NEWS & VIEWS determines which Bell state was encoded, and the nature of the photons themselves. In But even with experimental noise and errors, discovers the message. addition to polarization, photons possess Barreiro et al.3 have succeeded in surpassing But how is this ‘superdense’ coding if two additional physical properties, such as the linear-optics limit of 1.58 bits per photon. photons are needed to send the two classical momentum and frequency. Indeed, it One might think that the use of bits? The answer is that all the information is possible to produce photons that are additional entanglement would nullify the is carried by the second photon. In fact, ‘hyperentangled’, that is, entangled in multiple communication advantage of superdense when Bob sends the first photon to Alice, he degrees of freedom simultaneously5. In the coding. But it is important to note that might not even know which two-bit message case of superdense coding, one can use the Bob manipulates only the polarization of he will send on the following day. If Alice extra entanglement to assist in the Bell-state the second photon, and thus the orbital measures the first photon alone, she will measurement6. Barreiro et al.3 harness this angular momentum contains no information obtain no clue whatsoever as to what Bob’s property in their experiment. They use a about the message Bob sends. The extra message is (or will be). She needs to perform process known as spontaneous parametric entanglement is used only to mediate the a joint measurement on the entangled pair, down-conversion to produce pairs of interaction between the polarization qubits, because the message is encoded in their photons that are entangled in two degrees of allowing a measurement that can be, in combined state. freedom — polarization and orbital angular principle, 100% efficient. This experiment3 To read the message, Alice must momentum7. As in the example given above demonstrates a considerable technical determine which of the four Bell states Bob (Fig. 1a), the polarization state encodes the advantage as well. Because each photon is sent. This requires the joint measurement four Bell states, whereas the orbital angular analysed separately, Alice does not need of the two photons, and the measurement, momentum helps in the measurement of the a quantum memory device to store the in turn, requires an interaction between the polarization state. The first photon is sent to quantum state of the first photon while she photons. The photon–photon coupling could, Alice (Fig. 1b), where it passes through its waits for the second to arrive. Inventive in principle, be implemented with a nonlinear own analyser, a cleverly crafted arrangement schemes like these should help make optical medium2. But the nonlinear coupling of holographic plates, polarizing beam quantum communication a future reality.� between single photons is very weak, splitters and photodetectors. Bob encodes his rendering any such measurement device two-bit message by rotating the polarization References 1. Bennett, C. H. & Wiesner, S. J. Phys. Rev. Lett. 69, 2881–2884 (1992). tremendously inefficient. Linear optics by of the second photon using computer- 2. Kim, Y. H., Kulik S. P. & Shih, Y. Phys. Rev. Lett. 86, itself can produce only a partial measurement controlled liquid crystals, and then sends it 1370–1373 (2001). at best, allowing Alice to identify no more to Alice where it passes through an analyser 3. Barreiro, J. T., Wei, T.-C. & Kwiat, P. G. Nature Phys. 4, than three out of the four states4. Therefore, in similar to the first one. The analysers act on 282–286 (2008). 4. Mattle, K., Weinfurter, H., Kwiat, P. G. & Zeilinger, A. Phys. Rev.���� a linear-optics experiment, Bob can only send both the polarization and orbital angular Lett. 76, 4656–4659 (1996). log23 = 1.58 bits to Alice, which is not enough momentum of each photon. Based on her 5. Barreiro, J. T., Langford, N. K., Peters, N. A. & Kwiat, P. G. Phys. to relay the entire two-bit message. joint detection results for both photons, Rev. Lett. 95, 260501 (2005). This type of setback has led to the Alice can identify all of the four different Bell 6. Kwiat, P. G. & Weinfurter, H. Phys. Rev.���� A� 58, R2623–R2626 (1998). development of a number of ingenious states, which gives, ideally, a transmission 7. Molina-Terriza, G., Torres, J. P. & Torner, L. Nature Phys. 3, solutions, one of which is made possible by rate of two bits of information per photon. 305–310 (2007). ASTROPHYSICS Black is black In cosmology and astrophysics, it is the rotating star) and uncharged. Whether era of data. Measurements made using the Kerr metric does describe the various telescopes and satellites are external spacetime of an astrophysical revealing the Universe and its history in a black hole could soon be testable, using, degree of detail few could have anticipated; for example, data collected in searching the general theory of relativity that once for gravitational waves or in high- seemed untestable is now under scrutiny. energy observations. All tests of general relativity so far have However, Psaltis et al. show that found no flaws in Einstein’s logic. It is, at several types of extension to the general very least, a successful effective theory of theory — adding scalar, vector or tensor gravity at the scales that can be probed at degrees of freedom, such as might exist present. But what if it is only an effective in a theory of quantum gravity — still theory, and there exists some more result in the Kerr metric. The exciting fundamental theory beyond it — how experimental possibility of proving the might that be detected? Writing in Physical Kerr metric would, in fact, offer little scope Review Letters, Dimitrios Psaltis and for discrimination between approaches to colleagues consider what we might learn a fundamental theory. from those seemingly least informative The flipside, of course, is that should MEHAU KULYK / SCIENCE PHOTO LIBRARY PHOTO SCIENCE / KULYK MEHAU of astrophysical objects — black holes any deviation from the Kerr metric be (Phys. Rev. Lett. 100, 091101; 2008). metric corresponds to a black hole that detected, then there’s definitely something Of the four exact black-hole solutions is rotating (as may be expected from its else out there. to Einstein’s field equations, the Kerr formation following the collapse of a Alison Wright

nature physics | VOL 4 | APRIL 2008 | www.nature.com/naturephysics 269 © 2008 Nature Publishing Group