Quantum communication

Reliable teleportation

Two complementary experiments have demonstrated deterministic quantum teleportation of quantum bits. The results could find applications in quantum communications and computing. See Letters p.315 & p.319

The ability to teleport quantum states from one place to another by means of classical communication systems is both counter-intuitive and potentially useful. However, experimental demonstrations of such quantum teleportation have tended to be hit-and-miss, with few successful attempts. Two studies in this issue now report deterministic quantum teleportation: one involves the teleportation of the quantum state of a single photon, by Takeda et al.1 (page 315); the other is teleportation of the quantum state of a superconducting circuit, by Steffen et al.2 (page 319).

Quantum teleportation3 relies on the ability to produce and measure entangled states. Entanglement refers to strong correlations between distinct quantum systems that defy classical explanation. Generically, teleportation protocols proceed by three steps (Fig. 1): a pair of quantum systems in an entangled state is produced and distributed, one to a sender (Alice) and the other to a receiver (Bob); Alice makes a joint measurement of her member of the entangled pair and the unknown state she wishes to teleport, and sends the measurement result to Bob; Bob uses the measurement result he receives from Alice to manipulate his quantum system in a predetermined way. After this manipulation, Bob's quantum system ends up being in the unknown state, that teleported from Alice to Bob, with the only direct communication being a classical message — Alice's measurement result.

Figure 1: A quantum teleportation protocol.
figure1

A qubit, here either a single photon1 in a superposition of two pulses or a superconducting circuit2, is prepared in an unknown state. A pair of quantum systems in an entangled state has previously been prepared and distributed between a sender (Alice) and a receiver (Bob). Alice makes a joint measurement between the unknown state, which she wishes to teleport, and her member of the entangled pair. The measurement result is sent to Bob via a classical channel. He uses it to manipulate his member of the entangled pair, in the process recreating the unknown state at his station. The quantum link that allows the entangled pair to be distributed to Alice and Bob is optical in the case of the photon and uses microwave photons in the superconducting case.

Most previous quantum teleportation experiments have been non-deterministic because some steps of the protocol, in particular producing the initial entanglement and carrying out the joint measurement, have been difficult to perform efficiently. For example, in single-photon quantum optics, which most earlier teleportation studies have used, the following restrictions apply: first, entangled pairs are produced by a spontaneous process that typically succeeds less than once in 100 attempts; second, with linear optics, the required joint measurement can be performed with a probability of success of only 50%. In their studies, Takeda et al. and Steffen et al. take very different approaches to overcoming this problem of limited efficiency.

Takeda and colleagues' experiment teleports a single-photon quantum bit (qubit) with much higher efficiency than all earlier optical studies. A qubit is a quantum system that has two distinct outcomes — in this case, a single photon in a superposition of two different arrival times at a detector. The trick the authors use is to perform a more general form of teleportation that works for any optical-field state, rather than only for single-photon qubit states. This sounds, and is, hard to achieve, but it turns out to make a big difference because deterministic sources of optical-field entanglement and efficient joint field measurements are both available. This 'continuous-variable' teleportation protocol4 has previously been demonstrated for multi-photon states5,6, but this is the first time that a single-photon qubit has been teleported by this method. The result is the deterministic teleportation of a single-photon qubit with a quality that exceeds the limits set by any classical protocol — that is, one that does not use entanglement but in which Alice instead tries to directly measure (necessarily imperfectly) the state of the qubit and sends this information to Bob.

Steffen and colleagues tackled the efficiency problem in a different way. They implemented teleportation of a solid-state qubit — a superconducting circuit. The qubit circuit is on the 100-micrometre scale and is held at temperatures of around 20 millikelvins. Qubits are formed from small currents running in the circuit7, and at these sizes and temperatures, the currents behave quantum mechanically. As in Takeda and colleagues' study, photons are still used to move information around, but these are now at microwave (rather than optical) frequencies8. Alice and Bob are separated by about 5 millimetres. Strong interactions between the superconducting qubits and the microwave photons allow entanglement to be produced deterministically between the superconducting qubits held by Alice and Bob. Such interactions can also be used to make a joint measurement deterministically between Alice's qubit and the unknown qubit, as required by the teleportation protocol. Again, the result is deterministic teleportation with a quality that exceeds the classical limit. Deterministic teleportation has been achieved previously in ion-trap experiments, but in that situation the distances between the qubits were 1,000 times smaller9,10.

Both experiments are 'hero' experiments, in the sense that they only just exceed classical limits when they are running in the fully deterministic mode. In Takeda and colleagues' study, this is mainly due to the limited strength of the field entanglement, whereas for Steffen and co-workers' study the main problem is imperfect differentiation of the outcomes of the joint-measurement results. However, both groups are also able to run their experiments in a non-deterministic mode. In Takeda and colleagues' experiment, the photon qubit can be successfully teleported about 40% of the time (compared with a value of much less than 1% for all previous optical studies), and reproduced at Bob with about 88% quality (similar to the best achieved in previous experiments). Steffen et al. can arrange for their qubit to be teleported 25% of the time with a quality of about 82%. For both teams, the limitations in their experiments are clearly understood and shown not to be fundamental impediments to future improvements.

The advances made in these experiments should flow on to allow improved quantum-information protocols. Takeda and co-workers' optical 'flying qubits' have potential applications in quantum communications. But there is a caveat. Improving quantum communications using teleportation requires the purification of entangled states sent through a noisy channel. Teleportation could therefore be used to transfer quantum states between distant locations with better quality than sending them directly. Purification can be achieved by distillation techniques. However, distillation techniques for field entanglement are not as advanced as those for qubit entanglement. Nevertheless, promising advances in distilling field entanglement have been made11. By contrast, the solid-state 'standing qubits' in Steffen and colleagues' experiment are more likely to find applications in quantum computing12. Notably, they demonstrate the increasing sophistication and quality of the manipulations possible with superconducting qubits coupled to microwave transmission lines, and raise the profile of such qubits as potential building blocks for large-scale quantum computation.

More progress is needed before deterministic quantum teleportation under practical conditions, and with quality approaching 100%, becomes a reality. But these experiments represent significant steps along that path.

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Correspondence to Timothy C. Ralph.

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Ralph, T. Reliable teleportation. Nature 500, 282–283 (2013). https://doi.org/10.1038/500282a

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