Multiplexed single photon source using one-way switching elements

A method for quasi-deterministically generating single photons at a predetermined generation time Tg=NTd, with the total number of lap cycles N being a natural number and Td being a delay time, by means of a multiplexed single photon source, the multiplexed single photon source comprising:

• a pulsed random single photon source capable of randomly producing single photons with a probability p at a production rate corresponding to the inverse of a production period, wherein, when a single photon is randomly produced, it is heralded by a heralding signal and is output on an output mode of the random single photon source;
• a first optical switch and a last optical switch, each having a first input mode and a second input mode and a first output mode and a second output mode, wherein the first input mode of the first optical switch is connected to the output mode of the random single photon source, wherein the first and last optical switch are configured to realize a unitary mapping from the input modes to the output modes, the unitary mapping depending on a state of the respective optical switch, wherein in a first state, the unitary corresponds to a SWAP operation, wherein in a second state the unitary mapping corresponds to an Identity operation, and wherein the first and last optical switch are capable of being switched from the first to the second state and/or from the second state to the first state by a control signal;
• a closed optical path comprising a delay loop introducing a delay Td to a photon and connecting the second output mode of the last optical switch to second input mode of the first optical switch;
• a control unit controlling each of the first and last optical switches based on the heralding signal and a cycle number N, the cycle number N being a natural number corresponding to a number of, the method comprising:
• before or at the start of a first lap cycle, initializing the first optical switch in the first state and, if applicable, the last optical switch in the second state, such that, upon generation of a first photon by the random single photon source, that first photon enters the first input mode of the first optical switch and, by being output on the second output mode of the first optical switch, is routed onto the closed optical path;
• based upon the heralding signal heralding the first photon, switching the first optical switch from its first state to its second state after the photon has been routed onto the closed optical path, thereby ensuring that the photon loops around the closed optical path as long as the first and last optical switches remain in their respective second state;
• and
• in an Nth lap cycle, switching the last optical switch from the second state to the first state thereby causing the first photon on the closed optical path to be shunted to the first output mode of the last optical switch and thus being output on the output mode of the multiplexed single photon source.

A method of quasi-deterministically generating single photons with a multiplexed single photon source wherein heralded photons generated by random single photon source are sent through a series of optical switches each having first and second input and output modes and each capable of being switched from a first state corresponding to a SWAP operation to a second state corresponding to an Identity operation on the mode space, wherein the first and second input and output modes of the optical switches are connected in series to form a first and second optical path respectively, and wherein a first output mode of a last optical switch forms the output mode of the multiplexed single photon source and a second output mode of the last optical switch is connected by a delay loop introducing a time delay Td to the second input mode of a first optical switch, wherein the first and last optical switches may be the same component, the method comprising:

• initializing, before or at the start of a first lap cycle, the first switch in the first state and all subsequent switches in the second state;
• switching, when the generation of a random single photon is heralded, the first switch to the second state after that photon has been routed onto the closed optical path formed by the second optical path and the delay loop, thereby ensuring that the photon may loop around the closed optical path;
• and
• switching, at the start of the Nth cycle, a last switch of the series of optical switches into the first state, thereby causing the photon to be routed out of the closed optical path and into the output mode of the multiplexed single photon source, such that the photon is output quasi-deterministically at a time NTd after the start of the first lap cycle.