Teleportation of Massive Particles

13 June 2007

Theorists from the UQ (Ashton Bradley, Simon Haine, Murray Olsen) and ANU Faculties (Joseph Hope) nodes of the ARC Centre of Excellence for Quantum-Atom Optics (ACQAO) have come up with a scheme to teleport quantum states of collections of atoms from one position to another by converting the quantum state to light and back again.  This work has been highlighted in a recent article in New Scientist.

When an object is transferred from one location to another, by a method other than physically moving the object itself, it is said to have undergone "teleportation". A fax machine might be said to teleport a piece of paper, but it doesn't have perfect resolution, so the paper you send to the receiver is always a little different to the piece you started with. You might think that measuring the paper with increasing accuracy might eventually lead to the perfect 'teleporter'. If you get all the atoms exactly positioned, it doesn't matter if they are the 'same' atoms as the original piece of paper, because quantum mechanics dictates that all particles of a given "type" are fundamentally indistinguishable (i.e., the universe is exactly the same if you swap two electrons).

Unfortunately, quantum mechanics, in particular the Heisenberg uncertainty principle, dictates that we can never measure the quantum state of a system perfectly. This "quantum noise" will make it impossible to accurately teleport something.

However, in 1993, a team of Physicists (C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres and W. K. Wootters (read the original paper here), devised a scheme to avoid this problem by using quantum entanglement. Quantum entanglement is a bizarre property of quantum mechanics. When two particles are entangled, measurements on one of the particles instantaneously effect the quantum state of the other particle, no matter how far away it is. Entanglement is used to implement quantum teleportation as follows:

The sender (usually referred to as "Alice") combines the particle she wants to teleport with one half of an entangled pair of particles, and then measures the properties of the system, disrupting the quantum state of the system in the process. The added quantum noise from this process 'hides' the original quantum state from Alice. She then sends the results from her measurement to the receiver (bob), who possesses the other half of the entangled pair. Bob then uses this information to perform operations on his half of the entangled pair to retrieve the original quantum state. This works because the quantum noise of his half of the entangled pair is exactly right to cancel the noise added by Alice's half. Experiments using this scheme, or variations of it, have been performed with single photons, beams of light (also done in the first Australian teleportation experiment) , trapped ions, and the nuclear spin states of an ensemble of atoms.

In theory this scheme can lead to the quantum state being perfectly teleported. However, in practice quantum entanglement is never perfect, and this limits the fidelity of current teleportation experiments. So far, teleportation experiments have been limited to a fidelity of 85%.

Our scheme is different as it does not rely on Alice and Bob sharing quantum entanglement. We have shown that it may be possible to teleport a group of about 5 thousand cold atoms by transferring their quantum state onto a laser beam, which is then 'beamed' to a new location where the receiver can use this laser beam to recreate the original group of atoms almost exactly. The scheme relies on the sender and receiver each having a reservoir of extremely cold atoms, known as a Bose-Einstein condensate (BEC). BEC is a state of matter that occurs when atoms become very cold, (about 100 Billionths of a degree about absolute zero). Due to a phenomenon known as Bose-Enhancement, all the atoms like to act the same way. This causes the atoms to act as one macroscopic matterwave, rather than a collection of individual atoms.

Our proposed scheme is illustrated above. We consider atoms that have three internal electronic states (which we will label (|1>, |2> , and |3>). Atoms in state |1> experience a force due to magnetic fields. By applying an appropriate configuration of magnetic fields, it is possible to contain state |1> atoms in a magnetic “trap”. BEC is usually confined in a trap by this method. Atoms in state |2> do not experience a force due to magnetic fields. State |3> represents an excited atomic state. The separation in energy of state |3> from state |1> and |2> is about the same as the energy carried by an optical photon. By transferring atoms from state |1> to state |2> in a controlled fashion, atoms can be made to slowly leak out of the trap and form an Atom Laser.

We begin by sending a pulse of atoms in state |2> towards a trapped condensate (made up of state |1> atoms). It is the pulse of state |2> atoms that we wish to teleport. The condensate is illuminated with a laser beam (called the control beam) which is of the correct polarization to couple atoms from state |2> to state |3>. However, the control beam is slightly detuned from the exact resonance. When the atoms encounter the control beam, they absorb a photon, transferring them into state |3>. This state is unstable, and ordinarily the atoms would rapidly emit a photon in a random direction from this state, and end up back in state |2> or state |1>. However, due to the presence of the BEC, and that the control beam is slightly detuned, the atoms are stimulated to join the BEC, and thus emit a photon such that they all end up in state |1>. As all the BEC atoms have a very well defined momentum, conservation of momentum dictates that all the photons must be emitted in the same direction, forming our signal beam. By careful adjustment of the intensity and wavelength of the control beam, we can arrange it such that the quantum state of the atomic pulse (i.e., the position and momentum of each atom, or, equivalently, the amplitude and phase of the atomic matterwave) are encoded onto the signal beam. Ideally, the number of photons in the signal beam is exactly equal to the number of atoms in our original atomic pulse.

This information is then 'beamed' to a second BEC, which is also illuminated with a control laser. The atoms in the BEC absorb a photon from the signal beam, and are forced to emit it into the control beam due to stimulated emission, transferring the some of the BEC atoms into state |2>, where they are kicked out of the condensate due to the momentum of photons. As the information of the original atomic pulse is transferred to the new pulse, we have effectively teleported our original blob of atoms.

Here is a simulation of our teleportation scheme. The dark blue line represents the density of the input atomic matter wave (not to scale), the cyan lines represent the densities of the BEC's, and the red line represents the photon density of the signal beam.

Our scheme is quite different from what is usually coined quantum teleportation because it gets around the need for the sender and receiver to share entanglement, as the quantum state to be teleported is never actually measured. As our scheme doesn't rely on the quality of the entanglement, it may be possible to achieve more accurate teleportation via this method.

For more details read our paper.


For the New Scientist news item on this work click here.