Bures metric explained

In mathematics, in the area of quantum information geometry, the Bures metric (named after Donald Bures)^[1] or Helstrom metric (named after Carl W. Helstrom)^[2] defines an infinitesimal distance between density matrix operators defining quantum states. It is a quantum generalization of the Fisher information metric, and is identical to the Fubini–Study metric^[3] when restricted to the pure states alone.

Definition

The Bures metric may be defined as

[D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) tr ( d \rho G ), where G is the Hermitian 1-form operator implicitly given by\rho G + G \rho = d \rho, which is a special case of a continuous Lyapunov equation.

Some of the applications of the Bures metric include that given a target error, it allows the calculation of the minimum number of measurements to distinguish two different states^[4] and the use of the volume element as a candidate for the Jeffreys prior probability density^[5] for mixed quantum states.

Bures distance

The Bures distance is the finite version of the infinitesimal square distance described above and is givenby

D _{B(\rho} _{1,\rho} ^{2} ^{2 )} = 2 ( 1- \sqrt{F(\rho _{1,\rho} _{2)}),} where the fidelity function is defined as^[6] F(\rho _{1,\rho} _{2)} = \left[ tr ( \sqrt{ \sqrt{\rho _{1}\rho} _{2\sqrt{\rho} ^{2 .} ^{1 }})\right]} Another associated function is the Bures arc also known as Bures angle, Bures length or quantum angle, defined asD _{A(\rho} _{1,\rho} _{2)} = \arccos \sqrt{F(\rho _{1,\rho} _{2)},} which is a measure of the statistical distance^[7] between quantum states.

Wootters distance

When both density operators are diagonal (so that they are just classical probability distributions), then let

\rho _{1} = diag(p _{1,} ...) and similarly \rho _{2} = diag(q _{1,} ...) , then the fidelity is$$\backslash sqrt\; =\; \backslash sum\_i\; \backslash sqrt$$with the Bures length becoming the Wootters distance \arccos \left(\sum _{i} \sqrt{p _{i} q _{i}\right)} . The Wootters distance is the geodesic distance between the probability distributions p, q under the chi-squared metric ds ^{2} = (12)/(\sum) _{i} (dp ^{2} _{i})/(p _{i}) .^[8]

Perform a change of variables with

x _{i} : = \sqrt{p _{i}} , then the chi-squared metric becomes ds ^{2} = \sum _{i} dx ^{2} _{i} . Since \sum _{i} x ^{2} _{i} = \sum _{i} p _{i} = 1 , the points x are restricted to move on the positive quadrant of a unit hypersphere. So, the geodesics are just the great circles on the hypersphere, and we also obtain the Wootters distance formula.

If both density operators are pure states,

\psi, \phi , then the fidelity is \sqrt{F} = |\langle \psi | \phi \rangle | , and we obtain the quantum version of Wootters distance\arccos (|\langle \psi | \phi \rangle |) .^[9]

In particular, the direct Bures distance between any two orthogonal states is

\sqrt 2 , while the Bures distance summed along the geodesic path connecting them is \pi/ 2 .

Quantum Fisher information

The Bures metric can be seen as the quantum equivalent of the Fisher information metric and can be rewritten in terms of the variation of coordinate parameters as

[D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) tr \left( (d \rho)/(d \theta ^{\mu}) L _{\nu} \right) d \theta ^{\mu} d\theta ^{\nu} , which holds as long as \rho and \rho + d\rho have the same rank. In cases where they do not have the same rank, there is an additional term on the right hand side.^{[10] [11]} L _{\mu} is the Symmetric logarithmic derivative operator (SLD) defined from^[12] (\rho L _{\mu} + L _{\mu} \rho)/(2) = (d \rho)/(d \theta ^{\mu}) .

In this way, one has

[D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) tr \left[ \rho (L _{\mu} L _{\nu} + L _{\nu} L _{\mu})/(2) \right] d \theta ^{\mu} d\theta ^{\nu} , where the quantum Fisher metric (tensor components) is identified asJ _{\mu} = tr \left[ \rho (L _{\mu} L _{\nu} + L _{\nu} L _{\mu})/(2) \right].

The definition of the SLD implies that the quantum Fisher metric is 4 times the Bures metric. In other words, given that

g _{\mu\nu} are components of the Bures metric tensor, one hasJ _{\mu\nu} = 4 g _{\mu} .

As it happens with the classical Fisher information metric, the quantum Fisher metric can be used to find the Cramér–Rao bound of the covariance.

