Physical Observables

Observables are physical quantities that can be measured on ensembles. We distinguish between primary observables and derived quantities, using language from Ref. [16]. A primary observable can be measured directly on a single configuration. A derived quantity is a generally nonlinear function of primary observables which can only be estimated using a whole ensemble.

For example, while the baryon number is a primary observable, the two-point fluctuations

\[\left\langle \delta n*\delta n \right\rangle = \left\langle n*n \right\rangle - \left\langle n \right\rangle * \left\langle n \right\rangle\]

is not because it requires combining different expectation values.

Primary observables are descriptors of the GrandCanonical ensemble (and therefore also of the Canonical ensemble). Derived quantities, because they can only be estimated from expectation values, are descriptors of the Bootstrap. You can add custom observables without modifying the internals of these classes.

We adopt a convention where local densities start with a lowercase letter and global quantities with an Uppercase.

• The Propagator
  • G()
  • G_momentum()
• The Action
  • Action
• Fermi Momentum and Friends
  • Fermi Momentum
  • Binding Energy
  • Temperature
  • α
• Number
  • Baryon Number
  • Bosonic Estimators
  • Density-density two-point functions
• Spin
  • Local and Global Spin
  • Spin-spin two-point functions
• Momentum Occupancy
  • n_momentum()
  • spin_momentum()
• Energies
  • Kinetic Energy
  • Potential Energy
  • Internal Energy
• The Contact
  • Double Occupancy
  • Contact
• Currents
  • Baryon number current
• Vorticity
  • Baryon Number Vorticity
• Pairing
  • Singlet Pairing
  • Triplet Pairing
  • Pairing Matrices
• Adding Custom Observables