Temporally controlled optical logic provides a promising route toward ultrafast information processing. However, implementing conditional multi-bit logic in non-equilibrium systems with ultrashort lifetimes remains highly non-trivial. Here, we demonstrate a classical 2-bit controlled-NOT (CNOT) gate operated in the time domain in an exciton–polariton microcavity at room temperature. The logic functionality is enabled by the interplay between a transient excitonic reservoir and bosonic cascading dynamics, which together provide a short-lived but controllable memory linking two sequential optical inputs. By encoding logical states in the temporal sequence and energy redistribution of polariton condensates, we realize all four truth-table operations of a classical CNOT gate on picosecond timescales. The gate operation exhibits clear tolerance to timing variation and reproducible recovery dynamics. Our results establish a physical mechanism for temporally conditional logic in non-equilibrium polaritonic systems, offering a general framework for multi-bit optical operations mediated by reservoir-assisted nonlinear dynamics.
Temporally controlled optical logic provides a promising route toward ultrafast information processing. However, implementing conditional multi-bit logic in non-equilibrium systems with ultrashort lifetimes remains highly non-trivial. Here, we demonstrate a classical 2-bit controlled-NOT (CNOT) gate operated in the time domain in an exciton–polariton microcavity at room temperature. The logic functionality is enabled by the interplay between a transient excitonic reservoir and bosonic cascading dynamics, which together provide a short-lived but controllable memory linking two sequential optical inputs. By encoding logical states in the temporal sequence and energy redistribution of polariton condensates, we realize all four truth-table operations of a classical CNOT gate on picosecond timescales. The gate operation exhibits clear tolerance to timing variation and reproducible recovery dynamics. Our results establish a physical mechanism for temporally conditional logic in non-equilibrium polaritonic systems, offering a general framework for multi-bit optical operations mediated by reservoir-assisted nonlinear dynamics.
Summary form only given. Bosons can populate quantum states macroscopically, forming various kinds of thermal-equilibrium, and quasi-equilibrium condensates, which exhibit a wide range of collective phenomena including superfluidity and macroscopic coherence in matter. Interferometry has been employed to demonstrate long-range off-diagonal order in cold-atom systems, and, recently, in the rapidly developing field of exciton-polariton condensates. However, all such interferometric measurements have been performed on steady-state condensates, unable to access the temporal off-diagonal order and non-equilibrium dynamics. Moreover, in solid-state systems, interference between two separate condensates has not been observed before. Here we use optical injection to resonantly generate two dynamic exciton-polariton condensates with distinct momenta and at different times. We characterize the interference between condensates, demonstrating their long-range temporal order. We show that the build-up of the temporal long-range order strongly depends on the strength of polariton interactions, as well as the temperature and particle density. Moreover, polariton interactions yield a blue shift of the dynamic condensate energy, acting as an extremely large optical nonlinearity. These results provide direct evidence of temporal long-range order in dynamic condensates as well as demonstrate a new approach for probing the ultrafast coherent dynamics of exciton-polaritons, opening new directions in the fundamental study of coherence in matter as well as in practical applications such as quantum information processing and ultrafast logic.
The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.
Quantum computers promise great improvements in solving problems such as factoring large integers, simulating quantum systems, and database searching. Using a photon as a quantum bit (qubit) is one of the most promising ways to realize a universal quantum computer because the coherent superposition state of a photon is very robust against various sources of decoherence. However, it is too difficult to realize two-qubit (photon) gates because it requires huge nonlinearity between photons. Here we show the realization of a controlled-NOT (CNOT) gate, the most important and elemental two-qubit gate for quantum computation, by extending our previous research. The heart of our experiment is the conditional measurement of two-photon coincidences in the Franson-type experiment[7]. The photon counting measurement plays the same role as the nonlinearity required for the two-qubit gate, and our system reproduces the truth table of the CNOT gate. Furthermore, we create an entangled state from the superposition state by our gate, which is clear evidence that our gate works as a quantum logic gate. Our results make it possible to manipulate the quantum state of photons including entanglement and represent significant progress in the operation of various algorithms in quantum computation.
Strongly-coupled light-matter exciton-polaritons constitute an on-chip solid-state platform where the macroscopic quantum phenomenon of condensation is not only achievable at elevated temperatures, but can also be optically controlled, including through their interaction with inaccessible "dark" states. A recent study has shown that polariton condensation can be established under nonlinear two-photon pumping, paving the way towards dark state-condensate coherent control and highly-efficient terahertz (THz) lasing. In this letter, we show for the first time a temporal signature of dark exciton induced bosonic stimulation by investigating one- and two-photon pumped condensation with time-resolved photoluminescence spectroscopy. Our results show a clear difference in the detuning dependence of the buildup and relaxation rates of the condensate-induced blueshifts under one- and two-photon pumping. This difference is associated with the stronger exciton-fraction dependence expected for one-photon pumped condensation, where polariton-polariton stimulation dominates, compared with two-photon pumped condensation, where a 2p-exciton-to-lower-polariton THz transition can provide another stimulation channel, together with various spin-flip, electron-hole exchange and phonon-based mechanisms. These observations indicate the presence of a new dark state originated stimulation channel that could facilitate highly-efficient THz lasing in semiconductor microcavities.
We consider bosonic condensates of exciton-polaritons optically confined in elliptical traps. A superposition of two non-degenerated p-type states of the condensate oriented along the two main axes of the trap is represented by a point on a Bloch sphere, being considered as an optically tunable qubit. We describe a set of universal single-qubit gates resulting in a controllable shift of the Bloch vector by means of an auxiliary laser beam. Moreover, we consider interaction mechanisms between two neighboring traps that enable designing two-qubit operations such as CPHASE and CNOT gates. Both the single- and two-qubit gates are analyzed in the presence of error sources in the context of polariton traps, such as pure dephasing and spontaneous relaxation mechanisms, leading to a fidelity reduction of the final qubit states and quantum concurrence, as well as the increase of Von Neumann entropy. We also discuss the applicability of our qubit proposal in the context of DiVincenzo's criteria for the realization of local quantum computing processes. Altogether, the developed set of quantum operations would pave the way to the realization of a variety of quantum algorithms in a planar microcavity with a set of optically induced elliptical traps.
Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum information processing. However, photons, one of the best candidates for qubits, suffer from a lack of strong nonlinear coupling, which is required for quantum logic operations. Here we show how this drawback can be overcome by reporting a proof-of-principle experimental demonstration of a nondestructive controlled-NOT (CNOT) gate for two independent photons using only linear optical elements in conjunction with single-photon sources and conditional dynamics. Moreover, we exploit the CNOT gate to discriminate all four Bell states in a teleportation experiment.