Explicit formulas

The actual computation of the Bures metric is not evident from the definition, so, some formulas were developed for that purpose. For 2x2 and 3x3 systems, respectively, the quadratic form of the Bures metric is calculated as^[13]

[D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(4) tr \left[ d \rho d \rho + (1)/(\det(\rho)) ( 1 - \rho)d\rho ( 1 - \rho)d\rho \right], [D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(4) tr \left[ d \rho d \rho + (3)/(1- tr \rho ^{3}) ( 1 - \rho)d\rho ( 1 - \rho)d\rho + (3 \det{\rho)/(}{ 1- tr) \rho ^{3}} ( 1 - \rho ^{-1} )d\rho ( 1 - \rho ^{-1} )d\rho \right].

For general systems, the Bures metric can be written in terms of the eigenvectors and eigenvalues of the density matrix

\rho = \sum ^{n λ} _{j|j\rangle\langle} j| as^{[14] [15]} [D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) \sum ^{n} _{j,k =1} (|\langle j| d\rho | k\rangle | ^{2})/(λ _{j+λ} _{k}) , as an integral,^[16] [D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) \int ^{inf ty tr [e} _{0} ^{-\rho} d\rho e ^{-\rho} d\rho] dt, or in terms of Kronecker product and vectorization,^[17] [D _{B(\rho,} \rho + d\rho)] ^{2} = (1)/(2) vec [d\rho] ^{\dagger(\rho} ^{* ⊗} 1 + 1 ⊗ \rho ) ^{-1} vec [d\rho], where ^{*} denotes complex conjugate, and ^{\dagger} denotes conjugate transpose. This formula holds for invertible density matrices. For non-invertible density matrices, the inverse above is substituted by the Moore-Penrose pseudoinverse. Alternatively, the expression can be also computed by performing a limit on a certain mixed and thus invertible state.

Two-level system

The state of a two-level system can be parametrized with three variables as

\rho = (1)/(2) ( I + \boldsymbol{r ⋅ \sigma} ), where \boldsymbol{\sigma} is the vector of Pauli matrices and \boldsymbol{r} is the (three-dimensional) Bloch vector satisfying r ^{2\stackrel{def} } \boldsymbol \le 1 . The components of the Bures metric in this parametrization can be calculated asg = (I)/(+ 4) (\boldsymbol{r ⊗ r)/() }.The Bures measure can be calculated by taking the square root of the determinant to finddV _{B} = (d ^{3\boldsymbol{r})/() },which can be used to calculate the Bures volume asV _{B} = \iiint _{r ^{2\leq} 1} (d ^{3\boldsymbol{r})/() } = \frac .

Three-level system

The state of a three-level system can be parametrized with eight variables as

\rho = (1)/(3) ( I + \sqrt{ 3 } \sum ^{8 \xi} _{\nu λ} _{\nu),} where λ _{\nu} are the eight Gell-Mann matrices and \boldsymbol \xi \in R ^{8} the 8-dimensional Bloch vector satisfying certain constraints.

See also

• Fubini–Study metric
• Fidelity of quantum states
• Fisher information
• Fisher information metric

Further reading

• Book: Uhlmann, A. . The Metric of Bures and the Geometric Phase . Groups and Related Topics . Proceedings of the First Max Born Symposium . Gielerak . R. . Lukierski . J. . Popowicz . Z. . 267–274 . 1992 . 94-010-5244-1 . 10.1007/978-94-011-2801-8_23 .
• Sommers . H. J. . Zyczkowski . K. . Bures volume of the set of mixed quantum states . Journal of Physics A . 36 . 39. 10083–10100 . 2003 . 10.1088/0305-4470/36/39/308 . quant-ph/0304041 . 2003JPhA...3610083S . 39943897 .
• Dittmann . J. . On the Riemannian Geometry of Finite Dimensional Mixed States . Seminar Sophus Lie . 73 . 1993 .
• Paul B. . Slater . Quantum Fisher-Bures information of two-level systems and a three-level extension . J. Phys. A: Math. Gen. . 29 . 10 . 1996 . L271–L275 . 10.1088/0305-4470/29/10/008 .
• Book: M. A. . Nielsen . I. L. . Chuang . Quantum Computation and Quantum Information . Cambridge University Press . 2000 . 0-521-63235-8 .

