Five States At Once: How Physicists Just Multiplied The Power Of Quantum Teleportation

The quantum state exists in one place. Then it exists in another. Nothing physical moved between them — not an electron, not a photon carrying the information. Just the spooky correlations of entanglement doing what Einstein famously refused to believe they could do. Quantum teleportation is, in this sense, genuinely strange every time it works.

But for most of its three-decade history, the trick has come with a frustrating limitation: you can only teleport one quantum state at a time. Building a useful quantum network this way is a bit like trying to run a telephone exchange with a single line. Each call needs its own dedicated connection, each transmission its own entangled pair. Scale up to something practical — a quantum internet, distributed quantum computing, secure communication across cities — and the resource costs balloon fast.

Now a team at Shanxi University in China has found a way around this bottleneck, and the solution turns out to hinge on something deceptively simple: the relationship between phase and frequency.

The work, led by Xiaolong Su at the university’s Institute of Opto-Electronics, demonstrated quantum teleportation of five quantum states simultaneously using a single entangled resource. All five arrived at their destination with fidelities around 70 per cent, clearing the threshold that distinguishes genuinely quantum transmission from anything a classical system could achieve. The results appear in Science Bulletin.

To understand what the team did, it helps to understand what quantum teleportation actually involves. Despite the science fiction associations, it doesn’t move objects. What it transfers is quantum information — the precise quantum state of a system, including all the superpositions and correlations that classical physics cannot capture. The process requires three ingredients: a pair of entangled particles shared between sender and receiver, a classical communication channel, and a measurement by the sender that effectively collapses their entangled particle in a way that allows the receiver to reconstruct the original state.

In the continuous-variable version of teleportation that the Shanxi team uses, quantum information is encoded in the amplitude and phase quadratures of light — roughly speaking, the two complementary ways you can measure the oscillations of an electromagnetic wave. The entangled resource is an EPR state, named for Einstein, Podolsky and Rosen, generated by a device called a non-degenerate optical parametric amplifier. What comes out is a pair of light beams whose quantum fluctuations are correlated in a way that enables teleportation.

The clever part of the new approach lies in how classical information travels between sender (conventionally called Alice) and receiver (Bob). When Alice measures her half of the entangled state, she sends the results to Bob via two classical channels. Bob uses this information to displace his own beam in phase space, reconstructing the teleported state. The key insight is that the phase accumulated in those classical channels depends on frequency. A signal travelling down a cable acquires a phase shift that scales with how fast it oscillates.

Su’s team realised this frequency dependence, usually treated as a nuisance to be carefully calibrated away, could instead be turned into a feature. By setting the classical channel phase for one specific “basic” sideband frequency and then exploiting the mathematical regularity of how phase shifts scale across other frequencies, they found that multiple quantum states at harmonically related frequencies get teleported simultaneously — almost as a side effect of the single calibration.

The number of states you can send at once is controllable. By adjusting the basic sideband frequency and the measurement angles at Alice’s station, the team could dial up three, four, or five simultaneous teleportations within their 24 megahertz bandwidth. In one configuration, four states at 5, 10, 15 and 20 megahertz were teleported together. In another, five states at 2.5, 7.5, 12.5, 17.5 and 22.5 megahertz came through at once. To verify the results, the team reconstructed Wigner functions — phase-space representations of quantum states — for all five output states simultaneously, confirming that what arrived at Bob’s end genuinely reflected what Alice had sent.

There’s an obvious analogy to classical telecommunications, where frequency-division multiplexing has long allowed multiple phone calls to share the same cable. The quantum version is considerably more delicate — the squeezing levels the team achieved ranged from 3 to 5 decibels across the bandwidth, limiting fidelity — but the principle translates. One entangled channel doing the work that previously required five.

Previous attempts to enhance quantum teleportation’s throughput went in different directions. Researchers have teleported photons carrying orbital angular momentum, squeezed states across broadband channels, and last year demonstrated teleportation of four simultaneous degrees of freedom of a single photon. What distinguishes the Shanxi approach is the combination of controllability and determinism: you can choose which frequencies to target, how many states to send, and the whole process runs without post-selection or probabilistic filtering.

The scalability question is straightforward on paper. Wider bandwidth entanglement sources would allow more sideband frequencies; better squeezing would push fidelities higher. The current system is limited by the bandwidth of the optical parametric amplifier and the homodyne detectors. Both are engineering challenges rather than fundamental barriers.

What the demonstration points toward is a particular vision of a quantum network: a server node able to simultaneously distribute quantum states to multiple users, each operating at their own frequency band, without requiring separate entangled links for each. Add classical information to the mix — the team notes that multiple images encoded at different sideband frequencies could in principle be teleported simultaneously — and you have something that starts to look like a genuinely practical communication architecture rather than a laboratory curiosity.

Quantum teleportation has always carried a faint air of implausibility about it. The idea that you can transfer a quantum state without physically transmitting it, exploiting correlations that span space and yet somehow respect causality, still stops people short the first time they encounter it. The Shanxi result doesn’t make it less strange. It just makes it five times more useful.

Study link: https://www.sciencedirect.com/science/article/pii/S2095927325013246