Experimental Realization of Multipartite Entanglement of 60 Modes of a Quantum Optical Frequency Comb

We report the experimental realization and characterization of one 60-mode copy and of two 30-mode copies of a dual-rail quantum-wire cluster state in the quantum optical frequency comb of a bimodally pumped optical parametric oscillator. This is the largest entangled system ever created whose subsystems are all available simultaneously. The entanglement proceeds from the coherent concatenation of a multitude of Einstein, Podolsky, and Rosen pairs by a single beam splitter, a procedure which is also a building block for the realization of hypercubic-lattice cluster states for universal quantum computing.

Figure 1

Generation of a dual-rail quantum wire in the QOFC. (a) EPR pairs created by $zzz$ and $yyy$ interactions in the QOFC of a polarization-degenerate OPO (at each frequency $n$ the $z$ and $y$ modes are denoted by the double black lines). The vertical arrows mark the half-frequencies of the pumps; the curved arrows denote the $zzz$ (bottom) and $yyy$ (top) EPR pairs. (b) Quantum graph states [30]: The initial EPR pairs from the OPO (top) turn, after a single beam splitter (gray ellipses), into a dual-rail CV cluster state (bottom), whose $\pm 1/2$-weight edges are color coded (contrary to the qubit case, weighted cluster CV states are still stabilizer states [30, 31]).

Figure 2

Visualization of the measured nullifiers of Eqs. (2, 3, 4, 5) [blue (left) and red (right) boxes] on the dual-rail graph state of Fig. 1. As shown in the text, simultaneous squeezing of $Q_{-2,3}(r_z)$ and $Q_{-2,1}(r_y)$ is equivalent to squeezing of the canonical nullifiers of Eqs. (6) and (7).

Figure 3

Generation of two dual-rail quantum wires. The only difference with Fig. 1 is that the pump frequency difference is $4\mathrm{\Delta }\omega$ instead of $2\mathrm{\Delta }\omega$. The frequency sequences of the wires are totally distinct: $(\dots ,-8,7,-4,3,0,-1,4,-5,8,\dots )$ for the orange wire and $(\dots ,-7,6,-3,2,1,-2,5,-6,9,\dots )$ for the purple wire.

Figure 4

Experimental setup. PLL: phaselock loop; HWP: half wave plate; PZT: piezoelectric transducer; PBS: polarizing beam splitter; SA: spectrum analyzer; AOM: acousto-optic modulator; EOM: electro-optic modulator; PDH: Pound-Drever-Hall lock loop.

Figure 5

Zero-span spectrum analyzer traces of raw squeezing measurements for $m=1$ and $m=2$ quantum wires. For each case, the QOFC is at the top, with the pump half-frequencies denoted by the blue (left) and red (right) arrows, and the quantum graph is beneath it. The highlighted modes indicate the LO sidebands. The black traces indicate the vacuum noise level. Center frequency: 1.25 MHz. Resolution bandwidth: 30 kHz. Video bandwidth: 30 Hz.