Photon blockade is a quantum optical phenomenon in which the presence of a single photon in a nonlinear optical system suppresses the absorption or transmission of additional photons, resulting in strongly nonclassical light with photon antibunching. The effect arises when light–matter interactions introduce sufficient anharmonicity into the excitation spectrum of an optical mode, preventing resonant multi-photon excitation. Photon blockade is commonly regarded as the photonic analogue of Coulomb blockade in mesoscopic electronic systems.

Photon blockade is central to quantum nonlinear optics and underpins applications such as single-photon sources, quantum optical switches, and single-photon transistors.

The concept of photon blockade was introduced by [Imamoğlu] et al. in (1997), who proposed that strong effective photon–photon interactions could be engineered in cavity quantum electrodynamics (QED) systems, leading to suppression of multi-photon transmission in direct analogy with Coulomb blockade in electronic transport.^[1]

Subsequent theoretical work explored photon statistics and blockade mechanisms in realistic systems. Grangier, Dan Walls, and Gheri (1998) analysed nonclassical photon statistics in nonlinear optical systems, identifying regimes closely related to blockade behaviour.^[2] Rebić, Tan, Parkins, and Walls (1999) investigated photon statistics in cavity QED with multilevel atoms, highlighting the role of atom–field correlations and quantum interference in suppressing multi-photon excitation.^[3] Werner and Imamoğlu (2000) further clarified the connection between spectral anharmonicity and effective photon–photon interactions in strongly coupled cavity systems.^[4]

Photon blockade was also studied in more complex atomic configurations, including multilevel and multi-atom systems, demonstrating that blockade is not restricted to the simplest two-level atom scenario.^[5]

The first experimental observation of photon blockade was reported by Birnbaum et al. (2005), who demonstrated antibunched transmission from an optical cavity containing a single trapped atom in the strong-coupling regime.^[6]

Photon blockade arises when the excitation spectrum of a driven optical system becomes anharmonic, such that the energy required to add a second photon differs from that required to add the first. When the system is driven resonantly at the single-photon transition and dissipation is sufficiently weak, higher-order photon states are detuned, suppressing multi-photon occupation.

The simplest realization of photon blockade is a single two-level atom strongly coupled to a single cavity mode, described by the Jaynes–Cummings model. The dressed eigenstates form a ladder with energies

${\displaystyle E_{n,\pm }=n\hbar \omega _{c}\pm \hbar g{\sqrt {n}},}$

where ${\displaystyle n}$ is the number of excitations, ${\displaystyle \omega _{c}}$ is the cavity frequency, and ${\displaystyle g}$ is the atom–cavity coupling strength.

The characteristic ${\displaystyle {\sqrt {n}}}$ dependence produces intrinsic anharmonicity: the transition from zero to one excitation occurs at a different frequency from the transition between higher rungs. As a result, a laser tuned to the one-photon transition is detuned from the two-photon transition, suppressing simultaneous multi-photon occupation and producing antibunched light.

Photon blockade is inspired by Coulomb blockade, in which electron transport through a small conducting island is suppressed due to the charging energy associated with adding a single electron. In photon blockade, effective photon–photon interactions induced by strong light–matter coupling play an analogous role, despite photons being bosons and non-interacting in free space.

Photon blockade is conceptually related to Dipole blockade, which occurs in strongly interacting atomic ensembles, particularly involving Rydberg states. In dipole blockade, excitation of one atom shifts the energy levels of nearby atoms, preventing their simultaneous excitation. Although dipole blockade suppresses atomic rather than photonic excitations, it can mediate strong photon–photon interactions and has been used to realise single-photon nonlinearities.

Photon blockade mechanisms are commonly classified as:

• Conventional photon blockade, arising from strong spectral anharmonicity and typically requiring nonlinear interaction strengths exceeding dissipation rates.
• Unconventional photon blockade, where strong antibunching arises from destructive quantum interference between excitation pathways even in weakly nonlinear systems.

Photon blockade enables the generation of antibunched light with suppressed second-order correlations ${\displaystyle g^{(2)}(0)<1}$, providing a mechanism for single-photon sources in quantum communication and metrology.

Photon blockade underlies proposals and demonstrations of single-photon transistors, in which a single photon controls the transmission of many others. Implementations include cavity QED and Rydberg-mediated systems.

The Jaynes–Cummings–Hubbard model extends photon blockade physics to arrays of coupled cavities, predicting strongly correlated phases of light such as photonic Mott insulators and nonequilibrium quantum phase transitions.

• Quantum nonlinear optics
• Cavity quantum electrodynamics
• Jaynes–Cummings model
• Rydberg blockade
• Circuit quantum electrodynamics