Experimental demonstration of a hyper-entangled ten-qubit Schrödinger cat state

Abstract

Coherent manipulation of a large number of qubits and the generation of entangled states between them has been an important goal and benchmark in quantum information science, leading to various applications such as measurement-based quantum computing 1 and high-precision quantum metrology 2 . However, the experimental preparation of multiparticle entanglement remains challenging. Using atoms 3,4 , entangled states of up to eight qubits have been created, and up to six photons 5 have been entangled. Here, by exploiting both the photons' polarization and momentum degrees of freedom, we experimentally generate hyper-entangled six-, eight-and ten-qubit Schrödinger cat states with verified genuine multi-qubit entanglement. We also demonstrate super-resolving phase measurements enhanced by entanglement, with a precision to beat the standard quantum limit. Modifications of the experimental set-up would enable the generation of other graph states up to ten qubits. Our method offers a way of expanding the effective Hilbert space and should provide a versatile test-bed for various quantum applications.Control of single photonic qubits using linear optics has been an appealing approach to implementing quantum computing 6 . Experiments in recent years have demonstrated the photons' extremely long decoherence time 7 , fast clock speed 8 , a series of controlled quantum logic gates 9,10 and algorithms 8,11,12 and the generation of multiqubit entangled states 5 . A significant challenge, however, lies in making experimentally accessible sources of photonic multi-qubit states. This is because, on the one hand, the probabilistic nature of spontaneous parametric down-conversion 13 represents a bottleneck with regard to the attainable brightness and fidelity of multiphoton states based on it: manipulating seven photons or more seems an insurmountable challenge with present technology. On the other hand, triggered single-photon sources from independent quantum dots or other emitters still suffer from spectral and temporal distinguishability, which prevents up-scaling.There is, however, a way to experimentally control more effectively qubits, by exploiting hyper-entanglement 14 -the simultaneous entanglement in the multiple degrees of freedom that naturally exist for various physical systems. For instance, one can encode quantum information not only in the polarization of a single photon, but also in its spatial modes 15 Although the largest hyper-entangled state 15 realized so far has expanded the Hilbert space up to 144 dimensions, it is a product state of two-party entangled states and does not involve multipartite entanglement. Other schemes 20,21 for creating hyperentanglement have been limited by the technical problem of photonic subwavelength phase stability and seem infeasible to generate larger states than the two-photon four-qubit ones. Here, we will describe a method that overcomes these limitations, and the experimental generation of hyper-entangled six-, eight-and ten-qubit photonic Schrödinger cat states.The Schröding...

Multipartite dense coding versus quantum correlation: Noise inverts relative capability of information transfer

“…This is due to the fact that the preparation of multiparticle states with quantum coherence enables us to realize several quantum information protocols like quantum dense coding [5], quantum teleportation [6], secure quantum cryptography [7], and one way quantum computation [8], in a way that is better than their classical counterparts. This increasing interest is further boosted by the latest advances in experiments to realize multipartite states in different physical systems including photons, ion traps, optical lattices, and nuclear magnetic resonances [9][10][11][12][13][14].…”

Experimental generation of an eight-photon Greenberger–Horne–Zeilinger state

“…In particular, polarization-entangled Einstein-Podolsky-Rosen (EPR) pairs are often used as the basic blocks for generating multi-photon entangled states or as resources for performing quantum teleportation tasks. A number of experiments involving two photon pairs [1][2][3][4][5][6][7][8] and a few experiments involving five photons (two photon pairs plus one single photon) 9 or three photon pairs [10][11][12][13] have been carried out to demonstrate various protocols in quantum communication and linear optical quantum computing. Typical systems for two-photon-pair experiments use a frequency-doubled Ti:sapphire mode-locked laser as a pump, with a repetition rate of ~80 MHz, a pulse duration of ~150 fs, and an operating wavelength of ~400 nm.…”

Experimental replication of single-qubit quantum phase gates

“…We utilize time-correlated photon pairs generated in the process of spontaneous parametric downconversion in a nonlinear crystal pumped by a laser diode. The two signal qubits are encoded into the spatial and polarization degrees of freedom of the signal photon [24][25][26][27], respectively, while the auxiliary qubit is represented by polarization state of the idler photon [13,27]. The spatial qubit is supported by an inherently stable Mach-Zehnder interferometer formed by two calcite beam-displacers BD which introduce a transversal spatial offset between vertically and horizontally polarized beam.…”