Researchers have successfully used quantum teleportation to transfer a complete quantum logic operation from one ion to another — a significant step towards the development of a large scale quantum computer.
Physicists at the National Institute of Standards and Technology (NIST) have successfully teleported a computer circuit instruction known as a quantum logic operation between two separated ions — electrically charged atoms. The test demonstrates how quantum computer programs could carry out tasks in future large-scale quantum networks.
Quantum teleportation refers to the transfer of data from one quantum system — in this case, an ion — to another. This can be carried out even if the two systems are completely isolated from each other, like two books in the basements of separate buildings.
The difference between this real-life form of teleportation and something that would be seen in Star Trek or other sci-fi is in quantum teleportation only information, not matter, is transported. So no beaming to work to beat the morning rush, unfortunately.
This isn’t the first time quantum teleportation has been demonstrated with ions and a variety of other quantum systems, but it is the first to teleport a complete quantum logic operation using ions. This is particularly significant because ions are a leading candidate for the architecture of future quantum computers.
The experiments are described in the May 31 issue of Science.
NIST physicist Dietrich Leibfried explains: “We verified that our logic operation works on all input states of two quantum bits with 85 to 87% probability — far from perfect, but it is a start.”
If a full-scale quantum computer can be built, could solve certain problems that are currently intractable. To work towards this NIST has contributed to global research efforts to harness quantum behaviour for practical technologies.
Quantum computers will probably need millions of quantum bits, or “qubits,” to operate as researchers hope, they will also need ways to conduct operations between qubits distributed across large-scale machines and networks. Teleportation of logic operations is one way do that without direct quantum mechanical connections. In addition to this, physical connections for the exchange of classical information will still be needed.
How quantum teleportation works.
The NIST team teleported a quantum controlled-NOT (CNOT) logic operation — logic gate — between two beryllium ion qubits, located more than 340 micrometres (millionths of a meter) apart in separate zones of an ion trap. This is a distance that rules out any substantial direct interaction for quantum systems.
A CNOT operation flips the second qubit from 0 to 1, or vice versa, but only if the first qubit is 1 — nothing happens if the first qubit is 0. In typical quantum fashion, both qubits can be in “superpositions” in which they have values of both 1 and 0 at the same time.
The NIST teleportation process relies on entanglement linking the quantum properties of two or more entangled particles even when they are separated by large distances.
A “messenger” pair of entangled magnesium ions are used to transfer information between the beryllium ions.
What the NIST team found, is that its teleported CNOT process entangled the two magnesium ions — a crucial early step — with a 95% success rate, while the full logic operation succeeded 85% to 87% of the time.
Leibfried continues: “Gate teleportation allows us to perform a quantum logic gate between two ions that are spatially separated and may have never interacted before.
“The trick is that they each have one ion of another entangled pair by their side, and this entanglement resource, distributed ahead of the gate, allows us to do a quantum trick that has no classical counterpart.”
The scientist adds that the entangled messenger pairs could be produced in a dedicated part of the computer and shipped separately to qubits that need to be connected with a logic gate but are in remote locations.
The NIST work also integrated into a single experiment several operations that will be essential for building large-scale quantum computers based on ions. This includes control of different types of ions, ion transport, and entangling operations on selected subsets of the system. Once again, this is the first time a team of researchers have achieved such a feat.
To verify that they performed a CNOT gate, the researchers prepared the first qubit in 16 different combinations of input states and then measured the outputs on the second qubit. This produced a generalized quantum “truth table” showing the process worked.
In addition to generating a truth table, the researchers checked the consistency of the data over extended run times to help identify error sources in the experimental setup.
This technique is expected to be an important tool in characterizing quantum information processes in future experiments and represents another step towards successful quantum computing.