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Quantum physics

Push-button teleportation

Two groups have succeeded in teleporting quantum states between different atoms — a spectacular advance in the quest to achieve quantum computation.

In 1993, Charles Bennett and colleagues described a remarkable protocol for transporting a quantum state from one location to another1, a protocol that succeeds even when the quantum state is completely unknown at the respective sites. Such quantum teleportation makes use of an extraordinary quantum resource, namely entanglement between two systems. Moreover, it also requires ordinary classical information obtained by performing a joint measurement on the system that carries the quantum state to be teleported and one component of the entangled state (as outlined in Fig. 1). Strangely, neither classical nor quantum channels individually carry any information about the quantum state, leading to the characterization of teleportation as the disembodied transport of quantum states. Initial experimental demonstrations of quantum teleportation, from 1997 onwards, involved the quantum states of beams of light2,3,4. Now, in a landmark advance, two teams have achieved teleportation for the quantum states of massive particles5,6.

Figure 1: Quantum teleportation, step by step.
figure1

Although the details of their experiments differ, Riebe et al.5 and Barrett et al.6 have followed the protocol suggested by Bennett et al.1 to achieve deterministic teleportation of a quantum state between trapped ions. First, an entangled state of ions A and B is generated, then the state to be teleported — a coherent superposition of internal states — is created in a third ion, P. The third step is a joint measurement of P and A, with the result sent to the location of ion B, where it is used to transform the state of ion B (step 4). The state created for P has then been teleported to B.

As described on pages 734 and 737 of this issue, Riebe et al.5 and Barrett et al.6 have generated coherent superpositions of two internal states for a single trapped ion (P in Fig. 1), and have teleported these quantum states to a second ion (B), with the help of a third, auxiliary ion (A). The import of these experiments goes well beyond the demonstration of teleportation per se, because both schemes incorporate many complex procedures that are required for scalable quantum computing. Indeed, the ion-trap set-up is generally considered one of the most promising implementations for quantum computing, as is once again confirmed by these experiments.

Moreover, quantum teleportation has emerged as an essential operation for diverse tasks in quantum information science. For example, if entangled particles were distributed throughout various sectors of a quantum computer, then quantum teleportation could provide a means for distant quantum bits (or qubits) to interact without the requirement of physical proximity — effectively ‘quantum wiring’, with desirable scaling properties. In addition, disposable quantum software could be delivered from a remote location using a generalized form of quantum teleportation to enhance the capabilities of rudimentary quantum hardware7.

Remarkably, the two groups5,6 have used quite different techniques for achieving teleportation, and yet both reach very similar values of so-called fidelity. Fidelity is a figure of merit that quantifies how well the quantum state that appears in the second ion after teleportation resembles the original quantum state; fidelity is 1 in the ideal case. Both teams report values around 0.75, which exceeds the ‘classical’ value of 2/3 that can be reached without quantum entanglement. For classical teleportation, the original quantum state is simply measured, and a new quantum state recreated by using only the classical information obtained from the measurement.

Both experiments have thereby reached the milestone of unconditional, or deterministic, teleportation of atomic qubits. The initial quantum state is prepared on demand, then teleported from one ion to another with high efficiency at the push of a button (which in fact triggers a computer-controlled array of complex operations). The teleported state is then available for further experiments. Such bona fide teleportation of quantum bits, following the original proposal of Bennett et al.1, has not been achieved before — not in experiments with polarization states of light, and certainly not for any material system. The only other setting in which deterministic teleportation has been realized is that of continuous quantum variables (roughly, the amplitude and phase of a beam of light)4.

In terms of the actual physical systems, Riebe et al.5 employ ground and metastable states of trapped calcium ions as qubits; Barrett et al.6 utilize two ground states in the hyperfine structure of beryllium ions. As for the implementation of quantum operations, the two experiments differ in several important aspects. First, crucial elements of both teleportation and quantum computing are joint operations for two qubits that cannot be performed by simply manipulating the qubits separately. Such two-body interactions are required for the creation of entanglement between two ions (step 1 in Fig. 1), and for the implementation of joint or Bell-state measurements (step 3). Riebe et al. use a version of the Cirac–Zoller two-qubit gate8, which relies on the common centre-of-mass motion of the ions. Barrett et al. adopt a recently developed geometric method to perform two-qubit gating9.

A second difference concerns how the authors addressed individual ions for manipulating quantum states, including projective measurements. Riebe et al. are able to address any specified target ion using tightly focused laser beams and have developed a technique to ‘hide’ the remaining ions from the target ion's fluorescence by changing their internal states so that they are insensitive to the fluorescent light. Barrett et al. have developed the capability to move groups of ions selectively to separate zones in a segmented trap, thereby isolating any target ion while still maintaining entanglement within the system.

The details of implementation aside, these two experiments represent a magnificent confluence of experimental advances, ranging from precision spectroscopy and laser cooling to new capabilities for controlled two-body interactions. The techniques developed and employed by these groups will no doubt prove important in the quest to build large-scale quantum computers based on trapped ions. Indeed, the fact that such diverse procedures performed so superbly in two separate laboratories attests to the flexibility and great potential of ion trapping for processing quantum information.

References

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    Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895–1899 (1993).

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    Bouwmeester, D. et al. Nature 390, 575–579 (1997).

  3. 3

    Boschi, D., Branca, S., De Martini, F., Hardy, L. & Popescu, S. Phys. Rev. Lett. 80, 1121–1125 (1998).

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    Furusawa, A. et al. 282, 706–709 (1998).

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    Riebe, M. et al. Nature 429, 734–737 (2004).

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    Barrett, M. D. et al. Nature 429, 737–739 (2004).

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    Gottesman, D. & Chuang, I. L. Nature 402, 390–393 (1999).

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    Cirac, J. I. & Zoller, P. Phys. Rev. Lett. 74, 4091–4094 (1995).

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    Leibfried, D. et al. 422, 412–415 (2003).

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