A symmetrical approach is suggested (Chang DE et al. Nat Phys 3:807, 2007) to realize a single-photon transistor, where the presence (or absence) of a single incident photon in a ‘gate’ field is sufficient to allow (prevent) the propagation of a subsequent ‘signal’ photon along the nanowire, on condition that the ‘gate’ field is symmetrically incident from both sides of an emitter simultaneously. We present a scheme for single-photon transistors based on the strong emitter-surface-plasmon interaction. In this scheme, coherent absorption of an incoming ‘gate’ photon incident along a nanotip by an emitter located near the tip of the nanotip results in a state flip in the emitter, which controls the subsequent propagation of a ‘signal’ photon in a nanowire perpendicular to the axis of the nanotip.
KeywordsSingle-photon transistor Nanotip Surface plasmon
The fundamental limit of a photonic transistor  is a single-photon transistor where the propagation of a single photon in the ‘signal’ field is controlled by the presence or absence of a single photon in the ‘gate’ field. Such a nonlinear device may find many interesting applications in fields such as optical communication , optical quantum computer , and quantum-information processing . However, its physical realization is extremely demanding because photons rarely interact. To achieve strong interaction between photons, several schemes based on either the resonantly enhanced nonlinearities of atomic ensembles [5–8] or individual atoms coupled to photons in cavity quantum electrodynamics (CQED) have been proposed [9–12]. Recently, a robust, practical approach based on the tight concentration of optical fields associated with guided surface plasmons (SP) on conducting nanowires has emerged . However, this scheme works on condition that the optical ‘gate’ is split into two completely same parts and having them incident from both sides of the emitter simultaneously.
In this paper, we present a scheme for a single-photon transistor consisting of a nanotip, a nanowire, and an emitter. A single ‘gate’ photon propagating along a nanotip is coherently stored under the action of a classic control field, which results in an internal state flip in the emitter. This conditional state flip can change the propagation of a subsequent ‘signal’ photon traveling along the nanowire. In our scheme, the aforesaid condition can be released, the single ‘gate’ photon is incident from one side of the nanotip and travels toward the emitter which locates near the tip of the nanotip.
Recently, as a new scheme to achieve strong coupling between light and an emitter, surface plasmons which are propagating electromagnetic modes confined to the surface of a conductor-dielectric interface, have attracted intensive interests [13–21]. Surface plasmons can reduce the effective mode volume V eff for the photons, thereby achieving a substantial increase in the coupling strength . An effective Purcell factor in realistic systems may be achievable according to the theoretical results in [18, 22], where Γ pl is the spontaneous emission rate into the surface plasmons (photons) and Γ′ describes contributions from both emission into free space and non-radiative emission via ohmic losses in the conductor. Furthermore, this strong coupling is broadband .
and the transmission coefficient t(δ k ) = 1 + r(δ k ), where . Here, c denotes the group velocity of the SPs and ω e is the energy difference between an excited state and a ground state . On resonance, r≈−(1−1/P), and thus the emitter in state works as a nearly perfect mirror for large P. The bandwidth Δω of the process determined by the total spontaneous emission rate Γ can be quite large. However, at high incident powers, the emitter rapidly saturates, as it cannot scatter more than one photon every time . Two photons directly interact very weakly, but we can, first, let one photon change the state of an emitter, and then such change will significantly affect the propagation of another one. According to this principle a single-photon transistor may be realized physically .
In the stage of photon storage, the ‘gate’ photon propagating along the nanotip is on resonant with the transition and the frequency ω L of the control field Ω(t) satisfies the resonance condition . In this stage, the emitter does not excite the fundamental plasmon mode of the nanowire because and p 2 is off resonant with the ‘gate’ field . Thus, the aforesaid storage protocol can be applied to the system comprising the nanotip, the nanowire, and the emitter. In the second stage, the ‘signal’ field containing one photon resonant with the transition propagates along the nanowire. This field will not excite the fundamental plasmon mode in the nanotip since and p 1 is off resonant with the ‘signal’ field . Thus, the propagating property of SPs can be used in this situation.
Combining the techniques of state-dependent conditional reflection and single-photon storage, a single-photon transistor can be realized . First, the emitter is initialized in state . Under the action of the control field Ω(t), the presence or absence of a photon in a ‘gate’ pulse with frequency ω1 traveling along the nanotip flips the internal state of the emitter to state or remains in state during the storage process. Then, this conditional flip can control the propagation of subsequent ‘signal’ photons with frequency ω2 propagating along the nanowire. Thus, the interaction of subsequent signal pulse and the emitter depends on the internal state of emitter after the storage. If the emitter is in the state , the signal field is near, completely reflected by the emitter. Otherwise, the emitter is in the state , then the field is near-completely transmitted because does not interact with the surface plasmon. The storage and conditional spin flip makes the emitter either highly reflecting or completely transparent depending on the gate field containing none or one single-photon. Thus, the presence or absence of a single incident photon in a ‘gate’ field is sufficient to control the propagation of the subsequent ‘signal’ field, and the system therefore can serve as an efficient single-photon switcher or transistor.
As a summary, we have presented a scheme for a single-photon transistor, where the ‘gate’ field propagates along a nanotip and the ‘signal’ field travels along a nanowire perpendicular to the nanotip. A single ‘gate’ photon can control the propagation of a single ‘signal’ photon through changing the internal state of an emitter assisted by classic control field. This transistor may find many important applications in areas such as efficient single-photon detection  and quantum information science. Based on this scheme, the controlled-phase gate  for photons can be made; furthermore, a CNOT gate which is a key part of an optical quantum computer  is available. This system may also be a promising candidate for realizing electromagnetically induced transparency-based nonlinear schemes [5–8].
This work was supported by the State Key Programs for Basic Research of China (2005CB623605 and 2006CB921803), and by National Foundation of Natural Science in China Grant Nos. 10474033 and 60676056.