in favorable cases, be handled in terms of dynamical Lie algebras, discussed in Section 6.9.