Griffith University Summary: Researchers have overcome one of the key challenges to quantum computing by simplifying a complex quantum logic operation. They demonstrated this by experimentally realizing a challenging circuit -- the quantum Fredkin gate -- for the first time. They demonstrated this by experimentally realising a challenging circuit -- the quantum Fredkin gate -- for the first time. However, if larger bricks are used the same wall could be built with far fewer bricks," said Dr Patel. This is a gate where two qubits are swapped depending on the value of the third. The research team used the quantum entanglement of photons -- particles of light -- to implement the controlled-SWAP operation directly.
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Erasing the which-path information. Section S2. Generation of three-photon GHZ states. HOM dip measurements.
Abstract Minimizing the resources required to build logic gates into useful processing circuits is key to realizing quantum computers.
Although the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems have made more complex operations intractable. This is exemplified in the classical Fredkin controlled-SWAP gate for which, despite theoretical proposals, no quantum analog has been realized.
By adding control to the SWAP unitary, we use photonic qubit logic to demonstrate the first quantum Fredkin gate, which promises many applications in quantum information and measurement. We implement example algorithms and generate the highest-fidelity three-photon Greenberger-Horne-Zeilinger states to date.
The technique we use allows one to add a control operation to a black-box unitary, something that is impossible in the standard circuit model. Our experiment represents the first use of this technique to control a two-qubit operation and paves the way for larger controlled circuits to be realized efficiently.
Keywords: quantum computers, circuits, Fredkin controlled-SWAP gate, downconversion, qubits, quantum optics, GHZ states, entanglement, logic gate, photons INTRODUCTION One of the greatest challenges in modern science is the realization of quantum computers 1 — 3 , which, as they increase in scale, will allow enhanced performance of tasks in secure networking, simulations, distributed computing, and other key tasks where exponential speedups are available.
Processing circuits to realize these applications are built up from logic gates that harness quantum effects such as superposition and entanglement. At present, even small-scale and medium-scale quantum computer circuits are hard to realize due to the need to sufficiently control enough quantum systems to chain together many gates into circuits.
One example of this is the quantum Fredkin gate, which requires at least five two-qubit gates 4 to be implemented in the standard circuit model. Thus, despite featuring prominently in quantum computing 5 — 7 , error correction 8 , 9 , cryptography 10 — 12 , and measurement 13 , 14 , no such gate has been realized to date.
The quantum Fredkin gate, as shown in Fig. The original, classical version of the gate first proposed by Edward Fredkin 15 also serves as one of the first examples of a reversible logic operation where the number of bits is conserved and no energy is dissipated as a result of erasure. In the framework of universal quantum computation, gates are also reversible, so it may seem natural to ask whether it is possible to construct a quantum version of the Fredkin gate.
The first design of the quantum Fredkin gate was proposed by Milburn 16 and was to use single photons as qubits and cross-Kerr nonlinearities to produce the necessary coherent interactions.
Further schemes utilizing linear optics developed these ideas further 4 , 17 — 20 by using ancilla photons, interference, and multiple two-qubit 21 , 22 and single-qubit gates. Thus, it would be desirable to be able to construct a quantum Fredkin gate directly without decomposition and avoid the associated resource overhead.
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A quantum Fredkin gate
Erasing the which-path information. Section S2. Generation of three-photon GHZ states. HOM dip measurements. Abstract Minimizing the resources required to build logic gates into useful processing circuits is key to realizing quantum computers.