Short answer: A Fock state is a state of a resonator with an exact number of photons. Three experiments from the same line of work show how a superconducting phase qubit can prepare them in a microwave resonator: Fock states with up to 6 photons, Fock states with up to 15 photons and their decay, and arbitrary superpositions characterised by Wigner tomography. These are laboratory experiments.
Why are Fock states hard to make?
An author-hosted copy of the first paper explains the background. A two-level spin can be prepared in an arbitrary state using classical excitations, but classical excitations applied to an oscillator generate a coherent state, which the authors say is nearly indistinguishable from a classical state. Quantum behaviour in an oscillator is most obvious in Fock states, but such states are hard to create. The difficulty is overcome by placing a nonlinear quantum system between a classical source and the resonator, preparing a state in the nonlinear system and transferring it to the resonator, repeated several times. The venue of this author-hosted copy was not confirmed from the page.
What did the first experiment show?
The authors say they demonstrate the first controlled generation of multi-photon Fock states in a solid-state system. They use a superconducting phase qubit, a close approximation to a two-level spin system, coupled to a microwave resonator acting as the harmonic oscillator, to prepare and analyse pure Fock states with up to 6 photons. They contrast these with coherent states generated by classical pulses applied directly to the resonator. They state that analogous deterministic creation in electrodynamic resonators had previously been shown only for one or two photons.
What did the 2008 Physical Review Letters paper add?
The paper demonstrates controlled generation of Fock states with up to 15 photons in a microwave coplanar waveguide resonator coupled to a superconducting phase qubit. The authors then monitor the decay of the Fock states due to dissipation by varying the delay between preparation and a number-state analysis. They find that the decay dynamics can be described by a master equation in which the lifetime of the n-photon Fock state scales as 1/n, in agreement with theory. They also generate a coherent state, and show that it keeps a Poisson distribution as it decays, with an average photon number that decreases with the same characteristic time as the one-photon Fock state.
What did the arbitrary-state paper add?
A second author-hosted paper, also of unconfirmed venue, says superposed states are the basis of quantum computation and simulation, and that creating complex superpositions in harmonic systems such as trapped-ion motion, microwave resonators or optical cavities has been a challenge because classical control cannot produce them. The authors demonstrate preparation and measurement of arbitrary quantum states in an electromagnetic resonator, superposing states with different numbers of photons in a controlled and deterministic way. They use a superconducting phase qubit to phase-coherently pump photons into the resonator with an algorithm that generalises their earlier Fock-state method, and characterise the states completely with Wigner tomography, which they say is equivalent to measuring the full density matrix.
How to read this
The photon counts are the authors' own, and the 6 and 15 photon figures come from different papers and setups. The 1/n decay scaling is a statement from the 2008 abstract. This post is based on abstracts and introductions, and full texts were not checked. It makes no claim about products.
FAQ
What is a Fock state?
A resonator state with a definite number of photons, per the author-hosted text.
How many photons were prepared?
Up to 6 in the first paper and up to 15 in the 2008 letter.
How does a Fock state decay?
The authors report that the n-photon lifetime scales as 1/n.
What is Wigner tomography?
A method used to characterise the full quantum state of the resonator in the third paper.
All field notes