1. D-Wave Systems Inc.
D-Wave Systems Inc.
THE QUANTUM COMPUTING COMPANYTM
A.M. Zagoskin (D-Wave Systems and UBC)
A.Yu. Smirnov (D-Wave Systems)
M.H.S. Amin (D-Wave Systems)
Alec Maassen van den Brink (D-Wave Systems)
E. Il’ichev (IPHT)
A. Izmalkov (IPHT)
M. Grajčar (Comenius University)
Th. Wagner (IPHT)
H.-G. Meyer (IPHT)
Evidence for entangled states formation in a system of two coupled flux qubits
Quantum Condensed Matter Meeting, Vancouver, January 2004

2. D-Wave Systems Inc. 3JJ flux qubit
Orlando et al. (1999); van der Wal et al. (2000)

3. D-Wave Systems Inc.
3JJ flux qubit

4. D-Wave Systems Inc. Coupled 3JJ flux qubits
Majer et al. (2003) Izmalkov et al. (2003)

5. Current experimental status of two-qubit entanglement
D-Wave Systems Inc.
Current experimental status of two-qubit entanglement
NEC (!) - charge qubits
Pashkin et al., Nature 421, 823 (2003)
Yamamoto et al., Nature 425, 941 (2003)
Maryland - CBJJ qubits
Berkley et al., Science 300, 1548 (2003)
Delft (?) - 3JJ qubits
Majer et al., cond-mat/ (2003)

6. Impedance Measurement Technique
D-Wave Systems Inc.
Impedance Measurement Technique
Greenberg et al. (2002)

7. Impedance Measurement Technique
D-Wave Systems Inc.
Impedance Measurement Technique
Voltage-current angle in the tank
tan 

8. Coherent tunneling in a 3JJ qubit: IMT dip
D-Wave Systems Inc.
Coherent tunneling in a 3JJ qubit: IMT dip
Voltage-current angle in the tank: solid line is a fit for /h = 650 MHz 

9. 3JJ flux qubit coupled to a tank circuit
D-Wave Systems Inc.
3JJ flux qubit coupled to a tank circuit
Nb coil is prepared on oxidized Si substrates lithographically.
The line width of the coil windings was 2 m, with a 2 m spacing.
Various square-shape coils with between 20 and 150 m windings were designed.
External capacitance CT.

10. D-Wave Systems Inc. Rabi spectroscopy M L C h  T
Il’ichev et al. (2003)

11. The voltage spectral density at different HF amplitudes
D-Wave Systems Inc.
The voltage spectral density at different HF amplitudes
at f=868  2 MHz

12. Fit to the experimental data
D-Wave Systems Inc.
Fit to the experimental data

13. Two qubits inductively coupled to a resonance tank
D-Wave Systems Inc.
Two qubits inductively coupled to a resonance tank
Sq = 80 m2
Lq = 39 pH
Ic  400 nA
EC  3.2 GHz
Mab = 2.7 pH
CT = 470 pF
LT  130 nH
fT = MHz
QT = 1680 (at 10 mK)
Izmalkov et al., cond-mat/ (2003)

14. Two qubits inductively coupled to a resonance tank
D-Wave Systems Inc.
Two qubits inductively coupled to a resonance tank
Izmalkov et al. (2003)

15. Two qubits inductively coupled to a resonance tank
D-Wave Systems Inc.
Two qubits inductively coupled to a resonance tank
tan   -2 (QT/LT) (T)
Izmalkov et al. (2003)

16. Signature of entanglement
D-Wave Systems Inc.
Signature of entanglement
Izmalkov et al. (2003); Smirnov, cond-mat/ (2003)

17. D-Wave Systems Inc. IMT dips (experiment)
T = 10 (nominally), 50, 90, 160 mK
Izmalkov et al. (2003)

18. D-Wave Systems Inc. IMT dips (theory) IMT deficit T = 50, 90, 160 mK
Izmalkov et al. (2003)

19. Temperature dependence of IMT dips
D-Wave Systems Inc.
Temperature dependence of IMT dips
Squares: qubit a, triangles: qubit b,
circles: coincident
Izmalkov et al. (2003)

20. Experimentally determined parameters
D-Wave Systems Inc.
Experimentally determined parameters
a = 550 MHz
b = 450 MHz
Ia  Ib = Ip = 320 nA
J = 420 MHz
Izmalkov et al. (2003)

21. Measure of entanglement
D-Wave Systems Inc.
Measure of entanglement
Concurrence at the co-degeneracy point:
C1 = C4 = 0.39
C2 = C3 = 0.97
For the equilibrium density matrix at 10 mK
Ceq = 0.33
Izmalkov et al. (2003)

22. D-Wave Systems Inc.
Conclusions:
IMT approach allows to observe the signature of entanglement
in two coupled flux qubits
Quantitative agreement between the theory and experiment
confirms that the system is in an equilibrium mixture of entangled
two-qubit states
The quantitative measure of entanglement can be calculated
Only a lower limit on the appropriate coherence time can be
established
Rabi spectroscopy experiments on two qubits are under way