Quantum multiplexing in single photons

The main challenge for the emerging technology of quantum information lies in the development of an effective quantum ‘hardware,’ that is, a set of physical methods to implement many qubits (the elementary units of quantum information, analogous to classical bits) and to process them simultaneously. In photonic implementations, qubits are realized using photon quantum states by exploiting different properties of light, such as polarization, wavelength, and spatial modes. Usually each photon carries a single qubit, but in order to expand the available quantum memory or to enhance the efficiency of a quantum communication channel, it may be convenient to squeeze more than one qubit into each photon. Although it is relatively simple to write multiple qubits directly in each photon, there has been no straightforward way to transfer qubits initially carried by two distinct photons into a single photon. Such a quantum-state joining process, depicted in Figure 1, could be used to multiplex quantum information and transport it in a reduced number of photons. Similarly, there has been no way to take a photon carrying multiple qubits and split them among separate photons, for example, to demultiplex the quantum information across separate channels. The problem with implementing these tasks is that they require an interaction between photons. But photons do not interact in vacuum, and in ordinary nonlinear media they exhibit exceedingly weak interactions. A way to introduce an effective interaction is the KLM method1 (named after Emanuel Knill, Raymond Laflamme, and Gerard Milburn), which exploits twophoton interferences and a subsequent ‘wavefunction collapse’ occurring on measurement. This idea enabled the first experimental demonstrations of controlled-NOT (CNOT) quantum logical gates among qubits carried by different photons,2 and it is the basis of our demonstration of quantum joining.3 Figure 1. Artistic representation of the quantum joining concept. Two input photons, each carrying a qubit, are turned into a single output photon carrying both qubits. The demonstrated device is probabilistic, but the user is informed when the process succeeds (represented by the green LED turning on).