Notes and References

1. Bures . Donald . An extension of Kakutani's theorem on infinite product measures to the tensor product of semifinite \omega *-algebras . Transactions of the American Mathematical Society . American Mathematical Society (AMS) . 135 . 1969 . 0002-9947 . 10.1090/s0002-9947-1969-0236719-2 . 199. free.
2. Helstrom . C.W. . Minimum mean-squared error of estimates in quantum statistics . Physics Letters A . Elsevier BV . 25 . 2 . 1967 . 0375-9601 . 10.1016/0375-9601(67)90366-0 . 101–102. 1967PhLA...25..101H .
3. Facchi . Paolo . Kulkarni . Ravi . Man'ko . V.I. . Marmo . Giuseppe . Sudarshan . E.C.G. . Ventriglia . Franco . Classical and quantum Fisher information in the geometrical formulation of quantum mechanics . Physics Letters A . 374 . 48 . 2010 . 0375-9601 . 10.1016/j.physleta.2010.10.005 . 4801–4803. 1009.5219. 2010PhLA..374.4801F . 55558124 .
4. Braunstein . Samuel L. . Caves . Carlton M. . Statistical distance and the geometry of quantum states . Physical Review Letters . American Physical Society (APS) . 72 . 22 . 1994-05-30 . 0031-9007 . 10.1103/physrevlett.72.3439 . 3439–3443. 10056200 . 1994PhRvL..72.3439B .
5. Slater . Paul B. . Applications of quantum and classical Fisher information to two‐level complex and quaternionic and three‐level complex systems . Journal of Mathematical Physics . AIP Publishing . 37 . 6 . 1996 . 0022-2488 . 10.1063/1.531528 . 2682–2693. 1996JMP....37.2682S .
6. Unfortunately, some authors use a different definition, F(\rho _{1,\rho} _{2)} = tr ( \sqrt{ \sqrt{\rho _{1}\rho} _{2\sqrt{\rho} _{1}})}
7. Wootters . W. K. . Statistical distance and Hilbert space . Physical Review D . American Physical Society (APS) . 23 . 2 . 1981-01-15 . 0556-2821 . 10.1103/physrevd.23.357 . 357–362. 1981PhRvD..23..357W .
8. Braunstein . Samuel L. . Caves . Carlton M. . 1994-05-30 . Statistical distance and the geometry of quantum states . Physical Review Letters . 72 . 22 . 3439–3443 . 10.1103/PhysRevLett.72.3439. 10056200 . 1994PhRvL..72.3439B .
9. Deffner . Sebastian . Campbell . Steve . 2017-11-10 . Quantum speed limits: from Heisenberg's uncertainty principle to optimal quantum control . Journal of Physics A: Mathematical and Theoretical . 50 . 45 . 453001 . 10.1088/1751-8121/aa86c6 . 1705.08023 . 2017JPhA...50S3001D . 11603/19391 . 3477317 . 1751-8113.
10. Šafránek . Dominik . Discontinuities of the quantum Fisher information and the Bures metric . Physical Review A . 95 . 5 . 2017-05-11 . 2469-9926 . 10.1103/physreva.95.052320 . 052320. 1612.04581 . 2017PhRvA..95e2320S .
11. Rezakhani. A. T.. Hassani. M.. Alipour. S.. 2019-09-12. Continuity of the quantum Fisher information. Physical Review A. 100. 3. 032317. 1507.01736. 10.1103/PhysRevA.100.032317. 2019PhRvA.100c2317R . 51680508 .
12. Paris . Matteo G. A. . Quantum estimation for quantum technology . International Journal of Quantum Information . 07 . supp01 . 2009 . 0219-7499 . 10.1142/s0219749909004839 . 125–137. 0804.2981 . 2365312 .
13. Dittmann . J . Explicit formulae for the Bures metric . Journal of Physics A: Mathematical and General . 32 . 14 . 1999-01-01 . 0305-4470 . 10.1088/0305-4470/32/14/007 . 2663–2670. quant-ph/9808044 . 1999JPhA...32.2663D . 18298901 .
14. Hübner . Matthias . Explicit computation of the Bures distance for density matrices . Physics Letters A . Elsevier BV . 163 . 4 . 1992 . 0375-9601 . 10.1016/0375-9601(92)91004-b . 239–242. 1992PhLA..163..239H .
15. Hübner . Matthias . Computation of Uhlmann's parallel transport for density matrices and the Bures metric on three-dimensional Hilbert space . Physics Letters A . Elsevier BV . 179 . 4–5 . 1993 . 0375-9601 . 10.1016/0375-9601(93)90668-p . 226–230. 1993PhLA..179..226H .
16. PARIS . MATTEO G. A. . Quantum estimation for quantum technology . International Journal of Quantum Information . 07 . supp01 . 2009 . 0219-7499 . 10.1142/s0219749909004839 . 125–137. 0804.2981 . 2365312 .
17. Šafránek . Dominik . Simple expression for the quantum Fisher information matrix . Physical Review A . 97 . 4 . 2018-04-12 . 2469-9926 . 10.1103/physreva.97.042322 . 042322. 1801.00945 . 2018PhRvA..97d2322S .